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DEVELOPMENT OF SOME NOVEL LIGNIN DERIVATIVES FOR
ADSORPTIVE REMOVAL OF HEAVY METALS AND RECOVERY
OF PRECIOUS METALS
Durga Parajuli
A thesis submitted in partial fulfillment of the
requirements for the degree of
Doctor of Engineering
Department of Energy and Materials Science
Graduate School of Science and Engineering
SAGA UNIVERSITY
2006, September
ABSTRACT
DEVELOPMENT OF SOME NOVEL LIGNIN DERIVATIVES FOR ADSORPTIVE
REMOVAL OF HEAVY METALS AND RECOVERY OF PRECIOUS METALS
by Durga Parajuli
Chairperson of the Supervisory Committee: Professor Katsutoshi INOUE
Department of Energy and Materials Science, Saga University
Lignophenol was prepared from waste wood powder by a phase separation method
which involved the immobilization of lignin with phenol followed by separation of cellulose
matrix by treating with sulphuric acid solution. Different polyphenolic adsorption gels
namely crosslinked lignophenol, crosslineked lignocatechol and crosslinked lignopyrogallol
were prepared by immobilizing phenol, catechol and pyrogallol onto lignin and subsequent
crosslinking with paraformaldehyde. The study of adsorption behavior of the gels for some
precious and base metals revealed that they were highly selective for Au(III) with
competitive adsorption capacity. Crosslinked lignophenol exhibited outstanding selectivity
for Au(III) since it was almost inert towards other metals. Furthermore, it reduced Au(III) to
Au(I) and finally to elemental gold making it easier for recovery. On the other hand,
crosslinked lignocatechol exhibited high selectivity for various heavy metals with maximum
capacity for Pb(II). During column experiment also it exhibited efficiency, recyclability and
stability. In addition, the crosslinked lignophenol gel was further functionalized with
ethylenediamine and primary amine groups to get EN–lignin and PA–lignin, respectively.
They were found be selective for Au(III), Pd(II) and Pt(IV) possessing very high adsorption
capacity for Au(III) and almost no adsorption for other metal ions like Cu(II), Sn(IV),
Fe(III), Ni(II), Zn(II) and so on that are commonly occur together with precious metals.
Also, EN–lignin was found to have adsorption ability for some heavy metal oxyions like
vanadates, tungstate and molybdate. In all cases, the gels were found to be superior to
activated carbon in selectivity in effective separation and recovery of precious metals. While
using un-crosslinked lignophenol for adsorption of Au(III), beautiful gold microplates of
hexagonal or triangular symmetry were observed. Thus, by the present study, novel uses of
wasted wood material have been found in the form of low cost, environment friendly,
biodegradable and convenient lignin based adsorption gels which can be effectively applied
for separation and recovery of precious metals and removal of heavy metals from industrial
effluents.
DEVELOPMENT OF SOME NOVEL LIGNIN DERIVATIVES FOR ADSORPTIVE
REMOVAL OF HEAVY METALS AND RECOVERY OF PRECIOUS METALS
by
Durga Parajuli
A thesis submitted in partial fulfillment of the requirements for the degree of
Doctor of Engineering
Faculty of Science and Engineering
Department of Energy and Materials Science
SAGA UNIVERSITY
Approved by __________________________________________________
Prof. Katsutoshi INOUE
Supervisor
__________________________________________________
Prof. Tohru MIYAJIMA
__________________________________________________
Prof. Takanori WATARI
__________________________________________________
Assoc. Prof. Keisuke OHTO
Program Authorized
to Offer Degree _________Doctor of Engineering _____________
Date: 2006-06-22
TABLE OF CONTENTS
List of Figures List of Tables List of Schemes Acknowledgement
iv v v
vi
1. Research Background 1.1 Lignin and Lignophenol
1.1.1. Lignin 1.1.2. Lignophenol
1.2 E-waste 1.2.1. Introduction 1.2.2. What are e-wastes 1.2.3. Composition of WEEE 1.2.4. Problems caused by e-waste
1.3 Precious Metals 1.3.1. Introduction 1.3.2. Application of precious metals 1.3.3. Chloride chemistry of gold, palladium and platinum 1.3.4. Precious metal recovery: brief literature review
1.4 Heavy Metals 1.4.1. Impacts of heavy metals in human health 1.4.2. Methods of heavy metal removal: adsorption & ion exchange
1.5 Objective of the Present study References
1-241-7
7-10
10-16
16-19
19 19-24
2. Adsorption of Heavy Metals on Crosslinked Lignocatechol 2.1 Introduction 2.2 Experimental
2.2.1. Preparation of crosslinked lignocatechol 2.2.2. Surface analysis 2.2.3. Solution preparation 2.2.4. Batch wise adsorption experiments 2.2.5. Chromatographic separation of metal ion pair 2.2.6. Monitoring the adsorption–elution cycles
2.3 Results and Discussion 2.3.1. Effect of shaking time 2.3.2. Effect of pH on the adsorption behavior of crosslinked
lignocatechol
25-3925-27 27-31
31-38
ii
2.3.3. Effect of initial concentration of metal ions: adsorption isotherm 2.3.4. Chromatographic separation of metal ion pair 2.3.5. Monitoring the adsorption–elution cycles 2.3.6. Mechanism of adsorption
2.4 Conclusion References
38 38-39
3. Selective Recovery of Gold by Lignin Based Novel Adsorption Gels 3.1 Introduction 3.2 Experimental
3.2.1. Reagent 3.2.2. Adsorption gels 3.2.3. Preparation of crosslinked lignophenol 3.2.4. Solid state analysis 3.2.5. Batch wise adsorption tests 3.2.6. Column experiment
3.3 Results and Discussion 3.3.1. Surface analysis 3.3.2. Effect of shaking time 3.3.3. Effect of HCl concentration on the adsorption of some metal ions 3.3.4. Adsorption isotherms of gold 3.3.5. Solid state analysis of gels after adsorption 3.3.6. chromatographic separation of Au(III) away from Pd(II) and
Pt(IV) 3.4 Conclusion
References
40-6040-42 42-45
46-58
59 59-60
4. Recovery of Gold(III), Palladium(II) And Platinum(IV) by Aminated Lignin Derivatives
4.1 Introduction 4.2 Experimental
4.2.1. Preparation of crosslinked lignophenol 4.2.2. Preparation of aminated crosslinked lignophenol 4.2.3. Identification of aminated lignin gels 4.2.4. Surface Analysis 4.2.5. Metal species for adsorption tests 4.2.6. Batch wise adsorption tests 4.2.7. Breakthrough followed by elution experiments
4.3 Results and Discussion 4.3.1. Protonation capacity of EN–lignin and PA–lignin 4.3.2. Kinetics of adsorption of Au(III), Pd(II), and Pt(IV)
61–79
61-62 63-67
67-77
iii
4.3.3. Batch wise adsorption tests 4.3.4. Adsorption isotherms of Au(III), Pd(II), and Pt(IV) 4.3.5. Breakthrough followed by elution tests for Pd(II).
4.4 Conclusion References
78 78-79
5. Adsorption of Some Heavy Metal Oxides on Ethylenediamine Functionalized Crosslinked Lignophenol Gel
5.1 Introduction 5.2 Methods and Materials 5.3 Results and Discussion 5.4 Conclusion
References
80-84
80-81 81
81-83 83 84
6. Prospective Application of Lignophenol for the Fabrication of Gold Single Crystals
6.1. Introduction 6.2. Process and observations
6.2.1. Preparation of lignophenol powder 6.2.2. Reduction of Au(III)
References
85-87
85 85-87
87
7. Adsorption of Antimony (Sb) by Crosslinked Lignophenol Gels 7.1. Introduction 7.2. Experimental 7.3. Results and Discussion References
88-9288-89
90 90-91
92
Conclusion
List Of Publications
Contributions To Conferences
Résumé
93-9596
97
98
iv
LIST OF FIGURES
Figure Number Page Number Figure Number Page Number
1.1 3
1.2 3
1.3 4
1.4 5
1.5 9
1.6 13
1.7 16
2.1 29
2.2 30
2.3 32
2.4 32
2.5 34
2.6 35
2.7 36
3.1 42
3.2 46
3.3 47
3.4 48
3.5 49
3.6 49
3.7 51
3.8 51
3.9 52
3.10 53
3.11 54
3.12 55
3.13 55
3.14 56
3.15 58
4.1 65
4.2 68
4.3 69
4.4 70
4.5 71
4.6 72
4.7 74
4.8 75
4.9 76
4.10 77
5.1 82
6.1 86
6.2 87
7.1 90
7.2 91
v
LIST OF TABLES
Table Number Page Number 1.1
1.2
4.1
11
17
65
LIST OF SCHEMES
Scheme Number Page Number
1.1
2.1
2.2
3.1
4.1
4.2
4.3
4.4
6
28
37
43
63
64
72
75
vi
ACKNOWLEDGEMENT
Thank you very much.
Durga Parajuli Adhikari
Saga,
September, 2006
1
Chapter 1
RESEARCH BACKGROUND
1.1. LIGNIN AND LIGNOPHENOL
Wood as a primary source of renewable biomaterials forms the basis of chemical
energy flow on earth. Present chemical interest in the wood science consists, for example,
elucidation of the biochemistry of wood formation, studies exploiting the wood as a pool
of chiral auxiliarity and suppressing the environmental concerns connected to technical
wood biomass applications by means of "green chemistry" approaches.
1.1.1. Lignin. Lignin is a large macromolecule with molecular mass in excess of 10,000
atomic mass unit. It is hydrophobic and aromatic in nature. The molecule consists of
various types of substructures which repeat in random manner. It is most commonly
derived from wood and is an integral part of the cell walls of plants, especially in tracheids,
xylem fibres and sclereids. It is the second most abundant organic compound on earth after
cellulose. Lignin makes up about one-quarter to one-third of the dry mass of wood. It fills
the spaces in the cell wall between cellulose, hemicellulose and pectin components and
confers mechanical strength to the cell wall and therefore the entire plant. It is particularly
abundant in compression wood, but curiously scarce in tension wood. Lignin plays a
crucial part in conducting water in plant stems. The polysaccharide components of plant
cell walls are highly hydrophilic and thus permeable to water. Lignin makes it possible to
form vessels which conduct water efficiently. Lignin resists attack by most
microorganisms, and anaerobic processes tend not to attack the aromatic rings at all.
2
Aerobic breakdown of lignin is slow and may take many days. Therefore it is an efficient
physical barrier against pathogens which would invade plant tissues. For example an
infection by a fungus causes the plant to deposit more lignin near the infection site. Lignin,
along with hemicellulose is nature's cement which exploits the strength of cellulose and
confers flexibility. Highly lignified wood is durable and therefore a good raw material for
many applications. It is also an excellent fuel, since lignin yields more energy than
cellulose when burned. However, lignin is detrimental to paper manufacturing process and
must be removed from pulp before making paper. In the sulfite and sulfate (also called
kraft) chemical pulping processes, lignin is removed from wood pulp as sulphates. These
materials have several uses like dispersants in high performance cement applications, water
treatment formulations and textile dyes; additives in specialty oil field applications and
agricultural chemicals; raw materials for several chemicals, such as vanillin, DMSO,
ethanol, torula yeast, xylitol sugar and humic acid and environmental friendly dust
supression agent for roads.
Type of tissues, cell wall layers and the stages and conditions of development
determine the total content of lignin in any plant body, even within the same plant species.
On this basis lignin is classified as soft wood lignin and hard wood lignin. In general, hard
woods are rich in lignin followed by soft woods and the cryptogams virtually lack lignin.
The lignification process is recognized as a terminal and an in situ, nonreversible step,
which determines both the ultrastructural organization and the properties of not only
woods but also other lignocellulosic plant components. [1-4]
Structure and biosynthesis of lignin. Lignin biosynthesis begins with the synthesis of
monolignols. The starting material is the amino acid phenylalanine. The first reactions in
the biosynthesis are shared with the phenylpropanoid pathway, and monolignols are
considered to be a part of this group of compounds. There are three types of monolignols:
coniferyl alcohol, sinapyl alcohol and paracoumaryl alcohol, as shown in Figure 1.1.
Different plants use different monolignols. For example, spruce lignin is almost entirely
coniferyl alcohol while paracoumaryl alcohol is found almost exclusively in grasses.
Monolignols are synthetised in the cytosol as glucosides. The glucose is added to the
monolignol to make them water soluble and to reduce their toxicity. The glucosides are
3
transported through the cell membrane to the apoplast. The glucose is then removed and
the monolignols are polymerised into lignin.
OH
OCH3
OH
OH
OCH3
OH
H3CO
OH
OH
21
34
5
6
αβ
γ
p-coumaryl alcohol Coniferyl alcohol Sinapyl alcohol
Figure 1.1. Structures of the three commonly occurring monolignols
OH
OCH3
HO
OH
OCH3
O
OH
OCH3
O
Lignin
1/2 H2O
1/4 O2
Oxidase
1/2 H2O2
1/2 H2O
Peroxidase
Figure 1.2. Routes of polymerisation of coniferyl alcohol to lignin.
4
The polymerisation step is catalysed by oxidative enzymes, oxidase or peroxidase
and hence the reaction has two alternative routes catalysed by these different oxidative
enzymes, as shown in Figure 1.2. Both peroxidase and laccase enzymes are present in the
plant cell walls, and it is not known whether one or both of these groups participates in the
polymerisation. The oxidative enzyme catalyses the formation of monolignol radicals.
These radicals then undergo chemical coupling to form the lignin polymer. The details of
this final step are being debated, since it is not known how the abundance of various
possible bond types between monolignols in controlled. Some theories favor pure chemical
coupling, while other state that dirigent proteins control this step. An early stage in the
condensation of various monomers to form lignin is shown in Figure 1.3.[1-4]
CHCHCHO
OO
CH2C
OH
CHOH
CH2OH
OOHC
CH2OH
O
CHOH
OCHOH
CCH2OH
O
OHOHC
C O
OHH3C
H3C
A
Figure 1.3. Early stage of polymerization of monolignols to lignin.
There are several groups shown in bold that can react further. Some will simply
extend the polymer while others would establish cross linking. The monomer ‘A’ has three
of its functional groups linked to other monomers, so it is starting a branch or cross link.
The large lignin molecules possess heavily cross linked three dimensional structures as
shown in Figure 1.4. Sometimes lignin is isolated as a brown powder, but more often it is a
gummy mixture of lignins with a wide range of molecular weights.
5
Figure 1.4. Representative chemical structure of lignin.
1.1.2. Lignophenol. As mentioned earlier, lignin is an amorphous, aromatic network
polymer, second to cellulose in natural abundance. The biosphere is estimated to contain
3x1011 tons of lignin, with an annual biosynthesis rate of about 2x1010 tons.5 Although
lignin is one of the most durable biopolymers, unlike synthetic polymers, it is perfectly
biodegraded in nature, and there have been no reports of the accumulation of lignin in the
ecological system. Because of this unique function which synthetic polymers do not have,
together with its abundance as available biomass, there has been great interest in its
degradation and potential application in the manufacture of various chemicals and other
products. However, in contrast to the importance and potential of lignin in nature, lignin-
based products have little been used by man. This strange situation of non-recognition of
lignin is because lignin molecules lack stereoregularity and the repeating units in its
molecule are heterogeneous and complex. In addition, non-selective modifications during
isolation from the cell wall make lignin molecules much more heterogeneous.
Native lignins have only 0.1 – 0.2 mol/C9 of hydroxyl groups, although lignin has
been classified as a phenolic polymer.6,7 This low frequency is not enough to make lignin
O C
H C
H 2C H O
O H
O H 3C
H O H O
O
O H 3C H O
OH
OH3 C
H O O H 3C
O
HOHO
O CH3
HO
O
OH3C
O
OO
CH3
O
OH
OH
OCH3
O O
HOHO
OH
O CH3
H3C
HO O
O
O
CH3
OO
OCH3
OH
OH
OHHO
OH3C
O
O CH 3
O O H
OHO
HO
OO
O
H3C
O H O H
OCH CH
H2CHO
OH3C
O
O H O H
H O O C
HC
H O
H 2C O H
6
act as a phenolic material. Almost all functional groups of lignin building units Cα are
reactive towards phenol derivatives.8 Selective phenol grafting at Cα positions leads to
highly improved phenolic functionality and decreased heterogeneity in structure and
reactivity. It is difficult to immobilize any functional group without loosing its original
characteristics for the preparation of new functional polymeric materials by means of
chemical modifications onto lignin. The limited use of lignin compared to cellulose is
attributable to its molecular structure which lacks stereo-regularity and possesses
heterogeneous repeating units. In addition, lignin in wood forms interpenetrating polymer
units with cellulose.9 Hence, with conventional methods, strong reagents are necessary to
separate hydrophobic lignin from hydrophilic cellulose, which results in the condensation
of lignin on itself giving complicated molecular networks.10,11
Scheme 1.1. Preparation of lignophenol by Phase Separation Method.
Funaoka et al. of Mie University, Japan, have developed a novel separation system
to recover lignin compounds from waste wood as a new type of phenolic lignin-based
linear polymer (lignophenol derivatives) based on sophisticated molecular design away
from polysaccharides such as cellulose and hemicellulose.11-13 This process includes a
phase-separation reaction system that consists of organic phase of phenol derivatives and
+ Phenol
+H2SO4
Hydrolysed Cellulose
Lignophenol Standby
Cellulose & Hemicellulose
Lignin
Wood
7
aqueous phase of concentrated acid. During the reaction, polysaccharides are
depolymerized into water-soluble sugar compounds that can be dissolved in aqueous phase
completely which can be easily separated from lignophenol derivatives formed in the
organic phase, as shown in Scheme 1. The lignophenol derivatives are characterized by
their unique functions of retention of the original inter unit linkages unlike conventional
lignin products.
Most of the researches related to lignin deal with the kraft lignin or black liquor of
paper industry. Many other types of lignin were also designated as having dark black
appearance. This is solely due to the self condensation of various functional groups of
lignin macromolecule during separation from lignocellulosic matrix. Hence, identical
performance can not be expected in the metal ion recovery by lignin and lignophenol
because of the original structure maintained by the later. Zuman, P. et al. have extensively
studied the sorption of various compounds like bile salts, nitosamines and pesticides as
well as some metal ions by lignin.14-21 But, application of lignophenol in environmental
remediation and metal recovery has not been yet been reported.
1.2. E-WASTE
The success of the electrical and electronic industries over the last decade in
developing a mass consumer market for cell phones, computers, and other personal
equipments has been phenomenal. Attention must be given to develop methods for safely
and economically recovering the materials that are embedded in these products so as to
challenge the towering heaps of electrical and electronic wastes: e-waste.
1.2.1. Introduction. The use of electronic devices has proliferated in recent decades, and
proportionately, the quantity of electronic devices, such as PCs, mobile telephones and
entertainment electronics that are disposed of, is growing rapidly throughout the world.
The high pace of technological changes and competitive market strategies that encourage
8
people to buy the latest models before their old appliances stop functioning has caused an
alarming increase in electronic and electrical wastes. In 1994, it was estimated that
approximately 20 million PCs (about 7 million tons) became obsolete. By 2004, this figure
was to increase to over 100 million PCs.22 Cumulatively, about 500 million PCs reached
the end of their service lives between 1994 and 2003. 500 million PCs contain
approximately 2,872,000 t of plastics, 718,000 t of lead, 1363 t of cadmium and 287 t of
mercury.23 This fast growing waste stream is accelerating because the global market for
PCs is far from saturation and the average lifespan of a PC is decreasing rapidly.24 PCs,
however, comprise only a fraction of all e-waste. Every year hundreds of millions of
mobile phones are also expected to retire. Similar quantities of electronic waste are
expected for all kinds of portable electronic devices such as PDAs, MP3 players, computer
games and peripherals.25
1.2.2. What are e-wastes? Electronic waste or e-waste also called city mine is a generic
term embracing various forms of electric and electronic equipment that have ceased to be
of any value to their owners. Waste electrical and electrical equipment is abbreviated as
WEEE. According to the EU WEEE Directive, electrical or electronic equipment which is
termed as waste includes all components, sub-assemblies and consumables, which are part
of the product at the time of discarding.26 According to the Basel Action Network, e-waste
encompasses a broad and growing range of electronic devices ranging from large
household devices such as refrigerators, air conditioners, cell phones, personal stereos, and
consumer electronics to computers which have been discarded by their users.23
1.2.3. Composition of WEEE. Waste Electrical and Electronic Equipment, WEEE,
comprises large household appliances, small household appliances, IT and
telecommunications equipment, consumer equipment, lighting equipment, electrical and
electronic tools, toys and sports equipment, medical devices, monitoring and control
instruments, automatic dispensers, and so on.
From Figure 1.5 it is clear that more than half of these wastes consist of metals that
include a significant proportion of valuable metals or their compounds, which indicates not
only the loss of huge amount of resources but also the threat of environmental pollution.
Along with other useful materials, precious metals, which are mainly used in making
9
printed circuit boards (PCB) that furnishes approximately 1.71% of total weight of WEEE,
are also being wasted out. In a rough estimate, the percentage composition of different
metals by weight in a mobile phone, as for example, is as follows: copper – 15%, iron –
3%, zinc – 1%, and less than 1% of a number of metals like tin, palladium, and gold.28
Although the portion of precious metals is very low compared to the other metals in one
set of a device, the amount disposed in this form is much higher than the content in ore
itself.29 Another study showed that the content of precious metals in one tone of mobile
phone scrap is: gold-1430 g, silver-5700 g, and palladium 430 g because of which
discarded mobile phones are called ‘El Dorado in the city’.30 For a sustainable society and
strong economy, it becomes necessary to recycle and reuse such precious metal resources.
Metal Plastic Mixture 4.97%
Plastics 15.21%
Metals 60.2%
Pollutants 2.7%
Cables 1.97%
Screens 11.87%
PCB 1.71%Others 1.38%
Figure 1.5. Approximate composition of e-waste.27
1.2.4. Problems caused by e-waste. E-waste is both valuable as source for secondary raw
material, and toxic if treated and discarded improperly. Rapid technology change, low
initial cost and even planned obsolescence have resulted in a fast growing problem around
10
the globe. Technical solutions are available but in most cases a legal framework, a
collection system, logistics and other services need to be implemented before a technical
solution can be applied. Due to lower environmental standards and working conditions in
developing countries like China and India, e-waste is being sent to these countries from the
developed countries like Japan. Uncontrolled burning and disposal are causing
environmental problems.
The toxicity related to e-waste is due in part to Pb, Hg, Cd, and a number of other
substances. A typical computer monitor may contain more than 6% Pb by weight. Upto 36
separate chemical elements are incorporated into e-waste items. Among them Pb, Cd, Cr,
Zn and Hg are in considerable amounts. Elements in trace amounts includes Ge, Ga, Ba, Ni,
Ta, In, V, Tb, Be, Au, Eu, Ti, Ru, Co, Pd, Mn, Ag, Sb, Bi, Se, Nb, Y, Rh, Pt, and As.
Others are Si, C, Fe, Al, Sn, Co, lead in glass (CRT) and nickel-cadmium batteries. Many
problems associated with the environmental contamination of heavy metals will be
discussed in section 1.4. Since both valuable and toxic heavy metals are present in most of
the electronics, an appropriate recovery and remediation strategy is needed to control the
environmental contamination and for the recover and reuse of valuable metals.
1.3. PRECIOUS METALS
A precious metal is a rare metallic chemical element of high, durable economic
value and plays unique role in the society; least reactive towards atoms or molecules at
the interface with a gas or a liquid but forms stable alloys with many other metals.
1.3.1. Introduction. Chemically, the precious metals are less reactive than most elements,
have high luster, and have higher melting points than other metals. Historically, precious
metals were important as currency, but they are now regarded mainly as investment and
industrial commodities. Gold, silver, platinum and palladium, however, are still
internationally recognized as equivalents of currency under ISO 4217.
11
The best-known precious metals are gold (Au) and silver (Ag). Other precious
metals include the Platinum group metals: ruthenium (Ru), rhodium (Rh), palladium (Pd),
osmium (Os), iridium (Ir), and platinum (Pt). Plutonium (Pu) and uranium (U) are also
considered precious metals in various contexts. Among all the noble or precious metals
gold is the noblest. This is because of its unique role in society and its inertness towards
other atoms or molecules at the interface with a gas or a liquid. The inertness of gold,
however, does not reflect a general inability to form chemical bonds. Gold forms very
stable alloys with many other metals. Since the present research is associated with recovery
of gold, palladium, and platinum only, the following sections are focused on the properties
and separation methods of these three metals.
Some properties of gold, palladium and platinum are given in Table 1.1.
Table 1.1. Properties of gold, palladium and platinum.
Metal Melting
point / ºC Form Solvent
Common
oxidation states
Au 1063 Yellow, lustrous,
malleable, ductile
Aqua regia, halogens,
cyanide + air or H2O2 Au(I), Au(III)
Pd 1552 Gray-white, lustrous,
malleable, ductile conc. HNO3, HCl + Cl2 Pd(II)
Pt 1769 Gray-white, lustrous,
malleable, ductile Aqua regia Pt(II), Pt(IV)
1.3.2. Applications of precious metals. Traditional uses of precious metals are jewelry,
bullion or currency. With the technological advancements, their application had been
extended to electrical & electronic equipments, various bio-medial tools and medicines.
Gold has been known and highly valued from ancient era, not only because of its
beauty and resistance to corrosion, but also because its ease to work with than all other
metals, i.e., molding to any shape or size. Major portion of gold is still used in coinage and
jewelry. Because of high electrical conductivity and the high corrosion resistance of gold
and many of its alloys, a considerable portion of gold is being used for electromechanical
devices such as connectors, switches or relays. Chloroauric acid is used in photography for
toning silver images. Potassium gold cyanide is used in electrogilding. Gold is also used in
12
dentistry and its radioisotopes are used in biological research and in the treatment of
cancer.31
One of the main uses of platinum is in the automotive industry in the autocatalyst
for diesel engines to remove undesirable exhaust emissions. It is also used in the chemical
industry in process catalysis. In the electrical industry, platinum is used to enhance the
magnetic qualities of the cobalt alloy which coats the hard-disk surface where the data is
stored on personal computers. An increasing need of high data-storage capacity, and high
quality graphics and sound have led to a rapid increase in the use of platinum in the
manufacture of hard disks.
The demand of palladium is ever increasing especially in the electronics industry. It
is used in the production of multi-layer ceramic capacitors (MLCC) which are used in
mobile phones and automotive electronics. Palladium is also used in surge-resistor
networks that protect telecommunications equipments from damage by large voltage. It is
also being used in the autocatalyst industry and is used as an alloying component and as a
whitening agent in jewelry.
Thus the precious metals have become, in one way or the other, essential parts of
our everyday life. Hence their judicious use is necessary in order to tackle the ever
increasing demand.
1.3.3. Chloride chemistry of gold, palladium and platinum. The stability constant data
of Au(III) ion in chloride medium are log K1 = 8.51, log K2 = 8.06, log K3 = 7 and log K4 =
6.07. For Pd(II), log K1 = 4.7, log K2 = 3, log K3 = 2.6 and log K4 = 1.6. For Pt(IV), (log K1
to log K4 values are not reported to the date) log K5 = 3.7 and log K6 = 2.25.32 From these
data the stepwise formation of various metal–chloride complexes can be understood. From
this study, the most stable form within weak to strong hydrochloric acid medium for
Au(III) ion is found to be AuCl4- , for Pd(II) ion is PdCl4
2-, and for Pt(IV) ion is PtCl62- as
shown in Figure 1.6.
From Figure 1.6. it is clear that Au(III), Pd(II) and Pt(IV) ions exist as chloride
complex anions over a wide range due to which the anion exchangers can be suggested as
the best sorbents for their recovery from chloride medium. Considering this fact, various
adsorption gels, resins or solvent extraction reagents having anion exchanging functional
groups have been developed and some of them are being utilized for the recovery of these
13
precious metals. In the following section, a brief review of literature related to the recovery
of Au(III), Pd(II), and Pt(IV) is made.
0
20
40
60
80
100
-12 -10 -8 -6 -4 -2
log [Cl-]
% A
u
Au3+
AuCl2+
AuCl2 +
AuCl3
AuCl4-AuCl4−
AuCl3
AuCl2+
AuCl2+
Au3+
0
20
40
60
80
100
-8 -6 -4 -2 0log [Cl-]
% P
t
PtCl4
PtCl5-
PtCl62-
PtCl4
PtCl5−
PtCl62−
0
20
40
60
80
100
-10 -8 -6 -4 -2 0 2log [Cl-]
% P
d
Pd2+
PdCl+
PdCl2
PdCl3-
PdCl42-
Pd2+
PdCl+
PdCl3−
PdCl42−
PdCl2
Figure 1.6.Chemical speciation diagrams of Au(III), Pd(II), and Pt(IV) in chloride medium.
1.3.4. Precious metal recovery: brief literature review. Studies on the separation and
preconcentration of precious metals by adsorption have been very extensive. Several
14
traditional sorbents, such as activated carbon, sulfhydryl cotton and polyurethane foam, are
still widely used and efforts are being made to establish improved procedures. Many new
sorbents have been synthesized, some of which are specific to certain precious metals.
The main characteristics of the type of sorbent with the trade name of Polyorgs
were reviewed, in which sorbents containing pyrazole, imidazole, 2-
mercaptobenzothiazole and thioglycolanilide groups exhibit high selectivity for precious
metals and have been applied to the preconcentration of the metals in rocks, ores, industrial
products and natural waters.33 A macroporous poly(viny1thiopropionamide) chelating resin
was synthesized and applied to the preconcentration of Au(III), Pt(IV), and Pd(II).34
Investigations by Fourier transform IR spectrometry and electron spectroscopy revealed
that Au, Pt and Pd ions were chelated mainly with the thioketo form of the
thiopropionamide group in the resin, forming a quadridentate chelate. The other resins
recommended by various researchers are as follows: a newly synthesized macroporous
poly(viny1aminoacetone) chelating resin for the preconcentration of Au(III), Pd(II),
Rh(III) and Ru(III);35 a chelating resin containing a quinaldinic acid amide group for the
separation of Pt(IV) and Pd(II) from each other and from noble-base mixtures, and also
from native Ag solutions;36 new chelating sorbents of the pyrazolone type for the
separation of Au, Pd and Ag from base metals.37 The enrichment of Pd (as PdC142-) from
aqueous solution was carried out by means of a liquid membrane containing trioctylamine
in the form of an emulsion.38 N,N-dialkyl-N'-benzoylthiourea was used for the
preconcentration and separation of Pd, Pt, Ru, Rh and Ir.39 An anion-exchange resin with a
quaternary phosphonium chloride group in the structure exhibiting high sorption selectivity
for precious metal ions such as Au(III) and Pt(IV) was prepared and applied to the
separation of Au(III) from Cu(II).40 The macroporous styrene anion-exchange resin D371
was used for the preconcentration of Au.41 The positively charged mixed complexes of Au
with ethylenediamine, chloride and bromide are readily transported through the cation-
exchange membrane and can be separated from Pt(IV), Ir(III) and Rh(III).42 Adsorption
behavior of resin duolite GT-73 for Au(III) and Pd(II) from hydrochloric acid medium was
investigated.43 Au(III) was selectively extracted in the presence of Pt(IV) and Pd(II) from a
mixture of 2-propanol and water by using salting out method.44 Bio-Rad AG1-8X and 100-
200 mesh anion exchange resins were used for the recovery of gold, palladium, and
platinum in chloride medium obtained after sodium peroxide fusion of silicate rocks.45
15
Separation of Pd(II), Pt(IV) and Au(III) in chloride medium from a number of other metals
such as Ag, Al, Co, Cu, Fe, Ni, Mn, Pb, and Zn was achieved by using a column packed
Amberlite IRA-35 anion exchange resin.46 Selective recovery of platinum and palladium
wasted from industrial metal refinery effluents was successfully achieved by using a silica-
based (poly)amine ion exchanger having ligands like 3-(trimethoxysilyl)propylamine, N-
[3-(trimethoxysilyl)propyl]ehtylenediamine and N-[3-(trimethoxysilyl)propyl] diethylene-
triamine.47 Competitive adsorption of gold from chloride solution in the presence of Fe(III)
or Te(IV) was conducted by using a polymeric adsorbent XAD-7 having polymethacrylic
ester group. The resin has high capacity for gold.48 Phosphine sulphide type chelating
polymer with spacer arms synthesized from chloromethylated polystyrene is reported to be
effective in the selective adsorption of Au(III) and Pd(II) from base metals.49,50
These all adsorption or ion-exchange studies for the recovery of precious metals:
Au(III), Pd(II), and Pt(IV) use synthetic resins or artificial substrates. Although the
problem of loss of valuable metal can be solved by their use, a new problem of their safe
disposal after use or the preparation of their original matrix is always crucial. For this
reason, attention is being given in the development of effective measures of metal recovery
by using recyclable resources, i.e. bio-materials. There are some examples of researches
carried out for the recovery of precious metals by using bio-degradable polymeric
materials such as chitin & chitosan, persimmon tannin, saw dust, and so on.
Among the biomass wastes chitin and chitosan derivatives, tannin products and
some algae species has been studied for the recovery of precious metals. Dithiocarbamate-
chitin51 and N-(2-pyridylmethyl)chitosan52 were synthesized and the former was used for
the preconcentration of Au(III), Pd(II), and Ru(III), while the later exhibited absorptive
activity for Pd(II) and Pt(IV). Adsorption of gold, platinum and palladium and subsequent
removal of traces of mercury in chloride medium was carried out by using cross-linked
chitosan synthesized from chitosan and ethylene glycol diglycidyl ether.53 Sulphur
derivatives of chitosan,54 chitosan crosslinked with gluteraldehyde or hexamethylene
diisocyanate55 were synthesized. Chitosan with sulphur moiety was reported to be effective
for palladium recovery while the others were used in gold adsorption. Tannin gels prepared
from persimmon peel or condensed tannin were found to be highly effective in the
adsorption of gold.56-58 In an effort to develop cost effective and nature friendly recovery
method to recover gold, alfalfa biomass was used. Compared to other metal ions it has
16
highest binding affinity for Au(III).59 Similar adsorption property was shown by polyvinyl
alcohol immobilized fungal biomass.60
From this brief review of researches conducted for the recovery of precious metals,
it is clear that the trend of study is shifting from synthetic to bio polymers. This suggests
an increasing importance of the use of sustainable, recyclable resources.
1.4. HEAVY METALS
The increase in industrial activities has intensified environmental pollution and
the deterioration of ecosystems, with the accumulation of pollutants such as heavy metals,
synthetic compounds, etc. Mining and metallurgical waste waters are considered to be
the major sources of heavy metal contamination, and the need for economic and effective
methods for their removal is attracting attention of researchers.
Figure 1.7. Movement of heavy metals within the environment.
Anthropogenic activity
Precipitation Rivers Lakes Oceans
Natural Rocks
Air
Aquatic plants
Aquatic animals
Man
SoilsLand plants
Land animals
Sediments
Glacial activity CurrentsVolcanic
Industrial, commercial, agricultural
Hydrosphere Atmosphere Lithosphere
Winds, currents, climate
Erosion Weathering
17
1.4.1. Impacts of heavy metals in human health. The movement of heavy metals in the
environment is shown in Figure 1.7. It is clearly seen that the edible parts of the plants, as
important parts of the food chain, provides the route for heavy metals to become
incorporated into the body components of animals and humans. Along this natural pathway
water is a significant source of heavy metals input. Water, through natural weathering and
leaching processes, and by dissolving and reacting with materials, mobilizes and
distributes heavy metals. But, the natural geological and biological alternations of the
earth’s surface are slow. Dramatic increase in heavy metal level especially in water is
resulted by various activities of human beings, i.e., different kinds of industries.
Table 1.2. Health effects associated with a deficiency or excess of heavy metals.
Health effect Heavy metals Deficiency Excess Fe
Anemia Cirrhosis of liver, haemochromatosis
Cu Anemia and changes in ossification Wilson’s disease Zn Growth retardation, hypogonadism,
mental lethargy, poor appetite, skin lesions
Nausea, anemia, neutropenia
Se Keshan disease, cardiomyopathy, white muscle disease
Lassitude, skin depigmentation, hair loss
Cr Impaired glucose tolerance Eczema and linked to cancer V Reduction in red blood cell, upset iron
metabolism, lack of bones, teeth, and cartilage growth
Kidney failure, linked to manic depression
Pb - Impaired haemesynthesis, insomnia, headaches, irritability, impaired mental activity, reproductive and developmental problems
Cd - Hypertension, renal damage, osteomalacia, anosmia
The threat of ever increasing elemental pollution, especially of toxic heavy metals,
may be directly related to the increased incidences of various diseases, including asthma,
18
birth abnormalities, impaired mental development, cancer and Alzheimer’s disease.
Although at present numerous trace elements are known to be essential for animals and
humans (As, Cr, Co, Cu, F, I, Fe, Mn, Mo, Ni, Se, Si, Sn, V and Zn) under conditions of
excessive dietary or environmental exposures, they are all capable of becoming toxic and
impairing their respective biological functions, resulting in poor health status. Cd, Hg, Pb,
and Th are known as toxic elements, and even at small increase in magnitude they can
dramatically impair health.
Moreover, the impact of many trace elements on animal and human health is not
only related to magnitude, but also chemical form, and the ‘cocktail effect’. Many trace
elements are more toxic in organic rather than in inorganic forms: CH3Hg+ and (CH3)2Hg,
for example, are more toxic than Hg2+. A ‘cocktail of trace elements’ can include inter-
element interactions whereby an excess of one will induce a deficiency of another element.
Also, in some cases elements will compete for specific enzyme binding sites or carrier
ligands, and thereby impair the absorption, transportation, or utilization of other element.
This results in altered physiological activity and poor health status. An environmental or
dietary excess of Pb, as for example, interferes with the absorption of Ca, Fe, Cu, and Zn,
and may induce health problems like osteoporosis, anemia, reproductive and growth
depression. Some examples of health effects associated with deficiency or excess of heavy
metals are given in Table 1.2.
1.4.2. Methods of heavy metal recovery: Adsorption and ion-exchange. Effective
removal of toxic heavy metals, in connection with a comprehensive wastewater treatment
strategy, still remains a major problem. Increased awareness on the toxicity of metals
prompted the implementation of strict regulations for its disposal causing traditional
treatment processes such as chemical precipitation to undergo major changes. Modern
metal removal technologies such as adsorption and ion exchange are now becoming
components in integrated systems that produce effluents of better quality while allowing
for the recovery and reuse of metals. Currently used commercial ion exchange resins use
huge amount of artificial plastics which, besides being expensive, are creating heavy load
to the environment. In this regard, development of biodegradable, effective and low cost
sorption materials is an urgent need. Recently, much attention has been paid to the
utilization of various biomass wastes like sea food wastes, various fruits wastes obtained
19
after juicing or peeling, waste wood generated during lumbering or similar processes,
paper wastes, and so on in developing effective sorption active materials for heavy metal
removal.
1.5. OBJECTIVES OF THE PRESENT STUDY
The short term objectives of the study are:
1. To explore the scope of waste wood powder in the form of useful materials.
2. To prepare and characterize various lignophenol derivatives, i.e., lignophenol,
lignocatechol and lignopyrogallol by immobilizing phenol, catechol and pyrogallol
onto wood lignin.
3. To determined the adsorption behavior and to estimate the maximum adsorption
capacity of the gels to various base metals and precious metals.
4. To know the effectiveness of further functionalization of novel gels for example with
amine groups.
5. To analyze the results.
The long term objectives of the study are:
1. To establish waste wood material as an useful biomass in the form of adsorption gels
and substitute to plastics materials in the preparation of ion exchange resins.
2. To suggest ways to recover precious metals from e-waste by using novel lignin gels.
3. To devise suitable means to treat industrial effluents for decontamination of heavy
metals by using lignin gels
4. To contribute to the protection of environment and improvement in human health.
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35. Chang, X.; Luo, X.; Zhan, G.; Su, Z. Synthesis and characterization of a macroporous
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traces of gold, palladium, rhodium and ruthenium. Talanta 1992, 39(8), 937.
36. Konar, B.; Basu, S.; Das, J. Separation of platinum and palladium using a chelating
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38. Wang, Z.; Li, Y.; Li, Y.; Guo, Y. The separation and enrichment of trace levels of
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39. Szczepaniak, W. Chelating ion exchanger containing N,N-dialkyl-N'-benzylthiourea as
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40. Fujiwara, M.; Matsushita, T.; Kobayashi, T.; Yamashoji, Y.; Tanaka, M. Preparation of
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behaviour for noble metal ions. Anal. Chim. Acta 1993, 274(2), 293.
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application. Anal. Sci. 1991, 7 (Suppl., Proc. Int. Congr. Anal. Sci.,1991, Pt. 2), 1297.
42. Pyrzynska, K. Membrane method for preconcentrating and separating gold complexes
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43. Iglesias, M.; Antico, E.; Salvapo, V. Recovery of palladium(II) and gold(II) from
diluted liquors using the resin duolite GT-73. Anal. Chim. Acta 1999, 381, 61.
44. Chung, N.H.; Tabata, M. Selective extraction of gold(III) in the presence of
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rocks by the anion exchange separation of chloro complexes after a sodium peroxide
fusion: an investigation of low recoveries. Talanta 1995, 42, 1411-1418.
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47. Kramer, J.; Driessen, W.L.; Koch, K.R.; Reedijk, J. Highly selective extraction of
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25
Chapter 2
ADSORPTION OF HEAVY METALS ON CROSSLINKED
LIGNOCATECHOL
Crosslinked lignocatechol gel was prepared by immobilizing catechol onto wood
lignin followed by crosslinking. The adsorption behavior of crosslinked lignocatechol was
studied batch wise by varying different parameters like pH, initial concentration of metal
ions and shaking time. Based on the results obtained in batch wise experiments,
breakthrough profiles were examined using a column packed with crosslinked
lignocatechol gel for the separation of small concentration of Pb(II) from an excess of
Zn(II) followed by elution tests. Ten consecutive adsorption-elution cycles were
conducted so as to check the stability and efficiency of the gel for recycling purpose. A
constant efficiency was observed that reveals the feasibility of recycling of the gel. Cation
exchange mechanism is postulated as the predominant process of the adsorption.
2.1. INTRODUCTION
In recent years, it has become very necessary to establish a sustainable society
instead of the present society, which is typified by huge amounts of production,
consumption and waste, consumption of large amounts of limited fossil resources such as
petroleum and coal and pollution of the global environment. For a sustainable society in
future, we human beings should rely not on limited fossil resources but on renewable
resources such as biomass. That is, various biomass wastes generated in huge amounts in
various fields like agriculture, forestry and fishery at present should be much more
effectively and largely utilized for various purposes not only for biomass energy, composts
and so on but also as feed materials for various advanced functional materials making use
26
of their own original and excellent characteristics for a sustainable society in the future.
Waste wood from tree thinning and that generated from lumbering activities and from
housing demolition are the most abundant among various kinds of biomass wastes, and,
therefore, their effective use is very important for the above-mentioned purposes. In
addition, effective use of waste wood in large amounts will activate forestry and increase
the area of wood on the earth, which protects the global environment.
The main components of wood are cellulose, hemicellulose and lignin; i.e.,
depending on the species, wood contains 10-30% lignin.1,2 A big amount of lignin is
wasting out from the paper industries as black liquor. Compared with the polysaccharides,
cellulose and hemicellulose, the effective use of lignin has not been progressed owing to
the difficult separation of lignin from the other components of wood without their
denaturation, as will be discussed in detail later. Lignin is an amorphous multifunctional
phenolic network polymer, as shown in Figure 1.4, containing hydroxyl, ether and
carbonyl groups.3 It is difficult to immobilize any functional group for the preparation of
new functional polymeric materials by means of chemical modifications of the groups
contained in lignin without loosing its original characteristics. The limited use of lignin
compared to cellulose is attributable to its molecular structure; i.e., the lignin molecule
lacks stereo-regularity and the repeating units of the polymer chain are heterogeneous.
Also, lignin in wood forms interpenetrating polymer units with cellulose.4 Hence, with
conventional methods, strong reagents are necessary to separate hydrophobic lignin from
hydrophilic cellulose, which results in the condensation of lignin on itself giving
complicated molecular networks.5,6
In recent years, Funaoka et al. developed a novel separation system to recover
lignin compounds from waste wood as a new type of phenolic lignin-based linear polymer
(lignophenol derivatives) based on sophisticated molecular design away from
polysaccharides such as cellulose and hemicellulose.6-8 As will be described in detail later,
this process includes a phase-separation reaction system consisted of organic phase of
phenol derivatives and aqueous phase of concentrated acid. During the reaction,
polysaccharides are depolymerized into water-soluble sugar compounds to be dissolved in
aqueous phase completely separated from lignophenol derivatives formed in the organic
phase. The lignophenol derivatives are characterized by their unique functions different
from conventional lignin in spite of retention of the original inter unit linkages.
27
Although lignin is one of the most durable biopolymers, it is perfectly
biodegradable in nature in the presence of some enzymes unlike synthetic polymers that
present serious environmental problems in post-treatment after use.9 Trees containing
lignin are converted into humic substances in soil after their death. Humic substances
adsorb and immobilize nutrients and metal ions, which are necessary for growth of plants
in soil. The adverse effects caused by the lignin in soil or lignin in foodstuffs in the
bioavailability of pesticides or essential metals and bile salts were widely studied by
Zuman et al.10-13 The adsorption tendencies of various types of lignin for a number of base
metals have also been reported.14-17
Consequently, it is expected that it will be easy to prepare some advanced
adsorption gels for metal ions, which are biodegradable and, therefore, environmentally
benign, by incorporating some functional groups with high affinity and selectivity for
specified metal ions by means of simple chemical modification. In this regard, the
application of lignin compounds to adsorption of metal ions is of prime importance from
the viewpoint of reproducing the working in nature as materials for human society.
In the present work, we have prepared a novel adsorption gel, crosslinked
lignocatechol, from wood powder according to the phase-separation process and its
adsorption behavior was examined for some metal ions.
2.2. EXPERIMENTAL
2.2.1. Preparation of crosslinked lignocatechol. Prior to the preparation of lignin
derivatives, the extractives contained in the wood powder generated in the lumbering of
Japanese cedar trees in Miyazaki, Japan, were removed as follows. Dried wood powder 12
g was mixed together with a mixture of 150 ml ethanol and 300 ml benzene (1:2 v/v ratio).
The mixture was stirred thoroughly for 48 h at room temperature followed by filtration and
the residue was dried in a hot air oven. From the extractives-free wood powder,
lignocatechol was prepared by interacting with catechol according to the phase separation
method of Funaoka et al.6-8
The reaction scheme is shown in step 1 of Scheme 2.1. In a 500 ml beaker, 5 g
extractives-free wood powder was taken together with 10 g catechol and mixed in the
28
presence of excess acetone. The mixture was allowed to stand for about 24 h, after which
the acetone was completely removed by using a dropper followed by vigorous stirring to
make the sample completely dry. To the dried mixture, 60 ml of 72 wt % sulphuric acid
was slowly added with continuous stirring. The mixture was thoroughly stirred until the
viscosity of the mixture became a maximum followed by a return to the normal value. The
mixture was further stirred continuously for 1 h at 30 °C. When a homogeneous phase was
observed, water was added to complete dispersion followed by continuous stirring for half
an hour. The brown colored precipitates were collected by centrifugation. The product thus
obtained was washed with distilled water until neutral pH was achieved followed by
vacuum drying. The dried product was dispersed in excess acetone and kept standing for at
least 24 h in order to extract lignocatecol as the final product. After 24 h, the mixture was
filtered. The filtrate was concentrated by means of vacuum evaporation. The concentrated
solution was added dropwise to 400 ml diethyl ether in an ice bath with constant stirring.
After stirring for a few minutes the mixture was filtered to collect lignocatechol in fine
powder form.
OCH3
C
O
HCCH2OH
LigninOH
H2SO4
OCH3
CH
O
HCCH2OH
Lignin
+30 oC
Lignin Catechol Lignocatechol
OH
H OH OH
HO
OCH3
CH
O
HCCH2OH
Lignin
Lignocatechol
OH
HO
H2SO4
CH
OCH3
O
HC Lignin
CH2OH
CH2
+ (HCHO)n
Crosslinked lignocatechol
100 oCOH
HO
Paraformaldehyde Scheme 2.1. Preparation of crosslinked lignocatechol gel.
29
In order to avoid the dissolution of lignocatechol prepared as described above, it
was crosslinked by using para-formaldehyde according to the reaction scheme as shown in
step 2 of Scheme 2.1. The crosslinking takes place at grafted phenol unit or at C5th
position of aromatic nucleus via methylene group. For this, 5 g lignophenol was taken
together with 50 ml 72 wt % sulphuric acid in a 300 ml eggplant flask and stirred for a few
minutes. Then 6.5 g para-formaldehyde was added and stirred continuously for about 24 h
at 100 °C. After cooling, a 5% sodium hydrogen carbonate solution was slowly added to
the reaction mixture and stirred for 3 h followed by filtration. The residue obtained was
then washed with hot water and then by 1 g/l hydrochloric acid solution. Finally the
product was washed with cold distilled water until neutral pH was achieved and dried at
90°C for 48 h in a hot air oven.
2.2.2. Surface analysis. SEM images of the crosslinked lignocatechol were obtained using
a JEOL model JSM 5200 scanning microscope operated at an acceleration voltage of 25
kV. The images at a magnification of 500x and 2000x are shown in Figures 2.1 (a) and (b)
respectively. The surface of the resin shows a remarkable lack of holes and cracks.
Figure 2.1. SEM images obtained at (a) 500x magnification and (b) 2000x magnification
for crosslinked lignocatechol. Acceleration voltage = 25 kV. Scale = 50 µm and 10 µm
respectively.
2.2.3. Solution preparation. Aqueous solutions of Al(III), Cd(II), Co(II), Fe(III), La(III),
Ni(II), Pb(II), and Zn(II) ions for the batch wise experiments were prepared by dissolving
corresponding analytical grade individual metal chlorides in 0.1 M hydrochloric acid
(a) (b)
30
solution and in a solution of 0.1 M N-[2-Hydroxyethyl] piperazine-N’-[2-ethanesulfonic
acid] (HEPES), a buffer reagent, followed by mixing these two solutions at an ordinary
volume ratio to adjust pH, while dilute sodium hydroxide solution was used to prepare
higher pH solutions. In addition, analytical grade nitrate salts of Pb(II) and Zn(II) were
used to prepare sample solution of chromatographic separation test.
2.2.4. Batch wise adsorption experiments. Dried gel 20 mg was taken together with 15
ml of 0.2 mM test solutions at various pH values in a stoppered flask. The flask was
shaken vigorously in a thermostated shaker maintained at 30°C for about 24 h. The mixture
was then filtered and the filtrate was taken as the sample solution for the metal ion
concentration analysis. The effect of shaking time was checked for Pb(II) at pH 5 in a
number of flasks. Different solutions were sampled by varying the shaking time. The effect
of initial metal concentration was studied by sampling a number of test solutions of
varying concentration at constant pH values corresponding to various metals.
The pH of the sample solution was measured by using a BECKMAN model ф-45
pH meter and the concentrations of corresponding metal ions before and after the
adsorption were measured by using a Shimadzu model AA-6650 atomic absorption
spectrophotometer.
Figure 2.2. A schematic diagram of adsorption column.
31
2.2.5. Chromatographic separation of metal ion pair. A mutual separation test between
typical coexisting metal ion pair that may be expected in many practical applications, in
the present case, Pb(II) and Zn(II) pair to simulate the practical application of removal of
impurities as for example impurity lead from a zinc plating bath, was carried out using
crosslinked lignocatechol gel. That is, breakthrough curves were produced using a column
packed with cross-linked lignocathechol gel placed between the layers of glass beads as
shown in Figure 2.2 followed by elution tests. The packed column was washed with
distilled water followed by dilute nitric acid solution of pH 2.5, for a few hours. Feed
solution was prepared by mixing two solutions of 0.1 M nitric acid and 0.1 M HEPES
containing 110 ppm Zn(II) and 11 ppm Pb(II) each to adjust the pH to 2.5. This was fed
to the column at 3.0 ml/h by using an EYELA model micro tube pump. The sample
solutions from the outlet were collected using a BIO-RAD model 2110 fraction collector.
Similarly, after complete Pb(II) breakthrough, the column was washed with
distilled water for a few hours and then, the elution test was carried out using 1M
hydrochloric acid fed at 3.0 ml/h. Again, the outlet solutions were collected in a similar
way. The metal concentrations were measured by using the Shimadzu model AA-6650
atomic absorption spectrophotometer.
2.2.6. Monitoring of the adsorption – elution cycles. The possibility of regeneration of
the used gel was examined chromatographically. For this, 10 ppm Pb(II) solution was fed
at pH 3 to a column packed with crosslinked lignocatechol at a rate of 10 ml/h. The
column set up and conditioning was the same as described in the previous section. After
passing a definite volume of sample solution, the column was washed with distilled water
and then eluted with 1M hydrochloric acid solution. After all the loaded Pb(II) was eluted,
the column was washed with distilled water and conditioned with pH 3 nitric acid solution.
In the same way ten consecutive cycles were produced.
2.3. RESULTS AND DISCUSSION
2.3.1. Effect of shaking time. The effect of shaking time on the adsorption of Pb(II) ion,
as an example, on crosslinked lignocatechol is shown in Figure 2.3. More than 50%
adsorption occurred within half an hour. But for a complete uptake under the given
32
conditions, 24h shaking appears to be necessary. Therefore, all adsorption tests were
carried out by shaking for 24h.
0
20
40
60
80
100
120
0 10 20 30 40 50
Time/ h
% A
dsor
ptio
n
Figure 2.3. Effect of shaking time in the adsorption of Pb(II) on crosslinked lignocatechol.
2.3.2. Effect of pH on the adsorption behavior of crosslinked lignocatechol.
0
20
40
60
80
100
1 2 3 4 5pHe
% A
dsor
ptio
n
Pb (II)
Cd (II)
Zn (II)
Co (II)
Ni (II)
La (III)
Al (III)
Fe (III)
Figure 2.4. Adsorption of different metal ions from 0.1M HCl-0.1 M HEPES solutions on
crosslinked lignocatechol at varying pH. Initial metal ion concentration = 0.2 mM, dry
weight of gel = 20 mg, volume of test solution = 15 ml, shaking time = 24 h, temperature =
30 ˚C.
33
Figure 2.4 shows the adsorption behavior of crosslinked lignocatechol for various
metal ions wherein the % adsorption of metal ions are plotted against equilibrium pH. The
effect of pH is distinctive. For all metal ions tested a similar variation of increase in %
adsorption with increasing pH is observed in acidic pH range. As the metal ions shown in
Figure 2.4 exist as cationic species at pH values less than 5, the result directly supports the
cation exchange mechanism, as will be discussed in detail in the subsequent section.11 The
order of the selectivity with respect to the width of pH range among the tested metal ions is
as follows: Pb(II) ∼ La(III) > Fe(III) > Al(III) > Ni(II) ∼ Zn(II) ∼ Cd(II) ~ Co(II). This fact
suggests the feasibility of mutual separation of metals discussed herein themselves by
specifying pH values.
2.3.3. Effect of initial concentration of metal ions: Adsorption isotherms. Figure 2.5(a)
shows a plot of the adsorption isotherms for Cd(II), Co(II), La(III) and Fe(III) on the
crosslinked lignocatechol gel, i.e., the amount of adsorption against the equilibrium
concentration of metal ions. The pH of the system for Cd(II), Co(II) and La(III) was
maintained at 5.2. But, in the case of Fe(III), a pH value of 2.5 was maintained in order to
escape from the precipitation error of Fe(OH)3 at higher pH values. From this figure, it
appears that the adsorption of the metal ions took place according to a Langmuir type
adsorption; that is, adsorption increases with increasing metal ion concentration at low
concentration while it tends to approach constant values corresponding to each metal ion at
high metal ion concentration, from which the maximum adsorption capacities were
evaluated as 1.35, 1.15, 0.74 and 0.74 mol/kg-dry gel for Fe(III), Cd(II), La(III) and Co(II)
respectively.
In Figure 2.5(b), the adsorption isotherm and the Langmuir plot are plotted for the
adsorption of Pb(II) at pH 5.2. From the plateau of the isotherm and from the slope of the
Langmuir curve, the maximum adsorption capacity of crosslinked lignocatechol was
evaluated as 1.79 mol / kg. This value is the highest among all the metal ions tested. Such a
high capacity of the gel for Pb(II), a typical toxic heavy metal ion, is noteworthy together
with the higher selectivity as shown in Figure 2.4.
34
0
0.3
0.6
0.9
1.2
1.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0Ce x 103 / M
q (m
ol /
kg)
Cd (II) La (III) Fe (III) Co (II)
1.35
1.15
0.74
y = 0.5564x + 0.0145
0.0
0.5
1.0
1.5
2.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Ce x 103 / M
q (m
ol /
kg)
0.0
0.5
1.0
1.5
Ce x
103 /
M
1.79
qmax = 1/slope = 1.79
Figure 2.5. (a) The adsorption isotherms of Cd, La, Co and Fe ions on crosslinked
lignocatechol from 0.1M HCl – 0.1M HEPES solutions at pH values of 5.2 and 2.5 for Fe.
(b) The adsorption isotherm and the Langmuir plot for Pb(II) at pH 5.2. Dry weight of gel
= 20 mg, volume of test solution = 15 ml, shaking time = 24 h, temp. = 30 ˚C.
2.3.4. Chromatographic separation of a metal ion pair. Based on the batch wise
adsorption tests of various metal ions on the crosslinked lignocatechol gel as shown in
Figure 2.4 and a remarkable adsorption capacity for Pb(II) observed in Figure 2.6(b), a
mutual chromatographic separation test was carried out between Pb(II) and Zn(II); i. e., a
small amount of Pb(II) was separated from a large excess of Zn(II) using a column packed
35
with the crosslinked lignocatechol gel. Figure 2.6 (a) shows the breakthrough profile of
Pb(II) and Zn(II) under the conditions described in the figure legend. From the figure it is
clear that zinc breaks through immediately after the feed flow is started, since Pb(II) is
much more selectively adsorbed than Zn(II), as shown in Figure 2.4.
(a)
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80Time / h
CZn
/ pp
m
0
2
4
6
8
10
12
CP
b / p
pm
Zn (II)
Pb(II)
(b)
0
50
100
150
200
0 2 4 6 8 10Time / h
C /
ppm Pb (II)
Zn (II)
Figure 2.6. (a) Breakthrough profiles of Pb(II) and Zn(II) from the column packed with
crosslinked lignocatechol. Feed concentration of Pb = 11 ppm and that of Zn = 110 ppm,
pH = 2.5, weight of the adsorbent packed in the column = 0.40 g, feed rate = 3.00 mL/h.
(b) Elution profiles of Pb and Zn from the loaded column of crosslinked lignocatechol by 1
M hydrochloric acid. Feed rate = 3.00 mL/h.
36
Figure 2.6 (b) shows the elution profiles of both metal ions with 1M hydrochloric
acid solutions from the loaded column after the breakthrough of Pb(II). It is apparently
seen that Pb(II) is eluted at very high concentration, as high as 20 times compared with the
feed concentration. Also, almost equal areas were observed in both breakthrough and
elution curves of Pb(II). This means 100% elution was achieved by using 1M hydrochloric
acid solution. These results suggest that effective mutual separation and pre-concentration
of Pb(II) away from Zn(II) using crosslinked lignocatechol can be satisfactorily achieved.
2.3.5. Monitoring of the adsorption – elution cycles.
0
20
40
60
80
100
0 2 4 6 8 10
Number of Cycles
% A
dsor
ptio
n
0
20
40
60
80
100
% E
lutio
n
%Adsorption
%Elution
Figure 2.7. Adsorption – elution profile of crosslinked lignocatechol in a number of
consecutive cycles for the adsorption of Pb(II) at pH 3. Eluting reagent – 1 M hydrochloric
acid. Feed concentration of Pb(II) = 10 ppm; Feed rate = 10 ml/h.
In order to analyze the feasibility of re-use of the used crosslinked lignocatechol
gel, a number of consecutive adsorption-elution cycles were carried out for Pb(II) ion using
a column packed with crosslinked lignocatechol. At pH 3 and a feed rate of 10 ml /h an
almost constant efficiency was observed up to 10 cycles. For every 100% adsorption,
almost 100% elution was observed by calculating the curve areas. This result intensifies
the effectiveness of crosslinked lignocatechol gel for the separation and pre-concentration
of Pb(II) from any acidic aqueous systems.
37
2.3.6. Mechanism of adsorption.
OH
OH
M 2+
OH
OH
M 3+
OH
OH
M 3+
HO
or
O
O
O
O
O
O
M
M
M
O
OH
2H+
3H+
+
Crosslinked lignocatechol Metal chelate of crosslinked lignocatechol
+
+
Crosslinked lignocatechol Metal chelate of crosslinked lignocatechol
+2
+
Crosslinked lignocatechol Metal chelate of crosslinked lignocatechol
+ 3H++
O
Scheme 2.2. Mechanism of adsorption.
Because natural lignin itself has only small number of hydroxyl groups (∼10%), the
immobilization of polyphenolic ligand, catechol, onto the lignin molecules followed by
cross-linking, greatly increases the number of hydroxyl groups and produces branching of
the molecule as well. The long flexible polymeric chain that can adopt any required
configuration to form stable metal chelates is an important factor controlling the high
adsorption efficiency of this lignin derivative. As the adsorption behavior of crosslinked
lignocatechol is predominantly pH dependent, the mechanism involved is simply inferred
as cation exchange chemical adsorption. A number of cation exchangeable monophenolic
and polyphenolic hydroxyl groups introduced during the chemical modification or those
naturally existing on the original lignin molecule exhibit excellent adsorption
38
characteristics for cations i.e. metal ions according to a mutual exchange mechanism. For
example, the divalent and trivalent metal ions are considered to be adsorbed via the
formation of stable five-membered ring chelates as shown in Scheme 2.2.
2.4. CONCLUSIONS
Crosslinked lignocatechol exhibited excellent adsorption characteristics for the
metal ions tested, especially for Pb(II). With comparable efficiency, the method of its
preparation is very simple and the production cost is cheap. Hence, this novel lignin based
adsorption gel that is easy for repeated use is suggested as an efficient adsorbent for
environmental remediation.
REFERENCES
1. Sarkanen, K. V.; Ludwig, C. H. Lignin: Occurance, Formation, Structure and
Reactions. New York, Jhon Wiley & Sons, 1971, pp 916.
2. Musha, Y.; Goring, D. A. I. Kalson and acid soluble lignin content of wood. Wood Sci.
1974, 7, 133-134.
3. Brunow, G. Methods to reveal the structure of lignin. In, Biopolymers: Lignin, Humic
Substances and Coal, Edited by Hofricher, M.; Steinbuchel, A. Wiley-VCH, 2001, pp
93-106.
4. Cathala, B.; Lee, L. T.; Aguie-Beghin, V.; Douillard, R.; Monties, B. Organization
behavior of guaiacyl and guaiacyl/syringyl dehydrogenation polymers (lignin model
compounds) at the air/water interface. Langmuir 2000, 16 (26), 10444-10448.
5. Cathala, B.; Saake, B.; Faix, O.; Monties, B. Association behaviour of lignins and
lignin model compounds studied by multidetector size-exclusion chromatography. J
Chromatogr. A 2003, 1020(2), 229-239.
6. Funaoka, M. A new type of phenolic lignin-based network polymer with the structure-
variable function composed of 1,1-diarylpropane units. Polym. Inter. 1998, 47, 277-
290.
39
7. Funaoka, M.; Fukatsu, S. Synthesis of functional lignophenol derivatives from native
lignin in the two-phase system composed of phenol derivative and concentrated acid.
Netsu Kokasei Jushi 1994, 15 (2), 77-87.
8. Funaoka, M.; Nagamatsu, Y. Design and application of functionality controllable
lignin-based materials. Transactions of the Materials Research Society of Japan 2001,
26 (3) 821-824.
9. Zia, Z.; Yoshida, T.; Funaoka, M. Enzymatic degradation of highly phenolic lignin-
based polymers (lignophenols). Eur. Polym. J. 2003, 39, 909-914.
10. Kulik, F.; Wieber, J.; Pethica, B.; Zuman, P. Binding of copper(II) and zinc(II) ions on
various lignins. J. Electroanal. Chem. 1986, 214(1-2), 331-342.
11. Zuman, P.; Ainso, S.; Paden, C.; Pethica, B.A. Sorptions on Lignin, Wood and
Celluloses. I. Bile Salts. Colloid Surface 1988, 33, 121-132.
12. Ainso, S.; Paden, C.; Pethica, B.A., Zuman, P. Sorptions on Lignin, Wood and
Celluloses. II. Nitrosamines. Colloid Surface 1988, 33, 133-139.
13. Wieber, J.; Kulik, F.; Pethica, B.A.; Zuman, P. Sorptions on Lignin, Wood and
Celluloses. III. Copper(II) and Zinc(II) Ions. Colloid Surface 1988, 33, 141-152.
14. E.B. Rupp, P. Zuman, I. Sestakova, V. Horak, Polarographic determination of some
pesticides. Application to a study of their adsorption on lignin. J. Agr. Food Chem.
1992, 40 (10), 2016-2021.
15. Zuman, P.; Rupp, E. Electrochemical techniques in the investigation of interactions of
lignin with pesticides and the use of lignin as decontaminant. Proceedings-
Electrochemical Society 1995, 95-12, 267-278
16. Ludvık, J.; Zuman, P. Adsorption of 1,2,4-triazine pesticides metamitron and
metribuzin on lignin. Microchemical Journal 2000, 64, 15-20.
17. Rupp, E.B.; Zuman, P. Lignin as Adsorbent and Detoxicant. J Environ. Conscious
Design Manufact. 2001, 10, 23-30.
18. Deovkar, N. V.; Tavlarides, L. L. A chemically bonded adsorbent for the separation of
antimony, copper and lead. Hydrometallurgy 1997, 46, 121-135.
19. Matheickal, T. J.; Qiming,Y. Biosorption of lead (II) and copper (II) from aqueous
solutions by pre-treated biomass of Australian marine algae. Bioresource Tech. 1999,
69, 223-229.
40
Chapter 3
SELECTIVE RECOVERY OF GOLD BY LIGNIN BASED
NOVEL ADSORPTION GELS
Three different kinds of adsorption gels viz: crosslinked lignophenol, crosslinked
lignocatechol, and crosslinked lignopyrogallol were prepared by the chemical modification
of wood lignin. The adsorption behaviors of these gels for Au(III) along with some other
metals were studied and compared with that of activated carbon. All three gels have
proved to be more selective for Au(III) than activated carbon with comparable adsorption
capacities. Among the lignin gels, crosslinked lignophenol exhibited the highest
selectivity for Au(III) and was found to be almost inert towards other metals tested. All
three novel lignin gels as well as activated carbon were found to be efficient to reduce
Au(III) to elemental gold, which was proved by XRD analysis of the sorbents taken after
adsorption. However, a big difference between the novel sorbents and activated carbon
was found; i.e. the latter exhibited no selectivity among the metal ions tested whereas the
novel gels have high selectivity only to Au(III). In addition, the gold aggregates were
visually observed in the case of lignin gels and not in the case of activated carbon. This
result paves a new way for effective gold recovery.
3.1. INTRODUCTION
Millions of tons of spent electrical and electronic devices are being disposed every
year. More than half of these wastes consist of metals that include a significant proportion
of valuable metals or their compounds, which indicates not only the loss of huge amount of
41
resources but also the threat of environmental pollution. The high pace of technological
changes and competitive market strategies that encourage people to buy the latest models
before their old appliances stop functioning has caused an alarming increase in electronic
and electrical wastes. Along with other useful valuable metals, gold, which is mainly used
in making gold-coated edge contacts on printed circuit boards, is also being wasted out. In
a rough estimate, the % composition of different metals by weight in a mobile phone, as
for example, is as follows: copper – 15%, iron – 3%, zinc – 1%, and less than 1% of a
number of metals like tin, palladium, and gold.1 Although the portion of gold is very low
compared to the other metals in one set of a device, the amount of gold disposed in this
form is much higher than the content in gold ore itself.2 For a sustainable society and
strong economy, it becomes necessary to recycle and reuse such precious metal resources
in order not to waste them.
The history of extraction of gold and its use is as old as the human civilization.
With the technological advances, many methods of gold recovery were formulated.3 The
most common processes at present are chloride leaching and cyanide leaching methods.
Because of toxicity associated with cyanide and its ineffectiveness in refractory ores and
concentrates, cyanide leaching is not as common as chloride leaching. Some of the widely
used chloride leaching methods are chlorination, electrolytic refining and wet chemical
processing. Among them, a more extensively employed method of gold extraction is the
wet chemical refining. Raw gold containing a number of other metals is first dissolved in
hydrochloric acid in the presence of some oxidizing agent like chlorine or nitric acid. This
separates silver as silver chloride precipitate while the supernatant liquid is treated by
means of a number of chemical processing. Gold is separated by solvent extraction or ion
exchange method.4,5 Dibutyl carbitol is commonly employed as solvent extraction
reagent.6 Although this solvent has high selectivity for gold, it is not completely
satisfactory because of its water solubility which is associated with some problems like
solvent loss and waste water treatment. As mentioned above, either for safety or for
effective extraction, hydrochloric acid is used along with other accessory chemicals. In
this context a more cost effective and environmentally benign technique for the selective
gold recovery would be highly preferred.
For the purpose of developing an environmentally benign and cost effective process
for the selective recovery of gold from a mixture of many metals, we found a lignin based
42
adsorption gel, gel of lignophenol, that is efficient to uptake Au(III) as elemental gold. In
this chapter the development scheme of various types of lignin gels, the structures of which
are shown in Figure 3.1, their adsorption behavior and a comparison of their efficiency and
selectivity for Au(III) with wood derived activated carbon will be discussed. Because the
structure of lignin is complex, irregular, and consists of heterogeneous repeating units, the
structures given in Figure 3.1 are only tentative.
Figure 3.1. Tentative structures of crosslinked lignophenol, crosslinked lignocatechol, and
crosslinked lignopyrogallol.
3.2. EXPERIMENTAL
3.2.1. Reagents. Analytical grade chloride salts of copper, iron, palladium, tin, and zinc
were used to prepare the test solution of respective metals. Analytical grade HAuCl4.4H2O
and H2PtCl6.6H2O were used to prepare test solutions of gold and platinum, respectively.
3.2.2. Adsorption gels. Lignophenol, lignocatechol and lignopyrogallol were prepared by
immobilizing phenol, catechol, and pyrogallol onto wood lignin, respectively. All of these
lignin derivatives were prepared by phase separation method.8 As shown in their
structures in Fig. 3.1, phenol, catechol and pyrogallol are bonded to the α-carbon of the
aromatic nucleus. In order to avoid the dissolution in aqueous solutions, these lignin
compounds were crosslinked with paraformaldehyde, where crosslinking occurs at C5th
CH
OCH3
O
HC Lignin
CH2OH
CH2
Crosslinked lignocatechol
HO
HO
CH
OCH3
O
HC Lignin
CH2OH
CH2
Crosslinked lignopyrogallol
HO
HO
HO
CH
OCH3
O
HC Lignin
CH2OH
CH2
Crosslinked lignophenol
OH
H2C
43
position of benzene ring. Finely powdered, wood derived activated carbon (Sigma -
Aldrich) was used for comparison.
3.2.3. Preparation of crosslinked lignophenol.
OCH3
C
O
HC
CH2OH
Lignin
H OH
OH
H2SO4 OH
OCH3
CH
O
HC
CH2OH
Lignin
H2SO4
H2C
OH
OCH3
CH
O
HC
CH2OH
Lignin
CH2
+30 oC
Lignin Phenol Lignophenol
Crosslinked lignophenol
100 oC+ (HCHO)nOH
OCH3
CH
O
HC
CH2OH
Lignin
Lignophenol Paraformaldehyde
Scheme 3.1. Preparation of crosslinked lignophenol from wood powder.
In order to remove extractives, 12 g dried wood powder was mixed together with a
mixture of 150 ml ethanol and 300 ml benzene (1:2 v/v ratio) and stirred thoroughly for 48
h at room temperature followed by filtration and drying. From the extractive free wood
powder, lignophenol was prepared according to the reaction shown in Scheme 3.1 as
follows. 5 g of the wood powder was mixed with 50 mL phenol in a 500 mL beaker and
vigorously stirred mechanically for 5 min at 60 °C. The mixture was then cooled to 30 °C
and 100 mL of 72 wt. % sulphuric acid was slowly added with constant stirring. The
44
mixture was vigorously stirred till the viscosity became a maximum followed by a return
to the normal value, and it was further stirred mechanically for 1 h at 30 °C, which gave
rise to a greenish brown homogeneous mixture. It was then subjected to centrifuging for
20 min at 2000 rpm. After centrifuging, two separate layers of organic and aqueous phases
were obtained. The organic layer was retained while the aqueous layer was discarded.
The product thus collected was added drop wise to 300 ml diethyl ether in an ice-bath, with
continuous stirring. After stirring for a few minutes, the mixture was decanted and the
lignophenol portion settled at the bottom. Then entire ether portion was discarded and the
lignophenol portion was dissolved in acetone and filtered so as to remove acetone
insoluble impurities. The mixture of acetone and lignophenol was then concentrated by
vacuum evaporation and the concentrated solution was again subjected to decantation. The
final product obtained was identified as lignophenol by means of its FT-IR spectrum.
Lignophenol thus prepared was crosslinked with paraformaldehyde. 5 g of the
lignophenol was taken together with 50 ml of 72 wt. % sulphuric acid in a 300 mL
eggplant flask and stirred for a few minutes. Then 6.5 g paraformaldehyde was added and
stirred continuously for about 24 h at 100°C. After cooling, a 5 % sodium hydrogen
carbonate solution was slowly added to the reaction mixture and stirred for 3 h followed by
filtration. The obtained residue was washed with hot water and then by 1 g/L hydrochloric
acid solution. Finally the product was washed with cold distilled water until neutral pH
was achieved and was dried at 90 °C for 48 h in a convection oven.
Preparation methodology of the crosslinked lignocatechol is described in section
2.2.1. Crosslinked lignopyrogallol was prepared by following the same method as for
crosslinked lignocatechol by using pyrogallol in place of catechol.
3.2.4. Solid-state analysis. The scanning electron microscopy (SEM) analysis of the
adsorbents were done by using the JEOL model JSM 5200 scanning microscope. Rigaku
RINT – 8829 X-ray diffractometer was used to take x-ray diffraction spectrum (XRD).
The transmission electron microscopy (TEM) of crosslinked lignophenol and activated
carbon after adsorption of gold were taken by using JEOL model JSM – 2010 transmission
electron microscope. The SEM images of the adsorption gels employed in this study are
shown in Figure 3.2.
45
3.2.5. Batch wise adsorption tests. The adsorption behavior of the lignin derived
adsorption gels employed in the present work for Au (III), Cu(II), Fe(III), Pd(II), Pt(IV),
Sn(IV), and Zn(II) were individually examined at varying hydrochloric acid concentration.
Adsorption behavior of activated carbon for each of these metals was also tested for
comparison. For each metal ion, 0.2 mM metal solutions were prepared by varying the
concentration of hydrochloric acid (0.1 M to 10 M). 15 ml of each of the test solutions
were mixed together with 20 mg adsorption gels in stoppered flasks. The mixtures were
shaken for 24 h using a thermostated shaker maintained at 30 °C to attain equilibrium as
will be described later. The concentrations of metal ions before and after adsorption were
analyzed by Shimadzu model AA–6650 atomic absorption spectrophotometer.
Adsorption isotherms of Au(III) on the lignin-derived adsorption gels and activated
carbon were also examined. A number of test solutions of varying gold concentration from
0.2 to 10 mM were prepared in 0.5 M hydrochloric acid solution. 15 ml of each of these
solutions were mixed with 10 mg of adsorption gels and shaken for 100 h at 30 °C to attain
equilibrium. All the adsorption tests were repeated at least twice and the results were
reproducible with negligible differences.
3.2.6. Column experiment. A flow experiment for the separation of low concentration
Au(III) away from high concentration Pt(IV) and Pd(II) was conducted taking account of
the possibility of their co-existence in a number of practical applications. The column
setup was performed as similar to our previous work.8 A column packed with 0.2 g
crosslinked lignophenol was prepared. Prior to passing the test solution, the column was
conditioned with distilled water followed by 0.5 M hydrochloric acid solution. A test
solution containing 10 ppm Au(III), 100 ppm Pt(IV) and 100 ppm Pd(II) was prepared in
0.5 M hydrochloric acid. The mixture solution was then pumped at a constant flow rate of
6 ml/h through the column. The effluent solution was collected at definite time interval
using a fractional collector.
46
3.3. RESULTS AND DISCUSSION
3.3.1. Surface analysis. The surface of crosslinked lignophenol, as shown in Figure 3.2(a)
is very smooth as cracks and holes are apparently absent. On the contrary, crosslinked
lignocatechol and lignopyrogallol surfaces are rough and porous as shown in Figures
3.2(b) and 3.2(c) respectively. Similarly, the surface of activated carbon, as shown in
Figure 3.2(d) is characterized by a large number of micro-pores and cracks. The degree of
roughness of the adsorbent’s surfaces discussed herein is in the following order: activated
carbon » crosslinked lignocatechol › crosslinked lignopyrogallol › crosslinked lignophenol.
Figure 3.2. SEM micrographs of (a) crosslinked lignophenol, (b) crosslinked
lignocatechol, (c) crosslinked lignopyrogallol, and (d) activated carbon taken before
adsorption at 500 X magnification at 25 kV acceleration energy.
3.3.2. Effect of shaking time. Prior to the batch wise adsorption experiment, the
adsorption rate of gold on crosslinked lignophenol, lignocatechol and lignopyrogallol as
(a) (b)
(c) (d)
47
well as activated carbon was preliminarily measured so as to know the optimum time
required to attain the equilibrium. Time variation of the adsorption of Au(III) on
crosslinked lignophenol, lignocatechol and lignopyrogallol as well as activated carbon are
shown in Figure 3.3, which clearly shows that equilibrium is attained within one hour in
the case of activated carbon whereas it takes longer time in the case of lignin gels. Hence,
for batch analysis the test samples were shaken for 24 h. However, the test samples for the
measurement of adsorption isotherms were shaken for 100 h to attain optimum
equilibrium.
0.00
0.05
0.10
0.15
0.20
0 10 20 30 40
time / h
q / (
mol
/kg)
crosslinkedlignophenol
crosslinkedlignocatechol
crosslinkedlignopyrogallol
activatedcarbon
Figure 3.3. Kinetics of adsorption of Au(III) on crosslinked lignophenol, crosslinked
lignocatechol, crosslinked lignopyrogallol, and activated carbon. Initial concentration of
Au(III) = 0.2 mM, wt. of adsorbent = 20 mg, [HCl] = 0.5 M.
3.3.3. Effect of hydrochloric acid concentration on the adsorption of some metal ions.
3.3.3.1. Crosslinked lignophenol. Figure 3.4 shows the % adsorption of Au(III), Cu(II),
Fe(III), Pd(II), Pt(IV), Sn(IV) and Zn(II) on crosslinked lignophenol at varying
hydrochloric acid concentration. As seen from this figure, crosslinked lignophenol has
proved to be selective only for Au(III) and the % adsorption of gold is nearly equal
regardless of hydrochloric acid concentration. This result is quite interesting because the
gel is almost completely inert not only towards ferric, stannic, cupric and zinc ions, which
48
usually coexist together with gold, but also towards Pt(IV) and Pd(II) ions, which means
that crosslinked lignophenol has the highest affinity for Au(III). In order to elucidate the
effect of other metal ions on the adsorption of Au(III), mixtures containing Au(III) and
Cu(II) at varying hydrochloric acid concentration were prepared to carry out the adsorption
tests as a typical coexisting system. The result in this case was the same as that for the
individual system; that is, a similar % adsorption has been observed for Au(III) and was
still inert for Cu(II). Such a high selectivity towards Au(III) as mentioned above has not
been reported so far.
0
10
20
30
40
50
0 2 4 6 8[HCl] / M
%Ad
sorp
tion
Au(III)
Fe(III)
Cu(II)
Pd(II)
Zn(II)
Pt(IV)
Sn(IV)
Figure 3.4. Variation of adsorption behavior of crosslinked lignophenol for different
metal ions with hydrochloric acid concentration. Initial concentration of metal ions = 0.2
mM, wt. of gel = 20 mg, shaking time = 24 h, temperature = 30 °C.
3.3.3.2. Crosslinked lignocatechol. As shown in Figure 3.5, similar to crosslinked
lignophenol, lignocatechol showed highest selectivity for Au(III) and remained almost
inert towards Pd(II), Cu(II), and Zn(II). However, unlike crosslinked lignophenol, it
exhibited some selectivity towards Pt(IV) and Fe(III). In addition, a small amount of
Sn(IV) was also adsorbed. Although % adsorption of Au(III) decreased with increasing
hydrochloric acid concentration, nearly constant % absorption was observed for Pt(IV)
over all hydrochloric acid concentration whereas significant adsorption of Fe(III) was
observed at concentration greater than 4M and it increased with increasing hydrochloric
acid concentration. Although crosslinked lignophenol and lignocatechol are different only
49
by the immobilized phenol group, the later exhibited some different adsorption behavior in
strongly acidic system.
0
20
40
60
80
100
0 2 4 6 8[HCl] / M
%Ad
sorp
tion
Au(III)
Cu(II)
Sn(IV)
Pd(II)
Fe(III)
Pt(VI)
Zn(II)
Figure 3.5. Variation of adsorption behavior of crosslinked lignocatechol for different
metal ions with hydrochloric acid concentration. Initial concentration of metal ions = 0.2
mM, wt. of gel = 20 mg, shaking time = 24 h, temperature = 30 °C.
3.3.3.3. Crosslinked lignopyrogallol.
0
20
40
60
80
100
0 2 4 6 8[HCl] / M
%A
dsor
ptio
n
Au(III)
Cu(II)
Sn(IV)
Pd(II)
Fe(III)
Pt(VI)
Zn(II)
Figure 3.6. Variation of adsorption behavior of crosslinked lignopyrogallol for different
metal ions with hydrochloric acid concentration. Initial concentration of metal ions = 0.2
mM, wt. of gel = 20 mg, shaking time = 24 h, temperature = 30 °C.
50
The adsorption behavior of crosslinked lignopyrogallol for Au(III), Cu(II), Fe(III),
Pd(II), Pt(IV), and Zn(II) as a function of concentration of hydrochloric acid is shown in
Figure 3.6. Similar to crosslinked lignophenol and lignocatechol, it exhibited highest
selectivity for Au(III) over whole hydrochloric acid concentration and was almost inert
towards Cu(II), Pd(II), and Zn(II) though it also has shown some adsorption for Pt(IV) and
Fe(III). Also, in this case a small amount of Sn(IV) was adsorbed. Adsorption of Pt(IV)
slightly increases with increasing hydrochloric acid concentration while Fe(III) adsorption
took place at concentration higher than 4M, which means the gel is selective only for
Au(III) and Pt(IV) at the hydrochloric acid concentration between 0.5 and 2 M.
From the above-mentioned experimental results we can draw some conclusions as
follows. The most important is that crosslinked lignophenol is the most suitable gel for
selective recovery of gold compared to crosslinked lignocatechol and lignopyrogallol.
Despite having some adsorption tendencies towards metals ions other than Au(III),
crosslinked lignocatechol and lignopyrogallol also can be used for the selective recovery of
Au(III) because both of them are highly selective for Au(III) owing to their ability of
reduction and formation of elemental gold. Differing only by the immobilizing groups, all
the novel lignin gels are capable of recovering gold in elemental form.
3.3.3.4. Activated carbon. Adsorption tests were also carried out for comparison for
activated carbon, the most versatile adsorbent, which has been commercially employed for
the recovery of gold, for example, from spent gold plating solution containing cyanide.
Figure 3.7 shows the % adsorption of Au(III), Cu(II), Fe(III), Pd(II), Pt(IV), Sn(IV) and
Zn(II) at varying hydrochloric acid concentration. Unlike the lignin gels, activated carbon
exhibited considerable adsorption for all metals tested except for Cu(II). For example, at
0.5 M hydrochloric acid concentration, almost complete adsorption was observed not only
for Au(III) but also for Pd(II), Pt(IV), and Sn(IV) though Au(III) adsorption was nearly
quantitative over the whole hydrochloric acid concentration. Also, in this case adsorption
of Fe(III) increased with increasing hydrochloric acid concentration at the concentration
greater than 2 M.
51
0
20
40
60
80
100
0 2 4 6 8[HCl] / M
%A
dsor
ptio
n
Au(III)
Cu(II)
Sn(IV)
Pd(II)
Fe(III)
Pt(IV)
Zn(II)
Figure 3.7. Variation of adsorption behavior of activated carbon for different metal ions
with hydrochloric acid concentration. Initial concentration of metal ions = 0.2 mM, wt. of
gel = 20 mg, shaking time = 24 h, temperature = 30 °C.
3.3.4. Adsorption isotherms of gold.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0Ce / mM
q / (
mol
/kg)
crosslinked lignopyrogallol
crosslinked lignophenol
crosslinked lignocatechol
activated carbon
Figure 3.8. The adsorption isotherm of the adsorption of Au(III) on crosslinked
lignophenol, crosslinked lignocatechol, crosslinked lignopyrogallol, and activated carbon.
[HCl] = 0.5 M, shaking time = 100 h, temperature = 30 °C, wt. of gel = 10 mg.
52
Figure 3.8 shows the adsorption isotherms of gold on crosslinked lignophenol,
lignocatechol, and lignopyrogallol, as well as activated carbon. From the batch wise
adsorption tests of gold as presented in Figures 3.4 – 3.7, we came to know that for the
hydrochloric acid concentration more than 0.1 M, the effect of acid concentration in the
adsorption of Au(III) is not very apparent. Hence, 0.5 M hydrochloric acid was selected as
the representative medium to carry out the adsorption isotherm test. In all cases, the
adsorption exhibited Langmuir type adsorption, i.e., it increased with increasing gold
concentration in its low concentration region and reached constant values corresponding to
each adsorbent in its high concentration region. From the constant values, the maximum
adsorption capacities for Au(III) were evaluated as 1.9, 2.4, 1.9, and 2.5 mol/kg dry gel,
for crosslinked lignophenol, lignocatechol, lignopyrogallol, and activated carbon,
respectively. These values are comparable to each other. Even though the Langmuir type
adsorption isotherm was depicted in the adsorption of Au(III) on lignin gels and
activated carbon, because of the formation of elemental gold during adsorption it was
difficult to correlate the observed data with the linearized Langmuir adsorption model,
which basically represents the monolayer adsorption.
3.3.5. Solid state analysis of gels after adsorption.
37.98
44.26
64.5277.50
0
200
400
600
0 20 40 60 80
inte
nsity
/ a.
u
2θ / degree
Figure 3.9. XRD pattern obtained for crosslinked lignophenol after adsorption of Au(III).
53
Figure 3.10. SEM images of the crosslinked lignophenol recorded after the adsorption of
Au(III). (a) 500x magnification and (b) 5000x magnification; acceleration voltage = 25
kV; scale = 50 µm and 5 µm respectively.
Some instrumental analysis of the gels as well as activated carbon after adsorption
were carried out to ascertain the formation of elemental gold during Au(III) adsorption.
The X–ray diffraction analyses of crosslinked lignophenol, lignocatechol, and
lignopyrogallol as well as activated carbon were performed after adsorption of Au(III).
Almost same patterns were observed for all the adsorbents. XRD-spectrum obtained in the
case of crosslinked lignophenol is as shown in Figure 3.9. The existence of sharp peaks
close to the scattering angles of gold viz: 38, 44.5, 64, and 77 degrees in the figure proves
the formation of elemental gold during the adsorption of Au(III). The SEM images of
crosslinked lignophenol after Au(III) adsorption is shown in Figures 3.10(a) and (b) at
different magnifications. Gold particles of different dimension are clearly seen on the gel
surface. Since, micropores and cracks are seldom observed on the surface of lignophenol
gel, as shown in Figure 3.2, significant physical adsorption is not likely to occur. In fact,
such type of surface structure is considered to be beneficial for the recovery of gold
particles. After 100 h of shaking, fine gold particles floating on the surface of hydrochloric
acid solution were observed in all cases of crosslinked lignophenol, lignocatechol, and
lignopyrogallol as shown in Figure 3.11. Although, they were grown up to larger
aggregates and sank down at the bottom of the container, they were totally distinct and
separable from the gel particles. A different result was observed in the case of activated
(a) (b)
54
carbon. Although the formation of elemental gold is proved by XRD spectrum, the
aggregated particles were not observed. These differences are attributed to the highly
porous structure of activated carbon; that is, the fine gold particles are prevented from
forming big aggregates in limited space of micro pores therein.
In order to understand the aggregation behavior of gold nano particles on the
surface of crosslinked lignophenol gel and activated carbon, the transmission electron
microscopy (TEM) images the adsorbents after adsorption of Au(III) were observed.
Figures 3.12 and 3.13 show the TEM images for gold loaded crosslinked lignophenol gel
and activated carbon, respectively. It is clearly seen from these figures that the gold
particles in the activated carbon are of much smaller size than that on crosslinked
lignophenol surface. Because of very high physical adsorption tendency of activated
carbon caused by higher van der Waals force, the nano-sized particles on its surface are
inferred to be prevented from forming aggregates. Besides, the surface of activated carbon
consists of a number of micropores making up microchannels of similar size. Once the
gold particles are formed and get occupied in pores or channels, they are prevented from
aggregating to form bigger particles because of the size factor. In contrast, crosslinked
lignophenol gel seems to rarely possess micropores and hence exhibits lower physical
adsorption capacity that facilitates the formation of micro-aggregates from the nano
particles. In other words, the gold nano particles formed on crosslinked lignophenol
surface by the reduction of Au(III) are free to attract each other to form larger aggregates.
Figure 3.11. Fine gold particles are floating on the surface of adsorption mixture
containing hydrochloric acid solution, Au(III) and crosslinked lignophenol gel.
Gold Particles
55
Figure 3.12. TEM images of gold micro particles obtained after the adsorption of Au(III)
on crosslinked lignophenol at different magnifications.
Figure 3.13. TEM images of gold micro particles obtained after the adsorption of Au(III)
on activated carbon at different magnifications.
On the basis of the observation of the formation of gold particles from Au(III)
solution during adsorption on lignin gels as well as on activated carbon, it is supposed that
a redox system has been developed during the adsorption. Au(III) itself is a good
oxidizing agent with a standard reduction potential of +1.40 V. In addition, there are a
large number of phenolic and polyphenolic hydroxyl groups along with carbonyl and ether
56
functions in lignophenol molecule. These oxygen-containing functional groups like
hydroxyl groups in the present case always tend to be oxidized in aqueous medium. The
change in potential of the adsorption system may explain the mechanism in depth. But
prediction of the potential of lignin gels is certainly a more challenging task and needs to
perform more elaborated electrochemical studies.
400140024003400Wavenumber / cm-1
%T
before adsorption after adsorption
9501150135015501750
Figure 3.14. FT-IR spectrum of crosslinked lignophenol taken before and after the
adsorption of Au(III) and the enlarged view of 950 to 1750 cm-1 region.
In order to understand the structural change of crosslinked lignophenol after
adsorption, the FT–IR spectra were taken before and after the adsorption for comparison.
As shown in Fig. 3.14, a sharp difference is observed in the FT–IR spectra taken before
and after the adsorption of Au(III). Because the structure of crosslinked lignophenol is
very complex, a complete diagnosis of the spectra seems a tough job. A tentative
57
comparison made with reference to the structural change of humic substances during
Au(III)-adsorption signifies changes in –OH as well as aromatic and aliphatic ether
functions.10, 11 The typical –OH stretch region near 3350 to 3500 cm-1 has flattened and
the intensity also has decreased. Other changes are in the 950 to 1750 cm-1 range. Peaks
of 1450 and 1110 cm-1 probably ascribable to secondary alcohol and ether functions
completely disappear after the adsorption whereas the intensity of a broad band at 1190–
1230 cm-1 that is expected to be of phenol or of aromatic-aliphatic ether function has
sharply decreased. The sharp increase in the intensity of the peak at 1690–1720 cm-1 is
considered to be attributable to the increase in the number of C=O functions in the
crosslinked lignophenol molecule. Because of the complex and heterogeneous functional
structure of crosslinked lignophenol, it is very difficult to assign the peaks in Figure 3.14
precisely. Nevertheless, considering –OH groups as the major functional group of the gel,
the following reactions are proposed to be the driving actions for the reduction of Au(III).
R–CH2OH = RCHO + 2H+ + 2e– (1)
R–CHOH–CH3 = RCO–CH3 + 2H+ + 2e– (2)
Ph–OH = Ph=O + H+ + e– (3)
Or in general, (4)
Considering the potential application of these novel gels and especially crosslinked
lignophenol have proved far better than activated carbon for the selective separation of
gold particles from other base and precious metals. Although, activated carbon is also
capable of reducing Au(III) to elemental gold, it is costly and difficult to take out the
particles away from it. Charring the gold impregnated activated carbon involves the risk of
loss of some gold by evaporation. In contrast, the lignin based gels prepared by
immobilizing phenolic function in wood lignin can become very efficient in producing
gold nuggets from Au(III) in hydrochloric acid system. In addition, since the surface of
the gels lack cracks and holes, gold particles are free to form aggregates, which can be
easily separated.
P + H2O = + 2H+ + 2eOH P O
58
3.3.6. Chromatographic separation of Au(III) away from Pd(II) and Pt(IV). From the
batch wise tests, crosslinked lignophenol was confirmed as an outstanding innovation for
gold separation. To intensify the results, a breakthrough test was carried out in order to
separate low concentration of Au(III) from Pd(II) and Pt(IV) by using a column packed
with crosslinked lignophenol. In the breakthrough curve as shown in Figure 3.15, it is
clearly seen that Pt(IV) and Pd(II) were brokenthrough immediately after the start of the
operation, but breakthrough of Au(III) was observed after 20 h. From the calculation of
area of the breakthrough curve, adsorption capacity of the gel was evaluated as 0.05
mol/kg which is much lower than that evaluated in the batchwise test. The big difference
between the value of adsorption capacity calculated from adsorption isotherm and from the
area of breakthrough curve can be attributed to the short contact time; i.e. it may be
considered that the retention time in the packed bed was too short for Au(III) ions to be
reduced to gold particle and get aggregated.
0
20
40
60
80
100
0 10 20 30 40 50 60 70Time / h
[Pd(
II)] o
r [P
t(IV
)] / p
pm
0
2
4
6
8
10
[Au(
III)]
/ ppm
Pd(II)
Pt(IV)
Au(III)
Figure 3.15. Breakthrough profiles of Au(III), Pt(IV) and Pd(II) from the column packed
with crosslinked lignophenol. Feed rate = 6 ml/h, wt. of crosslinked lignophenol packed in
column = 0.2 g, initial concentration of Pt(IV) and Pd(II) ≈ 100 ppm, and that of Au(III) ≈
10 ppm, [HCl] = 1 M, temperature = 5~20 °C ( room temp).
59
3.4. CONCLUSIONS
The present study indicates that all of crosslinked lignophenol, lignocatechol, and
lignopyrogallol are capable of Au(III) uptake with significant capacity. All these gels act
as reducing agents of ionic gold species rather than as complexing adsorbents. The
capacity of the activated carbon that was used for comparison was also remarkable. It was
also found to act as reductant. Among the three lignin gels tested, crosslinked lignophenol
exhibited the highest selectivity. It was found to be much more selective than activated
carbon under the conditions studied. Although the complete mechanism is considered to
be tough to understand because of the complex chemical structure of lignin, the oxidation
of hydroxyl and ether functional groups was supposed to be the main electron donating
reaction. As such a high selectivity for gold has not been observed to the date, crosslinked
lignophenol can be superior over other commercial separation agents like activated carbon
and can be utilized for the selective and effective recovery of gold from aqueous chloride
system.
REFERENCES
1. http://www.btplc.com/society
2. http://www.docomo-tokai.co.jp/2003/normal-hp/main/profile/eco/recycle/
3. Dähne, W. GOLD – Progress in Chemistry, Biochemistry and Technology; ed. by
Schmidbaur, H., John Wiley and Sons, 1999, 119-141
4. Puvvada, G. V. K., Sridhar, R., Lakshmanan, V. I. Chloride Metallurgy: PGM
Recovery and Titanium Dioxide Production. JOM 2003, 55(8), 38.
5. Flett, D. S. HYDROMETALLURGY Research, Development and Plant Practice; ed. by
Osseo-Asare, K.; Miller, J. D. The Metallurgical Society of AIME, Warrendale, 1982,
39-64.
6. Morris, D. F. C.; Khan, M. A. Application of solvent extraction to the refining of
precious metals–III Purification of gold. Talanta 1968, 15, 1301.
60
7. Parajuli, D.; Inoue, K.; Kuriyama, M.; Funaoka, M.; Makino, K. Reductive Adsorption
of Gold (III) by Crosslinked Lignophenol. Chem. Lett. 2005, 34, 34.
8. Funaoka, M. A new type of phenolic lignin-based network polymer with the structure-
variable function composed of 1,1-diarylpropane units. Polym. Int. 1998, 47, 277.
9. Parajuli, D.; Inoue, K.; Murota, A.; Ohto, K.; Oshima, T.; Funaoka, M.; Makino, K.
Adsorption of Heavy Metals on Crosslinked Lignocatechol – A Modified Lignin Gel.
React. Func. Polym. 2005, 62, 129.
10. Gatellier, J. P.; Disnar, J. R. Kinetics and mechanism of reduction of Au(III) to Au(0)
by sedimentary organic materials. Org. Geochem. 1990, 16, 631.
11. Machesky, M. L.; Andrade, W. O.; Rose, A. W. Interactions of gold (III) chloride and
elemental gold with peat-derived humic substances. Chem. Geol. 1992, 102, 53.
61
Chapter 4
RECOVERY OF GOLD (III), PALLADIUM (II) AND
PLATINUM (IV) BY AMINATED LIGNIN DERIVATIVES
Conventional metallurgical processes of precious metals involve the use of large
amount of toxic chemicals. Realizing a need to develop environmentally benign
metallurgical technology for precious metals, two types of adsorption gels, containing the
functional group of primary amine and ethylene diamine abbreviated as PA–lignin and
EN–lignin, respectively, were prepared from wood powder. Both of the adsorption gels
were found to be effective for the adsorption of Au(III), Pd(II) and Pt(IV) from weak to
strong hydrochloric acid medium. However, base metals such as Cu(II), Fe(III), Ni(II) and
Zn(II) were almost not adsorbed on both the gels. The above mentioned precious metals
were adsorbed on the gels according to the Langmuir adsorption model where the highest
maximum adsorption capacity was observed for Au(III). The formation of ion pairs of
metal–chloro complex anions and protonated adsorption gels in acidic medium was
proposed to be the main adsorption process. But, in the case of adsorption of Au(III), a
reductive adsorption mechanism was confirmed to be taken place with reference to XRD–
spectrum and SEM images of the gels taken after adsorption.
4.1. INTRODUCTION
Nowadays, precious metals such as gold, palladium and platinum are in extensive
use not only as traditional jewelries but also as useful component in a variety of well
known advanced materials such as electric and electronic devices, catalysts, and medical
62
instruments. Since these metals have limited resources existing only in a small amount on
the earth; it must be ensured that such metals should be effectively recovered from various
wastes for recycling and reusing purposes. From the economical point of view, the
recovery process should be such that the precious metals are highly selectively separated
from excessively co-existing base metals like iron, copper and zinc.
The conventional and traditional separation and refining processes of precious
metals consist of a series of dissolution using aqua regia – conditioning – precipitation,
which is inefficient in terms of the degree of purification, yields, operational complexity,
energy consumption and labor costs. Instead of these traditional processes, attempts have
been made in recent years to develop new processes consisting of total leaching in
hydrochloric acid containing chlorine gas or hypochlorite followed by solvent extraction
and ion exchange and they have also been employed by some refineries.1 In the new
processes, solvent extraction reagents or ion exchangers with high selectivity and high
loading capacity are strongly required to simplify the process and to lower the costs. For
this purpose some new techniques for precious metals recovery have been developed and
some of them were employed for commercial purposes.2-11 However, the solvent extraction
reagents and chelating resins which have been developed to the date have not necessarily
accomplished satisfactory separation and recovery as expected. In addition, such reagents
and resins are expensive and are counterproductive for the environment.
In order to maintain a sustainable society it should be realized in the near future
that majority of reagents, solvents and resins produced from petroleum should be replaced
by environmentally benign alternatives derived from biomaterials. From this perspective,
our focus is now towards developing adsorption gels from a variety of biomass wastes for
metal separation. At present, we are developing different kinds of adsorption gels based
on plant lignin extracted from waste wood for the purpose of selective recovery of various
precious metals.
In this chapter, the incorporation of ligands of primary amine and ethylene diamine
onto crosslinked lignophenol to prepare two types of aminated crosslinked lignophenol
derivatives is discussed. These products were employed for the selective separation of
gold, palladium, and platinum from other coexisting metals taking into consideration that
these nitrogen containing compounds belonging to soft bases have high selectivity to
precious metals belonging to typical soft acids.
63
4.2. EXPERIMENTAL
4.2.1. Preparation of crosslinked lignophenol. The preparation process of crosslinked
lignophenol gel is discussed in section 3.2.3. of chapter 3.
4.2.2. Preparation of aminated crosslinked lignophenol. Two types of aminated
crosslinked lignophenol, those containing functional groups of primary amine and ethylene
diamine, which are abbreviated as PA–lignin and EN–lignin, respectively, were prepared
from the crosslinked lignophenol using different aminating reagents.
4.2.2.1. Preparation of PA–lignin.
H2C
OH
CH
OCH3
O
HC LigninCH2OH
CH2
+ SOCl2343 K
H2C
OH
CH
OCH3
O
HC LigninCH2Cl
CH2
348K
H2C
OH
CH
OCH3
O
HC LigninCH2NH2
CH2
Crosslinked lignophenol Chlorinated Crosslinked lignophenol
+ NH3 aq
Aminated Crosslinked lignophenol (PA-lignin)
H2C
OH
CH
OCH3
O
HC LigninCH2Cl
CH2
Chlorinated Crosslinked lignophenol
Thionyl Chloride
Scheme 4.1. Preparation of PA–lignin.
64
PA–lignin was prepared according to the reaction shown in Scheme 4.1. In a 3-
necked flask, 5 g crosslinked lignophenol was taken together with 30 mL of 0.84 M thionyl
chloride. Then 200 mL pyridine was added and stirred continuously for 5h at 70 °C. The
chlorinated product was washed with distilled water and dried for 5h at 50 °C in
convection oven. 5 g chlorinated crosslinked lignophenol was taken in a 3-necked flask
and kept in an ice bath. Then 40 mL of 1.18 M ammonia water was added. After stirring
for a few minutes, 100 mL dimethyl formamide (DMF) was added and the mixture was
stirred continuously for 48 h at 75 °C. The product obtained was washed with distilled
water and dried in a convection oven for 24 h at 50 °C.
4.2.2.2. Preparation of EN–lignin. EN–lignin was prepared according to the reaction
shown in Scheme 4.2. In a 300 mL 3-necked flask, 5 g crosslinked lignophenol was taken
together with 50 mL dimethyl sulfoxide (DMSO) and stirred for a few minute. Then 8 g
sodium carbonate and 22 mL ethylene diamine were added. The mixture was stirred
continuously for 24 h at 80 °C. Subsequently, the reaction mixture was filtered and the
residue was washed first with 0.1 M hydrochloric acid solution followed by distilled water
until neutral pH was achieved. The product was collected and dried in convection oven for
24 h at 70 °C.
H2C
OH
CH
OCH3
O
HC LigninCH2OH
CH2
NH2(CH2)2NH2DMSO, Na2CO3
353 K
H2C
OH
CH
OCH3
O
HC LigninCH2NHCH2CH2NH2
CH2
Crosslinked lignophenol Aminated Crosslinked lignophenol (EN-lignin)
+
Ethylenediamine
Scheme 4.2. Preparation of EN–lignin
4.2.3. Identification of aminated lignin gels. The amination of crosslinked lignophenol
was confirmed by FT-IR analysis and elemental analysis of the final products. The
65
observed and estimated percentages of different elements in PA–lignin and EN–lignin are
given in Table 1. The % contribution of individual elements cited in Table 1 was
evaluated from the tentative structure of aminated crosslinked lignophenol. From the
observed % and calculated % of nitrogen, the degree of amination or functionalization in
EN–lignin was calculated as 23.5 % and that in PA–lignin was calculated as 70.1 %,
according to equation (1).
)1(100%
%% ×=Nitrogenofwtcalculated
NitrogenofwtobservedizationFunctional
Table 4.1. Observed and calculated % of different elements in PA–lignin and EN–lignin.
Gels PA–lignin EN–lignin
Elements H C N H C N
Observed % 5.32 61.78 3.30 5.80 67.69 3.83
Calculated % 6.39 72.72 4.71 7.05 70.58 8.23
4.2.4. Surface analysis.
a. b.
Figure 4.1. SEM images obtained at 500x magnification of (a) EN–lignin and (b) PA–
lignin. Acceleration voltage = 20 kV and 30 kV, respectively.
66
SEM micrographs of PA–lignin and EN–lignin were taken using a JEOL model
JSM 5200 scanning electron microscope operated at an acceleration voltage of 25 kV. The
images at a magnification of 500x are shown in Figures 4.1(a) and 4.1(b). From these
figures, it is clear that the gel surfaces are relatively smooth with low degree of porosity.
4.2.5. Metal species for adsorption tests. For the preparation of test solutions of Cu(II),
Fe(III), Ni(II), Pd(II), and Zn(II), analytical grade chloride salts of the respective metals
were used. Analytical grade HAuCl4.4H2O and H2PtCl6.6H2O were used to prepare
Au(III) and Pt(IV) test solutions, respectively.
4.2.6. Batch wise adsorption tests. As mentioned earlier, commercial recovery or
separation of precious metals is carried out from hydrochloric acid solutions, adsorption
behaviors of EN–lignin and PA–lignin for Au(III), Pd(II) and Pt(IV) as well as for some
base metals were examined batch-wise at varying concentrations of hydrochloric acid.
Similarly, adsorption of behavior of DIAION WA 30, commercially available weakly
basic anion exchange resin produced and marketed by Mitsubishi Chemical Corp.,
containing functional group of dimethylamine incorporated on matrices of polystyrene was
tested under the same condition for comparison.
The adsorption tests of precious metals, namely Au(III), Pd(II) and Pt(IV), as well
as of some base metals from highly acidic medium were conducted using solutions of
nearly the same molarity of individual metal ions dissolved in varying concentrations of
hydrochloric acid solution. For the measurement of the adsorption isotherm of Au(III),
Pd(II) and Pt(IV), a range of test solutions (0.3 to 6 mM) were prepared by dissolving
weighed quantity of respective chloride salts in 0.5 M hydrochloric acid solution. 15 mL
of each test solutions were mixed together with 20 mg of adsorption gel in a stoppered
flask. The flasks were then shaken in a thermostated shaker maintained at 30 °C for 100 h
to attain equilibrium as will be mentioned later.
The pH of the test solution was measured by using a BECKMAN model ф-45 pH
meter. The concentration of hydrochloric acid before and after adsorption was measured
by titration using phenolphthalein as an indicator. The metal concentrations in aqueous
solutions before and after adsorption were measured by using Shimadzu model AA-6650
67
atomic absorption spectrophotometer and Shimadzu model ICPS-100 III ICP-AES
spectrometer.
4.2.7. Breakthrough followed by elution experiments using a packed column.
Breakthrough followed by elution tests using a column packed with PA–lignin gel was
carried out by using a column packed with 0.2 g gel fitted with EYELA model micro tube
pump and a programmable BIO-RAD model 2110 fraction collector, the details of which is
described in section 2.2.4. of chapter 2. The feed solution was prepared by mixing equal
volume of 0.4 mM Pd(II) and 4 mM Cu(II) prepared in 0.5 M hydrochloric acid solution.
The packed column was conditioned by passing distilled water for a few hours followed by
0.5 M hydrochloric acid. After about 12 h conditioning, the feed solution was pumped
through the column at a flow rate of 4 mL/h. Effluent samples for analysis were collected
at a required time intervals using the fraction collector. After the breakthrough of both
metal ions, a mixture of 0.2 M thiourea and 0.01 M hydrochloric acid solution was fed to
carry out the elution test. The concentrations of both the metal ions in the effluent samples
were measured by using a Shimadzu model ICPS-100 III ICP-AES spectrometer.
4.3. RESULTS AND DISCUSSION
4.3.1. Protonation capacity of EN–lignin and PA–lignin. Adsorption tests of varying
concentrations of hydrochloric acid on EN–lignin and PA–lignin were carried out in order
to know the protonation capacity of the gels. As shown in Figure 4.2, for both gels the
amount of adsorption increases with the increasing concentration of acid until tending to
reach a maximum value, from which PA–lignin and EN–lignin were evaluated to have
maximum adsorption capacities of 0.90 and 0.98 mol/kg dry weight of the gel for
hydrogen ion, respectively. Adsorption of hydrochloric acid on EN–lignin and PA–lignin
takes place according to the reactions described by Eqs. (2) and (3), respectively.
)2(2).)((2)( 3222222−++ −−−↔+−−− ClNHCHNHRHClNHCHNHR
)3().( 32−+−↔+− ClNHRHClNHR
68
0.980.90
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-8 -6 -4 -2 0log[H+] / M
q / (
mol
/ kg)
EN-lignin
PA-lignin
Figure 4.2. Proton adsorption properties of PA–lignin and EN–lignin.
The protonated gels get paired with co-existing anions, in the present case, with chloride
ions, which means that the higher the degree of protonation then higher will be the anion
exchange tendency of the gels. Although EN–lignin consists of two amino groups, its
capacity is only slightly higher than that of PA–lignin, which can be attributed to the low
degree of amination in the formers case as suggested from Table 4.1.
4.3.2. Kinetics of adsorption of Au(III), Pd(II), and Pt(IV). The kinetics of adsorption
of Au(III), Pd(II), and Pt(IV) on EN–lignin and PA–lignin are shown in Figures 4.3(a) and
4.3(b), respectively. As evident from these figures, in the case of Pd(II), and Pt(IV)
equilibrium has been attained within a few hours. But, an strange pattern has been
observed for Au(III). Different from the usual adsorption kinetics, a gradual decrease in the
amount of Au(III) adsorbed was observed at the initial stage of adsorption and a gradual
increase was observed after 100-200 min of the apparent steady state. In order to explain
this peculiar observation, the change in concentration of Au(III) was tracked by means of
UV spectroscopy.
69
0.00
0.10
0.20
0.30
0.40
0 50 100 150 200 250 300
Time / min
q / (
mol
/kg)
Pt(IV)
Pd(II)
Au(III)
(a)
0.00
0.15
0.30
300 400 500 600(a')
0.00
0.10
0.20
0.30
0.40
0 50 100 150 200 250 300Time / min
q / (
mol
/Kg)
Pt(IV)
Pd(II)
Au(III)
(b)
0.00
0.10
0.20
0.30
300 400 500 600(b')
Figure 4.3. Kinetics of adsorption of Au(III), Pd(II), and Pt(IV) on (a) EN–lignin and
(b) PA–lignin. The kinetic data taken after 5 hours for Au(III) are displayed in (a’) and
(b’) respectively. Initial metal concentration = 0.5 mM, wt. of gel = 10 mg, volume of test
solution = 10 mL, concentration of HCl = 0.5 M, temperature = 30 °C.
The peculiar nature of Au(III) adsorption kinetics on EN–lignin and PA–lignin can
be seen from Figure 4.4. As shown in the figure, although a gradual decrease in
concentration of Au(III) was evaluated from the absorbance observed at 314 nm (the λmax
for Au(III)), a gradual increase in total concentration of gold with increase in time was
observed from AAS measurement. The gradual decrease in equilibrium concentration of
Au(III) implies a gradual adsorption of Au(III) ion onto the gel confirming to an usual
adsorption kinetics. But the peculiar case of total gold concentration can be explained only
in terms of the change in oxidation state of some Au(III) once adsorbed leading to the
70
formation of new ionic species, most probably Au(I), to which the gels are not selective.
Due to this reason, a pseudo stationary state or pseudo equilibrium is observed in Figures
4.3(a) and 4.3(b). This state represents a state of equilibrium between adsorption of
Au(III) and elution of the new species. A further increase in the amount of metal adsorbed,
q, after some time leads ultimately to a true equilibrium state. The final section of kinetics
curve most probably represents the formation of elemental gold, which will be discussed in
detail in the following sections.
0.25
0.26
0.27
0.28
0.29
0.3
0 20 40 60
Time / min
Ce (
tota
l) / m
M
0.00
0.03
0.06
0.09
0.12
0.15
[Au(
III)]
/ mM
Ce(total) (PA-lignin)
Ce(total) (EN-lignin)
[Au(III)] (PA-lignin)
[Au(III)] (EN-lignin)
Figure 4.4. Variation of total equilibrium concentration of gold and equilibrium
concentration of Au(III) observed during the kinetics study. Initial metal concentration =
0.5 mM, wt. of gel = 10 mg, volume of test solution = 10 mL, concentration of HCl = 0.5
M, temperature = 30 °C.
4.3.3. Batch wise adsorption test. Figure 4.5(a) shows the adsorption behavior EN–
lignin for some metal ions at varying hydrochloric acid concentration. It is clear that the
gel is almost inert towards Cu(II), Fe(III), Ni(II), and Zn(II) while it absorbs Au(III), Pd(II)
and Pt(IV). Likewise, Figure 4.5(b) shows the adsorption behavior on PA–lignin at
varying hydrochloric acid concentrations. Also in this case, % adsorption observed for
base metals is negligible while there is remarkable adsorption of precious metals. A
number of anion exchange resins are reported to exhibit high selectivity to precious metals
over base metals and some anion exchange resin or gels are found to be selective only for
71
precious metals.12-14 Although PA–lignin contains primary amino group and EN–lignin
contains ethylene diamine group, both of them are found to exhibit quite similar adsorption
behavior. However, the selectivity order for Au(III), Pd(II) and Pt(IV) is different as
shown in Figures 4.5(a) and 4.5(b); that is the selectivity order observed for EN–lignin is
Au(III) › Pt(IV) › Pd(II) while that for PA–lignin is Au(III) › Pd(II) » Pt(IV).
a)
0
20
40
60
80
100
0 1 2 3 4 5 6 7[HCl] / M
% A
dsor
ptio
n
Au(III)
Cu(II)
Fe(III)
Ni(II)
Pd(II)
Pt(IV)
Zn(II)
b)
0
20
40
60
80
100
0 2 4 6[HCl] / M
% A
dsor
ptio
n
Au(III)
Cu(II)
Fe(III)
Pd(II)
Pt(IV)
Zn(II)
Figure 4.5. % adsorption of different metal ions on (a) EN–lignin and (b) PA–lignin
gels at varying equilibrium concentration of hydrochloric acid. Initial concentration of
metal ions = 0.2 mM, wt. of gel = 20 mg, volume of the test solution = 15 mL, temperature
= 30 °C, shaking time = 24 h.
72
Au(III), Pd(II), and Pt(IV) form anionic chloride complex of the form AuCl4–,
PdCl42–, and PtCl6
2– respectively in a wide range of hydrochloric acid concentration.
Based upon this principle the adsorption of metal chloro complexes on PA–lignin is
inferred to be in terms of the anion exchange reactions expressed by equation 4 and 5 of
Scheme 4 and that on EN–lignin is by equation 6 of Scheme 4.3, where the coordination
numbers (4 for Au(III) & Pd(II) and 6 for Pt(IV)) is satisfied by coexisting Cl– ions.
[ ] [ ] )4(. 33−−+−−+ +−↔+− ClMClNHRMClClNHR nn
[ ] [ ] )5(2.2 223
23
−−+−−+ +−↔+− ClMClNHRMClClNHR nn
( )[ ] ( )[ ] )6(22. 23222
23222
−−++−−++ +−−−↔+−−− ClMClNHCHNHRMClClNHCHNHR mm
Scheme 4.3. Adsorption mechanism
0
20
40
60
80
100
0 2 4 6[HCl] / M
% A
dsor
ptio
n
Au(III)
Cu(II)
Fe(III)
Pd(II)
Pt(IV)
Zn(II)
Figure 4.6. % adsorption of different metal ions on Diaion WA 30 at varying
equilibrium concentration of hydrochloric acid. Initial concentration of metal ions = 0.2
mM, wt. of gel = 20 mg, volume of the test solution = 15 mL, temperature = 30 °C,
shaking time = 24 h.
Figure 4.6 shows the similar plot of % adsorption against HCl concentration in the
case of the adsorption on DIAION WA 30 resin. The order of the selectivity to precious
73
metals was as follows: Au(III)>Pt(IV)>Pd(II). The order is the same with that of PA–
lignin gel. However, different from PA– and EN–lignin gels, appreciable amount of
adsorption of Zn(II) and Fe(III) was adsorbed on WA 30 resin while no adsorption of
Cu(II) was observed also on this resin. From this comparison, it is apparent that PA– and
EN–lignin gels exhibit much better selectivity to precious metals than WA 30 resin, a
commercially available resin.
4.3.4. Adsorption isotherms of Au(III), Pd(II), and Pt(IV). As both EN–lignin and
PA–lignin were found to be effective to uptake Au(III), Pd(II) and Pt(IV) ions over a wide
range of hydrochloric acid concentration (Figures 4.5(a) and 4.5(b)), the adsorption
isotherms of these metal ions on EN–lignin and PA–lignin were examined by conducting
adsorption tests from lower (0.2 mM) to higher (6 mM) concentration of metal ions at
constant hydrochloric acid concentration (= 0.5 M) as shown in Figure 4.7(a) and Figure
4.7(b), respectively. From these figures, it is clear that for both adsorption gels, adsorption
increases with increasing metal ion concentration at low concentration whereas a plateau
region is observed at high concentration irrespective of the metal ion concentration,
suggesting that the Langmuir type adsorption takes place under the given conditions.
From Figure 4.7(a) the maximum adsorption capacity of EN–lignin for Au(III), Pd(II) and
Pt(IV) were evaluated as 3.08, 0.213 and 0.536 mol/kg of dry gel, respectively. Similarly,
from Figure 4.7(b), the maximum uptake capacity of PA–lignin for Au(III), Pd(II) and
Pt(IV) are evaluated as 1.95, 0.38 and 0.22 mol/kg of dry gel, respectively. The results
suggest that both the lignin gels exhibited the highest capacity for Au(III) with
comparatively lower values for Pd(II) and Pt(IV). The capacity of PA–lignin is higher for
Pd(II) than for Pt(IV) while the reverse is the case for EN–lignin, which is in agreement
with the selectivity order shown in Figures 4.5(a) and 4.5(b).
Although both EN–lignin and PA–lignin are found to be effective adsorbents for all
the precious metals tested, their capacities for Au(III) are extraordinarily higher than that
for Pt(IV) and Pd(II). More exciting result was observed as shown in Figure 4.8. The
formation of gold particles was confirmed by physical analysis of the gels after 24 h
shaking and bigger aggregates of gold were observed after 100 h shaking. In Figure 4.8, as
will be described later, a layer of fine gold particles floating on the water surface was
74
observed; the layer formed aggregates on further shaking and dropped gradually to the
bottom.
a)
0.0
1.0
2.0
3.0
0.0 1.0 2.0 3.0 4.0 5.0Ce / mM
q / (
mol
/kg)
0
0.2
0.4
0.6
0.8Au(III)
Pd(II)
Pt(IV)
3.08
0.536
0.21
b)
0.0
0.5
1.0
1.5
2.0
2.5
0.0 1.0 2.0 3.0 4.0Ce / mM
q / (
mol
/kg)
0
0.1
0.2
0.3
0.4
0.5
Au(III)
Pd(II)
Pt(IV)
1.95
0.38
0.22
Figure 4.7. Adsorption isotherms of (a) EN–lignin and (b) PA–lignin for Au(III),
Pd(II), and Pt(IV) from 0.5 M hydrochloric acid and the corresponding Langmuir plots for
(c) EN–lignin and (d) PA–lignin.
This fact suggests that the adsorption of gold on aminated crosslinked lignophenol
is accompanied by the reduction of Au(III) ion to Au(0) similar to the result observed in
the adsorption of Au(III) on crosslinked lignophenol. In the study of adsorption kinetics, a
75
gradual increase in the amount of adsorbed gold was observed after the plateau region of
pseudo stationary state (Figures 4(a) and 4(b)), which indicates the gradual reduction of
Au(III) takes place after a few hours of shaking. AuCl4- is a powerful oxidizing agent in
solution.15 In addition, polyphenolic groups, which are the major constituents of lignin
molecule, are reported to reduce auric ion in aqueous acidic system.16 Furthermore,
oxidation of amino groups present in the aminated crosslinked lignophenol may take place
as shown in Scheme 4.4, which also contributes to the reduction of Au(III).17
R CH2
NH2 R CH
NH + 2H + 2e
P + H2O + 2H + 2eOH P O
−+−+− ++↔++ ClHAueHAuCl 4444 0
4
Scheme 4.4. Reduction of Au(III) by aminated crosslinked lignophenol in acidic medium.
Figure 4.8. Elemental gold formed during adsorption of Au(III) are observed as a
floating layer in on water surface the beginning which develops aggregates.
In order to confirm the formation of metallic gold, analysis by means of X–ray
diffraction (XRD) and scanning electron microscopy (SEM) of the adsorbent were carried
out after the adsorption. The 2θ values of the peaks as shown in Figure 3.9 were found to
be unerringly matching with those of gold, which proves the formation of elemental gold
during the adsorption. Similarly, SEM micrographs of the adsorbents before and after
76
adsorption have elucidated the morphology of the adsorbent along with that of gold
particles on the surface. Varying size of gold particles are observed in SEM micrographs
of EN–lignin gel taken after adsorption, as shown in Figure 4.9. In low concentration
region of gold (0.2 mM), micro level gold particles were observed whereas in high
concentration region gold micro particles were found to form aggregates resulting in the
formation of big gold particles that can be visually observed.
Figure 4.9. SEM image of EN–lignin taken after adsorption of Au(III) at 100x
magnification shows gold aggregates that are larger than the gel particles.
4.3.5. Breakthrough followed by elution tests. The lignin-based adsorption gels
developed in this work were found to be very selective for Au(III) and their capacity for
Pt(IV) and Pd(II) are also noteworthy. Among EN–lignin and PA–lignin, PA–lignin has
exhibited higher capacity for Pd(II), on the basis of which PA–lignin was used for the
mutual separation of traces of Pd(II) from large excess of Cu(II) using a packed column.
As shown in Figure 4.10(a), breakthrough of Cu(II) immediately took place after the start
of the feeding whereas breakthrough of Pd(II) took place after about 200 bed volumes.
After the complete breakthrough of Pd(II) ion, the elution test was carried out using an
aqueous mixture of 0.2 M thiourea and 0.01 M hydrochloric acid. Pd(II) was eluted in a
high concentration of about 10 times of the feed concentration as shown in Figure 4.10(b).
From the area of the breakthrough curve, the maximum adsorption capacity was evaluated
as 0.18 M/kg gel, which is about half of the value evaluated from the adsorption isotherm;
Au
Au
77
this might be attributable to the short contact time in the continuous flow because of slow
adsorption kinetics of Pd(II). Nevertheless, the chromatographic separation using PA–
lignin provides a promisingly effective separation and pre-concentration of traces amount
of Pd(II) from large excess of Cu(II).
(a)
0.0
0.4
0.8
1.2
1.6
2.0
0 200 400 600 800 1000 1200 1400 1600
B. V.
CC
u / m
M
0.00
0.04
0.08
0.12
0.16
0.20
CP
d / m
M
Cu(II)
Pd(II)
b)
0.0
0.5
1.0
1.5
0 50 100 150 200 250 300B.V
C /
mM
Pd(II)
Cu(II)
Figure 4.10. (a) Breakthrough profiles of Pd(II) and Cu(II) from the column packed with
PA–lignin. Feed concentration of Pd(II) = 0.186 mM, and that of Cu(II) = 1.914 mM,
[HCl] = 0.5 M, weight of adsorbent = 0.2 g, temperature = 30 °C, feed rate = 3.933 mL/l.
(b) Elution profiles of Pd and Cu from the loaded column by a mixture of 0.2M thiourea
and 0.01 M hydrochloric acid solutions. Feed rate = 3.94 mL/l.
78
4.4. CONCLUSION
The present study clearly establishes that both the PA–lignin and EN–lignin are
effective adsorbents for Au(III), Pd(II) and Pt(IV) over the wide range of hydrochloric acid
concentration. The formation of stable anionic chloro-complexes of these metal ions and
the protonation of active functional groups of the adsorption gels in acidic medium causing
into the anion exchange adsorption are concluded to be the driving force for the selective
adsorption of precious metals. Hence, these adsorbents can be precisely recommended for
the recovery of the valuable metals from mixtures containing the base metals.
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10. Sancher, J.M.; Hidalgo, M.; Salvado, V. The selective adsorption of Au(III) and
Pd(II) on new phosphine sulphide type chelating polymers bearing different spacer
arms: Equilibrium and kinetic characterization. Reac. Func. Polym. 2001, 46(3),
283–291.
11. Topp, K.D.; Grote, M. Synthesis and characterization of a 1,2,4,5-tetrazine modified
ion exchange resin. React. Func. Polym. 1996, 31(2), 117–136.
12. Diamond, R.M.; Whitney, D.C. Resin Selectivity in Dilute to Concentrated Aqueous
Solutions. In Ion Exchange, Marinsky, J.A., Ed.; Marcel Dekker: New York, 1966, 1,
277-351.
13. Inoue, K.; Yoshizuka, K.; Baba, Y. Adsorption of Metal Ions on a Novel Amine-
Type Chelating Resin. Solvent Extraction and Ion Exchange 1990, 8(2), 309-323.
14. Inoue, K.; Yoshizuka, K.; Ohto, K. Adsorptive separation of some metal ions by
complexing agent types of chemically modified chitosan. Anal. Chim. Acta 1999, 388
(1-2), 209-218.
15. Cotton, F.A.; Wilkinson, G., Advanced inorganic chemistry, 5th edition, John Wiley
& Sons: New York, 1988, pp. 951 and 920.
16. Machesky, M.L.; Andrade, W.O.; Rose, A.W. Interaction of gold (III) chloride and
elemental gold with peat-derived humic substances. Chem. Geol. 1992, 102 (1–4),
53–71.
17. Bronstein, L.M.; Sidorov, S.N.; Gourkova, A.Y.; Valetsky, P.M.; Hartmann, J.;
Breulmann, M.; Crlfen, H.; Antonietti, M. Interaction of metal compounds with
‘double-hydrophilic’ block copolymers in aqueous medium and metal colloid
formation. Inorg. Chim. Acta 1998, 280, 348-354.
80
Chapter 5
ADSORPTION OF METAL OXOIONS ON ETHYLENEDIAMINE
FUNCTIONALIZED CROSSLINKED LIGNOPHENOL GEL
Ethylenediamine modified crosslinked lignophenol gel, EN–lignin can adsorb
vanadate, tungstate and molybdate effectively from aqueous solution containing salts of
VO3− or VO2+, WO4
2 −, and MoO42 − or Mo7O24
6−, respectively. Stable complex formation
of VO2+ with polyphenol groups of lignophenol matrix is attributed to its adsorption
whereas the adsorption of remaining oxyanions is explained on the basis of combined
effects created by anion exchanging amine groups and the gel matrix.
5.1. INTRODUCTION
High concentrations of less toxic anions, as tungstate, molybdate and vanadates are
becoming a major water quality problem in many areas, mainly due to the production of
high volumes of mining effluents without any recycling. On the other hand, since their use
is extending in various industrial activities it is necessary to conserve these metals, because
the resources are fast depleting. Therefore removal and recovery of these ions from
wastewaters and ground waters using low cost adsorbents is of importance from both
environmental and economic point of view. Hence their removal and recovery from water
using low cost treatments is of high concern.
Molybdenum (Mo) is an essential trace element for both plants and animals.
Though its deficiencies for many crops have often been reported throughout the world, a
high concentration of Mo may be potentially toxic. Vanadium (V) has numerous uses in
81
industrial processes, e.g., processing of steels, synthesis of polymers, and production of
ceramics and electronics and has been released into the environment mainly from
industrial sources, especially oil refineries and power plants using vanadium-rich fuel oil
and coal. Such sources can mobilize appreciable amounts of vanadium and increase the
natural background level. It is highly useful to examine the recovery and removal of
vanadium from the viewpoint of exploitation of undeveloped resources and environmental
control.
In the present investigation, crosslinked lignophenol modified with
ethylenediamine is used for the removal of various metal oxoions as a low cost sorbent.
5.2. METHODS AND MATERIALS
Ethylenediamine aminated crosslinked lignophenol gel, EN–lignin, was prepared
according to the method explained in section 4.2. Various metal oxide salts namely:
Na2MoO4.2H2O, Na2WO4.2H2O, NH4VO3, VOSO4.nH2O, and (NH4)6Mo7O24.4H2O were
used to prepare MoO42 −, WO4
2 −, VO3−, VO2+, and Mo7O24
6− sample solutions, respectively.
0.2 mM of metal oxide solutions were prepared in 0.1 M HEPES and 0.1 M hydrochloric
acid medium. For the adsorption test, 15 mL of individual metal solution adjusted to
required pH and 20 mg of EN–lignin were mixed and shaken for 24 h. The concentrations
of metal ions before and after adsorption were measured by using atomic absorption
spectrophotometer, AAS or inductively coupled plasma, ICP, spectrophotometer.
5.3. RESULTS AND DISCUSSION
From Figure 5.1 it is clear that EN–lignin exhibits extractability for all metal-oxy
ions viz: MoO42 −, WO4
2 −, VO3−, VO2+, and Mo7O24
6− in acidic pH. Adsorption of
chromate ion is increasing with pH. Although two different kinds of molybdate salts of
normal molybdate, Na2MoO4 and paramolybdate, (NH4)2Mo7O24 were used for adsorption
test, the adsorption trend looks almost same with a maximum at around pH 3. This may be
due to the formation of polymeric anion by both MoO42 − and Mo7O24
6 − in weakly acidic
82
pH. In the case of WO42 −, almost complete recovery was observed from pH 1 – 5. Even
though the aqueous chemistries of molybdate and tungstate ions are similar, the variation
of percentage adsorption for tungstate ion appears slightly different. A similar adsorption
pattern with comparatively narrower selectivity range of pH is observed for vanadates.
0
20
40
60
80
100
0 1 2 3 4 5 6 7pH
% A
dsor
ptio
n
MoO4
Mo7O24
VO3
VO
WO4
MoO42-
Mo7O246-
VO3-
VO2+
WO42-
Figure 5.1. Variation of % adsorption of various metal oxy-ions on EN–lignin with pH.
[M] = 0.2 mM, Volume of solution = 15 mL, Wt. of gel = 20 mg, Shaking time = 24 h.
The chemistry of molybdenum resembles with that of tungsten because of their
prominent feature of forming several polymolybdate(VI) or polytungstate(VI). In the case
of Mo(VI), no polynuclear species containing fewer than seven molybdenum atoms is
observed in solution. This means, MoO42– anion in solution polymerizes to Mo7O24
6– and
hence a parallel adsorption pattern was observed for both the ions. As shown in Figure 5.1,
a broad peak within pH 2-4 was observed in molybdate adsorption. This type of adsorption
pattern has been observed by other authers.1-3 The highest adsorption observed around mild
acidic pH and a decreasing tendency with increasing pH might be associated with the high
pH dependency of the acidity of the gel. In contrast to molybdate ion, almost complete
adsorption was observed for tungstate even at low pH. The main cause for this difference is
the tendency of tungstate species to form tungsten oxide precipitate at low pH.
83
Interestingly a similar adsorption pattern was observed for oxy-anion, VO3−, of
vanadium (V) and oxy-cation, VO2+, of vanadium (IV). VO3− in aqueous medium forms
isopolyvanadate species, dimmers and cyclotetramers being the dominating in acidic pH.
Considering the pKa values of ethylenediamine, (pKa, K1 = 7.5, K2 = 10.09)4 at least a part
of the functional groups of EN–lignin would be protonated at a pH lower than 8, and the
gel would function as a weakly basic anion exchanger. The high adsorption ability in the
acidic solutions should mainly be ascribed to the strong interaction between the protonated
groups of EN–lignin and the anionic isopolyvanadates.5 In the case of VO2+, the increasing
adsorption with increase in pH reflects a cation exchange process which is further
supported by the formation of VO[(H2O)5]2+ around pH 4.6 Also, VO2+ being a hard acid
forms stable complexes with polyphenol groups of lignophenol. The result observed by
Nakajima, A. in the adsorption of VO2+ and VO3−on persimmon tannin gel, the reduction
of some parts of VO3− into VO2+ by polyphenol groups present in the gel attributes to the
similar result observed for both types of ions of vanadium.7 Since, reduction of Au(III) to
elemental form was observed both by crosslinked lignophenol and EN–lignin as mentioned
in chapter 3 and 4, similar effect can be assumed in the present case. Hence, adsorption of
VO2+ species is attributed to the formation of stable complex with –OH groups of the gel
where as anion exchange tendency of ethylenediamine group with anionic isopolyvanadate
species formed by VO3− in solution as well as its subsequent reduction to VO2+ leading to
the complexation with –OH corresponds to the similar adsorption trends of these two kinds
of vanadium oxides on EN–lignin.
5.4. CONCLUSION
EN–lignin exhibits extractability for a number of metal oxy-ions in a range of pH.
The selectivity range for vanadates is different from other ions. Hence, EN–lignin can be
of potential application for the recovery of metal-oxy ions and for the mutual separation of
tungstate, or molybdates from vanadates below pH 3.
84
REFERENCE
1. Xu, N.; Christodoulatos, C.; Braida, W. Adsorption of molybdate and
tetrathiomolybdate onto pyrite and goethite: Effect of pH and competitive anions.
Chemosphere 2006, 62, 1726–1735.
2. Hiemstra, T.; Van Riemsdijk, W.H. A surface structural approach to ion adsorption: the
charge distribution (CD) model. J. Colloid Interface Sci. 1996, 179, 488–508.
3. Moret, A.; Rubio, J. Sulphate and molybdate ions uptake by chitin-based shrimp shells.
Minerals Engineering 2003, 16, 715–722.
4. Jeffery, G.H.; Bassett, J.; Mendham, J.; Denney, R.C. Vogel’s Textbook of quantitative
chemical analysis. ELBS, 5th Ed. 1989.
5. Miyazaki, Y; Matsuoka, S.; Miura, Y.; Sakashita, H.; Yoshimura, K. Complexation of
vanadium(V) oxoanion with ethanolamine derivatives in solution and in a cross-linked
polymer. Polyhedron 2005, 24, 985–994.
6. Cotton, F.A.; Wilkinson, G. Advanced inorganic chemistry. JOHN WILEY & SONS.
5th Ed. 1988.
7. Nakajima, A. Electron spin resonance study on the vanadium adsorption by persimmon
tannin gel. Talanta 2002, 57, 537-544.
85
Chapter 6
PROSPECTIVE APPLICATION OF LIGNOPHENOL FOR
THE FABRICATION OF GOLD SINGLE CRYSTALS
6.1. INTRODUCTION
In the past few years, fabrication of two dimensional metal particles have been the
focus of much scientific researches because of their unusual electronic, optical, magnetic,
thermal, catalytic, and other properties that are distinctly different from their bulk
counterparts. Therefore considerable attention from both fundamental and applied research
has been paid to their synthesis and characterization.1-3 In this regard, finding an effective
but simple method of fabricating two dimensional gold particles is of great interest.
The unique property of lignophenol gels in reduction of Au(III) to elemental form
has been explained in previous chapters. Inspired by this finding, un-crosslinked
lignophenol was prepared in powder form and tested for the adsorption of Au(III) so that
elemental gold formed after reduction could be easily separated by dissolving lignophenol
with acetone. Successful results in this direction facilitate to devise a very simple and
effective method in the selective separation of gold away from any other metal ions.
6.2. PROCESS AND OBSERVATIONS
6.2.1. Preparation of lignophenol powder. The preparation methodology for lignophenol
is explained in detail in chapter 4. Lignophenol once obtained by phase separation method
was washed, dissolved in minimum amount of acetone and then mixed with excess of
water for several hours. After filtration, tan colored lignophenol powder was obtained
86
which was soluble only in acetone and was found to be stable even in strongly acidic
medium. Digital microphotograph of lignophenol powder is as shown in Figure 6.1.
Figure 6.1. Digital microphotograph of lignophenol powder.
6.2.2. Reduction of Au(III). For this 20 mL of 1 mM Au(III) solution prepared in 0.5 M
hydrochloric acid was mixed with 20 mg of lignophenol powder. After 24 h of shaking, the
mixture was filtered and the equilibrium concentration of gold was measured by using
AAS. Unlike crosslinked lignophenol, very low adsorption took place within 24 hours.
Thus, the shaking time was extended for 100 hours during which it was found that almost
complete recovery took place. Similar to the case of crosslinked lignophenol and its
aminated derivatives, twinkling gold particles were observed in the mixture solution.
Surprisingly, unlike the case of lignophenol gels, the gold particles were observed in the
form of single crystal, not in the form of aggregates. The mixture was filtered and the solid
part consisting of gold particles and un-crosslinked lignophenol was allowed to dry. Then
it was mixed with adequate volume of acetone that dissolved lignophenol and what was
left behind were beautiful single crystals of gold. The morphology of the particles was
studied by taking digital microphotographs, as shown in Figure 6.2.
Figures 6.2. (a) and (b) illustrate hexagonal gold microplates whereas Figures 6.2.
(c) and (d) show triangular microplates. Due to the lack of measurement scale in the micro
photographer, the size of the microplates could not be labeled. However, the images clearly
indicate the formation of hexagonal and triangular gold microplates. Besides the shape,
their color is different. Figures (a) and (c) are black where as (b) and (d) are golden. Black
appearance signifies very small thickness of the particles. It is most likely that, in the
beginning, gold plate of either hexagonal or triangular shape having thickness of nano
level were formed. Then subsequent addition of layers took place making it opaque and it
87
appeared golden as shown in Figures 6.2(b) and (d). Some golden spots are distinct on the
black crystals which demonstrated that layer formation process is in progress. Hence, the
possibility of fabrication of gold single crystals of fixed shapes and size seems stronger.
(a) (b)
(c) (d)
Figure 6.2. Digital microphotographs of gold single crystals
Thus, in conclusion, lignophenol can be developed as a prospective mean in the
recovery of gold from chloride medium directly in the form of hexagonal or triangular
single crystals or microplates that not only allows the selective recovery of this valuable
metal away from any other metals but also gives products which can be of suitable
parameters for certain technological applications.
REFERENCES
1. Kundu, S.; Pal, A.; Ghosh, S.K.; Nath, S.; Panigrahi, S.; Praharaj, S.; Pal, T. A new
rout to obtain shape-controlled gold nanoparticles from Au(III)-β-diketonates. Inorg.
Chem. 2004, 43(18), 5489-5491.
2. Chu, H-C.; Kuo, C-H.; Huang, M.H. Thermal aqueous solution approach for the
synthesis of triangular and hexagonal gold nanoplates with three different size ranges.
Inorg. Chem. 2005, 45(2), 808-813.
3. Sun, X.; Dong, S.; Wang, E. Large-scale, Solution-phase Production of Microsized,
Single-crystalline, Hexagonal Gold Microplates by Thermal Reduction of HAuCl4 with
Oxalic Acid. Chem.Lett. 2005, 34(7), 968-969.
88
Chapter 7
ADSORPTION OF ANTIMONY (Sb) BY CROSSLINKED
LIGNOPHENOL GELS
A number of crosslinked lignophenol gels namely lignophenol, lignocatechol,
lignopyrogallol and lignocresol were prepared and these chemically modified forms of
lignin were used for Sb(III) adsorption tests and the result obtained was compared with the
adsorption behavior of raw wood powder in acidic as well as basic pH.
7.1. INTRODUCTION
Antimony (Sb) is a naturally occurring trace element in most soils, but it is likely to
be a pollutant in industrial environments such as chemicals and allied products, electrical
and electronic equipments, bearings and power transmission equipments. Important
sources of Sb are emissions from smelters and from vehicles. Sb contamination has been
reported near mining and smelting areas in Japan. Also, from the viewpoint of nuclear
waste management, 125Sb (half life 2.76 y) is of interest because it is a fission product of 235U and is found in nuclear wastes. Presence of Sb elements and their compounds in the
environment is a major concern due to their toxicity to many forms of life. LD50 value as a
measure of Sb toxicity was reported to be 100 and 150 mg/kg intraperitoneally in rats and
guinea pigs, respectively. Antimony is composed of trivalent forms (SbO+, HSbO2, SbO2−,
Sb(OH)4−) or pentavalent forms (SbO2
+, SbO3−, Sb(OH)6
−), which are modulated by pH,
concentration, and co-dissolved compounds.1,2 The production of copper from
pyrometallurgical technologies leads to the production of an impure copper that is not of
the purity required for most applications of the metal, thus, copper must be electrorefined.
In the electrorefining process, copper and metals less noble than copper are oxidized and
89
go into solution where they form a variety of compounds. Arsenic and antimony are
included among the most undesirable elements typically found in these electrolytes. The
conventional process to control the antimony impurities involves multistage electrolytic
depositing cells. This process has many disadvantages, due to the limited rate at which the
electrolyte can be cycled through the cells, the loss of copper, the formation of toxic gas,
and the formation of toxic deposits which have to be disposed of. A number of methods
have been proposed for the removal of antimony from copper electrolyte solutions. These
include ion exchange, precipitation and solvent extraction. The use of organophosphorous
reagents for solvent extraction of antimony and bismuth has been investigated.3 The
reagents used showed limited solubility in organic solvents and low stability at the highly
acid conditions. Facon et al. used organothiophosphorous reagents for solvent extraction of
antimony(III) from hydrochloric acid solutions.4 Hydroxamic acids have been used for the
extraction of antimony but these reagents cannot be easily stripped.5 Ion exchange
processes have been studied and are currently being used in a number of refineries.6,7 Ariel
and Kirowadeveloped an anion exchange method for the separation of tin, antimony, lead,
and copper from alloys.8 Another predominant method of treatment is precipitation. In
glass industries, antimony and arsenic are precipitated using ferric ion and quaternary
ammonium ions from grinding waters and acid polishing waters, respectively.9 However,
this process requires disposal of hazardous sludge. Solvent extraction is a very useful
process because of the selectivity of the extracting reagent. However, solvent extraction
requires mixing the phases to provide sufficient interfacial area for satisfactory extraction.
In addition to processing problems, separations by solvent extraction processes are
generally considered to be economical in the range of aqueous metal concentration from
0.01 to 1.0 mol/l.10 Although ion exchange processes offer ease of column separation,
other processing problems, such as irreversible adsorption of organics on the organic
polymeric network, swelling of resin, slow kinetics, and low mechanical strength, may
have to be overcome.11 The removal of antimony from dilute aqueous solutions using
selective chemical bonded adsorbents is proposed here as a technological alternative to
solvent extraction/ion exchange processes. The chemically modified absorbents of lignin
are prepared by immobilization of phenol derivatives onto waste wood lignin.
90
7.2. EXPERIMENTAL
Preparation methods of lignocatechol and lignophenol are described in sections
2.2.1. and 3.2.3, respectively. Lignocresol was prepared by using cresol in place of phenol
and lignopyrogallol was prepared by using pyrogallol in place of catechol. All the products
were crosslinked by using the same experimental method as described in sections 2.2.1.
and 3.2.3. Batchwise adsorption tests and adsorption isotherm of antimony (III) were
studied.
7.3. RESULTS AND DISCUSSION
As environmental regulations for Sb(III) are expected to become much more severe
in Japan in the near future, adsorption tests for Sb(III) were carried out using all the lignin
gels prepared in this work as described earlier and also using crude wood powder, the feed
material, for comparison, to examine their adsorption behaviors for Sb(III) and their
feasibility for practical application to the removal of Sb(III) from waste water.
0
20
40
60
80
100
0 2 4 6 8 10 12 14pH
% A
dsor
ptio
n
CrosslinkedLignocresol
CrosslinkedLignopyrogallol
CrosslinkedLignocatechol
CrosslinkedLignophenol
Wood Powder
Figure 7.1. Adsorption of Sb (III) ion on different adsorbents from an aqueous mixture of
0.1M HCl and 0.1 M HEPES. Initial concentration of Sb3+ = 0.2 mM, wt. of gel = 20 mg,
volume of solution = 15 ml
91
Figure 7.1 shows the plot of % adsorption of Sb(III) against equilibrium pH on the
crosslinked lignophenol, lignocatchol, lignocresol and lignopyrogallol gels as well as crude
wood powder. It is seen from this figure that about 90% adsorption is achieved on all of the
gels tested at pH values less than about 5, while the % adsorption decreases with increasing
pH values greater than 5. On the other hand, the crude wood powder shows comparatively
lower adsorption though it appears efficient in the weakly basic pH region of 8-10. Sb(III)
does not exist as a simple cationic species in aqueous solution but as a partially hydrolyzed
species by complexing with hydroxyl ions.11
Since, as discussed in relation to the adsorption of other metal ions by crosslinked
lignocatechol in chapter 2, adsorption of Sb(III) on the polyphenolic lignin compounds is
considered to take place according to cation exchange reactions between the hydroxyl groups
of the lignin compounds and the cationic metal ion, the decrease in adsorption at pH values
greater than about 5 may be attributed to the formation of anionic species of Sb(III),
Sb(OH)4-, which cannot be adsorbed according to the cation exchange mechanism.
From figure 7.2., adsorption isotherm of Sb(III) ion appears to take place according
to the Langmuir type adsorption; that is, adsorption increases with increasing metal ion
concentration at lower metal concentration while it tends to approach constant values at high
concentration region, from which maximum adsorption capacity was evaluated as 0.50
mol/kg-dry gel.
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.5 1 1.5 2 2.5 3
Ce / mM
q / (
mol
/ kg
)
Figure 7.2. Adsorption isotherm of Sb crosslinked lignocatechol from 0.1M HCl – 0.1M
HEPES solutions at pH 1.0.
92
REFERENCES
1. Nakamaru, Y.; Tagami, K.; Uchida, S. Antimony mobility in Japanese agricultural
soils and the factors affecting antimony sorption behavior. Environmental Pollution
2006, 141, 321-326.
2. Kawakita, H.; Uezu, K.; Tsuneda, S.; Saito, K.; Tamada, M.; Sugo, T. Recovery of
Sb(V) using a functional-ligand-containing porous hollow-fiber membrane prepared by
radiation-induced graft polymerization. Hydrometallurgy 2006, 81, 190–196.
3. Dreisinger, D.B.; Leong, B.J.Y. CANMET Rep. 0748 (Jan. 1992)]
4. Facon, S.; Cote, G.; Bauer, D. Solvent extraction of antimony(Ill), bismuth(Ill), lead(ll)
and tin(IV)with bis(2,4,4 trimethylpentyl)-phosphinodithioic acid (Cyanex 301).
Solvent Extract. Ion Exch. 1991, 9, 717-734.
5. Schwab, W.; Kehl, R. Common separation of contaminating elements from electrolyte
solutions of valuable metals. U.S. Pat. 4. 1989, 834, 951.
6. Sasaki, Y.; Kawai, S.; Takasawa, Y.; Furaga, S. In: Proc. of Copper '91, Vol. III.
Pergamon Press, Elmsford (1991), pp. 245-254.
7. Takashi, O.; Hosaka, H.; Kasai, S. Process for the removal of bismuth and antimony
from aqueous sulfuric acid solution containing bismuth and/or antimony. U.S. Pat. 4
1985, 501, 666.
8. Ariel, M.; Kirowa, E. The anion exchange separation of tin, antimony, lead, and copper.
Talanta 1961, 8, 214-222.
9. Kappel, J.; Bischof, J.; Hutter, F.; Kaiser, A. Recent development for the removal of
arsenic and antimony from processing waters of the glass industry. Glastech. Ber. 1991,
64, 109-114.
10. Akita, S.; Taekuchi, H. Sorption and separation of metals from aqueous solution by a
macromolecu-lax resin containing tri-n-octylamine. J. Chem. Eng. Jpn. 1990, 23, 439-
443.
11. Deorkar, N.V.; Tavlarides, L.L. A chemically bonded adsorbent for separation of
antimony, copper and lead. Hydrometallurgy 1997, 46, 121-135.
93
CONCLUSION
One of the drawbacks of conventional metallurgical processes of precious metals is
that they involve the use of large amount of toxic chemicals. Thus a need to develop
environmentally benign metallurgical technology for precious metals was realized. Also,
the need of cheap, efficient, and environmentally benign adsorption gel was felt for the
effective remediation of waste water contaminated with heavy metals. With an objective
of recovery of precious metals and removal of heavy metals, a number of noble adsorption
gels have been prepared by the simple chemical modification of widely available biomass,
lignin. Four novel gels, namely: lignophenol, lignocatechol, lignocresol, and
lignopyrogallol were prepared by immobilizing phenol, catechol, cresol, and pyrogallol,
respectively onto wood lignin and crosslinked with paraformaladehyde to obtain
corresponding gels.
The adsorption behavior of crosslinked lignophenol, lignocatechol and
lignopyrogallol gels for Au(III) along with some other metals was studied and compared
with that of activated carbon. All three gels have proved to be more selective for Au(III)
than activated carbon with competitive adsorption capacities. Crosslinked lignophenol
exhibited outstanding selectivity for Au(III) while it was found to be almost inert towards
other metals tested. Such phenomena have not been reported before. Since gold can be
easily recovered in elemental form by simple filtration after adsorption, the novel lignin
gels can prove to be a step forward in the recovery and reuse of gold, a precious resource
that has been applied so extensively in a large number of electrical & electronic devices
and recently in bio-medical field also.
Furthermore, crosslinked lignocatechol has exhibited high selectivity for various
heavy metals with maximum capacity for Pb(II). From both batch and column
experiments, crosslinked lignocatechol gel was found to be effective in Pb(II) removal. In
the column tests, at least 10 consecutive adsorption–elution cycles were exhibited with
equal efficiency. This confirmed the stability and recyclability of the gel thus making its
use more economic. Similarly the possible application of various lignophenol gels
discussed herein in the adsorptive removal of Sb(II) was also studied.
94
Because crosslinked lignophenol gels were found to be effective only for the
recovery of gold, crosslinked lignophenol was functionalized with two types of functional
groups: primary amine and ethylene diamine, in order to make it useful for other recovery
of precious metals like palladium and platinum. Primary amine crosslinked lignophenol is
abbreviated as PA–lignin and ethylenediamine crosslinked lignophenol abbreviated as
EN–lignin were prepared and tested for adsorption of metal ions. Both of the adsorption
gels were confirmed to be effective for the adsorption of Au(III), Pd(II) and Pt(IV) from
weak to strong hydrochloric acid medium. The fact that other base metals such as Cu(II),
Fe(III), Ni(II) and Zn(II) negligibly adsorbed on both gels indicates the superiority of the
gels over other commercially available ion exchange resins. Both types of gels bearing
anion exchanger functions were found to have very high capacity for gold compared to
palladium and platinum ions. This is solely due to the combined effect of adsorption and
subsequent reduction of Au(III) to elemental form. In the case of Pd(II) and Pt(IV)
formation of ion pairs of metal–chloro complex anions with protonated adsorption gels in
acidic medium was proposed to be the main adsorption process.
Reduction of Au(III) to elemental form during adsorption on all kinds of
lignophenol gels was tracked by difference in concentration of Au(III) by AAS and by UV
spectrometry. From this study, the stepwise reduction of auric ion as: Au(III) → Au(I) →
Au(0) was confirmed.
In addition to its selectivity for Au(III), Pd(II), and Pt(IV) in strong acidic medium,
the tendency of EN–lignin for different oxyions was examined in low to higher acidic pH.
The gel has exhibited adsorption ability for vanadate, molybdate, and tungstate ions. In
case of oxyanions, anion exchange mechanism was found to be dominant whereas –OH
groups of lignophenol were found to be responsible for the adsorption of oxycations.
Since lignophenol gels were found to be extraordinarily effective for the recovery
of gold from chloride solution directly in elemental form without using any additional
reducing agents, the possibility of use of un-crosslinked lignophenol which is soluble in
acetone was studied. Unlike crosslinked gel, lignophenol obtained in very fine powder
form could not restrain adsorption capacity. For this reason, very slow reduction kinetics
was observed. Also, fine gold particles obtained after dissolving lignophenol in acetone
were found to have beautiful hexagonal or triangular geometry. For this reason, regardless
of the slow reduction kinetics, un-crosslinked lignophenol can be employed to obtain
hexagonal or triangular gold micro particles.
95
Thus in summary, we have been able to find novel use of wasted wood material in
the form of environment friendly, biodegradable and easy to use lignin based adsorbents
that can contribute to the environmental protection, reduce human misery caused by the
presence of toxic heavy metals in wastewater system and contribute to the recycling and
reuse of the most coveted precious metal resources.
96
List of Publications
1. Durga Parajuli, Katsutoshi Inoue, Keisuke Ohto, Tatsuya Oshima, Atsushi Murota,
Masamitsu Funaoka, and Kenjiro Makino. Adsorption of heavy metals on crosslinked
lignocatechol: a modified lignin gel. Reactive and Functional Polymers, 62, 2, 129-
139, 2005.
2. Durga Parajuli, Katsutoshi Inoue, Masayuki Kuriyama, Masamitsu Funaoka, and
Kenjiro Makino. Reductive adsorption of gold(III) by crosslinked lignophenol.
Chemistry Letters, 34, 1, 34-35, 2005.
3. Durga Parajuli, Chaitanya Raj Adhikari, Masayuki Kuriyama, Hidetaka Kawakita,
Keisuke Ohto, Katsutoshi Inoue, and Masamitsu Funaoka. Selective recovery of gold
by novel lignin-based adsorption gels. Industrial and Engineering Chemistry
Research, 45, 1, 8-14, 2006.
4. Durga Parajuli, Hidetaka Kawakita, Katsutoshi Inoue, and Masamitsu Funaoka.
Recovery of gold(III), palladium(II), and platinum(IV) by aminated lignin derivatives.
Industrial and Engineering Chemistry Research, 45, 19, 6405-6412, 2006.
5. Katsutoshi Inoue, Durga Parajuli, Kenjiro Makino, and Masamotsu Funaoka.
Functions of lignin as adsorbents of metals. In: Advanced technologies for woody
organic resources [ Mokushitsukei Yuki Shigen no Shintenkai]. Edited by Masamitsu
Funaoka, pp. 108-112, CMC press, Tokyo, 2005.
6. Hidetaka Kawakita, Katsutoshi Inoue, Keisuke Ohto, Kyoko Itayama, and Durga
Parajuli. Preparation of amine type adsorbent using spent paper and adsorption of
metal ions. Waste Management Research [Haikibutsu Gakkaishi Ronbunshi, Journal
written in Japanese], 17, 3, 243-249, 2006.
97
Contribution to Conferences
1. Durga Parajuli, Katsutoshi Inoue, Atsushi Murota, Masamitsu Funaoka, and Kenjiro
Makino. Ion exchange adsorption of metal ions on lignin derivatives. 20th Annual
Meeting of Ion Exchange Society of Japan. September, 2004. Yamanashi University,
Yamanashi, JAPAN.
2. Durga Parajuli, Katsutoshi Inoue, Masamitsu Funaoka. Polyphenolic lignin compounds
for the selective separation of heavy metals and precious metals. SORST – JST Joint
Symposium. June, 2005. Tokyo, JAPAN.
3. Durga Parajuli, Katsutoshi Inoue, Hiroyuki Harada, Keisuke Ohto, Hidetake Kawakita,
and Masamitsu Funaoka. Reductive adsorption of gold on some lignin based gel and
activated carbon. Autumn Meeting of the Society of Chemical Engineers Japan.
September, 2005. Okayama University, Okayama, JAPAN.
4. Durga Parajuli, Katsutoshi Inoue, Hiroyuki Harada, Keisuke Ohto, Hidetake Kawakita,
and Masamitsu Funaoka. Adsorption of precious metals on aminated lignophenol gels.
Autumn Meeting of the Society of Chemical Engineers Japan. September, 2005.
Okayama University, Okayama, JAPAN.
5. Durga Parajuli, Katsutoshi Inoue, Masayuki Kuriyama, and Masamitsu Funaoka.
Selective recovery of gold (III) by using crosslinked lignophenol gel. 16th Annual
Conference of the Japan Society of Waste Management Experts. October, 2005.
Sendai, JAPAN. [won the excellent poster presentation award]
6. Durga Parajuli, Katsutoshi Inoue, Masayuki Kuriyama, and Masamitsu Funaoka.
Selective uptake and recovery of gold (III) in the form of elemental gold by using
crosslinked lignophenol gel. 8th International Symposium on East Asian Resources
Recycling Technology (EARTH). November, 2005. Beijing, CHINA.
7. Durga Parajuli, Katsutoshi Inoue, Hiroyuki Harada, Keisuke Ohto, Hidetake Kawakita,
and Masamitsu Funaoka. Recovery of valuable metals by using chemically modified
wood lignin. Meeting of the Society of Chemical Engineers Japan. March, 2006.
Tokyo Institute of Technology, Tokyo, JAPAN.
8. Katsutoshi Inoue, Durga Parajuli, and Masamitsu Funaoka. Recovery of Precious
Metals by Using Lignin Derivatives. March, 2006. Osaka Prefecture University,
Osaka, JAPAN.
98
Department of Applied Chemistry Saga University, 1-Honjo, 840-8502, Saga, JAPAN Tel: +81-952-28-8669 E-mail: [email protected]
148-Rastra Bank Marg Ratna Chowk, Pokhara – 8, Kaski, NEPAL
Tel: +977-61-5-25027 E-mail: [email protected]
Durga Parajuli Adhikari
Date and Place of Birth 1977/09/21, Pokhara-2, Kaski, NEPAL.
Gender / Marital Status Female / Married
Experience 2003/03–2003/09 Teaching Associate School of Biomedical and Pharmaceutical Sciences Pokhara University, Pokhara, NEPAL.
Education 1983-1993 Bindhyabasini Secondary School, Pokhara NEPAL • S.L.C. School Leaving Certificate Examination.
1993–2002 Tribhuvan University NEPAL • I.Sc., Intermediate of Science and Technology. • B.Sc. Bachelor of Science and Technology (Chemistry). • M.Sc. Masters of Science and Technology (Physical Chemistry).
2003–2006 Saga University JAPAN • Doctor of Engineering (expected).
Present Status Doctor Degree Candidate, Saga University, JAPAN.
Area of Research Chemical engineering: Recovery of metals by using biomass waste.
List of Major Publications D. Parajuli, K. Inoue, et.al. React. Func. Polym, 62(2) 2005, 129-139.
D. Parajuli, K. Inoue, et.al. Chem. Lett. 34(1) 2005, 34-35.
D. Parajuli, K. Inoue, et.al. Ind.Eng. Chem. Res. 45(1) 2006, 8-14.
D. Parajuli, K. Inoue, et.al. Ind.Eng. Chem. Res. 2006 (Article in press).
Reference Prof. Dr. Katsutoshi INOUE, Department of Applied Chemistry, Saga
University, Saga, JAPAN. ([email protected])