Author’s Accepted Manuscript

Dye sequestration using agricultural wastes asadsorbents

Kayode Adesina Adegoke, Olugbenga SolomonBello

PII: S2212-3717(15)30005-6DOI: http://dx.doi.org/10.1016/j.wri.2015.09.002Reference: WRI55

To appear in: Water Resources and Industry

Received date: 27 November 2014Revised date: 27 August 2015Accepted date: 21 September 2015

Cite this article as: Kayode Adesina Adegoke and Olugbenga Solomon Bello,Dye sequestration using agricultural wastes as adsorbents, Water Resources andIndustry, http://dx.doi.org/10.1016/j.wri.2015.09.002

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DYE SEQUESTRATION USING AGRICULTURAL WASTES AS ADSORBENTS

Kayode Adesina Adegoke, Olugbenga Solomon Bello*

Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology,

P. M. B. 4000, Ogbomoso. Oyo State, Nigeria.

E-mail corresponding author: osbello06@gmail.com

Abstract

Colour is a visible pollutant and the presence of even minute amount of coloring

substance makes it undesirable due to its appearance. The removal of color from dye-bearing

effluents is a major problem due to the difficulty in treating such wastewaters by conventional

treatment methods. The most commonly used methods for color removal are biological oxidation

and chemical precipitation. However, these processes are effective and economic only in the case

where the solute concentrations are relatively high. Most industries use dyes and pigments to

color their products. The presence of dyes in effluents is a major concern due to its adverse effect

on various forms of life. The discharge of dyes in the environment is a matter of concern for both

toxicological and esthetical reasons. It is evident from a literature survey of about 305 recently

published papers that low-cost adsorbents have demonstrated outstanding removal capabilities

for dye removal and the optimal equilibrium time of various dyes with different charcoal

adsorbents from agricultural residues is between 4 and 5 hrs. Maximum adsorptions of acidic

dyes were obtained from the solutions with pH 8–10. The challenges and future prospects are

discussed to provide a better framework for a safer and cleaner environment.

Keywords: Agricultural wastes, Dyes, Environment, Pollutants, Removal.

Contents

Introduction

Available technologies for dye removal

Production of ACs from agricultural byproducts

Other important agricultural solid wastes as adsorbents for dyes sequestration

Effects of operational parameters on dye sequestration using agricultural wastes.

Challenges and future prospects

Conclusion

References

INTRODUCTION

Industrial developments in recent years have left their impression on the environmental

society. Many industries like the textile industry used dyes to color their products and thus

produce wastewater containing organics with a strong color, where in the dyeing processes the

percentage of dye lost wastewater is 50% of the dye because of the low levels of dye-fiber

fixation [1]. Discharge of these dyes in to effluents affects the people who may use these

effluents for living purposes such as washing, bathing and drinking [2]. Therefore it is very

important to verify the water quality, especially when even just 1.0 mg/L of dye concentration in

drinkingwater could impart a significant color, making it unfit for human consumption [3].

Furthermore dyes can affect aquatic plants because they reduce sunlight transmission through

water. Also dyes may impart toxicity to aquatic life and may be mutagenic, carcinogenic and

may cause severe damage to human beings, such as dysfunction of the kidneys, reproductive

system, liver, brain and central nervous system [4–6]. The removal of color from waste effluents

becomes environmentally important because even a small quantity of dye in water can be toxic

and highly visible [7]. Since the removal of dyes from wastewater is considered an

environmental challenge and government legislation requires textile wastewater to be treated,

therefore there is a constant need to have an effective process that can efficiently remove these

dyes [8].

Therefore adsorption by agricultural by-products used recently as an economical and

realistic method for removal of different pollutants has proved to be an efficient at removing

many types of pollutants such as heavy metals [9-10], COD [11-12], phenol [13-14], gasses [15]

and dyes [16-18]. In order to increase the adsorption capacity of the adsorbent, Researchers have

followed different activation methods and they usually used the Langmuir isotherm to indicate

the effectiveness of the activation process. Activation methods involve physical activation such

as carbonization of material and chemical activation such as using chemical activating agents.

Real textile wastewater is a mixture of dyes, organic compounds, heavy metals, total dissolved

solids, surfactants, salts, chlorinated compounds, chemical oxygen demand and biological

oxygen demand [19-20]. Therefore some studies tested the agricultural wastes as adsorbents for

these pollutants. Ahmad and Hameed [12] studied the reduction of color and COD using bamboo

activated carbon, and they found that the maximum reduction of color and COD were 91.84%

and 75.21%, respectively.

Many industries, such as dyestuffs, textile, paper, plastics, tannery, and paint use dyes to

color their products and also consume substantial volumes of water. As a result, they generate a

considerable amount of colored wastewater [21]. The presence of very small amounts of dyes in

water (less than 1 ppm for some dyes) is highly visible and undesirable [22-23]. According to

Chakrabarti et al. [24], nearly 40 000 dyes and pigments are listed, which consist of more than

7000 different chemical structures. Most of them are completely resistant to biodegradation

processes [25]. Over 100 000 commercially available dyes exist and more than 7 × 105 tons are

produced worldwide annually [26-27]. Recent studies indicate that approximately 12% of

produced synthetic dyes are lost during manufacturing and processing operations. Approximately

20% of these lost dyes enter the industrial wastewaters [28-29]. An indication of the scale of the

problem is given by the fact that 10–15% of the dye is lost in the effluent during the dyeing

process [30-31]. Dye molecules consists of two key components: the chromophores, which are

largely responsible for producing the color, and the auxochromes, which not only supplement the

chromophore but also render the molecule soluble in water and enhance its affinity (to attach)

toward the fibers [32]. Dyes may be classified in several ways, according to chemical

constitution, application class, and end use. Dyes are here classified according to how they are

used in the dyeing process. Main dyes are grouped as acid dyes, basic dyes, direct dyes, mordant

dyes, vat dyes, reactive dyes, disperse dyes, azo dyes, and sulfur dyes. Typical dyes used in

textile dyeing operations are given in Table 1 [33]. As a result of increasingly stringent

restrictions on the organic content of industrial effluents, it is necessary to eliminate dyes from

wastewater before it is discharged.

Many of these dyes are also toxic and even carcinogenic [34, 35]. Besides this, they also

interfere with the transmission of light and upset the biological metabolism processes, which

causes the destruction of aquatic communities present in various ecosystems [36-37].

Furthermore, the dyes have a tendency to sequester metal and may cause microtoxicity to fish

and other organisms [38]. However, wastewater containing dyes is very difficult to treat because

the dyes are recalcitrant organic molecules, which are resistant to aerobic digestion, and are

stable to light, heat, and oxidizing agents [39, 40 - 43]. So color is the first contaminant to be

recognized in the wastewater [23].

In this article, the feasibility of agricultural wastes adsorbents for dye removal from

contaminated water has been reviewed. The main goal of this review is to (i) presents a critical

analysis of these materials; (ii) describes their characteristics, advantages and limitations; and

(iii) discusses various mechanisms involved. Reported adsorption capacities are presented to give

some idea of their effectiveness. However, the reported adsorption capacities must be taken as an

example of values that can be achieved under specific conditions since adsorption capacities of

the adsorbents presented vary, depending on the characteristics of the material, the experimental

conditions, and also the extent of chemical modifications.

AVAILABLE TECHNOLOGIES FOR DYE REMOVAL

Methods of dye wastewater treatment have been reviewed recently [22-23, 43-46]. There

are several reported treatment methods for the removal of dyes from effluents and these

technologies can be divided into three categories: biological methods, chemical methods, and

physical methods [22]. However, all of them have advantages and drawbacks. Because of the

high cost and disposal problems, many of these conventional methods for treating dye

wastewater have not been widely applied at large scale in the textile and paper industries [47]. At

the present time, there is no single process capable of adequate treatment, mainly because of the

complex nature of the effluents [48-49].

In spite of the availability of many techniques to remove these pollutants from

wastewaters as legal requirements, such as coagulation, chemical oxidation, membrane

separation process, electrochemical and aerobic and anaerobic microbial degradation, these

methods are not very successfully due to suffering from many restrictions [50]. Table 2 shows

the advantages and disadvantages of different dye removal methods [51-55]. Among all of these

methods adsorption has been preferred due to its cheapness and the high-quality of the treated

effluents especially for well-designed sorption processes [51]. Adsorption by activated carbon is

an important way to clean up effluents and wastewater [52], where it used to polish the influent

before it is discharged into the environment [53]. However adsorption by activated carbon has

some restrictions such as the cost of the activated carbon, the need for regeneration after

exhausting and the loss of adsorption efficiency after regeneration [54].

PRODUCTION OF ACS FROM AGRICULTURAL BY-PRODUCTS

Any cheap material, with a high carbon content and low inorganics, can be used as a raw

material for the production of AC [56]. Agricultural byproducts are available in large quantities

and are one of the most abundant renewable resources in the world. These waste materials have

little or no economic value and often present a disposal problem. Therefore, there is a need to

valorize these low cost byproducts. Thus, conversion of waste materials into ACs would add

considerable economic value, help reduce the cost of waste disposal and most importantly

provide a potentially inexpensive alternative to the existing commercial ACs. These waste

materials have proved to be promising raw materials for the production of AC with a high

adsorption capacity, considerable mechanical strength, and low ash content [57].

Adsorption methods employing solid sorbents are widely used to remove certain classes

of chemical pollutants from wastewater. However, among all the sorbent materials proposed,

activated carbon (AC) is the most popular material for the removal of pollutants from

wastewater. AC is an amorphous carbon in which a high degree of porosity has been developed

during manufacture. It is this porosity, which governs the way in which AC performs its

purifying role, and the very large surface area provides many sites upon which the adsorption of

impurity molecules can take place [84-90]. The factors which favour the selection of agricultural

adsorbents are its low cost, widespread presence and organic composition which shows strong

affinity for some selected dyes.

Because of their low cost, after being expended, these materials can be disposed of

without expensive regeneration. A wide variety of ACs have been prepared from agricultural

byproducts such as corn straw [58], wheat straw[58], rice straw[59, 118], sawdust [61–63], corn

cob [64], bagasse [64, 65], cotton stalk [66], coconut husk [67], rice husks [63, 68, 69], tobacco

stem [70], nut shells [71, 72], soybean oil cake [73], oil palm shell [74], and oil palm fiber [75].

Basically, there are two main steps for the preparation and manufacture of AC: (1) the

carbonization of the carbonaceous raw material below 1000 °C, in an inert atmosphere, and (2)

the activation of the carbonized product (char), which is either physical or chemical.

Physical Activation

Physical activation is a process in which the precursor is developed into AC using gases

and is generally carried out in a two-step process [76]. Carbonization is the first stage and

involves the formation of a char, which is normally nonporous, by pyrolysis of the precursor at

temperatures in the range between 400 and 850 °C, and sometimes reaches 1000 °C, in an inert,

usually nitrogen, atmosphere. Activation is the second stage and involves contacting the char

with an oxidizing gas, such as carbon dioxide, steam, air or their mixtures, in the temperature

range between 600 and 900 °C, which results in the removal of the more disorganized carbon

and the formation of a well-developed micropore structure. The activation gas is usually CO2,

since it is clean, easy to handle and it facilitates control of the activation process because of the

slow reaction rate at temperatures around 800 °C [76]. It is worthwhile noting that the ACs

produced by physical activation did not have satisfactory characteristics to be used as adsorbents

or as filters [77].

Chemical Activation

Chemical activation involves impregnation with chemicals such as ZnCl2, KOH, NaOH,

H3PO4, or K2CO3 followed by heating under a nitrogen flow at temperatures in the range of 450–

900 °C, depending on the impregnant used. In the chemical activation process, carbonization and

activation are carried out simultaneously, with the precursor being mixed with chemical

activating agents, such as dehydrating agents and oxidants. Chemical activation offers several

advantages as it is carried out in a single step, performed at lower temperatures and, therefore,

resulting in the development of a better porous structure, although the environmental concerns of

using chemical agents for activation could be developed. Besides, part of the added chemicals

(such as zinc salts and phosphoric acid), can be easily recovered [56, 76, 78].

However, a two-step process (an admixed method of physical and chemical processes)

can be applied [60]. It is to be noted that there is also an additional one-step treatment route,

denoted as steam-pyrolysis as reported [57, 79–83], where the raw agricultural residue is either

heated at moderate temperatures (500–700 °C) under a flow of pure steam, or heated at 700–800

°C under a flow of steam. Many agricultural residues such as straw, bagasse, apricot stones,

cherry stones, grape seeds, nutshells, almond shells, oat hulls, corn stover, and peanut hulls have

been studied with this method. AC produced with this method will not be discussed here, but

more details are available [77, 84-120]. The adsorption capacities of agricultural wastes for dye

removal are summarized briefly [104-120] (Table 3). From the recent literature reviewed it is

demonstrated that agricultural waste showed comparatively significant removal efficiency than

the commercial adsorbents. Decolourisation process is not specific and depends upon many

factors. Although there are lots of agricultural adsorbents which can act as a substitute for the

expensive commercial activated carbon but complete replacement is not possible. The factors

which favour the selection of agricultural adsorbents are its low cost, widespread presence and

organic composition which shows strong affinity for some selected dyes.

Adsorbent from Lignin

Lignin a waste/by-product discharged from paper mills in large quantities has been used

in its raw state as well as modified state for removal of contaminants by researchers. A spherical

sulfonic lignin adsorbent was prepared by Liu and Huang [121] from a bamboo pulp mill by-

product. The adsorbent was investigated for the removal and recovery of cationic dyes cationic

red GTL, cationic turquoise GB, and cationic yellow X-5GL, from aqueous solutions. Authors

found the adsorption to be initial concentration and temperature dependent, following both the

Freundlich and the Langmuir isotherms. The process was found to be endothermic, and

appreciable Langmuir adsorption capacities of 576.0, 582.4 and 640.8 mg g_1 were observed for

cationic red GTL, cationic turquoise GB, and cationic yellow X-5GL, respectively. Various

other studies [122] on dye removal, and review [123] highlighting the utilization of lignin are

also available.

Lignins are polymers of aromatic compounds. Lignin is a natural polymer which together

with hemicelluloses acts as a cementing agent matrix of cellulose fibres in the structures of

plants. Their functions are to provide structural strength, provide sealing of water conducting

system that links roots with leaves, and protect plants against degradation [124]. Lignin is a

macromolecule, which consists of alkylphenols and has a complex three-dimensional structure.

Lignin is covalently linked with xylans in the case of hardwoods and with galactoglucomannans

in softwoods [125]. The basic chemical phenylpropane units of lignin (primarily syringyl,

guaiacyl and p-hydroxy phenol) are bonded together by a set of linkages to form a very complex

matrix. This matrix comprises a variety of functional groups, such as hydroxyl,methoxyl and

carbonyl, which impart a high polarity to the lignin macromolecule [126,127–131].

Adsorbent from Corncob

Corncob waste has been investigated as adsorbent for dyes [132-133]. Activated carbons

having surface area in the range of 538–943 m2 g_1

as prepared from corncob [132]were able to

remove 230–1060 and 432–790 mg g_1

of acid blue 25 (astrazon red) and basic red 22 (telon

blue), respectively. Compared to AC prepared from bagasse (surface area 607m2 g-1

) and plum

kernel (surface area 1162 m2 g

-1) the removal capacity of AC from corncob for acid blue 25 was

the highest.

Adsorbent from Sawdust

Batzias and Sidiras (2004) studied that beech saw dust as low cost adsorbent for the

removal of methylene blue and basic red 22. Further, in order to know the effect of chemical

treatment and to improve its efficiency the authors also tested the potential of the adsorbent by

treating it with CaCl2 [134] and using mild acid hydrolysis [135] and found it to increase the

adsorption capacity. Besides this, the simulation studies for effect of pH were also carried out by

[136]. The authors determined the point of zero charge p.z.c. (5.2) of the sawdust and suggested

that increase of the pH enhances the adsorption behaviour. The low adsorption of methylene blue

at acidic pH was suggested to be due to the presence of excess H+ ions that compete with the dye

cation for adsorption sites. With the increase of the pH of the system, the number of positively

charged sites decreases while the number of the negatively charged sites increases. It was also

suggested that the negatively charged sites favour the adsorption of dye (cationic like methylene

blue) due to electrostatic attraction. Namasivayam et al. [137] investigated coir pith, an

agricultural residue, as an adsorbent for the adsorption of rhodamine B and acid violet dyes. The

source material was used after drying, sieving and carbonizing at 700oC. It was found that

rhodamine B adsorption reached equilibrium stage at 5, 7, 10, and 10 min for dye concentration

10, 20, 30 and 40 mg L-1

, respectively while crystal violet was found to have equilibration time

of 40 min for all concentrations studied. Though this material is freely available the low

adsorption capacities viz. 2.56 mg and 8.06 mg g_1

of the adsorbent for rhodamine B and acid

violet, respectively make it poor adsorbent for the removal of dyes.

Sawdust is one of the most appealing materials among agricultural waste materials, used

for removing pollutants, such as, dyes, salts and heavy metals from water and wastewater [138].

The material consists of lignin, cellulose and hemicellulose, with polyphenolic groups playing

important role for binding dyes through different mechanisms. Generally the adsorption takes

place by complexation, ion exchange and hydrogen bonding. Various fruitful studies have been

done on the removal of dyes by sawdust [139-141]. Sawdust (from Indian rosewood) easily

available at zero or negligible price in India [139] was studied for the removal of methylene blue.

Authors studied the effects of different system variables, like adsorbent dosage, initial dye

concentration, pH and contact time, and found that as the amount of the adsorbent increased, the

percentage of dye removal increased accordingly. Low concentrations of methylene blue

favoured higher adsorption percentages, and optimum pH value for dye adsorption was found to

be 7.0. In a study on the removal of acid and basic dyes by sawdust, [140] reported the sorption

capacity of basic dye (basic blue 69) to be much higher than acid dye (acid blue 25), which they

suggested to be due to the ionic charges on the dyes and the character of the biomaterials, also

the sorption of dyes was found to be exothermic in nature. Similar to other materials the

adsorption capacity of sawdust can also be improved using chemical treatment [134, 142-145].

Hard wood (Mansonia wood) sawdust was studied by Ofomaja and Ho [146] to know the effect

of temperature on the equilibrium biosorption of methyl violet dye from aqueous solution onto

the material. The equilibrium biosorption data was analysed using Langmuir, Freundlich and

Redlich–Peterson isotherms. Best fits were yielded with Langmuir and Redlich–Peterson

isotherms. The process was found to be endothermic in nature and the biosorption was strongly

dependent on solution pH and percentage dye removal became significant above pH 7, which

was slightly higher than the pHPZC of the sawdust material. The dependency of dye sorption on

sawdust has also been noted by other workers too [145,147]. More about the role of sawdust for

the removal of unwanted materials from waters can be found in a review by Shukla et al. [138]

Adsorbent from Wool

Wool carbonizing waste obtained as a result of processing of wool was investigated by

Perineau et al. [148] for the adsorption of dyes. They reported that the surface properties of the

material are such that they tend to adsorb solutes of ionic nature. The authors observed that the

adsorption of basic dyes is 6–10 times higher than that of acid dyes. Wheat husk, an agricultural

by-product, was investigated by Gupta et al. [149] as an adsorbent for the adsorption of reactofix

golden yellow 3 RFN from aqueous solution. The authors suggested that adsorption process can

be considered suitable for removing colour, COD from wastewater. Besides utilizing natural

materials and/or materials obtained as agricultural wastes as alternative adsorbents for the colour

removal a number of wastes/by-products which are generated by many industries, such as

thermal power plants, steel and metal, sugar and fertilizer industries etc. have also been used.

Adsorbent from durian peel

Durian (Durio zibethinus), locally referred to as the ‘‘king of fruits’’ is a dicotyledonous tropical

seasonal plant species belonging to the member of family Bombacaceae and genus of Durio

[150]. Its shape is typically ranging from ovoid to nearly round-shaped, featured by a distinctive,

strong, pungent and penetrating odor. The edible aril is enveloped in a green to yellowish brown

semi-woody rind, with a spike-like spine and sharply pointed pyramidal formidable thorns [150].

Durian tree (25–50 m in height) thrive well in a warm and humid climate, ideally in the

temperature range from 25 to 30 oC and evenly distributed rainfall rate between 1.5 and 2

months/year. Its flowers are produced in 3–30 clusters, with each flower having a calyx (sepals)

and five (rarely four or six) petals [151]. Today, its growth has been phenomenal in the global

trade market, forecasted a total world’s harvest of 1.4 Mt, dominated by its major producers,

Thailand (781 kt), Malaysia (376 kt), Indonesia (265 kt), followed by Philippines (Davao

Region), Cambodia, Laos, Vietnam, Myanmar, India, Sri Lanka, Florida, Hawaii, Papua New

Guinea, Madagascar, and Northern Australia. Meanwhile, China (65 kt), Singapore (40 kt) and

Taiwan (5 kt) are among the key importers accounting for 65 % of the total exports from the top

supplying countries. Despite its prolific implementations in food manufacturing industries, such

exertions are hampered by the massive generation of durian residues, chiefly in the form of

durian shells, seeds, peels and rinds, which constitute 70 % of the entire fruit [152]. In the

common practice, durian residues are burned or sent to the landfills, without taking care neither

of the surrounding environment, nor considering any precaution to prohibit the percolation of

contaminants into the underlying water channels. Lately, environmental rules and regulations

concerning pollution from agricultural waste streams by regulatory agencies are more stringent

and restrictive; inevitably affecting the design, planning, and operation of the durian processing

industry. This has inspired a developing exploration to establish a leading selective, reliable and

durable alternative for judicious utilization of these pollutants called durian residues.

Mesoporous-activated carbon from durian seed (DSAC) was prepared; it was used as

adsorbent for the removal of methyl red (MR) dye from aqueous solution [153]. Textural and

adsorptive characteristics of activated carbon prepared from raw durian seed (DS), char durian

seed (char DS) and activated durian seed (DSAC) were studied using scanning electron

microscopy, Fourier transform infra red spectroscopy, proximate analysis and adsorption of

nitrogen techniques, respectively. Acidic condition favors the adsorption of MR dye molecule by

electrostatic attraction [153]. The maximum dye removal was 92.52 % at pH 6. Experimental

data were analyzed by eight model equations: Langmuir, Freundlich, Temkin, Dubinin–

Radushkevich, Radke–Prausnitz, Sips, Vieth–Sladek and Brouers–Sotolongo isotherms and it

was found that the Freundlich isotherm model fitted the adsorption data most. Adsorption rate

constants were determined using pseudo-first-order, pseudo-second-order, Elovich, intraparticle

diffusion and Avrami kinetic model equations. The results clearly showed that the adsorption of

MR dye onto DSAC followed pseudo-second-order kinetic model. Both intraparticle and film

diffusion were involved in the adsorption process. The mean energy of adsorption calculated

from D–R isotherm confirmed the involvement of physical adsorption. Thermodynamic

parameters were obtained and it was found that the adsorption of MR dye onto DSAC was an

endothermic and spontaneous process at the temperatures under investigation [153].

The use of durian shell for removal of methylene blue from aqueous solutions which was

prepared using chemical activation method with potassium hydroxide as the activating agent

[154]. The activation was conducted at 673.15K for 1 h with mass ratio of chemical activating

agent to durian shell 1:2. Batch kinetics and isotherm studies were conducted to evaluate the

adsorption behavior of the activated carbon from durian shell. The adsorption experiments were

carried out isothermally at three different temperatures. The Langmuir and Freundlich isotherm

model were used to describe the equilibrium data. The Langmuir model agrees with experimental

data well. The Langmuir surface kinetics, pseudo first order and pseudo second order models

were used to evaluate the kinetics data and the rate constant were also determined. The

experimental data fitted very well with the Langmuir surface kinetics and pseudo first order

model [155].

Hameed and Hakimi, (2008) [156] utilized durian peel (DP) for its ability to remove acid

dye from aqueous solutions. Adsorption equilibrium and kinetics of acid green 25 (AG25) from

aqueous solutions at various initial dye concentrations (50–500 mg/L), pH (2–10), and

temperature (30–50 ◦

C) on DP were studied in a batch mode operation. Equilibrium isotherms

were analyzed by Langmuir and Freundlich isotherm models. The equilibrium data were best

represented by Langmuir isotherm model with maximum monolayer adsorption capacity of

63.29 mg/g at 30 ◦C. Kinetics analyses were conducted using pseudo-first- order, pseudo-second-

order and intraparticle diffusion models. It was found that the adsorption kinetics of AG25 on DP

obeyed pseudo-second-order sorption kinetics. The results indicate the potential of DP as sorbent

for the removal of acid dye from aqueous solution [156].

Adsorbent from sludge

By-products/wastes of steel plants viz. blast furnace sludge, blast furnace dust and blast

furnace slag were investigated for the removal of acid (ethyl orange, metanil yellow, acid blue

113) and basic dyes (chrysoidine G, crystal violet, meldola blue, methylene blue) by Jain et al.

[157-160], the adsorption capacities were also reported [160-162]. The authors compared the

materials with a fertilizer industry waste and standard activated carbon. The adsorption of dyes

was reported to follow Langmuir adsorption model and was first order in nature. Authors

observed that compared to standard activated charcoal the removal capacities were less. Slag as

an adsorbent for removal of various other dyes has been investigated by other workers too [161-

164]. A sludge obtained by precipitation of metal ions in wastewater with calcium hydroxide in

electroplating industry has been investigated for the removal of azo dyes [165]. The authors

noted a maximum adsorption capacity (48–62 mg g-1

) for azo-reactive (anionic) dyes and

suggested that metal hydroxide sludge being a positively charged adsorbent can remove azo-

reactive (anionic) dyes and the charge of the dyes is an important factor for the adsorption due to

the ion-exchange mechanism.

Adsorbent from Rice husk

Rice husk is an agricultural waste and a by-product of the rice milling industry to be

about more than 100 million tonnes, 96% of which is generated in the developing countries. The

utilization of this source of agricultural waste would solve both a disposal problem as well as

access to a cheaper material for adsorption in water pollutants control system [166-168]. The

maximum cost of commercially available rice husk is approximately US$ 0.025/kg. Since, the

main components of rice husk are carbon and silica (15–22% SiO2 in hydrated amorphous form

like silica gel), it has the potential to be used as an adsorbent [169]. Mckay et al. [170] studied

the use of rice husk in the removal of MB from aqueous solutions then after Vadivelan and

Kumar [171] used the rice husk for the adsorption of MB. The operating variables studied were

initial solution pH, initial dye concentration, adsorbent concentration, and contact time. The

amount of dye adsorbed was found to vary with initial solution pH, adsorbent dose, and contact

time. The monolayer sorption capacity of rice husks for MB sorption was found to be 40.58 mg/g

at room temperature (32 ◦C). The dye uptake process was found to be controlled by external

mass transfer initially followed by intraparticle diffusion.

Rice husk as obtained from a local rice mill grounded, sieved, washed and then dried at

80oC was used for removal of two basic dyes safranine and methylene blue and adsorption

capacity of 838 and 312 mg g-1

was found [169]. Since disposal or regeneration of spent

adsorbent is one of the important economic factors in assessing the feasibility of an adsorption

system, authors suggested that as the purchase costs of material is negligible and it is primarily

carbonaceous and cellulosic, the preferred disposal method is by dewatering, drying and burning.

Also it was suggested that the heat of combustion can be recovered as waste heat and used for

adsorbent drying and steam generation.

Vadivelan and Kumar (2005) investigated the batch experiments for the sorption of

methylene blue onto rice husk particles [171]. The operating variables studied were initial

solution pH, initial dye concentration (Fig. 1b), adsorbent concentration, and contact time.

Equilibrium data were fitted to the Freundlich and Langmuir isotherm equations and the

equilibrium data were found to be well represented by the Langmuir isotherm equation. The

monolayer sorption capacity of rice husks for methylene blue sorption was found to be 40.5833

mg/g at room temperature (32 ◦C). The sorption was analyzed using pseudo-first-order and

pseudo-second-order kinetic models and the sorption kinetics was found to follow a pseudo-

second-order kinetic model. Also the applicability of pseudo second order in modeling the

kinetic data was also discussed. The sorption process was found to be controlled by both surface

and pore diffusion with surface diffusion at the earlier stages followed by pore diffusion at the

later stages. The average external mass transfer coefficient and intraparticle diffusion coefficient

was found to be 0.01133 min−1 and 0.695358 mg/gmin0.5

. Analysis of sorption data using a

Boyd plot confirms that external mass transfer is the rate limiting step in the sorption process

(Fig. 1a). The effective diffusion coefficient, Di was calculated using the Boyd constant and was

found to be 5.05×10−4

cm2/s for an initial dye concentration of 50 mg/L. A single-stage batch-

adsorber design of the adsorption of methylene blue onto rice husk has been studied based on the

Langmuir isotherm equation [171].

Rice husk ash a waste from rice mills has been used as an adsorbent for removal of acidic

dyes namely acid violet 54, acid violet 17, acid blue 15, acid violet 49 and acid red 119 and their

COD from aqueous solutions. The adsorption capacity was found to vary from 99.4 to 155 mg

g_1, making rice husk ash a good material. Further, a time period of 30–120 min was found to be

optimum for attaining equilibrium [172].

Adsorbent palm oil ash

Hasnain (2006) studied the possibility of using palm oil ash as a low-cost adsorbent for

the removal of the dyes disperse blue and disperse red from aqueous solution [173]. Both batch

as well as continuous flow experiments were performed, and the effects of different system

variables, including pH, initial dye concentration and agitation time, were studied in the batch

tests. It was suggested that acidic pH favoured dye removal, and the optimum pH and agitation

time for the removal of the two dyes were found to be 2 and 60 min, respectively. Besides this

the authors also suggested that ash can be used in its natural form in batch processes while

pelletisation, due to cost implications, is not recommendable for industrial application. Shale oil

ash, an inorganic residue, obtained after the combustion of shale oil was used as adsorbent [174]

for dyes. The author reported that the material which is obtained after a high temperature process

possesses good porosity and suggested that it can have good adsorptive behavior for both organic

and inorganic pollutants. A two resistance mass transfer model based on the film resistance and

homogeneous solid phase diffusion was also developed. Red mud a by-product of aluminium

industry obtained from bauxite processing by Bayer process has been investigated for the

removal of dyes. Namasivayam and Yamuna [175] studied the adsorption of congo red from

aqueous solution and suggested the process to be economical since the material is discarded

waste. The removal capacity of the red mud for the dye was 4.05 mg g_1

. The process was found

to follow first rate expression and the adsorption data obeyed both Langmuir and Freundlich

isotherms. Authors noted the mechanism of adsorption to be mostly ion exchange. Red mud has

also been used [176] who investigated it for the removal of basic dye, methylene blue, from

aqueous solutions and found the adsorption capacity for raw red mud as 7.8x10_6

mol g_1

. The

effect of physical (heat) and chemical treatment was also studied on as-received red mud and

was reported to have adverse effect on the adsorption capacity. The acid treatment (by nitric

acid) resulted in a decrease of adsorption capacity of red mud (3.2 x 10-6

mol g-1

). Some other

fruitful studies [177-178] have also been carried out on adsorption of various dyes such as

rhodamine B, fast green, methylene blue and congo red on red mud.

Aerobically digested sludge [179] from an urban wastewater was tested for producing

activated carbons and further used for dye removal in order to overcome the costs of ACs in

water treatment. The sludge based (SB) AC was mainly mesoporous in nature, with a surface

area of 253 m2 g

_1 and an average pore diameter of 2.3 nm. The ACs so produced were used for

the removal of three anionic dyes viz. acid brown 283, direct red 89 and direct black 168 and a

basic dye basic red 46 from aqueous solutions. The SBAC was found to perform nearly as well

as Chemviron, AC which was mainly microporous with a surface area of 1026 m2 g

_1 and an

average diameter of 1.8 nm. The greater capacity of AC sludge based compared with the

commercial AC to remove acid and a direct dye from solution is attributed to its wider pore size

distribution. The authors also reported that the presence of acidic surface functional groups in

SBAC also plays an important role in determining the extent of adsorption as a function of

solution pH. Sewage sludge has also been used to prepare carbonaceous materials using chemical

activation [180]. Carbonaceous sorbents so developed from sludge were used to remove copper

ion, phenol and dyes (acid red 18 and basic violet 4) from aqueous solution as well as volatile

organic compounds from the gas phase. Two experimental conditions were suggested by the

authors but in order to have a high mass yield and to reduce the energetic cost of the process, the

optimal condition having 1.5 g of H2SO4 g-1

of sludge, 700oC and 145 min was suggested to be

more appropriate. Waste carbon slurry generated during liquid fuel combustion in fertilizer

plants has been converted into an inexpensive activated carbon [181]. Authors prepared an

activated carbon and fruitfully employed it for a basic dye (malachite green) removal, a 100%

removal was observed at low concentrations. Presence of anionic surfactant (Manoxol-1B) was

tested and found not to affect the uptake of dye significantly. (≤5 x10-5

M) and by particle

diffusion at The dye removal was suggested to take place through a film diffusion mechanism at

lower concentrations higher concentrations (>5 x10-5

M). Authors found that the adsorbent can

be regenerated to quantitatively recover the dye with acetone and the developed adsorbent was

cheaper than commercially available carbons. A similar carbonaceous adsorbent was

investigated [182] who utilized it for the removal of dyes and phenols. The authors compared the

efficiencies of the carbonaceous adsorbent with blast furnace slag, dust and sludge (steel industry

waste) and found the carbonaceous adsorbent to be the best among them. Further, comparison of

the carbonaceous adsorbent with an activated carbon showed it to bew45% efficient for acid dyes

(ethyl orange, metanil yellow and acid blue 113) and 70–80% efficient for basic dyes

(chrysoidine G, crystal violet, and meldola blue) as compared to a commercial activated carbon.

Authors proposed that the carbonaceous adsorbent being a low-cost material need not be

regenerated after being loaded with pollutants and can be disposed of by burning.

Various other materials have also been put to use for preparing alternative adsorbents. An

attempt to remove methylene blue and methyl orange by alginate beads containing magnetic

nanoparticles and activated carbon [183], so as to develop a low-cost methodology having low

impact on environment. The adsorption capacity of beadswas found to be higher than non-

encapsulated AC for methylene blue and was of same order of magnitude for methyl orange. In a

study, Shimada et al. [184] used newspaper as raw material for the production of activated

carbon. The raw material mixed with 8% phenol resin resulted in activated carbons having good

surface area (1000 m2 g-1

) and yield (40%). In view of its high surface area this product

functioned as a good adsorbent, as evident by high iodine (1310 mg g-1

) and methylene blue

number (326 mg g-1

). Besides these various other materials such as waste tyre rubber [195],

layered double hydroxides [186], bottom ash [187-190], de-oiled soya [191-194], black tea

leaves, tree fern [195], pyrite and synthetic iron sulfide, alum-impregnated activated alumina,

sorel’s cement, calcined alunite [196] rosa canina seeds [197] etc. have also been explored as

adsorbents.

Adsorbent from Cocoa (Theobroma cacao)

The kinetics and equilibrium studies of Cocoa (Theobroma cacao) shell-based activated

carbon by CO2 activation in removing of cationic dye from aqueous solution was investigated by

Ahmad et al [198]. It was reported that the adsorption of 4-NP Cocoa shell based activated

carbons followed the Freundlich, Temkin and Langmuir isotherm models (R2 > 0.9) . The

pseudo-second-order-rate model and Boyd model fits for the adsorption. The monolayer

adsorption (212.72 mg/g) capacity was comparable with other adsorbents. The pseudo-second-

order-rate model fits better for the adsorption kinetics as compared to the pseudo-first-order-rate

model (Table 5).

The adsorption kinetic models suggest the adsorption process is complex, involving more

than one mechanism. The adsorption kinetic models suggest the adsorption process is complex,

involving more than one mechanism. The mechanism may involve adsorption of dye at an active

site on the surface, diffusion and adsorption into the interior pores of the AC particle. The results

of this study show that the activated carbons derived from cocoa shell can be used as potential

adsorbent for MB in water or wastewater treatments [208]. Utilization of agricultural solid waste

is of great significance for the preparation of activated carbon [199] and several researchers have

been studying the use of alternative materials, which, although less efficient, involve lower costs

[200-201]. The adsorption behavior of Reactive Yellow 2(RY2) from aqueous solution onto

activated carbon prepared from Cocoa (Theobroma cacao) shell was investigated under various

experimental conditions. To evaluate the adsorption capacity, initial dye concentration and

contact time, effect of solution pH and adsorbent dosage were investigated in a batch mode.

Experimental isotherm data was represented with Langmuir, Freundlich, Temkin and Harkins-

Jura isotherm models. The adsorption data were found to follow the Langmuir model better than

the other models. The data were also fitted to kinetic models such as pseudofirst order, pseudo-

second order, Elovich and intraparticle diffusion models. Results indicated that Cocoa shell

activated carbon (CSAC) could be a low cost adsorbent for the removal of RY2 from aqueous

solution [198].

OTHER IMPORTANT AGRICULTURAL SOLID WASTES USED AS ADSORBENTS

FOR DYES SEQUESTRATION

There have been many attempts to find inexpensive and easily available adsorbents to

remove the pollutants such as the agricultural solid wastes where according to their physic-

chemical characteristics and low cost they may be good potential adsorbents [202-203].

Agricultural productions are available in large quantities around the world; thus big amount of

wastes rejected. Agricultural wastes are lignocellulosic materials that consist of three main

structural components which are lignin, cellulose and hemicelluloses. These components

contribute mass and have high molecular weights. Lignocellulosic materials also contain

extractive structural components which have a smaller molecular size [204]. Different adsorbents

derived from agricultural solid wastes have been used for dye removal from wastewater and

many studies of dye adsorption by agricultural solid wastes have been published. Agricultural

and industrial sectors dispose of large amounts of untreated waste, which may pollute the land,

water and air, and as a result damage the ecosystem. On the other hand, improper treatment of

these wastes causes similar problems. Therefore legislative control of pollutants should be

enacted to prevent or to minimize the transfer of hazardous material to other areas [205].

Therefore within the last few years many ideas have been introduced in order to properly dispose

of these wastes, such as intensive use as adsorbents for pollutant removal especially for dye

removal where it showed high adsorption capacity [206]. Agricultural wastes are renewable,

available in large amounts and less expensive as compared to other materials used as adsorbents.

Agricultural wastes are better than other adsorbents because the agricultural wastes are usually

used without or with a minimum of processing (washing, drying, grinding) and thus reduce

production costs by using a cheap raw material and eliminating energy costs associated with

thermal treatment [207].

Other agricultural solid wastes from cheap and readily available resources such as papaya

seeds [208], grass waste [209,210], pomelo (Citrus grandis) peel [211], guava leaves [212,213],

gulmohar (Delonix regia) plant leaf powder [214], jackfruit peel [215], banana waste [216,217],

palm kernel fiber [218], rice straw [219], broad bean peels [220], rubber seed shell [221], castor

seed shell [222], pumpkin seed hull [223], pineapple stem [224], dehydrated peanut hull [225],

coconut husk [226], coffee husks [227], Parthenium hysterophorus [228], garlic peel [229], fallen

phoenix tree’s leaves [230], ground hazelnut shells [231-232], coconut bunch waste [233],

peanut hull [234], Luffa cylindrica fibers [235], yellow passion fruit waste [236] jute waste

[237], cereal chaff [238], orange peel [217], wheat shells [239], wheat straw [240], neem

(Azadirachta indica) leaf powder [241] have also been successfully employed for the removal of

dyes from wastewater. Table 5 also present some previous studies of the adsorption of dyes using

adsorbents based on agricultural solid wastes.

EFFECT OF OPERATIONAL PARAMETERS ON DYE SEQUESTRATION USING

AGRICULTURAL WASTES.

Effect of contact time and initial dye concentration

The efficiency of dye removal was increased as the contact time increased and lowers

initial dye concentration [265]. Optimal equilibrium time of various dyes with different charcoal

adsorbents from agricultural residues (bagasse, groundnut shells, pea shells, tea leaves and wheat

straw) is between 4 and 5 h [266]. In general, adsorption of dyes increased with increasing of

sorbent dosage. For acidic dyes the ratios of dyes sorbed had approached maximum values when

sorbent dose of 5 (g/L) was used [266]. The experimental results of adsorptions of Congo red

(CR), Malachite green (MG) and Rhodamine B (RDB) on the activated carbon at various initial

concentrations (5, 10, 15, 20, 25 and 30 mg/L) with contact time are reported [267]. The percent

adsorption decreases with the increase in initial dye concentration, but the actual amount of dye

adsorbed per unit mass of carbon increased with increase in dyes concentration. It means that the

adsorption is highly dependent on the initial concentration of dyes [267]. The effect of initial

concentration of Cocoa (Theobroma Cacao) Shell in the solution on the capacity of adsorption

onto the adsorbent was also studied and shown in Fig. 3 [268]. The experiments were carried out

using Reactive Yellow 2 (RY2) solution with initial concentrations 20, 40, 60 and 80 mg/L were

agitated with 100 mg of Cocoa shell activated carbon (CSAC) at 35°C. As shown in Fig. 3, the

adsorption at different dye concentrations was rapid at the initial stages and then gradually

decreases with the progress of adsorption until the equilibrium was reached. The rapid adsorption

at the initial contact time can be attributed to the availability of the positively charged surface of

activated carbon [269]. The contact time needed for RY2 solution to reach equilibrium was 90

min. [268]. At lower concentration, the ratio of the initial number of dye molecules to the

available surface area is low, subsequently, the fractional adsorption become independent on the

initial concentration [267]. In the process of dye adsorption initially dye molecules have to first

encounter the boundary layer effect and then it has to diffuse from boundary layer film onto

adsorbent surface and then finally, it has to diffuse into the porous structure of the adsorbent

[268].

Effect of temperature

Some structural changes in the dyes and the adsorbent occur during the adsorption. The

adsorption capacity of the activated carbon increases with the increase in experimental

temperature from 303K to 333K [267]. The adsorption capacity of adsorbent depends on the

thermodynamic parameters, such as ΔG◦, ΔH◦ and ΔS◦ [270]. The values ΔH◦ and ΔS◦ are

obtained from the slope and intercept of Van’t Hoff plots. The values are the range of 1 and 93

kJ/mol indicating the favorability of physisorption. The positive values of ΔH◦ show the

endothermic nature of adsorption and the possibility of physical adsorption. In the case of

physical adsorption, while increasing the temperature of the system, the extent of dye adsorption

increases, this rules out the possibility of chemisorptions. The negative values of ΔG◦ show that

the adsorption is highly favorable for Congo red. The positive values of ΔS◦ show increased

disorder and randomness at the solid solution interface of the adsorbent [267]. The effect of

temperature on the methylene blue removal on sawdust was investigated [271].

Effect of pH on dye uptake

The initial pH of solution can significantly influence adsorption of dyes. Maximum

adsorptions of acidic dyes were obtained from the solutions with pH 8–10 [266]. For instance,

Abbas [272] reported that using adsorbent material, the percent removal of Congo red was

decreased when the pH of simulated synthetic aqueous solution (SSAS) was increased at

constant other variables, while the percent removal of methylene blue, brilliant green and crystal

violent dyes were increased when the pH of SSAS was increased at constant other variables (Fig.

4). It is well recognized that the pH of the aqueous solution is an important parameter in

affecting adsorption of cationic dye [273]. High adsorption of Congo red at low pH can be

explained in both terms; the species of Congo red and the adsorbent surface. For this case, at low

pH, i.e. acidic conditions, the surface of the adsorbent (RH) becomes highly protonated and

favours adsorb of above group of Congo red dye in the anionic form. With increasing the pH of

SSAS, the degree of protonation of the RH surface reduces gradually and hence adsorption is

decreased [274]. Furthermore, as pH increases there is competition between hydroxide ion (OH-

)and species of Congo red, the former being the dominant species at higher pH values. The net

positive surface potential of sorbent media decreases, resulting in a reduction the electrostatic

attraction between the (sorbent) Congo red species and the (sorbate) adsorbent material surface

(RH), with a consequent reduced sorption capacity which ultimately leads to decrease in

percentage adsorption of Congo red dye [275].

On the other hand, the adsorption of methylene blue, brilliant green and crystal violent

dyes (each one alone) can be explained by ion-exchange mechanism of sorption in which the

important role is played by functional groups that have cation exchange properties. For this case

at lower pH values, dyes removal was inhibited, possibly as a result of the competition between

hydrogen and dyes molecules on the sorption sites, with an apparent preponderance of hydrogen

ions, which restricts the approach of metal cations as in consequence of the repulsive force. As

the pH increased, the ligand functional groups in adsorbent media (RH) would be exposed,

increasing the negative charge density on the adsorbent material surface, increasing the attraction

of dyes molecules with positive charge and allowing the sorption onto adsorbent material surface

[272].

Adsorption mechanisms of dye-adsorbent

The adsorption mechanism involves chemical bonding and ion exchange. The adsorption

mechanisms can be explained by the presence of several interactions, such as complexation, ion

exchange due to a surface ionization, and hydrogen bonds. One problem with sawdust materials

is that the adsorption results are strongly pH-dependent [276]. There is a neutral pH beyond

which the sawdust will be either positively or negatively charged. Reactive dyes attach to their

substrates by a chemical reaction that forms a covalent bond between the molecule of dye and

that of the fiber. Thus the dyestuff becomes a part of the fiber and is much less likely to be

removed by washing than are dyestuffs that adhere by adsorption. The most important

characteristic of reactive dyes is the formation of covalent bonds with the substrate to be colored.

Thus the dye forms a chemical bond with cellulose, which is the main component of cotton

fibers. Chemical pretreatment of sawdust has been shown to improve the adsorption capacity and

to enhance the efficiency of sawdust adsorption [277–278]. Another waste product from the

timber industry is bark, a polyphenol-rich material. Because of its low cost and high availability,

bark is very attractive as an adsorbent. Like sawdust, the cost of forest wastes is only associated

with the transport cost from the storage place to the site where they will be utilized [279]. Bark is

an effective adsorbent because of its high tannin content [280]. The polyhydroxy polyphenol

groups of tannin are thought to be the active species in the adsorption process. Morais et al. [280]

studied adsorption of Remazol BB onto eucalyptus bark from Eucalyptus globulus. Tree fern, an

agricultural by-product, has been recently investigated to remove pollutants from aqueous

solutions [281]. Other agricultural solid wastes from cheap and readily available resources such

as cotton stalks [282], palm tree [282],wood pulp [283], bagasse [284], date pits [257], corncob

[285], barley husk [285], wheat straw [286], wood chips [287], and many others [288-305) have

also been successfully employed for the removal of dyes from aqueous solution.

The wheat husk has been activated and used as an adsorbent for the adsorption of

Reactofix golden yellow 3 RFN from aqueous solution. The equilibrium adsorption level was

determined to be a function of the solution pH, adsorbent dosage, dye concentration and contact

time. The equilibrium adsorption capacities of wheat husk and charcoal for dye removal were

obtained using Freundlich and Langmuir isotherms [304].

Conclusively, certain waste products from agricultural sources have been tested and

proposed for dye removal. The question now is which low-cost adsorbent is better and

satisfactory? To be frank, there is no direct answer to this question because each low-cost

adsorbent has its own specific physical and chemical characteristics such as porosity, surface

area and physical strength, as well as inherent advantages and disadvantages in wastewater

treatment that makes a better adsorbent for a particular type of dyes (Table 1). In addition,

adsorption capacities of sorbents also vary, depending on the experimental conditions. Therefore,

comparison of sorption performance is difficult to make. However, it is clear from the present

literature survey that non-conventional adsorbents have potential as readily available,

inexpensive and effective sorbents. They also possess several other advantages that actually

make them excellent materials for environmental purposes, such as high capacity and rate of

adsorption, high selectivity for different concentrations, equilibrium time and also rapid kinetics

(Tables 3).

CHALLENGES AND FUTURE PROSPECTS

This review has attempted to cover a wide range of inexpensive, locally available and

effective materials that could be used in lieu of commercial activated carbon for the removal of

various classes of dyes from wastewater. Only little efforts have been made to carry out a cost

comparison between activated carbon and various non-conventional adsorbents. This aspect

needs to be investigated further in order to promote large-scale use of non-conventional

adsorbents. In spite of the scarcity of consistent cost information, the widespread uses of low-

cost adsorbents in industries for wastewater treatment applications today are strongly

recommended due to their local availability, technical feasibility, engineering applicability, and

cost effectiveness. If low-cost adsorbents perform well in removing dyes at low cost, they can be

adopted and widely used in industries not only to minimize cost, but also improve profitability

with maximum output. Undoubtedly, low-cost adsorbents offer a lot of promising benefits for

commercial purposes in the future.

Further investigation on pore size distribution of adsorbent, size of dye molecules,

functional groups present on surface of adsorbent, initial pH, batch or column conditions,

temperature, particle size of adsorbent, the furnace as well as sample holder size (in case

activation is performed) should be provided. The effects of these parameters on the adsorption of

dyes should be examined by optimal experimental conditions. The adsorbent cost (raw material,

final product at laboratory scale) must also be made available, so that other users can compare

the materials with commercially available ones.

Moreover, batch conditions, characterization of adsorbents, adsorptive capacity, the

uptake mechanism and immobilization of the waste material for enhanced efficiency and

recovery are highly needed to design and carry out some pilot-plant scale in order to study and

check their feasibility at industrial level. As the dye effluents contain several other pollutants,

attention is required to be given to adsorption of dyes from mixtures and the low-cost alternative

adsorbents should further be investigated for their efficiency using dye effluents from industries.

Research workers should be encouraged to work on real effluents at local level. Since a

vast number of the low-cost adsorbents are produced from locally available materials, the

researchers utilizing low-cost alternative adsorbents for dye removal can work and promote the

mantra ‘think globally, act locally’. Combination of methodologies/system should also be

encouraged since one perfect method/system is practically difficult. Besides these, strategies to

minimize dye and related chemical effluent at all levels and designing more environmentally

friendly chemicals are also needed to achieve a maximum removal process of various pollutants

such as dyes and other organic/or inorganic from wastewater.

In spite of bright future of low cost adsorbents, there are some issues related to their

success in near future. The management of the exhausted adsorbent is an important issue and has

not been not been taken care completely. Besides, there is no report available on the issue related

to the management of removed dyes. In our views, the removed dyes should be recycled. The

removed dyes should be filled in steel containers and treated as in case of nuclear wastes. Some

adsorbents are not effective working under natural conditions. Therefore, there is great need to

develop such adsorbent that can work at pH 7.0, ambient temperature with short contact time.

Normally, low cost adsorbents are effective for removing dyes at mg/mL concentration.

Attempts should be made to module them for working at g/mL concentration too. Besides,

these adsorbents should be prepared in an eco-friendly manner and used under control to avoid

any environmental hazards.

CONCLUSION

Recently, various low-cost adsorbents derived from agricultural wastes have been

investigated intensively for dye removal from contaminated wastewater. Locally available

agricultural wastes are easily converted to charcoals which can be used as activated carbons.

However, despite numerous papers published on low-cost adsorbents, there is as yet little

information containing a full study of comparison between sorbents. Although much has been

accomplished in the area of low-cost sorbents, much work is necessary (i) to predict the

performance of the adsorption processes for dye removal from wastewaters under a range of

operating conditions, (ii) to better understand adsorption mechanisms and (iii) to demonstrate the

use of inexpensive adsorbents in an industrial scale. Therefore removal of dye contaminants from

wastewaters using agricultural based activated carbon has offered promising results with

maximum efficiency in the field of adsorption technology because they show outstanding

removal capabilities for various class of dyes and could be used in place of commercial activated

carbon.

.

ACKNOWLEDGEMENTS

The authors thankfully acknowledge the support obtained from The World Academy of

Science (TWAS) in form of grant; Research Grant number: 11-249

RG/CHE/AF/AC_1_UNESCO FR: 3240262674 given to the corresponding author.

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LIST OF TABLES AND TABLE LEGENDS

Table 1: Classification of dyes according to colour index (C.I) application [33]

Class Principal

substrates

Method of

application

Chemical Types Examples

Acid Nylon, wool,

silk, paper, inks,

and leather

Usually from

neutral to acidic

dye baths

azo (including

premetalized),

anthraquinone,

triphenylmethane

, azine, xanthene,

nitro and nitroso

Acid Yellow 36

Azoic

components

and

Composition

Cotton, rayon,

cellulose acetate

and polyester

Fibre

impregnated

with coupling

component and

treated with a

Azo Bluish red azoic dye

s solution of

stabilized

diazonium salt

Basic Paper,

polyacrylonitrile

,

modified nylon,

polyester and

inks

Applied from

acidic dye baths.

Cyanine,

hemicyanine,

diazahemicyanine

,

diphenylmethane,

triarylmethane,

azo, azine,

xanthene,

acridine, oxazine

and

anthraquinone

Basic brown1,

methylene blue

Direct Cotton, rayon,

paper, leather

and nylon

Applied from

neutral or

slightly alkaline

baths containing

additional

electrolyte.

Azo,

phthalocyanine,

stilbene, and

oxazine

Direct orange 26

Disperse Polyester,

polyamide,

acetate, acrylic

and plastic

Fine aqueous

dispersion often

applied by high

temperature /

pressure or lower

temperature

carrier methods;

dye may be

padded on cloth

Azo,

anthraquinone,

styryl, nitro, and

benzodifuranone

Disperse yellow 3,

Disperse Red 4, and

Disperse Blue 27

and baked on or

thermofixed

Flourescent

brighteners

Soaps and

detergents, and

all fibers, oils,

paints and

plastics

From solution,

dispersion Or

suspension in a

mass.

Stilbene,

pyrazoles,

coumarin, and

Naphthalimides

4,4’-bis

(ethoxycarbonylvinyl

) stilbene

Food, drug,

and

cosmetics

Foods, rugs, and

Cosmetics

Azo,

anthraquinone,

carotenoid And

triarylmethane

Food Yellow 4,

Tartrazine

Mordant Wool, leather,

and Anodized

aluminium

Applied in

conjunction with

Cr salts

Azo and

anthraquinone

Mordant red 11

Oxidation

bases

Hair, fur, and

cotton

Aromatic amines

and phenols

oxidized on the

substrate

Aniline black and

indeterminate

Structures

Reactive Cotton, wool,

silk, and nylon

Reactive site on

dye reacts with

functional group

on fiber to bind

dye covalently

under influence

of heat and pH

(alkaline)

Azo,

anthraquinone,

phthalocyanine,

formazan,

oxazine and basic

Reactive blue 5

Solvent Plastics,

gasoline,

varnishes,

lacquers, stains,

inks, fats, oils,

waxes

Dissolution in

the Substrate

Azo,

triphenylmethane

, anthraquinone,

and

phthalocyanine

Sulphur Cotton and

rayon

Aromatic

substrate vatted

with sodium

sulphide and

reoxidized to

insoluble

sulphur-

containing

products on fibre

Indeterminate

structures

Vat Cotton, rayon

and wool

Water-insoluble

dyes solubilized

by reducing with

sodium

hydrogensulphite

, then exhausted

on fibre and

reoxidized

Anthraquinone

(including

polycyclic

quinines) and

indigoids

Vat Blue 4

(Indanthrene).

Table 2: Advantages and disadvantages of dyes removal methods [55].

Table 3: Adsorption capacities of agricultural wastes for dye removal

Dyes Adsorbents Equilibrium time qe(mg/g) Refs.

Rhodamine B Banana Bark 40 min 40.161 [104]

Direct F. Scarlet Rice husk 90 min 4.35 [105]

Direct red-23 Mangrove Bark 4 hrs 21.55 [106]

Malchite Green Pandanus Leaves 40 min 9.737 [107]

Reactive red 23 P.Oceanica leaf

sheath

10 hrs 0.31 [108]

Methylene blue Rice husk 30 min 690 [109]

Red Brown C4R Tapioca peel 45 min 121.47 [110]

Acid violet 17 Bagasse 4 hrs 38.32 [111]

Acid violet 17 Groundnut shell 100.57 [111]

Acid Red 119 Pea Shell 4 hrs 44.48 [111]

Acid Blue15 Used tea leaves 4 hrs 126.53 [111]

Acid Red 119 wheat straw 72.81 [111]

Malachite green Rice husk 40 min 63.85 [112]

Malchite Green Neem Bark 7 hrs 0.36 [113]

Malchite Green Mango Bark 7 hrs 0.53 [113]

Congo red Tamarind shell 4 hrs 10.48 [114]

Congo red Neem leaf powder 5 hrs 72 [115]

Reactive blue 19 Grape fruit peel 45 min 12.53 [116]

Methylene blue Teak tree bark 30 min 333.3 [117]

Basic Yellow 21 Wheat straw 48 hrs 71.43 [118]

Methylene blue Sunflower seed husk 4 hrs 45.25 [119]

Methylene blue Hazlenut shell 60 min 76.9 [120]

Acid Blue Hazlenut shell 60 min 60.2 [120]

Methylene blue Cherry saw dust 2 hrs 39 [120]

Methylene blue Walnut saw dust 60 min 59.17 [120]

Acid Blue Oak saw dust 60 min 29.5 [120]

Acid Blue Pitch Pine saw dust 60 min 27.5 [120]

Table 4 – Pseudo-first-order model and pseudo-second-order model equation constants and

correlation coefficients for adsorption of MB on CSAC at 300C [198].

Initial concentration

(mg/L)

Pseudo first-order kinetic

model

Pseudo second-order kinetic

model

Qe, cal (mg/g) K1 (h-1

) R2 Qe, cal

(mg/g)

K1 (g/mgh) R2

50 - - - 8.31 12.08 0.99

100 5.37e+4

6.02 0.95 13.39 0.22 0.98

150 35.49 1.18 0.99 25.19 0.98 0.99

200 120.23 0.022 0.76 33.33 9.00 0.99

250 3.39 2.6 0.82 41.67 5.76 0.99

Table 5: Previous studies of the adsorption of dyes using adsorbents based on agricultural

solid wastes.

Adsorbents Dyes References

Sugar beet pulp Gemazol turquoise blue-G [242]

Rice husk ash Indigo Carmine [243]

Chemically modified

peanut hull Methylene blue, Brilliant cresyl blue,

Neutral red, Sunset yellow and Fast green [244]

Peanut hull Methylene blue, Brilliant cresyl blue,Neutral red [245]

Coir pith AC Reactive orange 12,

Reactive red 2 and Reactive blue 4 [246]

Coir pith AC Congo red [247]

Coir pith carbon Methylene blue [248]

Coir pith Acid violet [249]

Rice husks AC Malachite green [250]

Rice husk-based

porous carbon Malachite green [251]

Rice husk Congo red [252]

Tea waste Methylene blue [253]

Coniferous pinus

bark powder Crystal violet [254]

Orange peel AC Direct N Blue-106 [255]

Neem sawdust Malachite green [256]

Guava seed carbon Acid blue 80 [257]

Peanut hull Reactive Black 5 [258]

Loofa AC Reactive orange [259]

Apricot stone AC Astrazon yellow (7GL) [260]

Almond shells Direct red 80 [261]

Lemon peel Malachite green [262]

Bagasse fly ash Methyl violet [263]

Polygonum

orientale Linn AC Malachite green [264]

RESEARCH HIGHLIGHTS

DYE SEQUESTRATION USING AGRICULTURAL WASTES AS ADSORBENTS

The study highlights the following:

1. presents a critical analysis of these materials;

2. describes their characteristics, advantages and limitations; and

3. discusses various mechanisms involved.

4. future prospects of using agricultural wastes as adsorbents are addressed.

LIST OF FIGURES AND FIGURE LEGENDS

Fig1a. Plot of Bt versus t (Boyd plot). Fig1b. Predicted q values for different initial dye

concentrations (conditions: pH, 8; rpm, 800 revmin−1

; temperature, 32 ◦C; V/X, 1 L/4 g).

Figure 2: Kinetic plot for the adsorption of RY2 onto CSAC (a) Pseudo-first order, (b) Pseudo-

second order, (c) Elovich, and (d) Intraparticle diffusion [198].

Fig. 3: Effect of initial concentration and contact time [268].

Fig. 4: Effect of pH on the percent removal of dyes @ C˳ = 1 mg/l, Tf =55°C, hb = 1 m, t=60

min. and F=5 ml/min [272].