comparative study of several kinds of adsorption … study of several kinds of adsorption materials...

International Summer Water Resources Research School

Dept. of Water Resources Engineering, Lund University

Comparative study of several kinds

of adsorption materials in Hg2+

adsorption performance

By

Johanna Norup

2013-07-19

Johanna Norup 7th Lingfeng Summer Research School

Abstract This report is a result of the 7th Lingfeng Summer Research School at Xiamen University. The project

has been focused on an investigation of different adsorbents efficiency to adsorb mercury gas. The

different kinds of adsorbents have been compared in a bench scale system with a nitrogen-mercury

gas mixture. A comparison has been made with the amount of mercury that has passed the

adsorbent without being adsorbed. This project has studied the types BPL, DARCO FGD, molecular

sieve (SBA-15) and modified adsorbents: PBL-CX, DARCO FGD-CX, molecular sieve-CX. The detection

of the concentration was done with an Atomic Fluorescence Spectrometry, AFS.

The results showed that FGD-CX was the best adsorbent in the study and adsorbed 96% of the

mercury. The worst tested adsorbent was molecular sieve that only adsorbed 34% of the mercury

gas. The comparison with and without modification concluded that an activated carbon modified

with benzoic acid increased the ability to adsorb mercury gas.

Keywords: mercury removal, mercury gas, adsorption, activated carbon, flue gas, AFS


Table of Contents Abstract .................................................................................................................................................. 2

1 Introduction ........................................................................................................................................ 4

1.1 Aim ............................................................................................................................................... 4

2 Background ......................................................................................................................................... 5

2.1 Activated carbon .......................................................................................................................... 5

2.2 Mercury ........................................................................................................................................ 6

2.3 Mercury - a hazardous chemical ................................................................................................... 6

3 Materials and method ......................................................................................................................... 7

3.1 Experimental setup ...................................................................................................................... 7

3.2 Sampling ....................................................................................................................................... 7

3.3 Detection of mercury concentration ............................................................................................ 9

4 Results ............................................................................................................................................... 10

5 Analysis ............................................................................................................................................. 14

5.1 Limitations .................................................................................................................................. 14

5.2 Sources of error .......................................................................................................................... 14

5.3 Conclusions ................................................................................................................................ 15

6 Acknowledgement............................................................................................................................. 16

7 References ......................................................................................................................................... 17


1 Introduction Mercury is one of the most hazardous chemicals that humans utilize. Mercury is emitted into the

atmosphere mainly through coal combustion and waste incineration (Hall B. et al, 1991). Mercury is

not degraded in any large extension in the atmosphere. Instead it accumulates in the ecosystem and

will mainly affect the top consumers in the food web, for example humans (Scala F. et al, 2011).

A lot of research has been made within this topic, and everyone can agree that activated carbon is a

good adsorbent for mercury gas. The questions left are how it can be cheaper and more efficient to

clean the gas from mercury so the emissions will reach zero.

The whole ecosystem is affected by the emissions of mercury. Both plants and animals and humans

will experience negative effects from the accumulation of mercury. It is higher dosage higher in the

food web, which make mercury more hazardous for predators.

In this project different kinds of activated carbons ability to adsorb the mercury ion Hg2+ are studied

in a bench scale testing system, which will be a simulation of a cleaning system in an incineration

fume.

1.1 Aim The project’s aim is to compare different kinds of adsorbents to see how efficient they adsorb

mercury gas in the ionic form, Hg2+. The different types of activated carbon that has been studied are

BPL, DARCO FGD, molecular sieve (SBA-15) and modified activated carbon: PBL-CX, DARCO FGD-CX

and molecular sieve-CX.


2 Background

2.1 Activated carbon Activated carbon is a type of carbon that has been pulverized so the surface area increases. Non-

polarized subjects that are in contact with this area will be adsorbed by the activated carbon. The

activated carbon´s pore size distribution and surface area are important properties for the activated

carbon to be a good adsorbent. A large surface area is good because it offers a lot of active sites for

the mercury to attach to. The area can be as large as 1500m2/g carbon (Diamantopoulou I. et al,

2009).

Activated carbon can be produced of any kind of carbon containing material. The carbon can be

activated by treatment with water vapor, CO2, metal chlorides, phosphates or air (Elding L I., 2013).

The activated carbon BPL, which is studied in this project, stands for bituminous coal-based activated

carbon material that has been activated at high temperature in a steam atmosphere. It has a surface

area of 995m2/g and a pore volume of 0.488cm3/g (Chen X., 2013).

The next activated carbon in this study is DARCO FGD, a steam activated carbon manufactured

specifically for the removal of heavy metals found in incinerator flue gas emission streams. FGD

stands for Flue Gas Desulfurization. It has a surface area of 702m2/g and a pore volume of

0.535cm3/g (Chen X., 2013).

The third studied adsorbent is molecular sieve (SBA-15), a material with similar distributed holes in it

with uniform size. It is designed so molecules that are larger than the size of the holes will get stuck

and not get through the material (Environmental Expert, 2013). Molecular sieve has a surface area of

711m2/g and a pore volume of 0.781cm3/g (Chen X., 2013).

A summarized table of the physical properties for some of the adsorbents can be seen in table 1.

Sorbent-CX means that the sorbent has been modified by benzoic acid.

There are many different kinds of functional groups on the surface of the activated carbon. There are

four that can bind to mercury. They are carbonyl, carboxyl, lactones and phenol groups. The most

efficient are carbonyl- and carboxyl groups. Half of the adsorbents in this study is modified with

benzoic acid, which is a carboxyl acid (Chen X., 2013).

Figure 1. Benzoic acid

Johanna Norup 7th Lingfeng Summer Research School Table 1. Physical properties of the adsorbents. SBA-15 is the name of the studied molecular sieve.

Carbon BET specific surface area Pore volume Aperture

(m2/g) (cm3/g) (nm)

FGD 702 0.535 N/A

BPL 995 0.488 1.96

SBA-15 711 0.781 5.29

BPL-CX 739 0.380 2.03

2.2 Mercury Natural sources of mercury emissions to the atmosphere are volcanoes, forest fires and weathering

from mercury containing sediment. A natural level of exposure in the atmosphere is about 1-2ng/m3.

Most of the anthropogenic mercury comes from coal combustion and waste incineration. In waste

incineration the mercury originates from batteries, medical products and lamps (Elding L I., 2013).

The mercury concentration in the flue gas is around 0.5mg/Nm3. In many countries the emission limit

is as low as 0.05 mg/ Nm3. This means that it is very important to reduce the emissions directly at the

combustion (Diamantopoulou, I. et al, 2009).

About 95% of the mercury in the atmosphere is elemental (Hall B. et al, 1991). But it can be oxidized

with oxygen or ozone so Hg2+ ions form. These ions can be captured by particles or water droplets

and follow them down to the ground when it rains. When the mercury ions come in contact with the

ground they will bind to organic compounds and follow streams and rivers to lakes and seas

(Elding L I., 2013).

2.3 Mercury - a hazardous chemical In high dosages mercury can damage the nervous system, since mercury can penetrate the blood-

brain barrier. It also affects the kidneys, the liver and the immune system (Sterner O., 2010).

The Minamata disease tells us how dangerous methyl mercury is. Methyl mercury can be formed

when anaerobic organism are in contact with inorganic mercury in aquatic systems (EPA, 2013). In

the 1950’s a chemical factory near Minamata in Japan emitted mercury to the sea were it was

accumulated as methyl mercury in fish and seafood. The people of Minamata, which lived on fishing,

were affected by the methyl mercury that was formed (Harada, M. 1995). Methyl mercury affects the

peripheral nervous system that can give the consumer motor and mental disorders. Since mercury

can bind to the fat tissue in a human body, it maintains there and cannot be excreted with the urine

(Swedish EPA, 2013).


3 Materials and method Mercury adsorption trials were performed in two steps. First the bench scale testing system was

used to take samples. Then an AFS machine was used to detect the mercury concentration.

3.1 Experimental setup To compare the different activated carbons a bench scale testing system was prepared as seen in

figure 2. The input was nitrogen gas (1 L/min) mixed with mercury gas, Hg2+ (500ng/min). The gas

mixture was heated to 140°C to be more similar to incineration gas temperature. The adsorption

reactor is in an oven where the adsorption of mercury occurs. The gas is lead into the adsorption

reactor or directly to two bottles filled with KCl solution, where the mercury binds to the chloride ion

so the concentration can be measured.

Figure 2. Sketch over the bench scale testing system.

3.2 Sampling The procedure starts with weighting the activated carbon (30 mg mixed with 5 g quartz) and put it

into the adsorption reactor. The bottles with KCl are replaced once every hour to collect five samples

in total. But at first the initial concentration was measured. The gas was then lead directly to the KCl

solutions without going through the adsorption reactor and samples were taken every 30 minutes.

The following procedure was then done both for the initial concentration and the samples.

Every sample with KCl-mercury solution was stabilized with 5% KMnO4 solution so the mercury ions

will be stable.


Figure 3. Bench scale testing system.

Three new samples, 2ml each to put in 25ml test tubes, were taken from every hourly sample. This

resulted in 15 new samples. All new sample was put out together with 0.5ml concentrated H2SO4,

0.25ml concentrated HNO3, one drop of 5% KMnO4 solution and 0.75ml 5% K2S2O8. Then the solution

was diluted to 20ml. Three blank samples with KCl solution were also prepared in the same way to

detect the natural mercury concentration or error in the machine. All the samples were then heated

to 94°C for two hours.

Figure 4. New samples.


3.3 Detection of mercury concentration After putting one drop of 10% hydroxylamine sulfate, ((NH2OH)2 ·H2SO4 ) into the samples to make

them transparent, the samples are ready to be tested in the Atomic Fluorescence Spectrometry, AFS.

Mercury sends out 253.7 nm wavelength of fluorescence under ultraviolet excitation. The intensity of

fluorescence is proportional to the concentration of mercury. In the AFS, KBH4 was used as reducing

agent and 2% HNO3 was used as current-carrying. KBH4 react with the mercury sample and the ionic

mercury turns into elemental mercury so the AFS can detect the concentration of mercury´s

wavelength. Argon gas was preceded as a carrying gas for the mercury gas to the instrument for

detection. The result appeared on a computer screen connected to the AFS where all the data was

collected and saved, see figure 5.

Figure 5. The Atomic Fluorescence Spectrometry

To prepare the machine, standard solutions were tested first and a standard curve was performed.

Then after the machine was cleaned with HNO3 and KBH4, the samples’ concentrations could be

measured.

To calculate the actual concentration, the measured mean value of the three samples was subtracted

with the mean value of the three blank samples.

The same procedure was then repeated for all different adsorbents in the study.


4 Results The different adsorbents have been compared with a blank test and then with each other. The y-axis

in figure 6 and 7 shows the relation between the initial mercury concentration of the nitrogen

mercury gas mixture, c0, and the mercury concentration that has been adsorbed of the current

adsorbent, cx. On the x-axis the adsorption time is represented. Figure 6 shows all the studied

adsorbents’ capacity to adsorb mercury over five hours. The lowest mercury concentration in the

output gas is found in FGD-CX. The average ratio for FGD-CX is 96%, which means that 96% of the

mercury is adsorbed when the gas has passed the FGD-CX adsorbent. The worst adsorbent in the

study is molecular sieve. It has an average ratio of 34%, see table 2.

Figure 6. Ratio of adsorbed mercury over time with studied adsorbents.

Table 2. The average ratio of adsorbed mercury for the different activated carbons.

Type Average

ratio Cx/Co

Initial concentration

0,00

BPL 0,67

BPL-CX 0,83

FGD 0,87

FGD-CX 0,96

Molecular sieve 0,34

Molecular sieve-CX

0,53

0,000

0,200

0,400

0,600

0,800

1,000

1,200

1 2 3 4 5

Cx/

Co

(%

)

Adsorption time (h)

BPL

BPL-CX

FGD

FGD-CX

molecular sieve

molecular sieve-CX

initial concentration


To get the results neater, extended values can be ignored and the graph’s lines will be straighter, see

figure 7. The conclusion will be the same but it is easier to see the difference between the

adsorbents. These extended values may be there due to errors in the procedure, see limitations and

sources of error.

Figure 7. Ratio of adsorbed mercury over time with studied adsorbents with ignored extended values.

The result with modified values gives the same conclusion; the best adsorbent is still FGD-CX that

adsorbs 98% of the mercury with modifications according to table 3. Molecular sieve is also still the

worst adsorbent in the study which only adsorbs 19% with the modifications.

Table 3. The average ratio of adsorbed mercury for the different adsorbents with modified values.

Type Average

ratio Cx/Co

Initial concentration

0,00

BPL 0,67

BPL-CX 0,83

FGD 0,90

FGD-CX 0,98

Molecular sieve 0,19

Molecular sieve-CX

0,60

0,000

0,200

0,400

0,600

0,800

1,000

1,200

1 2 3 4 5

Cx/

Co

(%

)

Adsorption time (h)

BPL

BPL-CX

FGD

FGD-CX

molecular sieve

molecular sieve-CX

Johanna Norup 7th Lingfeng Summer Research School To see the difference in the ability to adsorb mercury with or without modification with benzoic acid,

figure 8, 9 and 10 shows the same adsorbent with and without CX. This is done to see if it the

modification is an efficient way to improve the adsorption for the activated carbon.

Figure 8. BPL and BPL-CX’s ability to adsorb mercury.

Figure 9. FGD and FGD-CX’s ability to adsorb mercury.

0,000 0,100 0,200 0,300 0,400 0,500 0,600 0,700 0,800 0,900 1,000

1 2 3 4 5

Cx/

Co

(%

)

Adsobent time (h)

BPL comparison

BPL

BPL-CX

0,750

0,800

0,850

0,900

0,950

1,000

1 2 3 4

Cx/

Co

(%

)

Adsobent time (h)

FGD comparison

FGD

FGD-CX


Figure 10. Molecular sieve and molecular sieve-CX’s ability to adsorb mercury.

0,000

0,100

0,200

0,300

0,400

0,500

0,600

0,700

0,800

1 2 3

Cx/

Co

(%

)

Adsorbent time (h)

Molecular sieve comparison

Molecular sieve

molecular sieve-CX


5 Analysis As seen in table 1, BPL has larger surface area than FGD. That would indicate a good adsorbent. But

in this study this is not the case. This can be because of the limitations in this project. No conclusion

can be made which one of the studied that are the best. Other studies have shown that BPL is better

than FGD, while this study shows that FGD is better (Chen, X. 2013). Therefore more experiments

have to be done with the same procedure for a longer time to get more accurate results.

A reason for the molecular sieve to be the worst adsorbent is that molecular sieve has relatively poor

thermal and chemical stability at high temperatures (Lin Y.S. et al, 1994).

In a comparison with the pore volume, there is also hard to find any correlations. The molecular sieve

has the largest pore volume while BPL has the smallest. Since the best adsorbent in this study was

FGD, this result indicates that the pore volume does not have any impact on the adsorption ability.

The result makes it hard to find any correlations between the physical property of the adsorbent and

the ability to adsorb mercury.

The figures 8, 9 and 10 shows that the adsorption is improved if the adsorbents are modified with

benzoic acid. This was expected since more effective active sites for mercury to attach to occurs with

the modification. The benzoic acid is a carboxyl acid and it increases the sites that are easiest for

mercury to attach to on the adsorbent. This means that the less efficient active sites; phenol and

lactones groups are less on the activated carbon’s surface area. Therefore the adsorbents modified

with benzoic acid could be better adsorbents, which have been confirmed in the study.

5.1 Limitations This project lasted from the 24th of June until the 19th of July. This means that there was a limited

time to take all the samples and detect the concentrations. The project was delayed due to an

electrical outage and the fume carport stopped working.

Every adsorbent was studied for five hours, even if some could adsorb mercury for a longer time. The

adsorbents’ time depending ability to adsorb mercury has not been considered in this study.

It also has to be taken into consideration that the gas mixture is a prototype of a flue gas fume. The

gas mixture will not look the same in reality and other trace gases in flue gas can affect the

adsorbents’ ability to adsorb mercury. The bench scale testing system is only a simulation of a

cleaning system in reality, which also affect the result.

5.2 Sources of error Almost everything has worked out as planned, but there are always some human factors that have to

be taken into consideration. One example is that the KCl solutions once were installed before the gas

was lead into the adsorption reactor, and therefore the concentration would be rather misleading.

The flasks with KCl solution was once put in the wrong way, so the gas came in where it should go

out, so there was some solution that was dropped.


5.3 Conclusions Due to this study the DARCO FGD-CX is a better adsorbent than the others studied in this project. The

worst adsorbent in this study is molecular sieve. In comparison with all studied adsorbents, the ones

modified with benzoic acid are better than them without.

More studies have to be done to make a general conclusion about which adsorbents that has the

best ability to adsorb mercury and what it depends on.


6 Acknowledgement The author wants to thank Professor Jinjing Luo for supervising this project. A special thank to

assistant Xiaobao Chen for many valuable comments and explanations about the experiments. Many

thanks to Syuan Lei Jhao for a good cooperation in this project.

A last thank to the organizers at Lund University and Xiamen University for making the Lingfeng

Summer Research School possible with many memorable moments and cultural experiences.


7 References Cai Y., Atomic Fluorescence in Environmental Analysis, 2009,

Chen X., oral communication June-July 2013

Diamantopoulou, I., Skodras G., Sakellaropoulos G.P, Sorption of mercury by activated carbon in the

presence of flue gas components, 2009

Elding L I, The National Encyclopedia, http://www.ne.se/lang/kvicksilver, read 2013-07-03

EPA,

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/methylmercury/factsheet.cfm,

read 2013-07-08

Hall B. Schager P., Lindqvist O., Chemical reactions of mercury in combustion flue gases, 1991

Harada, M., Minamata Disease and the Mercury Pollution of the Globe, 1995

Lin Y.S., Deng S., Adsorption and desorption of sulfur dioxide on novel adsorbent for flue gas desulfurization, 1994 Ohlsson R., The National Encyclopedia, http://www.ne.se/aktivt-kol , read 2013-07-05

Scala F., Chirone R., Lancia A., Elemental mercury vapor capture by powdered activated carbon in a

fluidized bed reactor, 2011

Sterner O., Chemistry, Health & Environment, 2010

Swedish EPA,

http://www2.naturskyddsforeningen.se/Regional%20Office%20Files/Kretsar%20och%20l%C3%A4nsf

%C3%B6rbund/Sk%C3%A5ne/Lund/F%C3%B6reningsdokument%20LNF/ESS/Fakta%20om%20kvicksil

ver.pdf, read 2013-07-04

The Environmental Expert, http://www.environmental-expert.com/products/bpl-4x10-granular-

activated-carbon-60487, read 2013-07-12

http://www.ne.se/lang/kvicksilver

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/methylmercury/factsheet.cfm

http://www.ne.se/aktivt-kol

http://www2.naturskyddsforeningen.se/Regional%20Office%20Files/Kretsar%20och%20l%C3%A4nsf%C3%B6rbund/Sk%C3%A5ne/Lund/F%C3%B6reningsdokument%20LNF/ESS/Fakta%20om%20kvicksilver.pdf



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comparative study of several kinds of adsorption … study of several kinds of adsorption materials...

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