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
Johanna Norup
7th Lingfeng Summer Research School
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
Johanna Norup
7th Lingfeng Summer Research School
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 Hg 2+ 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.
Johanna Norup
7th Lingfeng Summer Research School
2 Background
2.1 Activated carbon
Activated carbon is a type of carbon that has been pulverized so the surface area increases. Nonpolarized 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.
Figure 1. 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).
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
(m2/g)
Pore volume
(cm3/g)
Aperture
(nm)
FGD
BPL
SBA-15
BPL-CX
702
995
711
739
0.535
0.488
0.781
0.380
N/A
1.96
5.29
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 bloodbrain 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).
Johanna Norup
7th Lingfeng Summer Research School
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.
Johanna Norup
7th Lingfeng Summer Research School
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.
Johanna Norup
7th Lingfeng Summer Research School
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, KBH 4 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.
Johanna Norup
7th Lingfeng Summer Research School
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.
1,200
1,000
BPL
Cx/Co (%)
0,800
BPL-CX
FGD
0,600
FGD-CX
0,400
molecular sieve
0,200
molecular sieve-CX
initial concentration
0,000
1
2
3
4
5
Adsorption time (h)
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
Initial
concentration
BPL
BPL-CX
FGD
FGD-CX
Molecular sieve
Molecular sieveCX
Average
ratio
Cx/Co
0,00
0,67
0,83
0,87
0,96
0,34
0,53
Johanna Norup
7th Lingfeng Summer Research School
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.
1,200
Cx/Co (%)
1,000
0,800
BPL
0,600
BPL-CX
FGD
0,400
FGD-CX
0,200
molecular sieve
0,000
molecular sieve-CX
1
2
3
4
5
Adsorption time (h)
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
Johanna Norup
7th Lingfeng Summer Research School
Cx/Co (%)
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.
BPL comparison
1,000
0,900
0,800
0,700
0,600
0,500
0,400
0,300
0,200
0,100
0,000
BPL
BPL-CX
1
2
3
4
Adsobent time (h)
5
Figure 8. BPL and BPL-CX’s ability to adsorb mercury.
FGD comparison
1,000
Cx/Co (%)
0,950
0,900
FGD
0,850
FGD-CX
0,800
0,750
1
2
Adsobent
time3(h)
Figure 9. FGD and FGD-CX’s ability to adsorb mercury.
4
Johanna Norup
7th Lingfeng Summer Research School
Molecular sieve comparison
0,800
0,700
Cx/Co (%)
0,600
0,500
Molecular
sieve
0,400
0,300
molecular
sieve-CX
0,200
0,100
0,000
1 Adsorbent
2 time (h) 3
Figure 10. Molecular sieve and molecular sieve-CX’s ability to adsorb mercury.
Johanna Norup
7th Lingfeng Summer Research School
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.
Johanna Norup
7th Lingfeng Summer Research School
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.
Johanna Norup
7th Lingfeng Summer Research School
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.
Johanna Norup
7th Lingfeng Summer Research School
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-granularactivated-carbon-60487, read 2013-07-12