EFFECT OF MOISTURE ON ADSORPTION OF ELEMENTAL MERCURY BY
ACTIVATED CARBONS
Y. H. Li*, S.D. Serre, C. W. Lee, and B. K. Gullett
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research Laboratory (MD-65)
Research Triangle Park, NC 27711
Abstract
Experiments using activated carbon to capture elemental mercury (Hg ) were
performed using a bench-scale fixed-bed reactor and a flow reactor to determine the role
of surface moisture in Hg° adsorption. Three activated carbon samples, which have
different pore structure and ash contents, were tested for Hg° adsorption capacity. From
both fixed-bed reactor and flow reactor experimental results, the moisture on activated
carbon surfaces has been found to have a significant effect on Hg° adsorption. A
common effect of moisture on Hg° adsorption was observed for all three samples, despite
extreme differences in their ash contents, suggesting that this effect is not associated with
ash content. Temperature programmed desorption (TPD) experiments performed on the
carbon samples after the Hg° adsorption experiments indicated that chemisorption of Hg°
is a dominant process over physisorption for the moisture-containing carbon samples, and
diminished for the heat-treated moisture-free samples. X-ray absorption fine-structure
(XAFS) spectroscopy results provide evidence that mercury bonding on the carbon
surfaces was associated with oxygen through a mechanism likely involving electron
transfer processes. The aromatic resonance-stabilized structures in equilibrium with the
oxygen surface functional groups take up electrons from the mercury atoms. The active
sites likely are hydrated functional groups that result from the hydrogen bond of the
adsorbed water (t^O). Removal of the adsorbed H2O inhibits the equilibrium conditions
that allow surface functionalities to become electron acceptors.
Introduction
Activated carbons are widely used in the removal of pollutants from water and gases.
Direct injection of an activated carbon into the flue gas stream has been used as a
relatively simple approach for controlling mercury emissions. Although research has
been performed to study the adsorption of mercury by activated carbons, current
knowledge of mercury adsorption by activated carbons is limited, presenting intriguing
scientific questions related to the nature of the adsorption (physisorption or
chemisorption) and the effects of mercury species type in the gas and solid phases.
Adsorption of Hg° by activated carbons at ambient temperatures (e.g., 23 °C) has
been suggested to be a combination of chemisorption and physisorption, whereas
chemisorption is prevalent at higher temperatures; e.g., 140 °C [1]. The Hg° adsorption
capacity of activated carbons decreases with increasing temperature and decreasing
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mercury concentration [1-3]. Many factors have been found to influence the efficiency of
mercury removal, including carbon characteristics, flue gas composition, and the
presence of active components (e.g., fly ash) [4]. The low concentrations of Hg° in the
flue gases (~ 10 |ig/m3) and short residence times (< 3 s) of the injected sorbent require a
large amount of activated carbon (high carbon/Hg0 ratio) in order to achieve adequate
mercury removal [5, 6].
Studies have shown that sulfur-impregnated activated carbons exhibit significantly
greater Hg° adsorption capacities compared to those of the thermally activated carbons
[1-3, 7-9]. However, heteroatoms such as sulfur (S) and chlorine (Cl), which are reactive
for Hg° capture, generally exist only as trace elements in thermally activated carbons. On
the other hand, appreciable amounts of oxygen (O) are almost invariably associated with
activated carbons in the form of surface carbon-oxygen complexes produced from the
activation process. Carbon-oxygen surface complexes are by far the most important
structure influencing the surface characteristics and adsorption behaviors of activated
carbons [10]. So far, no known research has been done to understand the role of carbon-
oxygen surface complexes in Hg° adsorption.
Krishnan et al. [1] showed that Hg° capture capacities of heat-treated [140 °C,
flowing nitrogen (N2)] activated carbon samples were far less than those of unheated
samples. In addition, the same heat treatment applied to a sulfur-impregnated carbon
showed no significant reduction of Hg° capture capacity. Based on these observations,
they suggested that the active sites causing Hg° adsorption in the thermally activated
carbons contained O, and these weakly bonded surface oxygen complexes were effective
in capturing Hg°. The reduction of Hg° capture capacity for the heat-treated carbons was
due to the loss of weakly bonded surface oxygen complexes.
However, it is generally believed that the major effect of heat treatment at low
temperatures (e.g., 25-150 °C) on carbon surfaces is the removal of adsorbed H2O
molecules [11]. Extensive studies on water adsorption by activated carbons found that
H2O is adsorbed on the carbon surfaces by means of hydrogen bonding [12-16]. The
oxygen complexes on carbon surfaces form primary adsorption centers, which bind the
H2O molecules at low relative pressures. Adsorbed H2O molecules can then become
secondary adsorption centers as the H2O vapor pressure increases. In general, carbon-
oxygen surface complexes are stable below 200 °C [10], but decompose to produce H2O,
carbon dioxide (€62), and carbon monoxide (CO) when heated to higher temperatures
under an inert gas atmosphere. From the above discussions, it appears that the reduction
of Hg° capture resulting from low-temperature heat treatment as observed by Krishnan et
al. [1] might have been caused by the removal of moisture from the carbon surfaces.
Infrared (IR) spectroscopic studies [17] have shown that the change of surface species
occurred during the adsorption of H2O. The surface oxygen species or oxidized ions,
which are influenced by TC electrons from the extensive aromatic systems in carbons, may
play an important role in surface reactions, oxidation reactions in particular [18].
The main objective of the work reported in this paper was to understand Hg°
adsorption by activated carbons and the effect of adsorbed H2O on Hg° adsorption. The
thermal stability of the adsorbed mercury was determined by temperature programmed
desorption (TPD), and X-ray absorption fine structure (XAFS) spectroscopy was used to
examine the nature of the mercury bonding on the carbon surfaces.
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Experimental
Three activated carbons, a lignite-based activated carbon (FGD, Norit Americas Inc.),
a bituminous-coal-based activated carbon (BPL, Calgon Carbon Corporation), and an
activated-carbon fiber (ACN, American Kynol, Inc.) were tested for their Hg° adsorption
uptake capacity in this study. The coal-based carbon samples as received (BPL, FGD)
were ground and sieved to the desired particle size range and sealed in plastic containers.
Selected characteristics of these carbons are shown in Table 1. BET (Brunauer-Emmett-
Teller) and DR (Dubinin-Radushkevich) surface areas were measured by N2 adsorption at
77K with relative pressure (P/Po) up to 0.99, and carbon dioxide (CO2) adsorption at
273K with P/P0 up to about 0.03, respectively, using an adsorption apparatus (ASAP
2400, Micromeritics). All samples were degassed at 300 °C under vacuum for 3 h prior
to the measurements. The total pore volume was evaluated from the N2 adsorption
isotherm at P/Po = 0.99, and the micropore volume was estimated from CCb adsorption at
273K using the DR equation. A CO2 molecular density of 1.03 g/cm3 and a cross-
sectional area of 0.187 nm2 were assumed for estimating the DR surface area [19]. The
moisture and ash contents of the samples were measured by using a thermogravimetric
analyzer (TGA-7, Perkin Elmer). The ACN sample is derived from a phenolic resin, in
the form of needled felt with over 92% carbon and the remainder oxygen and hydrogen
[20], with no ash. By employing the ash-free ACN sample, the effect of ash associated
with the FGD and BPL samples on Hg° adsorption, if it is significant, can be estimated.
Table 1 Characteristics of activated carbon samples
Characteristic
BET surface area (m2/g)
CO2 surface area (m2/g)
Total pore volume (cm'Vg)
Micropore volume (cm3/g)
Particle size/fiber diameter (jim)
Moisture (% wt)
Combustible (% wt)
Ash (% wt)
FGD
540
470
0.55
0.18
16-24
4.0
68.3
27.7
BPL
1136
976
0.58
0.37
125-177
2.5
93.7
3.8
ACN
1250
1248
0.51
0.48
10a
2.0
98.0
0.0
From manufacturer's data sheet
Figure 1 presents the schematic diagram of the fixed-bed reactor experimental setup.
Industrial grade N2 gas was used as a purge and Hg° carrier gas. A quartz fixed-bed
reactor (1.27 cm, I.D.) surrounded by a temperature-controlled electrical furnace was
used. A dryer and an O2 trap were used to remove trace H2O vapor and O2, respectively,
remaining in the purge and carrier gas stream. To determine whether mercury is in the
elemental (Hg°) or oxidized (Hg++) forms in the reactor outlet, an additional electrical
furnace operating at a temperature of 900 °C was added downstream of the reactor to
convert Hg++ to Hg° [21]. The Hg°-laden gas mixture was generated using a Hg°-
containing permeation tube in a constant temperature system (Dynacalibrator Model 190,
VICI Metronic). An Hg° concentration of 58±2 ppb (476 |ig/Nm3) in N2 at a total flow
rate of 340 mL/min was generated and used for the Hg° adsorption experiments. An
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ultraviolet (UV) mercury analyzer (Model 400A, BUCK Scientific) was used to
continuously measure the concentration of Hg° in the outlet stream. The experiments of
carbon heat treatment for moisture removal, Hg° adsorption, and desorption can be
performed consecutively in situ, using the experimental setup shown in Figure 1, so that
moisture uptake due to exposure of the carbon sample to the atmosphere could be
avoided.
Oxygen Trap
Dryer
Fixed Bed
Reactor
Furnace
Reduction
Furnace
I i
Figure 1. Schematic diagram of fixed-bed reactor setup
To perform a Hg° adsorption experiment on an as-received carbon sample, the Hg°-
laden gas mixture was first sent through the bypass to establish the baseline Hg°
concentration prior to adsorption. After loading the pre-weighed carbon sample (about
20-40 mg) into the reactor, distributed evenly on a fine frit and maintained at a
temperature of 27 °C (slightly above room temperature), the Hg°-laden N2 flow was
switched from the bypass to the reactor. The adsorption experiment was performed for 2
h for convenience. The Hg° uptake for an adsorption experiment was evaluated using the
area between the inlet Hg° concentration (baseline) and the breakthrough curve. Hg°
adsorption experiments were also performed on the moisture-free carbon samples after
heat treatment. To remove moisture from the as-received carbon sample (loaded into the
reactor prior to Hg° adsorption), the sample was heated at a constant rate (8 C°/min) to
110 °C under a N2 atmosphere, then held for 30 min. The reactor was then cooled to the
Hg° adsorption temperature (27 °C) under Ni flow, before the adsorption experiment,
similar to that performed for the as-received samples, was started.
Temperature programmed desorption (TPD) experiments were performed to measure
the thermal stability of the adsorbed mercury following the Hg° adsorption experiment.
The reactor, containing the Hg°-exposed carbon sample, was purged with N2 gas at 27 °C
until the concentration of Hg° measured by the UV analyzer fell to insignificant levels (<
2 ppb). The sample was then heated at a constant heating rate (8 C°/min) to the final
temperature of 420 °C. Blank TPD experiments using carbon samples without mercury
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showed that there is no interference on the mercury analyzer with the loss of CC>2 and
H2O from the carbon surfaces. The amount of Hg° desorbed during the TPD run was
estimated from the area under the outlet Hg° concentration curve. Preliminary TPD
experiments showed that more than 90% of the adsorbed Hg° was desorbed when the
temperature reached 420 °C, so this was used as the final temperature.
A schematic of the flow reactor is shown in Figure 2. The reactor is 310.5 cm long,
has an I.D. of 4 cm, and is constructed of quartz. Three gas sample ports are located
along the length of the reactor, labeled SP1, SP2, and SP3, which provided residence
times ranging from 5 to 18 s at a total flow rate of 0.014 in /inin [STP (standard
temperature and pressure)]. The reactor is heated with three Lindbcrg, 3-zone electric
furnaces in series. Once the baseline Hg° concentration is established, activated carbon is
fed into the top of the reactor using a fluidized-bed feeder. The gas-phase Hg°
concentration is measured at one of the sample ports by sub-isokinetically pulling a gas
sample through a 1 ^irn filter, through a reducing furnace and finally to the UV analyzer.
The detailed experimental procedures can be found elsewhere [22]. Dry carbon sample
was obtained by connecting the feeder to a vacuum system and heating at 110 °C for 1 h
prior to the test. To test the carbon sample with varied moisture contents, water was also
added to the sample by placing the carbon in an enclosed vessel with a container of water
for several hours, thereby increasing the amount of moisture in the carbon. The moisture
content of the sample was tested by using a Infrared Moisture Determination Balance
(AD-4714, A&DCo., Ltd.).
Fluidized-Bed Feeder
Lindberg
3-Zone
Furnaces
Flow Reactor
Hg° Permeation Oven
Buck UV
Analyzer
Filter Reducing
Furnace
L
Carbon Trap
3^"Exhaust
Carbon
Trap
Pump
Figure 2. Schematic diagram of flow reactor setup
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Both as-received and heat treated BPL and ACN carbon samples which had been
exposed to Hg° were selected for XAFS spectroscopy analysis, in order to provide
information on the nature of mercury bonding on carbon surfaces, except for the FGD
carbon due to its high ash content. Details of the XAFS experimental procedures can be
found elsewhere [23].
Results and Discussions
Figure 3 shows a typical Hg° concentration curve during adsorption breakthrough,
Hg° desorption curve during N2 purge, and TPD in the fixed bed reactor. Figure 4
presents amounts of Hg° adsorbed as a function of time during adsorption. Tests run with
two coal-based as received (-AR) samples (BPL-AR, FGD-AR) show that the Hg° uptake
of FGD-AR is slightly larger than that of BPL-AR: the latter has larger surface area and
lower ash content. However, the Hg° uptake of the as-received carbon fiber sample
(ACN-AR) is much larger than those of the coal-based carbon samples, although the total
surface area of BPL is only 10 % less than that of ACN-AR sample (see Table 1). The
very similar values of BET and CC>2 surface areas for the carbon fiber sample (see Table
1) suggest that ACN is a microporous (< 2 nm) carbon with homogeneous distribution of
microporosity. The CC»2 surface areas of BPL and FGD samples are lower than their
BET surface area, suggesting that the samples contain some mesopores. The average
micropore width evaluated from CC»2 adsorption with the DR method gives a similar
value of about 1.5 nm for both BPL and ACN samples, and 1.3 nm for the FGD sample.
From the above results, it appears that there are no clear correlations between Hg° uptake
capacity and carbon pore structure characteristics, such as surface area and porosity, as
well as ash content.
25
50
75 100 125
Time (min)
150
175
Figure 3. Mercury adsorption and desorption profile in fixed-bed reactor
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Figure 4 shows that the Hg° uptake of carbon samples was drastically reduced to
similar low values after low temperature (110 °C) heat treatment. Such heat treatment
effects were also observed by Krishnan et al. [1] for coal-based activated carbons. Heat
treatment at such low temperatures (110-140 °C) cannot change the pore structure, such
as microporosity and surface area. However, adsorbed H2O molecules on carbon
surfaces are removed when heated at low temperatures; e.g., 25-150 °C [11], suggesting
that adsorbed H2O plays an important role in Hg° adsorption.
250 -
20
40 60 80
Adsorption time (min)
100
120
Figure 4. Mercury uptake of activated carbons as a function of adsorption time
FGD carbon with varied moisture contents (0, 4, and 16% H2O) was tested in the
flow reactor. The results shown in Figure 5 were performed at 100 °C and a Hg°
concentration of 86 ppb. Mercury removal using dry carbon is very low. At a feed rate
of 9.5 g/hr, the removal is below 8%. The as-received carbon (4% moisture) performed
better with a removal of 10% at a feed rate of 3 g/hr and increased to 30% at 6.4 g/hr.
This shows that removal of moisture also has a strong effect on Hg° capture in the flow
reactor condition. Increasing the moisture content of the carbon increased the Hg°
removal significantly. At a feed rate of 6.4 g/hr, the removal doubled from 30% for the
as-received carbon to 60% for the 16% water carbon. Note that this effect might be a
result of evaporation of H2O from the carbon surface lowering the temperature of the
particle, subsequently causing higher removal. In the flue gas, however, the direction and
driving force for H2O evaporation will depend on the moisture content of carbon, as well
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as the temperature and H^O concentration. It can be found from previous study [4] that
the presence of HiO in a simulated flue gas does not seem to affect Hg° uptake capacity.
Tests were conducted to determine the moisture loss from the particle as it falls through
the reactor with a 10 s particle residence time. Approximately, 50% of the moisture was
lost. The extent to which moisture content of activated carbon affects Hg° removal, the
evaporation of HiO from the moisture-containing carbon, and the temperature profile of
the particle, needs further investigation. Nevertheless, the result from the flow reactor
shows that the moisture content of activated carbon is also an important factor in Hg°
capture.
70 -,
60
50 -
o
E
£30-
20 -
10 -
0
16% Moisture
4% Moisture
0% Moisture
0
8
10
1234567
Activated carbon feed rate (g/hr)
Figure 5. Mercury removal by FGD carbon with different moisture contents in
the How reactor at 100 °C and 86 ppb Hg°
To better understand the mechanism of Hg° adsorption by activated carbon and how
carbon surface moisture could affect Hg capture significantly, it would be important to
know the nature of mercury bonding on the carbon surface. As shown in Figure 3 from
the fixed bed reactor Ni purge and TPD results, the chemisorbed Hg° can be
differentiated from physisorbed Hg°. The physisorbed Hg° on the carbon surfaces is
evolved during the purge period when the reactor gas is switched from the Hg°-Iaden N2
at the end of the adsorption experiment to pure N2 (27 °C). The mercury evolved during
the subsequent TPD run is referred to as chemisorbed Hg°, since it is stable at the
adsorption temperature under the inert gas atmosphere (N2) and decomposes into gas
species as the temperature rises. The amounts of Hg° desorbed were calculated from the
area under the outlet Hg° concentration, and the results are presented in Table 2,
expressed as a percentage of the Hg° adsorbed that was measured during the adsorption
experiment prior to the N2 purge. Similar amounts of Hg° were measured between
duplicate runs with and without operating the reduction furnace, which suggested that the
mercury evolved during desorption was Hg° and not an oxidized form. The mass
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balances between the adsorbed and desorbed Hg° in all cases were greater than 90%, and
most of the Hg° adsorbed by the carbons was recovered by heating the samples to 420 °C.
For the as-received samples, more than 86% of the adsorbed Hg° was desorbed during
the TPD runs at higher temperatures versus > 54% for their heat-treated counterparts.
Hg° evolved from the coal-based, as received samples (BPL-AR, FGD-AR) during the N2
purge accounts for only a small fraction of the total Hg° adsorbed. Adsorption and
desorption processes on the carbon surface are known to be governed by a distribution of
adsorption energies [24]. Chemical bonds formed between the adsorbed molecules and
the adsorption sites of an activated carbon are often very energetic. Since the TPD
experiments show that Hg° is evolved over a wide temperature range, Hg° bonds on the
carbon surface probably involve different site types, which have varying bonding
energies, implying that the nature of the carbon surface active sites for bonding Hg°
depends on the sample characteristics. For the as-received samples, chemisorption of
Hg° appears to be the predominant process. Note from Table 2 that a relatively large
amount of Hg° for the ACN-AR sample was evolved during N2 purge. As described,
ACN is a microporous carbon with homogeneous distribution of microporosity. The
physisorbed Hg° of ACN-AR in this case could be explained by its microporosity. The
reduction of Hg° capture for the heat-treated samples is caused by the reduction of
chemisorbed Hg°. Thus, it could be deduced that the removal of t^O from the carbon
surfaces by low-temperature treatment eliminates the active sites that can chemically
bond Hg°.
Table 2 Mercury uptake from adsorption and recovery from desorption
Hg adsorption at 27°C Hg desorption
Sample Hg captured (ng/g)a N2 purge (%) TPD (%)
ACN-AR
ACN- 110
BPL-AR
BPL-110
FGD-AR
FGD-110
236
34
83
20
95
22
12
42
5
20
3
42
86
54
93
71
96
56
Hg recovery
(%)
98
96
98
91
99
98
a Amount of mercury adsorbed for 100 min on dry-carbon basis
Figure 6 shows the Mercury Lm-edge XANES (X-ray absorption near-edge structure)
spectra of the five carbon samples, and the inflection point difference (IPD, eV) derived
from the spectra. Sufficiently strong spectra have permitted the derivative analysis to be
performed in order to obtain the IPD values, except for ACN-110, which could be due to
its low mercury concentration. Huggins et al. [23] showed that the IPD values could be
used as an effective parameter to identify the nature of the mercury bonding on carbon
surfaces. It has been shown that the mercury surface compounds with the smallest and
most ionic anions (O2~, O") have the largest values of IPD (e.g., > 9.0), whereas those
with the largest and more covalent anions have the smallest IPD values [23]. As shown
in Figure 6, the IPD values derived from the XANES spectra are larger than 9.0. One of
the as-received ACN samples gives an IPD value of 9.8, close to that of mercury acetate
(10.6) [23]. Results of the XAFS analysis indicate that the Hg-O bond is significant for
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the as-received carbon samples, which had been exposed to Hg°. The XAFS results are
consistent with the TPD results in that most of the adsorbed Hg° was strongly bound on
carbon surfaces and evolved at higher temperatures in all cases.
The above XANES results suggest that carbon-bound oxygen on the surface captures
Hg° during the adsorption experiments. Different types of oxygen surface groups are
believed to exist on activated carbon surfaces. Depending on their history of formation
and activation temperature, they could be carboxyl, lactone, phenolic, and carbonyl
groups [25]. Electron transfer processes are likely to be involved during the
chemisorption of Hg°. Abundant evidence from the literature shows that different
oxygen groups can participate in the electron transfer processes on carbon surfaces [18].
Extended n bonding in the extensive aromatic network of carbons permits electron clouds
and charges to be highly delocalized. For example, the resonance-stabilized structures of
the aromatic network in equilibration with the surface functional groups (e.g., quinonoid
complexes) could take up electrons resulting in the functional groups' becoming anions.
80 100
Energy (eV)
Figure 6. Mercury Lm-edge XANES spectra of activated carbons
Based on the above-proposed mechanism for Hg° oxidation, the observations that the
desorbed mercury from the TPD experiments was found to be Hg°, and that the adsorbed
H2O has a significant effect on Hg° adsorption, could be explained as follows: when
thermal energy is applied to a chemisorbed Hg°, such as a mercury-carbonyl complex, it
would be favorable for the electron(s) to return to the mercury rather than breaking the C-
O bonds. Although certain oxygen complexes would also decompose to produce CO2
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and H2O at low temperatures (e.g., < 420 °C), it is not clear if Hg° is bonded to
complexes such as carboxyl and lactone groups. However, as described by Leon y Leon
and Radovic [18], before the oxidation reactions can proceed they all have specific
experimental requirements that must be met such as pH, oxygen exposure, and kinds of
surface functional groups. The fact that the presence of surface moisture promotes Hg°
bonding could imply that surface groups for such bonding would probably involve certain
hydrated surface functional groups, which arise from the hydrogen bond of adsorbed H2O
[26]. Simply removing the adsorbed H2O at low temperature (< 150 °C) as moisture
would inhibit the equilibrium conditions favoring the surface structures to be the electron
acceptors. One or more types of oxygen functional groups could act as electron acceptors
if specific conditions are met. It is not known what particular surface functional groups
are responsible for Hg° bonding from the current investigation. Further research is
needed to characterize the carbon oxygen surface groups that act as electron acceptors to
capture Hg°.
Conclusions
The moisture on activated carbon surfaces has been found to have a significant effect
on Hg° adsorption. The results from this study show that the Hg° adsorption capacities of
activated carbon samples were drastically reduced after removing their moisture. By
comparing the results of an ash-free activated carbon sample (ACN) to those of coal-
based carbons, it is concluded that mineral matter or metal oxides in the coal-based
carbons do not play a role in this effect. It also appeared that no correlation could be
established between the carbon pore structure and Hg° adsorption capacity. Results of
the TPD experiments show that chemisorption of Hg° is a dominant process during Hg°
adsorption for the moisture-containing carbon samples. XAFS analysis provides
evidence that Hg° bonding on the carbon surfaces is associated with oxygen.
Results from both mercury desorption (TPD) and XAFS analyses suggest that surface
oxygen complexes provide the active centers for mercury bonding. The mechanism of
mercury bonding on the carbon surfaces likely involves an electron transfer process, in
which the aromatic, resonance-stabilized structures in equilibrium with the surface
functional groups would probably take up electrons from the Hg°. The observation that
the presence of surface moisture promotes mercury bonding suggested that the oxygen
surface groups capable of mercury bonding might involve certain hydrated functional
groups, which could result from the hydrogen bond of the adsorbed water. Removal of
the adsorbed H2O at low temperature (< 150 °C) as moisture would inhibit the
equilibrium conditions that allow surface functionalities to become electron acceptors.
Acknowledgement
This research was supported in part by an appointment to the Postdoctoral Research
Program at EPA's National Risk Management Research Laboratory administered by the
Oak Ridge Institute for Science and Education through interagency agreement
DW89938167 between the U.S. Department of Energy and the U.S. Environmental
Protection Agency. The authors appreciate the assistance of Prof. Frank Huggins at the
University of Kentucky in providing the XAFS analysis. Appreciation is also extended to
Dr. Carlos A. Leon y Leon for the discussions and suggestions on this subject through
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personal communication, and Joe Hayes of American Kynol, Inc. for providing the
activated carbon fibers.
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N RM RL- RT P- P- 543
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2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Effect of Moisture on Adsorption of Elemental
Mercury by Activated Carbons
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Y.H.Li*, S. D. Serre, C. W. Lee, andB.K. Gullett
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
See Block 12
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
DOE LAG DW89938167 (Li)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 1/99 - 7/00
14. SPONSORING AGENCY CODE
EPA/600/13
15.SUPPLEMENTARY NOTES APpCD project officer is Brian K. Gullett, Mail Drop 65, 919/
541-1534. For presentation at Conference on Air Quality II, Tysons Corner, McLean,
VA, 9/19-21/00. (*) Li is ORISE Post-doctoral fellow.
16. ABSTRACT
paper discusses experiments using activated carbon to capture elemen-
tal mercury (Hgo), and a bench-scale fixed-bed reactor and a flow reactor to deter-
mine the role of surface moisture in Hgo adsorption. Three activated- carbon sam-
ples, with different pore structure and ash content, were tested for Hgo adsorption
capacity. From both fixed-bed and flow reactor experimental results, the moisture
on activated- carbon surfaces has been found to have a significant effect on Hgo ad-
sorption. A common effect of moisture on Hgo adsorption was observed for all three
samples, despite extreme differences in their ash content, suggesting that this ef-
fect is not associated with ash content. Temperature- programmed desorption (TPD)
experiments performed on the carbon samples after the Hgo adsorption experiments
indicated that chemisorption of Hgo is a dominant process over physisorption for
the moisture- containing carbon samples, and diminished for the heat-treated mois-
ture-free samples. X-ray absorption fine- structure (XAFS) spectroscopy results
provide evidence that mercury bonding on the carbon surfaces was associated with
oxygen through a mechanism likely involving electron transfer processes. The aro-
matic resonance- stabilized structures in equilibrium with the oxygen surface funct-
ional groups take up electrons from the mercury atoms.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTlFIERS/OPEN ENDED TERMS
COSATI Field/Group
Pollution
Mercury (Metal)
Activated Carbon
Adsorption
Moisture
Pollution Control
Stationary Sources
13 B
07B
11G
14G
07D
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
13
2O. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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