ROLE OF IIC! IN ADSORPTION OF ELEMENTAL MERCURY VAPOR BY
CALCIUM-BASED SORBENTS
Behrooz Ghorishi
ARCADIS Geraghly & Miller, Inc., P.O. Box 13109
Research Triangle Park, NC 27709
Brian K. Gullett
U.S. Environmental Protection Agency
National Risk Management Research Laboratory (MD-65)
Research Triangle Park, NC 27711
Wojciech Jozewicz and Wojciech Kozlowski
ARCADIS Gcraghty & Miller, Inc., P.O. Box 13109
Research Triangle Park, NC 27709
Abstract
Elemental mercury (Hg°) capture has been mostly investigated using high surface area, expensive
activated carbons. Previous investigations showed that the presence of hydrogen chloride (HC1)
in the flue gas had a pronounced positive effect on the adsorption of IIg° by a lignite-coal-based
activated carbon with high calcium (Ca) content. The objective of this study was to identify
active sites and surface functional groups that may contribute to the adsorption of Hg° by less
expensive Ca-based sorbents. The study investigated the formation of chlorine (CI) sites in Ca-
based sorbents as well as their role and reactivity in the adsorption of Hg°. Initially four Ca-based
sorbents were exposed to HC1 and subsequently to Hg°: calcium oxide (CaO), calcium hydroxide
[Ca(OH)2], calcium carbonate (CaCOs), and calcium sulfate dihydrate (CaS04*2H20; gypsum).
HCl-exposed CaS0,t-2H20 exhibited a superior IIg° sorption capability. Through comparison to
anhydrous calcium sulfate (CaS04) and calcium sulfate hemihydrate (CaSOvV&feO; plaster),
crystalline water molecules on the surface of CaS04*2H20 were confirmed to contribute indirectly
to Hg° uptake. These surface molecules may have adsorbed HC1 through hydrogen bond
formation between an oxygen atom of a crystalline water molecule and a hydrogen atom of an
HC1 molecule. Two adjacent, physically adsorbed HC1 molecules could then trap an Hg° molecule
through formation of a mercuric chloride (IIgCl2)-like molecule. Thus, two adsorbed CI atoms
that are spaced at a distance equal to the length of an HgCl2 molecule could be regarded as an
IIg° active site. Significant correlations between CI and Hg° uptake by CaS04*2H20 were noted.

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Introduction
Mercury present in hazardous/municipal waste and in coal is readily volatilized during incineration
and combustion processes1. The capability of the existing flue gas desulfurization (FGD) systems
to remove mercury from a given flue gas was thought to be affected by the form of mercury
present. Oxidized mercury compounds such as mercuric chloride (HgCl2) can be removed from
the flue gas by conventional sulfur dioxide (S02) scrubbers2. However, a major part of elemental
mercury (Hg°), the most volatile form among trace metal compounds, can pass through
particulate matter (PM) control devices3, if uncontrolled.
Injection of dry sorbents upstream of a PM control device is one potential method for controlling
mercury emissions from combustion processes. The importance of the mercury species form for
the efficiency of mercury capture by dry sorbents lies in the fact that the oxidized forms of
mercury, such as HgCh, seem to be more easily controlled than Hg° via solid adsorption4-6. Past
research and field experience have identified two different classes of sorbents to be effective in
mercury removal: activated carbons and alkaline sorbents such as calcium-based (Ca-based)
compounds. Activated carbons have been found to be efficient Hg° and HgCl2 sorbents4,7"11.
Alkaline Ca-based sorbents (in addition to activated carbons) were effective HgCl2 sorbents5,12*14
in addition to effectively controlling emissions of acid gases, such as S02 and hydrogen chloride
(IIC1). The dual role of Ca-based sorbents in controlling emissions of acid gases and IIgCl2 may
be considered as a cost-effective, multi-pollutant control strategy. This strategy could be further
enhanced by focusing on improving the adsorption of Hg° by Ca-based sorbents in order to
increase total mercury capture from flue gas.
Dry sorbents that can react with Hg° species shift them from the vapor phase to the particulate
(sorbent) phase, thus facilitating removal. Two cr itical steps need to be accomplished for
effective Hg° adsorption by injected dry sorbents The first step is film mass transfer by diffusion
of Hg° species from the bulk flue gas to the vicinity of the particle surface within the available
residence time (sorbent particles assumed to be adequately dispersed). The second step involves
the reaction of Hg° molecules with sorbent active sites at the vicinity of the surface. A
fundamental analysis15 concluded that only a veiy small surface area of active sites was
theoretically required to trap Hg°. The implication was that the number of reactive surface sites is
much more important than the amount of surface area.
The importance of the reactive surface sites underscores the need for comprehensive research on
characterizing the role of active sites and surface functional groups on the adsorption of different
mercury species Previous investigation13 illustrated that calcium hydroxide [Ca(OH)2] and
Ca(OH)2-fly ash sorbents had insignificant removal capacity for Hg° in the absence of S02 in the
simulated flue gas. However, in the presence of S02, IIg° capture was enhanced for selected
Ca(OII)2-fly ash sorbents. This enhancement was likely due to the effects of a combination of
parameters such as generation of sulfur-activated sites, surface area, and internal pore structure.
More studies are needed to conclusively determine the effects of these parameters.
Another study14 indicated that Ca(OH)2 had no affinity for Hg° in nitrogen (N2) but captured
HgCl2 effectively. Sorbent alkalinity was hypothesized to be the underlying reason for the capture
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of acidic HgCI2. The presence of acid gases (S02 and HC1) drastically decreased the uptake of
HgCl2 by Ca(OII)2. It was hypothesized that the inhibition effect was due to competition between
these acid gases and HgCh for the same available alkaline sites. Conversely, the same study14
showed that the presence of acid gases (especially HC1) had a positive effect on the capture of
Hg° by a high-Ca lignite-coal-based activated carbon. This enhancement effect was then attributed
to the high concentration of Ca (acid gas sorbent) in carbon. It was hypothesized that the
attraction of HC1 to Ca/carbon sites in the activated carbon created active chlorine (CI) sites.
These sites captured tig0 through formation of Cl-Hg bonds in the activated carbon. This
hypothesis was further confirmed by another study16.
In summary, our previous study14 was significant for two observations. These observations were:
an abundance of HgCl2 sites in Ca-based sorbents and the enhancement effect of HC1 on Hg°
uptake by the activated carbon. They suggest that generation of suitable CI sites in Ca-based
sorbents may promote adsorption ofHg0 as an HgCl2-like molecule. Therefore, the focus of this
study was to elucidate the following:
•	potential for facilitation of IIg° uptake (IIg° captured as HgCl2) by Ca-based sorbents
following their exposure to a CI source (HC1),
•	comparison of rates and capacities of Hg° uptake by fresh and HCl-exposed Ca-based
sorbents,
•	correlation between the amount of CI uptake and subsequent Hg° uptake by Ca-based
sorbents, and
•	the nature of active sites in HCl-exposed, Ca-based sorbents that are instrumental in Hg°
capture.
Experimental
Apparatus
Detailed descriptions of the experimental setup, a fixed bed reactor system (Figure 1), have been
presented previously13, l4. Briefly, pure Hg° liquid in a permeation tube was the source of Hg°
vapor. HC1 [500 ppm in nitrogen (N2)] was delivered using certified gas cylinders. The
concentration of Hg° in the gas stream was controlled by adjusting the temperature of the
permeation tube's water bath. Hg° vapor generated was delivered to the fixed bed by an N2
stream at a constant total system flow rate of 300 cmVmin (at standard temperature and pressure,
STP). The sorbent to be studied (0.5 g) was placed in the reactor between two quartz wool plugs
and maintained at the desired bed temperature by a temperature controller. A furnace kept at 850°
C was placed downstream of the reactor to convert any oxidized mercury (Hg44, as in HgCl2) to
Hg°. The presence of the furnace enabled detection of oxidized, non-adsorbed Hg° as gas-phase
Hg° by the on-line ultraviolet (UV) Hg° analyzer. Thus, continuous Hg° capture data by the fixed
bed of sorbent could be acquired.
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Methodology
In each test, the fixed bed was first exposed to 500 ppm HC1 in N2 for different exposure times.
The bed was then subsequently exposed to 32 ppb Hg° in N2 for 60 minutes or until 100 percent
breakthrough (saturation) was achieved (whichever came first). During the Hg° contact period,
the exit concentration of mercury was continuously monitored. The instantaneous removal of Hg°
at any time (Qt) was obtained as given by equation (1):
Qt ~ [(Hg° in"*Hg°out)/ Hg° in] X 100	(1)
Hg° uptake was determined by integrating and evaluating the area under the removal curve. Hg°
uptake was defined as a cumulative removal of Hg° up to time t and was expressed as a weight
ratio of mercury species to sorbent (|ig Hg°/g sorbent). Similarly, CI uptake was defined as a
cumulative removal of CI up to time t and was expressed as a weight ratio of CI species to sorbent
(ng Cl/g sorbent). Selected experiments conducted during this test program were run in duplicate
and indicated a range of ±5 percent about the mean in the experimental results.
Analytical
Sorbent samples to be analyzed for CI uptake were removed from the fixed-bed reactor after the
completion of the HC1 exposure period. Each sample was transferred to a beaker containing 20 ml
of deionized water and a small amount of 2N sulfuric acid. The sample was stirred for 15 min and
was then filtered into a 100-ml volumetric flask to remove the quartz wool residue. An aliquot
was taken from the original 100-ml dilution for determination of CI by ion chromatography.
Chlorination products of sorbent reaction with HC1 in the fixed-bed reactor were evaluated in a
Perkin Elmer DSC7 differential scanning calorimeter (DSC). A detailed description of the DSC
analysis can be found elsewhere17. Particle size distributions were determined in a Micromeritics
Sedigraph using a gravity sedimentation method (Sedisperse A-l 1 dispersant). Porosity and
surface area were measured in a Micromeritics ASAP 2600 using N2 adsorption and desorption
with a Brunauer-Emmett-Teller (BET) method. The amount of crystalline water in certain
sorbents was determined in a Perkin Elmer TGA7 thermogravinietric analyzer. A detailed
description of TGA analysis is outlined elsewhere17.
Results
Hg° Uptake by HCI-Exposed Sorbents
The effect ofHCl on the capture ofHg0 by Ca-bascd sorbents was studied by carrying out
sequential exposures to HC1 and Hg°. An initial exposure to HCI (for 24 hours) was followed by
an exposure to Hg° at a bed temperature of 60 °C. Initially, four Ca-based sorbents (all reagent
grade, Fisher Scientific) were chosen for this study: Ca(OH)2, calcium oxide (CaO), calcium
carbonate (CaCC^), and calcium sulfate dihydrate (CaS04'2H20, gypsum). The Hg° uptake (fig
Hg°/g sorbent) by the HCl-exposed sorbents is shown in Figure 2. It should be noted that no 14g°
4

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removal was obtained with fresh (not exposed to HCl) sorbents (detection limit of about 0.05 pg
Hg°/g sorbent). Table 1 summarizes IIg° uptakes by sorbents previously exposed to HCl.
Table 1
Hg° Uptake by the HCl-exposed Ca-based Sorbents
	Sorbent Type	CI Uptake, ng/g	Ilg0 Uptake, jig/g	
Ca(OH)2	60000	0.108
CaO	16000	0.828±0.046
CaC03	2400	0.587±0.065
CaS04*2H20	3600^200	2.27±0.056
The CI uptake did not correlate with the Hg° uptake. The sorbent with the highest CI uptake,
Ca(OH)2, exhibited the lowest Tig0 removal. Exposure to HCl clearly induced Hg° removal
capabilities in certain sorbents. HCl-exposed CaS04*2H20 exhibited superior IIg° removal
capability compared to other HCl-exposed sorbents. One obvious difference between
CaS04-2H20 and other sorbents considered here is that the former contains crystalline water
within its structure.
Effect of Crystalline Water
The important role of crystalline water in adsorption of HCl and Hg° on the surface of
CaS04*2H20 was indicated by results presented above. This role was further confirmed by
alternate tests using anhydrous CaS04 and calcium sulfate hemihydrate (CaS04 !/2H20, plaster) at
identical solid/gas-phase contacting conditions. Results are shown in Figure 3. HCl-exposed
CaS04 was not active in capturing Hg°, and HCl-exposed CaS04*1/2H20 was less active than
CaS04*2H20. This figure shows the importance of crystalline water and its quantity on the
adsorption of HCl and subsequent creation of a suitable active site for capturing Hg°.
To further investigate the effect of crystalline water, two more crystalline water containing
sorbents were tested in addition to CaS04*2H20. They were: sodium phosphate tribasic
(Na3P04-12H20) and sodium borate (Na^CVlOH^O). Table 2 summarizes the CI uptake (in
pg/g sorbent) versus Hg° uptake (in pg/g sorbent, after 40 minutes of exposure) together with the
physical properties of these sorbents. The higher amount of crystalline water in Na2B407- 10II2O
and Na3P04* 121120 led to higher uptakes of CI. No correlation existed between the IIg° and CI
uptake and the physical properties of the sorbents such as surface area, pore volume, average pore
diameter, and median particle size.
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f
Table 2
CI and Hg° Uptake by Sorbcnts Containing Ciystalline Water
Sorbcnt Type
CaS04-2H20
Na2B4O7*10H2O
Na3P0412H20
Total CI Fed, g/g
0.68
0.68
0.68
CI Uptake, ng/g
3600±200
24000
38000
H2O/CI, mole ratio
115
39
29
Hg° Uptake, jig/g
2.27±0.056
0.197
0.697
Cl/Hg°, mole ratio
8637
343147
153515
Surface Area, m2/g
6.6
26.3
2.8
Pore Volume, cm3/g
0.033
0.135
0.013
Average Pore Diameter, A
198
205
182
Median Particle Size, \im
29
47
>300
Saturation Capacity of HCI-exposed CaS04m2H20
A series of tests was performed to evaluate the saturation capacity (breakthrough points) of CI by
CaS04*2H20. Different batches of CaS04-2H20 were exposed to 500 ppm HC1 in N2 for different
exposure times, and resulting CI and subsequent IIg° uptakes were determined. The results are
summarized in Table 3.
Table 3
Effect of Exposure Time on CI and Hg° Uptake by CaS04-2II2Q
Exposure Time, h
0
1
4
8
12
24
48
Total CI Fed, g/g
0
0.028
0.114
0.220
0.340
0.680
1.36
CI Uptake, |ig/g
<80
1502
1640
2600
3200
3600
3600
H2O/CI, mole ratio
>5160
274
252
159
129
115
115
Hg° Uptake, \ig/g
<0.05
0.123
0.613
0.974
1.80
2.27
2.21
Cl/Hg°, mole ratio
-
67742
12432
12333
8181
7286
7846
As can be seen in Table 3, the CI saturation capacity of CaS04*2H20 batches was obtained at 24
hour exposure time. Hg° uptake, interestingly, reached its saturation point at the same exposure
time as IIC1. Total CI fed at the breakthrough point was 0.68 g Cl/g CaS04*2H20; whereas, the
total CI uptake at this point was 0.0036 g Cl/g CaS04*2H20. This indicated that only a small
fraction of the gas-phase HC1 that passed through the fixed bed was actually captured.
Temperature Effect on the Adsorption ofHCI by CaS04'2H20
The effect of bed temperature on the adsorption of IIC1 by CaS04*2Il20 and subsequent capture
of IIg° by the HCI-exposed CaS04*2H20 was studied next. In these studies, CaS04*2II20 was
exposed to a high concentration HC1 in N2 stream (10000 ppm) for a short period of 1.2 hours to
expedite the previous lengthy (24-hour) experiments. Consequently, these results may not be
directly comparable to the previous ones outlined above since they were obtained at a different
HC1 concentration. The capture of HC1 by CaS04*2H2Q was studied at three temperatures: 60,
6

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100, and 120 °C. The Hg° capture by the HCl-exposed CaS04*2H20 was studied at 60 °C. In a
separate study, CaS04*2H20 was exposed to HCI at 60 °C, and the resulting HCl-exposed
sorbent was exposed to Hg° (32 ppb in N2) at three temperatures: 60, 100, and 120 °C. Table 4
summarizes the effect of temperature on CI and Hg° uptake by HCI-exposed CaSO^IkO.
Increasing the bed temperature from 60 to 100 °C during the HCI exposure period increased the
CI uptake and subsequently resulted in a higher Hg° uptake.
Table 4
Effect of Bed Temperature on CI and Hg° Uptake by CaS04*2H:0
IICI /Ilg0 Exposure Temperatures, °C/ °C Hg° Uptake, ng/g	CI Uptake, ng/g
60/60
0.992±0.123
1380±300
100/60
1.934
4200
120/60
1.736
2640
60/100
.0-1
1380±300
60/120
0.1
1380i 300
Discussion
Mechanism of Hg° Uptake by HCI-exposed CaS04-2H20
Two observations were made following experiments investigating the uptake of Hg° by HCl-
exposed sorbents as previously summarized in Table 1:
~	CI uptake by Ca-based sorbents promoted their capability to remove Kg0, and
•	there was no correlation between CI and Hg° uptake
The second observation may indicate that the reactivity (and not the quantity) of captured CI is
the determining factor in the subsequent adsorption of Hg°. To investigate this hypothesis, DSC
analysis of the HCl-exposed sorbents was performed to determine the nature of captured CI by
these sorbents. When scanned by DSC, only Ca(OH)2 (and to a very limited extent CaO) revealed
an endotherm. An endotherm indicates occurrence of a chemical reaction (as opposed to simple
adsorption). An earlier study14 identified this endotherm as a solid solution of CaCl2 and
CaClOH. This high uptake of CI by Ca(OII)2 (DSC endotherm) can be, therefore, explained by
the formation of this solid solution through a chemical reaction between gas-phase HCI and
Ca(OH)2. This chlorinated compound was subsequently shown to be inactive in capturing IIg°;
thus, no correlation between the CI and Hg° uptake could be demonstrated. A similar DSC
analysis of HCl-exposed CaS04-2H20 and CaC03 indicated the absence of this endotherm,
suggesting that the uptake of CI by these two sorbents may have occurred through a physical
adsorption mechanism. However, IlCl-exposed CaS04*2H20 subsequently revealed hiah uptake
Ofllg0.
The high uptake of Hg° by the HCl-exposed CaS04*2H20 may be explained as follows.
Crystalline water on the surface of the CaS04*2II20 lattice may be regarded as an attractive site
7

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for the uptake of CI. Here, the nature of surface crystalline water (as differentiated from surface
moisture) needs to be discussed. Crystalline water was in solid form and was attached to the
crystalline structure of the bulk of CaS04-2H20. The oxygen atom (O) of a solid crystalline water
molecule located at the surface, as depicted in Figure 4, may have attracted an IIC1 molecule
through formation of hydrogen bonds between 0 and the hydrogen atom (H) of HC1. This bond is
a subset of bonds formed by an electron donor-acceptor (EDA) interaction, also known as Lewis
base-acid interactions.
Lewis base-acid interactions have been studied extensively. For example, the case of adsorption
of HC1 in the liquid phase by activated carbon was studied18. The study explored adsorption of
HC1 by basic sites on activated carbon. One of the hypotheses presented18, related this adsorption
to surface functional groups containing O and an EDA interaction. The assertion was made that
the H atoms of HC1 molecules could interact with electron pair donors, O atoms, on the surface of
activated carbon (a Lewis acid-base interaction). This electrostatic interaction would lead to
formation of hydrogen bonds. Similarly, in the case considered here, the O atoms of crystalline
water molecules on the surface of CaS04*2H20 interacted with HCI molecules, forming hydrogen
bonds. Hydrogen bonds, formed by attraction of H and O atoms, are only one-tenth to one-
thirtieth as strong as covalcnt chemical bonds (3 to 250 kcal/mole). Therefore, they could be
classified, in this case, as physical rather than chemical adsorption mechanisms. As shown in
Figure 4, two neighboring crystalline water molecules can attract and adsorb two IIC1 molecules
and, if these two crystalline water molecules (or the adsorbed IIC1 molecules) are located at an
optimum distance, they subsequently could attract and adsorb an Hg° molecule possibly through
attractive van der Waals forces to form an HgClj-like molecule. This was in agreement with our
previous investigation14 and results by others16. These results showed that HgCI2 was easier to
capture than Hg° and that mercury species captured in the presence of IIC1 were most probably in
the form of an HgCl2-type molecule.
CI and IIg° uptake analysis of the exposed CaS04*2H20 showed that for every 10,000 CI atoms
one Iig° molecule was captured (refer to Table 1). This illustrated the fact that an optimum
spacing between crystalline water molecules or CI atoms on the surface of CaS04*2H20 was
required to trap and capture molecules of Hg(l as HgCl2. Apparently, only one out of 5000 such
sites (two CI atoms constitute one site) had this optimum spacing. Optimum distance may be the
length of an IlgCh molecule from one chlorine atom to the other. Ilg-Cl bond distance in IlgCh
was reported16 to be approximately 2.25 A. Bond lengths and angles of selected chemical
compounds are also listed in the CRC handbook19. Such data are available for sulfur dichloride
(SCI2). S-Cl bond length is I.99±0.03 A and Cl-S-Cl angle is 101±4 . Assuming that SCI2 and
HgCl2 have similar bond angles, the distance between the two chlorine atoms can be calculated as
3.47 A. This distance (3.47 A) may be the optimum distance between two O atoms that, through
formation of hydrogen bonds with two H atoms of two HCI molecules, can attract and trap an
IIg° molecule.
It was further hypothesized that, through their O atoms, surface crystalline water molecules were
the active sites for the capture of HCI and subsequent physical adsorption of Hg°. It was of
interest to evaluate the surface coverage of the crystalline water (its projected area) and compare
that to the total surface area of CaS0.i*2H20. Assuming that crystalline water at the surface
8

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»
resembled an SCI2 molecule, the distance between the two H atoms (see Figure 4) could be
calculated as 3.07 A. The projected area of one crystalline water molecule would thus be 7.4* 10"
20 m2/molecule. As indicated in Tables 1 and 3, maximum (saturation) uptake of CI by
CaS04-2H20 (at 60 °C) was achieved after 24 hours of exposure (3600 jig Cl/g CaS04-2H20).
This corresponded to l.OlxlO-4 mole Cl/g CaSC>4*2H20. If, through hydrogen bond formation,
each surface crystalline water adsorbed one molecule of HC1 at the saturation point, then the
number of surface crystalline water molecules would be: (1.01xl0"4)x(6.02xl02 ) = 6.1*1019
molecules H20(s)/g. The total projected area occupied by these molecules would thus be
(6.1xl019)x(7.4*10"2°) = 4.5 m2/g. The total surface area of CaS04*2H20 was measured to be
6.6 m2/g (see Table 3). The close agreement between these two numbers was another indication
that a bond (most probably a hydrogen bond) between gas-phase HC1 and surface crystalline
water was formed.
As indicated above, the amount and spacing of crystalline water molecules on the surface of a
sorbent may have controlled the amount of HC1 uptake and subsequent capture of Hg°. Of the
two tested compounds with crystalline water content higher than CaS(V2H20: \Ta3P04-12H20
demonstrated greater IIC1 uptake than Na2B407* 10II2O. This observation may confirm the
"hydrogen bonding" theory outlined above. However, the IIg° uptake results (see Table 2)
indicate that not all of the captured CI was instrumental in capturing Hg°; CaS04*2H20 with the
lowest CI uptake exhibited the highest Hg° uptake. This observation may support the theory that
an optimum spacing between CI atoms is required to trap Hg° as HgCl2 on the surface; not all
captured CI can provide this optimum spacing.
Hydrogen Bond Formation
Since the fixed-bed sorption process was not an external mass transfer controlled process14, the CI
capture must have been controlled by an adsorption kinetic mechanism. This mechanism,
specifically, involved the formation of hydrogen bonds between the H atom of an HC1 molecule
and the O atom of a hydrated water molecule as depicted by equation (2):
HCl(g) + II20(s) - C1II	OII2(s)	(2)
The rate of this hydrogen bond formation can be expressed as the difference between the
adsorption (formation) rate and the desorption (dissociation) rate. The adsorption rate should be
proportional to the HCI concentration in the gas phase and to the difference between the number
of sites (crystalline water molecules) available for adsorption and the number of occupied sites.
The desorption rate should simply be proportional to the number of occupied active sites; hence
the rate of the overall process is expressed by equation (3):
r - k)(qmax-q)c - k?q	(3)
Where r is the overall rate of sorption, ki and k2 are rate coefficients of the adsorption and
desorption reactions (respectively), qmax is the asymptotic adsorbate concentration (fully occupied
9

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surface), q is CI concentration in the solid phase (adsorbate), and c is HC1 concentration in the gas
phase. Both rate coefficients are Arrhenius functions of the bed temperature.
It may be hypothesized that in the 60-100 °C range (data in Table 4), ki in equation (2) is greater
than k2. Thus, increasing the temperature increased the adsorption rate of HC1 through a higher
rate of hydrogen bond formation. Further increase in temperature (from 100 to 120 °C) caused a
decrease in CI and subsequent Hg° uptake. This behavior may be interpreted two different ways.
If it is assumed that in the 100-120 °C range desorption rate (k2) was larger than the adsorption
rate (ki), then an increase in temperature resulted in higher desorption of CI and subsequent lower
capture of Hg°. More likely, however, the loss of HC1 uptake capability by CaS(V2H20 was the
result of the compound's losing its crystalline water. According to literature, CaS04*2H20 loses
its crystalline water at 140 °C. However, TGA analysis performed on CaS04-2H20, used in this
study, indicated that the loss of crystalline water occurred as early as 80 °C and approached
completion at about 140 °C. The partial loss of crystalline water in the 100-120 °C range may be
the underlying reason for the loss of the HC1 uptake capability of CaSC>4*2H20 at these elevated
temperatures.
Increasing the temperature to 100 or 120° C during the Hg° exposure period completely
suppressed the Hg° capture capability of the HCl-exposed CaSCV2H20 As illustrated in Figure
4, the Hg° attraction to the CI sites may be of a weak physical adsorption nature (van der Waals
bonds). Increasing system temperature significantly decreased the stability of these bonds.
Therefore, no He0 capture could be observed at higher temperatures. This indicates that Hg° was
not strongly attached to the active sites and could be easily desorbed through thermal treatment.
Conclusions
A series of experiments was performed to simulate adsorption of IIg° by Ca-based sorbents in the
presence of HQ. The following conclusions are based on the results of those experiments:
1)	For the family of four Ca-based sorbents tested here, the amount of Hg° uptake did not
correlate with the amount of the preceding HC1 capture because of the different nature of HC1
interactions with those Ca-based sorbents. Chlorinated compounds, produced as a result of
chemical reaction between IIC1 and CaO or Ca(OII)2, did not promote subsequent IIg° capture.
2)	HQ physical adsorption, through hydrogen bond formation, in Ca-based compounds with
crystalline water (CaS04*2H20 and CaSCV'/^O) was shown to be responsible for promoting
Hg° uptake. It was hypothesized that a pair of physically adsorbed HQ molecules can attract and
adsorb an Hg° molecule possibly through attractive van der Waals forces to form an HgCl2-like
molecule. The amount of HgC) uptake was correlated to the amount of crystalline water for Ca-
based sorbents.
3)	Optimum spacing between crystalline water molecules, or between adsorbed CI atoms, was
required to trap Hg° as an HgCl2-typc molecule.
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4)	As indicated by the effect of temperature studies on the uptake of HC1 by CaSO^P^O, and
depending on the range of temperatures, the rate of hydrogen bond formation and/or dissociation
was the rate limiting step for CI and subsequent Hg° uptake by CaS04*2H20.
5)	Increasing the temperature during the Hg° exposure period completely suppressed the Hg°
capture capability of the HCl-exposed CaS04*2II20. As mentioned, the Hg° attraction to the CI
sites may have been of a weak physical adsorption nature (van der Waals bonds). Increasing the
system temperature significantly decreased the stability of these bonds. This indicated that Hg°
was not strongly attached to the active sites and could be easily desorbed through thermal
treatment.
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Elements Through Coal-Fired Power Plant." Environmental Science & Technology. Vol.
9, p. 973 (1975).
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M ercury
Source
Carbon Trap
1
Manifold
On-Off Valves I
Wate
r Batht
3-Way Valve
Reactor
By-Pass
e—-	o
3-Way Valves
HCI/N
Carbon Trap
Data Acquisition
Mercury ^
Analyzer |
Rotameter
Figure 1
Schematic of the Fixed-bed Sorption Reactor System
14

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Hg° uptake (jig/g sorbent)
Figure 2
IIg° Uptake by ITCl-exposed, Ca-based Sorbents
CaO
Ca(OH)2
CaS04.2H20
*
CaC03
~
15

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Hg° uptake (^g/g sorbent)
2.5
0.5
0
10 20 30 40 50 60 70
time (min)
Figure 3
Effect of Crystalline Water (0, V2, and 2) on Hg° Uptake by the HCl-exposed Forms of CaS04
16

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H3°
Hg?
van der Waals bond • ^ \ 2 25^
cjji -347A - (j:i
H	H
i
|
I Hydrogen bond
A
H H
CaSCX
Figure 4
Mechanism of IIC1 and IIg° Capture by CaS04'2H20
17

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VCMDT n j -i o TPrMWirAI OPPOOTHATA
NRMRJy RT • 418 (Plct fore completing)
1. REPORT NO. 2.
600/A-99/060
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Role of HC1 in Adsorption of Elemental Mercury
Vapor by Calcium-based Sorbents
5. REPORT OATE
6. PERFORMING ORGANIZATION CODE
7.author(s) a Ghorishi (ARCADIS), B. K. Gullett (EPA),
and W. Josewicz and W.Kozlowski (ARCADIS)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING OROANIZATION NAME ANO ADDRESS
ARCADIS Geraghty and Miller, Inc.
P. C. Box 13ICS
Research Triangle Park, North Carolina 2770S
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-C-99-201, WAs 5-026
and 0-014
12. SPONSORING AGENCY NAME ANO 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; 9/98-5/99
14. SPONSORING AGENCY CODE
EPA/600/13
15.supplementarynotesappcd project officer is Brian K. Gullett, Mail Drop 65, 919/541-
1534. For presentation at EPRI/DOE/EPA Combined Utility Air Pollutant Control
Symposium, 8/16-20/99, Atlanta, GA.
i6. abstract 'j-he paper gives results of a study to identify active sites and surface func-
tional groups that may contribute to the adsorption of elemental mercury (Hg°) by
relatively inexpensive calcium (Ca)-based sorbents. (NCTE: Hg° capture has been
mostly investigated using high-surface-area, expensive activated carbons. Previous
investigations showed that hydrogen chloride (HC1) in the flue gas had a pronounced
positive effect on the adsorption of lJg° by a lignite-coal-based activated carbon with
high Ca con;ent.) The study investigated the formation of chlorine (Cl) sites in Ca-
based sorbents as well as their role and reactivity in the adsorption of Hg°. HC1-
exposed calcium sulfate dihydrate (gypsum) exhibited a superior IIg° sorption capa-
bility. Crystalline water molecules on the surface of the gypsum were confirmed to
contribute indirectly to Hg° uptake. These surface molecules may have adsorbed
1JC1 through hydrogen bond formation between an oxygen atom of a crystalline water
molecule and a hydrogen atom of an HC1 molecule. Two adjacent, physically adsor-
bed HC1 molecules could then trap an IIg° molecule through formation of a mercuric-
chloride-like molecule. Significant correlations between Cl and Hg° uptake by gyp-
sum were noted.
17. KEY WORDS AND OOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution Gypsum
Mercury (Metal)
V apors
Sorption
Calcium
Hydrogen Chloride
Air Pollution Control
Stationary Sources
Elemental Mercury
13 B 08G
07B
07D
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (This pexe)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)

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