Investigation of Changes in Surface Chemistries on Virgin andBrominated Activated Carbon Sorbents during Mercury Capture:Before and After RegenerationArindom Saha*

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

*S Supporting Information

ABSTRACT: Pre-halogenated activated carbon (AC) sorbents are known to be effective in Hg capture when injected into theflue gas stream generated from the combustion of low-chlorine-containing sub-bituminous and lignite coals. In this study, virgin(unfunctionalized) and brominated AC powders were tested for Hg sorption in a low-HCl (4 ppm)-containing simulatedPowder River Basin (PRB)-fired flue gas matrix before and after regeneration. X-ray photoelectron spectroscopy (XPS) was usedto investigate changes in sorbent surface chemistries, which were then correlated to Hg breakthrough data obtained from bench-scale experiments. It was observed that Hg capture tests on brominated AC powders removed the surface-bound bromine, aknown Hg-binding site, to levels below XPS detection limits. Further, XPS studies also revealed an increase in the chlorinecontent after adsorption on the tested samples and the samples regenerated at a lower temperature (204 °C). The decrease ofbromine peaks coupled with the presence/increase of Cl, N, and S on the tested and some regenerated samples strongly indicatesthat the HCl, SO2, and NOx components in the flue gas play a collaborative role during Hg breakthrough. It is also observed fromthe breakthrough data that Hg adsorption on regenerated samples decreases significantly to 6.5−9.6% of the initial adsorptionvalues. This indicates that regenerating AC sorbents for further Hg capture is not an effective process.

1. INTRODUCTIONThe escape of volatile Hg0 into the atmosphere during coalcombustion is an issue of great concern to the U.S. electricpower industry because of the recent implementation of airquality standards by the Environmental Protection Agency(EPA). After an extensive study, the EPA has determined thatHg emissions from power plants pose significant hazards topublic health and, hence, need to be reduced. The EPA’sMercury and Air Toxic Standards (MATS), which goes intoeffect in April 2015,1 imposes restrictions on the amount of Hgemissions from coal-fired power plants.During the coal combustion process, only a portion of the

vaporized Hg can be captured through conventional pollutioncontrol technologies.2 Particle-bound Hg can be constrainedthrough particulate control devices (PCDs), and wet flue gasdesulfurization (FGD) units are employed for removal ofoxidized Hg. Nevertheless, the majority of Hg escapes into theatmosphere in volatile elemental form (Hg0), which isdetrimental to the environment as well as human physiology.Activated carbon injection (ACI) upstream of the air-pollutioncontrol device appears to be a cheaper and efficient way tooxidize and capture this volatile Hg0. Currently, injectingbrominated powdered activated carbon (PAC) to capture Hgand adding bromide into coal/boiler to oxidize mercury are thetwo most effective technologies used for mercury reduction.Engineered solutions, such as the injection of halogenated

activated carbon (AC) into the flue gas stream of low-chlorine-containing coals have been very effective in capturing Hg andhave the potential to reduce carbon injection rates and, thereby,the overall cost. Recent efforts in Hg emission reduction haveemployed the addition of chlorine3 or other oxidizing agentsdirectly to the flue gas. Application of functionalized AC

sorbents (Cl, Br, I, and S) can also improve Hg captureperformance.4 The mechanism of Hg adsorption with thesechemically modified ACs is not fully understood, but thebeneficial role of Cl,5 S,6 I,7 and Br8 in the capture of Hgspecies is well-documented. It has been observed that ACsimpregnated with chlorine have much improved Hg0

adsorption compared to virgin (unfunctionalized) ACs inboth fixed-bed and entrained-flow systems.9

This study is inspired by Electric Power Research Institute(EPRI)’s MercScreen technology,10 which is implementeddownstream of an electrostatic precipitator (ESP). When thetechnology is placed downstream of the ESP fewer problemsrelated to particulate matter increasing pressure drop across themercury capture device are expected. At this point in theprocess, the temperature cools down to a range where the ACdoes not combust and is also low enough to facilitate mercuryremoval. Most functional groups that are used to treat ACsorbents for mercury capture are stable at this temperature(except, some sulfur treatments). AC also tend to oxidize anymercury that passes through the reactor. Oxidized mercury iseffectively captured in wet flue gas desulfurization scrubbers(wet FGD); therefore, by placing the AC bed prior to the wetFGD unit, mercury removal from the gas could be furtherincreased.The objective of this research is to investigate the changes in

surface chemistry of virgin and brominated AC sorbents duringHg capture and on regeneration and to correlate theobservations to further efficacy in Hg removal by these

Received: January 26, 2014Revised: May 12, 2014Published: May 13, 2014

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sorbents. To achieve this, Hg breakthrough tests wereconducted on AC sorbents using a simulated low-HCl PowderRiver Basin (PRB)-fired flue gas. Recently, X-ray photoelectronspectroscopy (XPS) has been used effectively by researchers tostudy the Hg-binding mechanism on both AC powder11,12 andAC fiber supports.13,14 Hence, XPS studies were conducted tocharacterize the AC sorbents before and after the breakthroughtests and also after regeneration to understand the changestaking place on the sorbent surface. A second set ofbreakthrough tests was conducted on two of the regeneratedsamples, regenerated-tested virgin powder (RTVP) andregenerated-tested brominated powder (RTBP), to studytheir ability in Hg mitigation. Hg equilibrium adsorptioncapacity tests were performed on each sample using identicalflue gas conditions. XPS studies were conducted on eachsample listed in Table 1.

2. MATERIALS AND METHODS2.1. Material Characteristics. Two distinct types of AC sorbents

were tested at URS Corporation for Hg capture under the simulatedPRB-fired flue gas conditions: virgin powders and brominatedpowders. Untested samples, which were not exposed to simulatedflue gases, are referred to as untested virgin powder (UTVP) anduntested brominated powder (UTBP), respectively. The UTVP(DARCO Hg) and the UTBP (DARCO-Hg-LH) are commerciallyavailable and were procured from Norit Americas, Inc. Thesepowdered AC sorbents were prepared from lignite based coal andthe surface area was between 500 and 640 m2/g and pore volume of

0.42−0.52 cm3/g. The tested samples are referred to as tested virginpowder (TVP) and tested brominated powder (TBP), respectively.

Besides the untested and tested samples, two categories ofregenerated samples were also studied. “Regeneration” refers totreating the tested samples at elevated temperatures (204 and 354 °Chere) at which point all adsorbed Hg is expected to be released fromthe sorbent surface thereby enabling their possible reuse in further Hgcapture studies. In the first category, the samples were just regeneratedat elevated temperatures after testing. The samples in this category arereferred to as regenerated virgin powder at 204 °C (RVP1),regenerated virgin powder at 354 °C (RVP2), regenerated brominatedpowder at 204 °C (RBP1), and regenerated brominated powder at 354°C (RBP2). In the second category, the samples were tested oncemore under the same simulated flue gas conditions after regeneration.The two samples in this category are referred to as regenerated andtested virgin powder at 354 °C (RTVP) and regenerated and testedbrominated powder at 204 °C (RTBP).

2.2. Breakthrough Studies. All bench-scale Hg adsorption testswere conducted at URS Corporation’s Process Technologies Office inAustin, Texas. The schematic design of the test unit is shown in Figure1. The flue gas conditions used for testing were 12−17 μg of Hg/nm3,400 ppm of SO2, 200 ppm of NOx (95% NO), 4 ppm of HCl, 7%H2O, 12% CO2, 6% O2, and 75% N2. This composition simulates theflue gas downstream of an ESP for a plant burning PRB coal.

All samples were tested at 149 °C. Hg was introduced to the flue gasusing a nitrogen carrier stream through a temperature-controlledpermeation chamber containing Hg0. More detailed informationregarding bench-scale testing, sorbent configuration, and Hgadsorption test results is provided in the Supporting Information.

Table 1. Breakthrough Test Results of the Samples Exposed to the Simulated Flue Gases

sorbent

desorptiontemperature

(°F)

inlet Hg0

concentration(μg/Nm3)

initialbreakthrough

(%)Hg0 adsorption capacity

(μg/g at 50 μg of Hg/Nm3)initial adsorption capacity

(μg/g at 50 μg of Hg/Nm3)

slope of thebreakthrough curve

(%/min)

percentdesorption

(%)

TVP N/A 31.4 9 807 755 0.90 N/ATBP N/A 54.4 <5 1254 1119 0.85 N/ARVP1 400 48.9 <5 977 787 0.65 8RVP2 670 50.6 6 953 597 0.44 15RTVP 670 47.2 (40.5)a <5 (<5) 1047 (100) 889 (66) 0.77 (5.67) 10RBP1 400 46.7 <5 1578 1329 0.40 15RBP2 670 53.6 <5 1212 1061 0.86 21RTBP 400 49.6 (44.7) <5 (<5) 1200 (78) 1088 (51) 0.68 (3.41) 12

aValues in parentheses represent the data of the second breakthrough test performed after regeneration of the sample.

Figure 1. Schematic of the bench-scale test system used at URS.

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2.3. XPS Analysis. All samples were analyzed by XPS at theStanford Nanocharacterization Laboratory (SNL) at StanfordUniversity. The PHI 5000 VersaProbe scanning XPS microprobeused for analysis is equipped with a 350 W monochromatic Al Kα X-ray source (1486.6 eV) and is operated at a base pressure of ∼5 ×10−10 Torr. The X-rays are focused onto a spot size of 100 × 800 μmof each sample. The detection limits of all elements, except hydrogen,were ∼0.01 monolayers on the surface and ∼0.1% in the bulk.All untested AC powder samples were pressed onto a clean indium

foil and then mounted on a clean sample holder without any furtherpreparation prior to XPS analysis. The tested and regenerated powdersamples were diluted with sand during breakthrough tests at URS. Forthis reason, these samples were first sieved using a 106 μm sieve, andthe collected AC powder was pressed onto indium foils, mounted on aclean sample holder, and then submitted for XPS studies. All surveyand high-resolution scans were taken with the charge neutralizeroperating at 30 V. This was performed to avoid any charging as a resultof the insulating components of the sample or external factors, such asleftover sand and glass wool residue.

3. RESULTS

3.1. Hg Adsorption Tests Using a Bench-Scale TestingSystem. Key data pertaining to Hg equilibrium adsorptioncapacity testing of the powder AC sorbents are tabulated inTable 1. In general, the Hg adsorption capacity datameasurements have an error in the range of ±20%. The testedsamples TVP and TBP had normalized Hg capacities of 807μg/g of carbon (at an inlet concentration of 50 μg of Hg/nm3)and 1254 μg/g of carbon (at an inlet concentration of 50 μg ofHg/nm3), respectively.In the first adsorption breakthrough studies, the samples

RTVP and RTBP had normalized Hg capacities of 1047 and1200 μg/g of carbon (at 50 μg of Hg/nm3 inlet concentration),respectively. In the second adsorption breakthrough studies onRTVP and RTBP after regeneration, normalized Hg capacitiesof only 100 and 78 μg/g of carbon (at 50 μg Hg/nm3 inletconcentration) were observed. This demonstrates that Hg

adsorption values for RTVP and RTBP decrease significantlyafter regeneration to 9.6% and 6.5% of their respective firstadsorption values.

3.2. XPS Studies. The XPS spectra for all samples werecalibrated by shifting the main C 1s peak to 284.5 eV becausethis is the binding energy (BE) generally observed for theinherent sorbent material in pure carbon samples.15 Theelemental compositions of all samples were obtained from theirsurvey spectra (Table 2). To facilitate quantitative comparisonsamong samples, the intensity of the C 1s line was used tonormalize peak intensities, as shown in Table 3. The data inTable 3 will be the focus of the Results and Discussion of thisstudy. No unexpected elements were observed in the surveyspectra (see Figure S1 of the Supporting Information). Strongpeaks of indium were observed in the survey spectra because ofthe foil on which the powdered samples were pressed. Fordeeper analysis into the chemistries of carbon, oxygen, Hg,sulfur, bromine, silicon, nitrogen, aluminum, chlorine, calcium,and sodium, their strongest lines, as identified in the surveyspectra, were investigated further with high-resolution scans inthe binding energy region of interest.Hg lines were not observed in any scans, indicating that the

concentration of Hg was below the detection limit for allsamples. The possibility of interference between the Hg 4f linesand the Si 2p lines can further complicate detection of Hg.Hence, this study focuses on the change in surface chemistriesof oxygen, sulfur, nitrogen, chlorine, and bromine after Hgadsorption testing, because these are the most importantelements participating in the reaction chemistry. The C 1sspectra of all of the samples contain one sharp, dominant peak,corresponding to the inherent sorbent material, along withsmaller peaks/shoulders to the high BE side (see Figure S2 ofthe Supporting Information). The O 1s spectra in all samples(see Figure S3 of the Supporting Information) show a mainpeak at ∼532 eV, consistent with literature values for the O 1s

Table 2. Elemental Compositions of Various Samples from Their Survey Spectra under XPS

sample name C O Br Si S N Cl Hg Al Ca Na

UTVP 80.7 15.8 0.0 1.9 0.5 0.0 0.0 0.0 0.0 1.1 0.0UTBP 76.7 15.9 1.0 2.0 1.5 0.5 0.0 0.0 0.0 1.7 0.7TVP 53.5 34.8 0.0 8.2 2.7 1.4 0.6 0.0 0.0 1.8 0.0TBP 48.7 42.5 0.0 2.3 3.4 0.7 0.9 0.0 0.0 1.1 0.4RVP1 49.4 39.8 0.0 7.7 0.5 0.0 1.1 0.0 0.0 1.5 0.0RVP2 44.2 38.3 0.0 13.0 2.4 0.0 0.0 0.0 0.8 1.3 0.0RTVP 59.7 29.7 0.0 5.0 3.9 0.3 0.0 0.0 0.0 1.4 0.0RBP1 50.0 40.4 0.0 0.9 5.6 2.0 0.0 0.0 0.0 1.0 0.1RBP2 43.2 39.7 0.0 9.5 4.0 0.0 0.0 0.0 0.6 2.1 0.9RTBP 43.5 40.7 0.0 4.0 8.6 1.3 0.0 0.0 0.0 1.0 0.9

Table 3. Elemental Compositions of Various Samples from Their Survey Spectra under XPS Normalized to 100% C

sample name C O Br Si S N Cl Hg Al Ca Na

UTVP 100.0 19.6 0.0 2.4 0.6 0.0 0.0 0.0 0.0 1.4 0.0UTBP 100.0 20.7 1.3 2.6 2.0 0.7 0.0 0.0 0.0 2.2 0.9TVP 100.0 65.0 0.0 15.3 5.0 2.6 1.1 0.0 0.0 3.4 0.0TBP 100.0 87.3 0.0 4.7 7.0 2.9 1.8 0.0 0.0 2.3 0.8RVP1 100.0 80.6 0.0 15.6 1.0 0.0 2.2 0.0 0.0 3.0 0.0RVP2 100.0 86.7 0.0 29.4 5.4 0.0 0.0 0.0 1.8 2.9 0.0RTVP 100.0 49.7 0.0 8.4 6.5 0.5 0.0 0.0 0.0 2.3 0.0RBP1 100.0 80.8 0.0 1.8 11.2 4.0 0.0 0.0 0.0 2.0 0.2RBP2 100.0 91.9 0.0 22.0 9.3 0.0 0.0 0.0 1.4 4.9 2.1RTBP 100.0 93.6 0.0 9.2 19.8 3.0 0.0 0.0 0.0 2.3 2.1

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BE for a wide variety of materials.16 The samples TVP, TBP,and RVP1 also show shoulder peaks at low BE values. Thesepeaks most likely arise from indium oxide, which was used as asupport for XPS analysis.All samples, except UTBP and TBP, revealed the presence of

silicon upon XPS analysis. The Si 2p signal at ∼102 eV17 fromthe high-resolution spectra (Figure 2) confirms the presence of

SiOx species. Detection of the Hg 4f peak by XPS wasunsuccessful. For all of the tested and regenerated samples,detection of the Hg 4f doublet (∼10118/∼105 eV19) iscomplicated by the fact that it coincides with the large Si 2ppeak at ∼102−103 eV. Further, monitoring of the Hg 4d peak,which has no spectral interference from any other element alsodid not indicate the presence of Hg in any of the tested andregenerated samples (see Figure S4 of the SupportingInformation). Sulfur high-resolution scans for all samples areshown in Figure 3. Sulfur was detected in the S 2p spectra at adifferent BE of ∼169 eV,20 corresponding to the S (VI) (H2SO4

and SO42−) form. Of all untested and tested samples, bromine

was only detected on UTBP, where Figure 4 indicates thepresence of the Br 3p doublet at ∼18121 and ∼188 eV.22

Interestingly, no bromine was detected on the TBP sample.Chlorine was detected on tested samples TVP and TBP and theregenerated sample RVP1, as shown in Figure 5. No Cl 2p peakwas detected at ∼199 eV in any of the regenerated brominatedsamples. Nitrogen was observed in minute amounts on theuntested sample UTVP and the regenerated sample RTVP.Significant amounts of nitrogen were detected on the testedsamples TVP and TBP and the regenerated samples RBP1 andRTBP. The compiled spectra shown in Figure 6 show the N 1speak at ∼400 eV.23 Small amounts of aluminum, calcium, andsodium were detected on some samples before testing, aftertesting, and upon regeneration (see Figures S7−S9 of theSupporting Information).

4. DISCUSSIONThe initial goal for this study was to detect the presence of Hgand identify its chemical state. However, no Hg was detected byXPS as the Hg 4f doublet (∼101/105 eV) or the Hg 4d5/2 line(∼360 eV) on any of the samples. This result was disappointingbut not totally unexpected because surface concentrationcalculations for Hg, even for the high-capacity RBP1 sample,were predicted to be below the XPS detection limits. Even if allof the Hg in this particular sample was present on the surface, itwould amount to a surface coverage of only 0.0006 monolayersof Hg (see the Supporting Information). The detection limit ofthe XPS probe used for analyses is approximately 0.01monolayers of Hg.

Figure 2. Compiled Si 2p/Hg 4f spectra of all untested, tested, andregenerated samples.

Figure 3. Compiled S 2p spectra of all untested, tested, andregenerated samples.

Figure 4. Compiled Br 3p spectra of all untested, tested, andregenerated samples.

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Because Hg was not detected, all inferences were made bystudying changes in the surface chemistry of the other elementsafter exposure to the simulated flue gas and combining thisknowledge with information obtained from the Hg break-through curves. The XPS analysis of the untested samples willbe discussed first, followed by the XPS analysis of the testedsamples, and then the regenerated samples.4.1. Surface Chemistry of the Untested Samples. The

untested samples UTVP and UTBP contain predominantlycarbon (∼80 atomic %) indicated by a sharp and strong peak at284.5 eV from the AC and a tail extending out to 293 eV

presumably from oxidized and adventitious carbon. The tailingis more prominent in the UTBP sample. Both of these sampleswere observed to have similar oxygen content at ∼20 atomic %when normalized to carbon (Table 3). The virgin powders alsocontain impurities naturally present in AC sorbents, such assilicon (∼2.5 atomic %) and calcium (1.4−2.2 atomic %). Bothsamples also indicate the presence of oxidized sulfur S (VI) inthe 0.6−2.0 atomic % range. In addition to this, the UTBPsample contains nominal amounts of nitrogen (0.7 atomic %)and sodium (0.9 atomic %). The N 1s peak location, however,does not match that for either NO (405 eV) or NO2 (407 eV)and indicates the presence of a more reduced form of nitrogen,such as NH4

+ (∼401 eV), as suggested by Olson et al. in theirstudies.24 As expected, the UTBP sample was also observed tocontain 1.3 atomic % bromine from XPS analysis.

4.2. Surface Chemistry of the Tested Samples. Ingeneral, there are a number of similarities observed in thesurface chemistry of both of the tested samples TVP and TBP.Elemental compositions from Table 3 as well as high-resolutionspectra indicate that the intensities of oxygen, silicon, sulfur,nitrogen, and chlorine increase significantly after testing onboth of the samples. The increase of silicon is likely due to sandand glass wool residues left after testing. The increase in thesulfur content on both tested samples TVP (from 0.6 to 5.0atomic %) and TBP (from 2.0 to 7.0 atomic %) stronglyindicates an increase in sulfur oxide concentrations on thesurface and is particularly consistent with the assignment of thesulfur peak to S (VI) (SO4

2−/H2SO4) forms. The correspond-ing increase in oxygen intensity is observed to be more than 4times greater than the increase in the S (VI) intensity. Thisfurther indicates that not all of the oxygen detected throughXPS is in the sulfate form. The other likely sources for oxygenintensity are adventitious organic molecules on the surface,oxidized carbon groups resulting from the activation process,SiOx from the starting carbonaceous material, and the glasswool used to pack the sorbent during testing. A significantincrease in the nitrogen content from UTVP (0.0 atomic %) toTVP (2.6 atomic %) and from UTVP (0.7 atomic %) to TBP(2.9 atomic %) was observed on XPS analysis. The increase inN 1s features were observed at 401 eV on both of the samplesand definitely appeared from NOx in the simulated flue gasstream. The N 1s peak location, however, does not match thatfor either NO (405 eV) or NO2 (407 eV).Interestingly, a significant increase of the chlorine content

was observed on both TVP (1.1 atomic %) and TBP (1.8atomic %) compared to their untested counterparts UTVP andUTBP, on which no chlorine was present at all. Thisobservation is very interesting, because usually no chlorine isdetected on samples after long periods of testing whenoxidizing gases, such as SO2 and NOx, are present in the fluegas matrix.24 No bromine was detected on the TBP sample asper expectations. A significant amount of calcium was observedon both TVP (3.4 atomic %) and TBP (2.3 atomic %). Aminute amount of sodium was also detected on the TBP (0.8atomic %) sample.

4.3. Surface Chemistry of the Regenerated Samples.In general, elemental compositions from Table 3 as well ashigh-resolution spectra indicate that the intensities of oxygen,silicon, and sulfur increase significantly after regeneration. Theincrease of oxygen intensity is the most significant; as forregenerated samples, the O 1s peak intensity is observed in the∼50−94 atomic % range after normalization, as compared to∼20 atomic % in the untested samples. There is also a

Figure 5. Compiled Cl 2p spectra of all untested, tested, andregenerated samples.

Figure 6. Compiled N 1s spectra of all untested, tested, andregenerated samples.

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significant increase in Si 2p intensity after regeneration in mostsamples compared to the untested samples and, in some cases,also compared to the tested samples. This could be a result ofcumulative silica residues from glass wool and sand duringregeneration and retesting after regeneration. A significantincrease in the sulfur content is observed in most of theregenerated samples compared to the untested samples. Insamples RBP1 and RBP2, the sulfur contents detected are 11.2and 9.3 atomic %, respectively. In one particular sample, RTBP,the detected sulfur content is as high as 19.8 atomic %. In allsamples, the sulfur content is observed entirely in the S (VI)form (SO4

2−/H2SO4). Interestingly, the increase in the sulfurcontent on regenerated brominated samples (9.3−19.8 atomic%) is significantly higher than regenerated virgin samples (1.0−6.5 atomic %).The increase in the nitrogen content is also observed in the

three regenerated samples RTVP (0.5 atomic %), RBP1 (4.0atomic %), and RTBP (3.0 atomic %). Calcium is detected inthe range of 2.3−4.9 atomic % on all of the regeneratedsamples. Some aluminum was detected in two regeneratedsamples RVP2 (1.8 atomic %) and RBP2 (1.4 atomic %),respectively. Sodium was detected in the range of 0.2−2.1atomic % on all brominated regenerated samples. As expected,no bromine was detected on any of the brominated samplesafter regeneration. Interestingly, however, some chlorine wasdetected on the regenerated sample RVP1 (2.2 atomic %).4.4. Understanding the Hg Sorption Process on

Tested and Regenerated Powder AC Sorbents. Hgbreakthrough data on the tested samples TVP (807 μg ofHg/g of carbon) and TBP (1254 μg of Hg/g of carbon)indicate that the presence of the −Br group definitely abets inthe Hg capture process. It is also obvious from XPS studies thatthe increase of oxygen, SO4

2−/H2SO4 groups, and −N-basedgroups on the tested and regenerated samples is detrimental tothe Hg capture process. Significantly higher SO4

2−/H2SO4 and−N contents on brominated tested and regenerated samplescompared to their virgin counterparts indicate that the surfaceof the brominated samples probably becomes more deactivatedafter testing. This is corroborated in the second Hg adsorptionstudies when only 78 μg of Hg/g of carbon is captured on theregenerated brominated sample RTBP, as compared to 100 μgof Hg/g of carbon on the regenerated virgin sample RTVP. Inthe tested samples (TVP and TBP) and the regenerated sampleRVP1, the presence/increase of −Cl indicates that chlorinecould also be playing a transient role during breakthrough atlow temperatures (149−204 °C). Because no bromine isdetected after testing and oxygen, S(VI), −N, and in somecases, chlorine contents increase on the tested samples, itappears that bromine on the active sites is initially replaced by−Cl at low temperatures, which is then eventually replaced bySO4

2− and −N groups during breakthrough.The RBP1 sample shows the highest Hg adsorption capacity

(1578 μg of Hg/g of C) of all samples, even though it shows nopresence of −Br or −Cl in the XPS data compiled in Table 3.However, upon closer inspection, it can also be observed thatthe S (11.2%) and N (4.0%) contents in this sample are alsoconsiderably higher than most samples. From this, it appearsthat, initially, during the bench-scale test, the −Br active sitessignificantly aid in Hg binding on the sorbent surface, hence,the high Hg adsorption capacity. With the increase in time,these −Br active sites are replaced by S(VI), and −N groupsfrom the corresponding components in the flue gas, therebyleading to Hg breakthrough.

5. CONCLUSION

Two AC powder sorbents, virgin (unfunctionalized) andbrominated, were exposed to the same simulated PRB-firedflue gas composition and tested for Hg capture. Besides thesetested samples, two categories of regenerated samples were alsostudied. In the first category, the samples were just regeneratedat elevated temperatures after testing, and in the secondcategory, the samples were tested once more under the samesimulated flue gas conditions after regeneration. All untested,tested, and regenerated samples were subsequently subjected toXPS analysis to understand surface chemistries involved duringthe Hg capture process.On the tested samples, TVP and TBP normalized Hg

capacities of 807 and 1254 μg/g of carbon were observed at 50μg of Hg/nm3 inlet concentration, respectively. In the firstadsorption studies on the regenerated samples, RTVP andRTBP normalized Hg capacities of 1047 and 1200 μg/g ofcarbon at 50 μg of Hg/nm3 inlet concentration were observed,respectively. In the second adsorption breakthrough studies onthese two samples after regeneration, normalized Hg capacitiesof only 100 and 78 μg/g of carbon at 50 μg of Hg/nm3 inletconcentration were observed, respectively. This demonstratesthat Hg adsorption values for these two samples decreasesignificantly after retesting and that regenerating AC sorbentsfor further Hg capture tests is not an effective process.No Hg was conclusively detected on any of the samples from

XPS analysis, likely because of Hg concentrations being belowthe detection limits of the instrument. XPS studies also indicatethat the increase of oxygen, SO4

2−/H2SO4 groups, and −Ngroup on the tested and regenerated samples is detrimental tothe Hg capture process. Significantly higher SO4

2−/H2SO4 and−N contents on the brominated samples compared to theunfunctionalized samples after testing and regeneration indicatethat the samples of the former category probably become moredeactivated during the breakthrough studies. This observationis corroborated in the second Hg adsorption tests when aslightly lower amount of Hg (78 μg of Hg/g of carbon) iscaptured on the brominated sample RTBP compared to thevirgin sample RTVP (100 μg of Hg/g of carbon). In the testedsamples (TVP and TBP) and the regenerated sample RVP1,the presence/increase of −Cl indicates that it could also beplaying a fleeting role during breakthrough, especially at lowtemperatures (149−204 °C). Because no bromine is detectedafter testing and oxygen, S (VI) groups, −N, and in some cases,chlorine contents increase on the tested samples, it appears thatbromine on the active sites is initially replaced by −Cl, which isthen eventually replaced by SO4

2−- and −N-containing groupsduring breakthrough.In essence, this study helps to correlate the Hg adsorption

capacities of the virgin and brominated AC sorbents duringtesting, regeneration, and retesting after regeneration with thechanges in surface chemistries observed through XPS. Thisstudy, however, cannot account for any changes in the bulk ofthe sorbents or changes below the XPS detection limits.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional information on sorbent configuration, sampleanalytical methods, and data analysis, as discussed under theMaterials and Methods (continued), calculations explainingwhy Hg was not detected by XPS, as discussed in theDiscussion (continued), and several figures depicting additional

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XPS spectra. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +1-925-325-6189. E-mail: arisaha72@gmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe author thanks the EPRI for making this research possiblethrough their generous funding. The author also acknowledgescontributions from scientists at URS in conducting the Hgbreakthrough studies and also conveys his gratitude to ChuckHitzman for his help and guidance in operating the PHIVersaProbe scanning XPS microprobe at the SNL. The authoralso appreciates the inputs of Dr. Alejandro Andreatta duringthe final review process of this manuscript.

■ REFERENCES(1) For additional information regarding the health effects of mercuryexposure and details of the MATS, see http://www.epa.gov/mats/.(2) Kolesnikov, S. P.; Rubinskaya, T. Y.; Strel’tsova, E. D.; Leonova,M. Y.; Korshevets, I. K.; Zykov, A. M.; Anichkov, S. N. Solid FuelChem. 2010, 44, 50.(3) Laumb, J. D.; Benson, S. A.; Olson, E. A. Fuel Process. Technol.2004, 90, 1364.(4) Srivastava, S. K.; Hutson, N. D.; Martin, G. B.; Princiotta, F.;Staudt, J. Environ. Sci. Technol. 2006, 41, 1385.(5) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K.;Jozewicz, W. S. Environ. Sci. Technol. 2002, 36, 4454.(6) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Ind. Eng. Chem. Res.2000, 39 (4), 1020.(7) Lee, S. J.; Seo, Y.-C.; Jurng, J.; Lee, T. G. Atmos. Environ. 2004,38, 4887.(8) Hutson, D. N.; Attwood, B. C.; Scheckel, K. G. Environ. Sci.Technol. 2007, 41, 747.(9) Zeng, H.; Jin, F.; Guo, J. Fuel 2004, 83, 143.(10) Machalek, T.; Richardson, C.; Paradis, J.; Chang, R.; Looney, B.;Mathews, M.; Merritt, R.; Lu, S.; Palko, A. O. URS Power TechnicalPaper; URS Corporation: San Francisco, CA, 2010; Paper 85.(11) Wilcox, J.; Sasmaz, E.; Kirchofer, A.; Lee, S.-S. J. Air WasteManage. Assoc. 2011, 61, 426.(12) Sasmaz, E.; Kirchofer, A.; Jew, A. D.; Saha, A.; Abram, D.;Jaramillo, T. J.; Wilcox, J. Fuel 2005, 55, 747.(13) Yao, Y.; Velpari, V.; Economy, J. J. Mater. Chem. A 2013, 1,12103.(14) Yao, Y.; Velpari, V.; Economy, J. Fuel 2014, 116, 560.(15) Estrade-Szwarckopf, H.; Rousseau, B. J. Phys. Chem. Solids 1992,53, 419.(16) Batich, C. D.; Donald, D. S. J. Am. Chem. Soc. 1984, 106, 2758.(17) Anwar, M.; Hogarth, C. A.; Bulpett, R. J. Mater. Sci. 1990, 25,1784.(18) Nefedov, V. I.; Salyn, Y. V.; Solozhenkin, P. M.; Pulatov, G. Y.Surf. Interface Anal. 1980, 2, 170.(19) Humbert, P. Solid State Commun. 1986, 60, 21.(20) Christie, A. B.; Lee, J.; Sutherland, I.; Walls, J. M. Appl. Surf. Sci.1983, 15, 224.(21) Sharma, J.; Iqbal, Z. Chem. Phys. Lett. 1978, 56, 373.(22) Uwamino, Y.; Tsuge, A.; Ishizuka, T.; Yamatera, H. Bull. Chem.Soc. Jpn. 1986, 59, 2263.(23) Swartz, W. E.; Alfonso, R. A. J. Electron Spectrosc. Relat. Phenom.1974, 4, 351.(24) Olson, S. E.; Crocker, C. R.; Benson, S. E.; Pavlish, J. H.;Holmes, M. J. J. Air Waste Manage. Assoc. 2005, 55, 747.

Energy & Fuels Article

dx.doi.org/10.1021/ef500257m | Energy Fuels 2014, 28, 4021−40274027