Mineralogical Associations of Mercury in FGD Products - Energy

May 22, 2012 - Mineralogical Associations of Mercury in FGD Products ...... Proceedings of the International Conference on Air Quality VIII; UND EERC,...
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Mineralogical Associations of Mercury in FGD Products William Lee Beatty,* Karl Schroeder, and Candace L. Kairies Beatty U.S. Department of Energy/National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States ABSTRACT: The natural mode of retention of mercury (Hg) in flue gas desulfurization (FGD) gypsum used in wallboard manufacturing has been investigated using a series of phase-targeted reagents. The sequence of batch extractions, based on the geochemical literature, allowed for separation of particular mineral phases and subsequent testing for the appearance of Hg in the resulting extract. Results indicated that Hg was associated with two distinct phases. Most of the Hg was solubilized by the extraction targeting iron oxides and hydroxides. The reagent targeting the organic matter and sulfide minerals also typically yielded significant amounts of Hg. Analysis of the extract indicated the presence of another phase, possibly clay minerals, that may have been associated with the retained Hg in addition to, or instead of, the targeted phase. Hg was solubilized only under extremely acidic (pH < 1) and oxidizing conditions at high temperature. Virtually no Hg was found in the extracts produced by weaker reagents.

1. INTRODUCTION Flue gas desulfurization (FGD) units typically use lime or limestone to capture sulfur dioxide (SO2) as calcium sulfite, most of which is subsequently converted to gypsum (CaSO4·2H2O) in forced oxidation units. FGD-produced gypsum is most commonly used as a substitute for natural gypsum in the manufacturing of wallboard, and, to a lesser extent, as an additive in cement and concrete and as a soil amendment. The 2010 survey by the American Coal Ash Association found that close to 49% of FGD gypsum was utilized after production, with nearly 7.7 million tons used in the manufacture of wallboard.1 Coal utilization byproducts, including FGD gypsum, contain low concentrations of the trace elements that occur naturally in the coal fired in the electric utility boiler. It is anticipated that stricter emission control/reduction policies, particularly those focusing on mercury, would lead to an increase in trace element concentration in these byproducts.2 The first peer-reviewed report of mercury capture in FGD units appeared in 1991.3 Since then, it has become widely recognized that the FGD technologies used for the removal of SO2 can result in the coremoval of at least a portion of the flue-gas mercury (Hg).4 A large portion of this Hg can be incorporated into the FGD slurry and its solid byproducts, including synthetic gypsum. The amount of Hg in FGD products may increase if these units are retrofitted or optimized for cocapture in the future.5 The U.S. Environmental Protection Agency outlines potential retrofit options for both wet and dry FGD operations to enhance the recovery of Hg and as a way to reduce capital costs.6 Many factors influence the nature of FGD gypsum and the amount of Hg it contains. The details of the FGD unit, including the nature of the limestone reactant, the amount of recycle and blow-down, and the extent of solids washing affect the product gypsum. In addition, upstream operations including the type of coal(s) fired or cofired, the use or nonuse of other pollution control devices (selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), electrostatic precipitators (ESP), etc.), and even the exact placement of the devices in the flue gas stream (e.g., hot-side vs cold-side ESP) may also © 2012 American Chemical Society

affect the Hg in the FGD product. It is beyond the scope of this paper to review all these variables and we refer the interested reader to the DOE-NETL Web site (http://www.netl.doe.gov/ technologies/coalpower/ewr/mercury/index.html) for more information. The focus of this study is the starting gypsum, as delivered to the wallboard plant. Various issues surround the use of FGD gypsum, including the potential for atmospheric and water releases of Hg during manufacturing processes, releases from the manufactured products themselves, and postdisposal mobilization from the waste wallboard or other gypsum product. The potential for release of Hg from FGD products and the potential for FGD products to retain Hg in insoluble forms are key issues relating to the utilization of coal byproducts as environmentally acceptable resources. Determining Hg’s natural mode of retention in FGD products can help address these issues. Selective (involving one extraction step) and sequential (involving multiple, consecutive extraction steps) extractions have been applied to different materials, including soils, sediments, and coal combustion byproducts to assess mercury retention or release.7−15 Gustin and Ladwig7 showed little mercury was released from FGD materials in slightly acidic water (pH = 4.9) using the synthetic precipitation leaching procedure (SPLP; EPA method 1312). A modified toxicity characteristic leaching protocol (TCLP; EPA Method 1311) was used by Taerakul et al.9 to assess Hg release from lime spray dryer (LSD) ash and its fractionated components under acidic (pH 2.88) conditions. Results of their work indicated that Hg was retained in the bulk LSD ash and the calciumenriched fraction, but minor amounts of Hg were released from the unburned carbon fraction of the LSD ash. A sequential extraction was used by Noel et al.10 to identify mercury retaining phases in fly ash. Sahuquillo et al.11 and Sanchez et al.13 used a modified sequential extraction technique originally developed by the Standard, Methods and Testing (SM&T) Received: January 6, 2012 Revised: May 12, 2012 Published: May 22, 2012 3399

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2. EXPERIMENTAL SECTION

Programme (formerly the Community Bureau of Reference (BCR)) of the European Commission to assess Hg mobility in contaminated soils. Some shortcomings associated with sequential extraction procedures for Hg and various other metals include the poor selectivity of extractants,16−18 potential alteration of the material being extracted,19 and post extraction readsorption,17,20 which can lead to an overestimation of metals associated with the residual phase.21 Sladek and Gustin8 noted that selective and sequential extractions designed to target specific Hg phases should be used with caution and that these methods might overestimate mercury released from mine wastes. Additionally, the presence of organic matter greatly influenced the results. Issaro et al.22 provide a thorough discussion of the pitfalls of both selective and sequential extractions for mercury fractionation and highlight the need to develop specific protocols to account for the chemistry of Hg as well the properties of the soil or sediment to which the Hg is bound. In the particular case of FGD gypsum, some of the perceived shortcomings might be considered less important than they are for natural materials. The FGD product is formed in a well-agitated slurry, so there is some opportunity for Hg to equilibrate to its most-stable form. Steps in a sequential extraction experiment are typically “user defined”, that is, the putative targets of the extraction steps are the results of experimental interpretation rather than established reagent−target associations. Conclusions drawn from sequential extraction data should rely on analytical results rather than published or predetermined relationships. Although some researchers criticize sequential extraction procedures,17,23 such experiments can be used to provide important information on trace element associations by defining geochemical reservoirs, such as “exchangeable” or “iron (Fe) oxide bound”.20 A previous study by Kairies et al.24 highlighted several investigations designed to evaluate mercury in FGD materials (including leaching using a continuous, stirred-tank extractor (CSTX)). They postulated that Hg in FGD materials and wallboard made from FGD gypsum is held within an Fecontaining phase, such as Fe-coated clay minerals or Fe oxide/ hydroxide particles, probably introduced with the lime or limestone reagent used in the capture of SO2 in the FGD scrubber. Because of these results, an extraction procedure based on Kairies,25 itself a modification of a procedure developed by the SM&T Programme (Formerly the BCR) of the European Commission,26 was used. This method was selected over other sequential extraction procedures used to study Hg fractionation12−15 because it specifically targets Fe oxides as possible phases for Hg retention. The objective of this study was to identify the natural mode of Hg retention in several FGD products. Through a sequential extraction scheme, FGD materials were subjected to a series of dissolution reagents employed to target selected mineral phases. The amount of Hg extracted by each reagent could then be associated chemically with the mineral phases targeted by that reagent. In this manner, the mineral phases with the greatest affinity for Hg and mineral forms in which Hg was naturally immobilized were identified. Although it is known that loss to the gas phase and readsorption can occur, our intent was not to confirm or deny these findings, but to explore nature of the Hg retention.

2.1. Samples. Eight FGD gypsums and five associated FGD wallboards were obtained from commercial wallboard manufacturers

Table 1. Extraction Samples CaSO4 purity

sample

name

G4A G4B G5A G5E G6A G6E

28B Gypsum 29F Gypsum 33A Gypsum 33C Gypsum 28H Gypsum 33E Gypsum (w/TMT Additive) 16-C Gypsum B2 Gypsum CSTX 015 OFS residue CSTX 145 gypsum residue CSTX 043 gypsum residue 33B Wallboard (source: 33A Gypsum) 33D Wallboard (source: 33C Gypsum) 28G Wallboard (source: 28H Gypsum) 33F Wallboard (source: 33E Gypsum) 16-A Wallboard (source: 16-C Gypsum)

G7B G7E R2A R2B R7F W5B W5F W6B W6F W7A

Hg (μg/g)

wallboard Hg/ gypsum Hg

± 0.004

84.67 77.74 107.81 107.46 86.10 102.39

1.227 0.467 0.226 0.177 0.182 0.113

93.73 90.52 n/a n/a

0.134 ± 0.007 0.191 ± 0.010 34.169 ± 2.162 5.556 ± 0.408

n/a

5.833 ± 0.342

86.72

0.140 ± 0.004

0.622

92.78

0.110 ± 0.006

0.622

86.66

0.056 ± 0.005

0.305

88.88

0.073 ± 0.002

0.649

90.54

0.175 ± 0.006

1.309

± ± ± ±

0.013 0.005 0.006 0.012

and used without additional processing such as washing (Table 1). Paper backing was removed from wallboard samples before crushing. Three concentrated residues were also obtained from continuous, stirred-tank extractor (CSTX) leaching experiments24 that produced a small amount of solid residue enriched in metals. Two were residues of FGD gypsum; the third was a residue of a material referred to by some in the industry as OFS (orange fluffy stuff), a fine, Fe-rich slurry collected as a separate waste stream during the operation of some commercial FGD units. 2.2. Reagents, Materials, and Analytical Methods. Milli-Q water (MQW, 18 MΩ·cm) was used for the water-rinse steps of the procedure. All solutions were prepared with Ultrapure reagents and MQW. Extractions were carried out in 250-mL Teflon centrifuge bottles. Teflon and glassware were cleaned using a Milestone TraceCLEAN Acid Reflux Cleaning System. Oxidation−reduction potential (ORP) and pH data were collected using a Mettler Toledo InLab Redox Combination Electrode and an InLab490 pH Combination Electrode connected to a Mettler Toledo Seven Multi unit. Mercury analyses were performed using a Milestone DMA-80 analyzer for solids and cold vapor atomic fluorescence (CVAF) spectroscopy for solutions. Extraction solutions containing 30% H2O2 were analyzed by ICP-MS for Hg. Concentrations of other elements in the solids were determined using EPA Method 3052.27 The resulting solutions, as well as those obtained from the extraction procedure, were analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES) or inductively coupled plasma-mass spectrometry (ICP-MS). QA/QC included analysis of standard reference materials (National Institute of Standards and Technology (NIST) 1633b coal fly ash and NIST 1641d Mercury in water), method blanks, sample dilutions, and sample spikes as outlined in EPA method 1631, Revision B28 and in EPA methods 6010 and 602029. Values for all QA/QC samples were within the specified limits. 2.3. Sequential Extraction Procedure. The steps of the sequential extraction and their associated targeted phases are 3400

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presented in Table 2. They include the following: Step 1 - MQW rinse that targeted the water-soluble and loosely sorbed fraction; Step 2 -

sample and control bottles, which were then tumbled for 15 min, centrifuged, and the liquid was removed. The water washes were retained for analysis. Step 5 consisted of a hydrogen peroxide digestion followed by an ammonium acetate extraction. In the geochemical literature, these are often combined because the H2O2 is heated to boiling and little if any liquid remains at the end of the digestion. In this application however, the digestion and extraction were separated in an effort to control Hg losses. For the H2O2 digestion, 40 mL of 30% H2O2 was added to the remaining solid. Control and blank bottles were also prepared. Because of the pressure exerted by the CO2 generated during this step, the bottles were capped and briefly shaken manually. The bottles were then opened, covered with watch glasses, and allowed to sit at room temperature with occasional manual shaking. After 1 h, the uncapped bottles were placed in a bath at 80−85 °C for an additional hour. The bottles were then removed from the bath, capped, allowed to cool, centrifuged, and the liquid was removed as above. A second 40-mL portion of 30% H2O2 was then added to the remaining solid and controls and the sample was treated as in the first extraction. Another MQW blank was also prepared. No preservative was added to vials containing H2O2. In the final extraction of Step 5, 200 mL of 1 M ammonium acetate, which had been adjusted to pH 2 with HNO3, was added to the remaining solid. Control and blank bottles were also prepared. Bottles were tumbled for 16 h, centrifuged, and the liquid was removed as described in Step 1 above. An 80-mL MQW rinse was added to the sample and control bottles, which were then tumbled for 15 min, centrifuged, and the liquid was removed. Water washes were retained for analysis. After the final wash, the remaining solids were dried in their uncapped bottles in a desiccator at room temperature. After drying, the solid residue was removed from the bottle and retained for analysis. Because it was suspected that Hg was bound in the fine solid material, care was taken to obtain aliquots free of any suspended material during the extractions. Retrieving a sample free of solid material, however, proved to be the most challenging part of the experiment. The fine nature of the solids meant they were not always easily removed from suspension and that they were susceptible to being resuspended if the smallest turbulence was introduced into the bottle during pipetting. Occasionally, collecting fine material along with the supernatant was unavoidable. When this occurred, it was noted for later comparison with Hg analysis. During the course of the experiments, the number of repetitions of an extraction step, usually the step used to dissolve iron oxides and hydroxides, was adjusted based on observations from previous runs and the amount of sample available.

Table 2. Steps of the Sequential Extraction Procedure and Their Associated Target Phases step

reagent

1

MQW

2

0.11 M acetic acid

3

0.1 M hydroxylamine hydrochloride

4 5A

0.25 M hydroxylamine hydrochloride in 0.25 M HCl hydrogen peroxide

5B

hydrogen peroxide

5C

0.1 M ammonium acetate

target water-soluble and loosely sorbed ions carbonate minerals and exchangeable ions manganese oxides and hydroxides iron oxides and hydroxides organic matter and sulfide minerals organic matter and sulfide minerals organic matter and sulfide minerals

0.11 M acetic acid that targeted the carbonate and ion-exchangeable fraction; Step 3 - 0.1 M hydroxylamine hydrochloride that targeted manganese oxides and hydroxides; Step 4 - 0.25 M hydroxylamine hydrochloride in 0.25 M HCl that targeted iron oxides and hydroxides; and Step 5 - hydrogen peroxide and 0.1 M ammonium acetate that targeted organic matter and sulfide minerals. Between extraction steps, MQW was used to rinse the previous reagent from the remaining solids. Water washes were retained for analysis. For each step, control and blank samples were prepared using only the reagent or water without the addition of any solid. Extraction Steps 4 and 5, which targeted iron oxides and hydroxides and organic matter and sulfide minerals, respectively, were conducted at 80−85 °C in a Thermo Scientific reciprocal shaking water bath. All other extractions were conducted at room temperature in a Glas-Col variable speed Rugged Rotator. In the first step of the extraction sequence, 160 mL of MQW was added to 4 g of dry material in a 200-mL Teflon centrifuge bottle. The slurry was mixed end-over-end using the Glas-Col Rotator for 15 min. After mixing, the sample was centrifuged until all visible solids settled from suspension and the supernatant was removed using a pipet. A 35mL aliquot for Hg analysis was placed in a clean 40-mL glass vial and preserved using 0.3 mL of BrCl;30 the remainder was placed in a clean 250-mL HDPE bottle and preserved with 0.5 mL of concentrated trace metal grade HCl prior to ICP analysis. Laboratory blanks were prepared by tumbling 160 mL of MQW in a 200-mL Teflon centrifuge bottle and taking aliquots for Hg and metals analyses as described for the samples above. The sequential extraction continued with the addition of 160 mL of 0.11 M acetic acid to the remaining solid in the Teflon bottle. The sample was tumbled for 16 h, centrifuged, and the liquid removed as described above. A 160-mL 0.11 M acetic acid control and a 160-mL MQW blank were treated in similar fashion. Following the removal of the liquid phase, an 80-mL MQW rinse was added to the sample and control bottles, which were then tumbled for 15 min. After tumbling, the bottles were centrifuged, and the liquid was removed and retained for analysis. In Step 3, 160 mL of 0.1 M hydroxylamine hydrochloride was added to the remaining solid in the Teflon bottle. No BrCl was added to the vials containing hydroxylamine hydrochloride because BrCl is destroyed by the reagent. An 80-mL MQW rinse was added to the sample and control bottles, which were then tumbled for 15 min, centrifuged, and the liquid was removed. In Step 4, 160 mL of 0.25 M hydroxylamine hydrochloride in 0.25 M HCl was added to the remaining solid. Control and blank bottles were also prepared. Bottles were placed in a shaker bath at 80−85 °C for 16 h. After shaking, the bottles were centrifuged and the liquid was removed as above. Again, no BrCl was added to vials containing hydroxylamine hydrochloride. An 80-mL MQW rinse was added to the

3. RESULTS 3.1. Sample Characterization. Mercury content of the gypsum samples examined ranged from 0.113 to 1.23 ppm (Table 1). The sample with the lowest Hg concentration came from a power plant that used Degussa TMT in an attempt to remove mercury from the power plant flue gas.31 Wallboard samples contained lower concentrations of Hg, ranging from 0.056 ppm to 0.175 ppm. CSTX residue samples (vide supra) were substantially enriched in Hg, with concentrations ranging from 5.56 to 34.2 ppm. Comparison of initial Hg concentrations in paired gypsum and wallboard samples showed that gypsum typically loses Hg during the wallboard manufacturing process. Four of five gypsum/wallboard pairs showed Hg losses, with Hg concentrations in wallboard ranging from 30.5 to 64.9% of those found in the associated gypsums (Table 1). 3.2. Mercury Extraction. Extraction Step 4 (targeting the iron oxide and hydroxide phase) and Step 5 (targeting the organic matter/sulfide phase) released material that contained 3401

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Very little, if any, Hg was associated with material extracted during Step 4A. The second extraction performed on sample R2B utilized approximately 1/3 the solid sample amount used in the first extraction. Increasing the liquid/solid ratio in this way changed the solubilization pattern significantly, with significant amounts of Hg recovered in Steps 4A and 5A. The majority of Hg recovered from gypsum residue sample R7F came from Step 5, with Steps 4A and 4B solubilizing much smaller percentages of Hg than in other residue samples. 3.3. Mercury Associations. The elemental composition of the Hg-associated phases of the gypsum/wallboard pairs was investigated by examining solutions from the extraction steps that produced significant Hg recoveries (Figure 2). These were compared to determine if a common set of elements was solubilized along with Hg. Results from wallboard samples typically mirrored their parent gypsums with minor variations in both composition and proportions of elements. Leachates from Step 4 were typically greater than 99% Ca and S by weight. The remainder often contained iron (Fe), aluminum (Al), potassium (K), magnesium (Mg), silicon (Si), strontium (Sr), and phosphorus (P) in varying amounts ≥1% of the non-calcium, sulfur component (Figure 2A). Iron was the major component of this fraction in each sample. Leachates from Step 5 showed a similar range of constituent elements (Figure 2B). Sulfur was the major component in six of eight samples and the second most abundant element in the remaining two. Iron was present in each sample and was often the second most abundant element. Potassium and Al were present in most samples in varying quantities. 3.4. pH and Eh. pH and ORP measurements were recorded for all samples during Steps 1, 2, 3, 4, and 5C. ORP and pH were not measured during extraction steps involving 30% H2O2 because of the potential for damage to the probes. ORP data were converted to Eh with reference to the standard hydrogen electrode (SHE) using the formula Eh = ORP + 207 mV. Plots of Hg recovery as a function of pH and Eh (Figure 3) indicate that major (>20% of original mass) Hg solubilization occurred in very narrow ranges (pH ≤ 1.0 and 500 mV ≤ Eh ≤ 600 mV).

significant amounts of Hg (measured as percent of the total mass of Hg recovered) in the gypsum and wallboard samples (Figure 1A). The largest amount of Hg was typically observed

Figure 1. Distribution of recovered Hg in sequential extraction steps. (A) Gypsum (G) and associated wallboard (W) samples. Repetitions of specific extraction steps, when performed, are combined. (B) CSTX residue samples. Repetitions of specific extraction steps, when performed, are displayed separately.

during Step 4, but for several samples, greater amounts of Hg were solubilized in Step 5. Amounts >10% of the total Hg recovered were observed twice during Step 1 (MQW rinse). These results coincided with problems obtaining aliquots free of suspended material and were determined to be caused by contaminated samples. Wallboard samples mimicked the Hg solubilization pattern seen in the parent gypsum, with the largest amounts of Hg solubilized in Steps 4 and 5. Lower, though still significant, percentages of the total Hg were recovered from Step 4, with higher amounts typically recovered from Step 5. Mercury recovery was also significant for the three residue samples during Steps 4 and 5 (Figure 1B). Because these residues were concentrated with respect to suspected Hgcontaining materials, their extraction was expected to provide more details about the conditions needed for Hg solubilization. When feasible, these samples were subjected to the extraction process several times or run through extraction steps repeatedly. Both runs of OFS residue sample R2A and the first run of gypsum residue sample R2B exhibited similar Hg solubilization patterns. A majority of the Hg in each sample was solubilized during Step 4B, with smaller amounts solubilized during Step 5.

4. DISCUSSION 4.1. Gypsum and Wallboard. Patterns of Hg solubilization in gypsum samples suggest that most Hg is associated with the iron oxide and hydroxide phase, with the majority of the remaining Hg associated with the organic matter/sulfide phase. A small amount of Hg appears to be distributed among the other phases targeted by sequential extraction. Because little Hg remains in the final residue, we can infer that the remaining solid is not an effective sorbent of Hg. The extraction profiles of gypsums and their associated wallboards (Figure 1A) suggest a trend that may be linked to the initial sample concentrations of Hg shown in Table 1. The proportion of total recovered Hg for gypsum samples occurring in Step 4 is larger than the proportion of total recovered Hg for wallboard samples occurring in Step 4, while the opposite is true in Step 5. This suggests that the Hg loss during wallboard manufacturing occurs preferentially from the more easily extracted fractions. Trace elements present in both gypsum and wallboard appear to be the binding agents for the vast majority of Hg. Extracts saturated in Ca and S typically contained negligible amounts of Hg unless Fe-bearing phases were also attacked. 3402

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Figure 2. Composition of leachates recovered from sequential extraction steps associated with higher Hg recoveries. (A) Composition of leachates recovered from Step 4A. Note that these leachates typically contained >99% Ca and S. Other elements are shown as percent of the non-Ca,S fraction ≥1%. (B) Composition of leachates recovered from Step 5 ≥1%. In all cases, Hg was recovered in trace amounts compared to other elements.

Figure 3. pH and Eh of sequential extraction leachates and associated Hg recoveries.

Kairies et al.24 confirms the presence of phyllosilicate minerals (illite and kaolinite) in postleaching gypsum and wallboard residues. These minerals are common minor components of limestone32 and could easily be introduced into the FGD scrubbing process in the limestone slurry. Step 4 appeared to solubilize iron oxides, hydroxides, and phyllosilicate minerals while Step 5 appeared to solubilize iron sulfides.

Recurring groups of trace elements observed in leachates retrieved during major Hg-releasing steps suggest possible mineralogies of Hg-binding materials. The frequent appearance of Fe, Al, K, Mg, Si, and S suggests that iron oxides and hydroxides, phyllosilicate minerals (such as illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10 ((OH)2,(H2O))), glauconite ((K, Na)(Fe3+,Al,Mg,Fe2+)2(Si, Al)4O10(OH)2)), and iron sulfides are responsible for most Hg adsorption. X-ray diffraction analysis in 3403

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4.4. Comparison with Other Investigations. A comparison of these results with other investigations indicates that Hg shows a strong tendency to immobilize in FGD gypsum and its manufactured products, although the speciation of the Hg is unclear. Alvarez-Ayuso et al. found the Hg leached from FGD gypsum produced in Spain to be below detection limits.43 In a comparison of the leaching profiles of Hg-amended calcium sulfate, laboratory-prepared FGD and site-collected FGD samples, Meischen observed that Hg-amended calcium sulfate cannot be used as a surrogate for FGD material.44 Nearly all the Hg was leached from the Hg-amended samples; a result dissimilar to the lab-prepared or site-collected FGD samples. Using thermal desorption, different Hg species were reported to be present in gypsum samples from two Spanish power stations.45 Mercury−sulfur compounds (HgS, HgSO4) were thought to be the most probable Hg species in one case, whereas Hg halogenated compounds were thought to be the main compounds in the other. HgS would report to the Step 5 material in the work above. Lee et al.46 also suggested mercury chlorides as possible constituents of FGD products from Pennsylvania power plants based on thermal desorption. It is unlikely that the Hg in the FGD materials examined here was present as the chloride because only negligible amounts of Hg were found in the aqueous washes. Kim et al. have reported the sorption of Hg on goethite, αand γ- aluminas, and bayerite proceeded to form inner-sphere complexes.47,48 Little dependence of sorption on pH was observed in the 4 < pH < 8 range.47 The presence of chloride resulted in reduced uptake of Hg(II) on all three substrates whereas the presence of sulfate caused enhanced Hg(II) uptake.48 These results are consistent with our interpretation of the extraction data. The high concentrations of sulfate in the original FGD unit and in the subsequent gypsum processing would favor the formation of the inner-sphere complex. Blythe et al. reported a relationship between Eh and Hg solubilization similar to that observed in this study.49 Their data showed a strong correlation between ORP (Eh) and phase partitioning in absorber slurry samples, with lower ORP promoting greater partitioning of Hg to the solid phase and higher ORP promoting partitioning to the liquor. These results are consistent with our observations of Hg retention in solids at lower Eh and solubilization at higher Eh.

Mercury has a high affinity for each of these materials. The high cation exchange capacity of clays makes them good candidates for the sorption of metals, and the surfaces of iron oxides and hydroxides are the sites of many sorption reactions.33−36 Glauconite has been used as an effective filter to remove Hg from contaminated groundwater37 and illite and iron sulfides are demonstrated adsorbents of Hg.38−41 4.2. Residues. Extraction of concentrated residues produced results similar to those of gypsum and wallboard experiments, but provided some interesting data regarding the mechanism of Hg recovery. Because of the lower reagent-tosolid ratio present during some of the extractions of the enriched residues, the target pH and Eh conditions were not realized during Step 4A and the dissolution was incomplete. No mercury was released under these milder conditions. The additional reagent used in Step 4B was sufficient to attain the final pH (0.9 ± 0.1) and Eh (535 ± 15) values and only then did Hg appear in the extract. This behavior suggests several possible chemistries. The first involves the presence of two Fe oxide and/or hydroxide phases in the samples: one resistant component, responsible for most Hg binding, and one less resistant component, binding little or no Hg. In this scenario, the less resistant component, easily attacked by the leaching agent during Step 4A, would preferentially dissolve first. With this component removed, the leaching agent would then attack the resistant component during Step 4B, also solubilizing the associated Hg. This sequence would not be apparent during the gypsum and wallboard experiments because the lower concentration of the targeted phase would allow for both weak and resistant components to be attacked during the same extraction step, allowing the Hg bound in the phase to be easily mobilized. The second scenario involves readsorption of Hg. Mercury liberated during the extraction quickly bound to unfilled or newly formed sorption sites in the targeted phase or to sites in a different phase. In this manner, Hg released during Step 4A was sorbed to open sites in the targeted phase, only appearing in the leachate after most (or all) sites for adsorption had been attacked. The extracting agent itself could have been responsible for creating new sorption sites, as has been demonstrated during dissolution of glauconite in acidic solutions.42 It is also conceivable that in the same manner, Hg liberated by Steps 1, 2, and 3 could migrate from one phase to another and readsorb during the extraction. Procedures using spiked samples to investigate the readsorption of Hg during sequential extraction21 could be applied to these materials to provide a more detailed picture of their behavior. 4.3. Hg Immobilization. Regardless of the behavior of Hg among the various phases, the conditions required for release into an aqueous environment suggest that Hg is immobilized in FGD products, including residues of those products with high metals concentrations. Mercury release into solution occurred only under very low pH and/or oxidizing conditions at 80−85 °C. Plotting Hg solubilization as a function of both pH and Eh provides a three-dimensional view of the conditions under which large amounts of Hg (>20%) are recovered (Figure 3). This space is extremely limited for the materials tested. Even if Hg is mobilized during earlier extraction steps, results suggest that it likely readsorbs “up the line” onto more resistant fractions or escapes to the gas phase. The limited recovery conditions indicate that the majority of Hg in FGD products remains bound under typical environmental conditions.

5. CONCLUSIONS Evidence from sequential extractions suggests that several Febearing mineral phases are involved in the immobilization of Hg in FGD gypsums destined for manufactured products. The presence of iron oxides and hydroxides, phyllosilicate minerals, and iron sulfides was suggested by the pattern of Hg solubilization from FGD materials and analysis of Hg-rich solids retrieved during the sequential extraction. These minerals were most likely introduced as minor components of the limestone utilized in the scrubbing process. Although FGD gypsum appeared to have lost Hg during the wallboard manufacturing process, the associations of Hg in the material did not appear to be altered. Residues of FGD materials with high metals content provided evidence of either Hg readsorption during the extraction process or the presence of two or more components within the phase containing the majority of Hg. Further work 3404

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with these materials is needed to accurately characterize the behavior of Hg during the sequential extraction process. The conditions under which Hg was solubilized suggests that its natural mode of retention in FGD products is stable under most environmental conditions. Mobilization of Hg in solution occurred only when conditions of low pH, an oxidizing environment, and high temperature were combined.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Current address: Winona State University, Department of Geoscience, P.O. Box 5838, Winona, MN 55987. Notes

Disclosure. The mention of specific product names is to facilitate understanding and does not imply an endorsement by the U.S. government. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. DOE Fossil Energy Postgraduate Research Participation Program at the NETL administered by the Oak Ridge Institute for Science and Education. We thank the three anonymous reviewers for their efforts and insightful comments.

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Proceedings of the International Conference on Air Quality VIII; UND EERC, Arlington, VA, Oct 24−27, 2011.

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dx.doi.org/10.1021/ef300033u | Energy Fuels 2012, 26, 3399−3406