Energy & Fuels 2008, 22, 3949–3954
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Distribution of Mercury in a Gulf Coast Lignite Mine Jason C. Paul, Paul B. Steinbach, and David R. Ownby* DiVision of EnVironmental Science, Stephen F. Austin State UniVersity, Post Office Box 13073, Nacogdoches, Texas 75962 ReceiVed April 27, 2008. ReVised Manuscript ReceiVed September 4, 2008
Evaluation of mercury distribution throughout a coal seam allows for evaluation of alternatives to reduce mercury emissions from coal-burning electric utilities, such as selective mining. Mercury concentrations in coal samples from 31 cores of a 3-10 ft thick lignite seam in a 475 acre study area within Rusk County, TX, were measured. Detailed samples (0.1 ft sections) were collected from three of the cores, and 6-12 samples (including samples of over- and underburden) were collected from each of the other cores. ASTM method 6414-01 showed erroneously high mercury concentrations for three low-rank coal standards (NIST SRM 2682b, EPRI ES-5, and CANSPEX2003-1) using the Perkin-Elmer AAnalyst FIAS system. Accordingly, a modified assay method to measure the mercury concentration in Texas lignite was developed that recovered 97-108% of the mercury in the low-rank coal standards. Mercury results for the three detailed cores showed substantial vertical variation, with adjacent sections often differing by over 200 µg of Hg/kg of dry coal. Pyritic sulfur and total organic carbon were positively correlated with the mercury concentration. Mercury concentrations did not correlate with vertical or lateral position within the lignite seam within the study area. Consequently, selective mining to produce a lower Hg product is likely not feasible in this area.
Introduction Although there are many natural sources of mercury, mercury concentrations have increased dramatically since the beginning of the industrial era. The U.S. Environmental Protection Agency (USEPA) estimates that 50-57% of the total yearly atmospheric input of mercury is due to human activities, with combustion being the largest of these sources (87%).1 To comply with likely USEPA regulations on coal-burning power plants, utilities are actively working to identify viable methods to reduce their mercury emissions. Achieving these mercury reductions will require coal-fired utilities to adopt strategies for compliance that include installation of new or modification of existing emissions control equipment, selective mining, coal washing, or switching fuel sources. To understand appropriate control technologies for a coal source, an understanding of coal formation and composition is needed. As coal undergoes increased degrees of metamorphism, the concentration of carbon increases as does the energy output. Generally, lignites such as those found in the Gulf Coast province of the U.S. contain higher mercury and provide less energy content than the higher ranked bituminous and subbituminous coals found in other U.S. regions.2,3 This partially explains why controlling mercury emissions from Gulf Coast lignite may be more costly than that of higher ranked coals. This study examines the distribution of mercury in relation to the mineralogy and stratification of sublayers within a lignite * To whom correspondence should be addressed: Department of Chemistry, Towson University, 8000 York Road, Towson, MD 21252-0001. E-mail:
[email protected]. (1) U.S. Environmental Protection Agency (USEPA). Mercury study report to Congress, EPA-452/R-97-003, 1997. (2) Bragg, L. J.; Oman, J. K.; Tewalt, S. J.; Oman, C. L.; Rega, N. H.; Washington, P. M.; Finkelman, R. B. The U.S. Geological Survey Coal Quality (COALQUAL) Database, Version 2.0. U.S. Geological Survey Open-File Report 97-134, 1998. (3) Toole-O’Neil, B.; Tewalt, S. J.; Finkelman, R. B.; Akers, D. J. Fuel 1999, 78, 47–54.
seam in the Oak Hill mining area, Rusk County, TX. The reduction of mercury emissions from the combustion of lowrank coals, such as Gulf Coast lignite, will likely prove difficult and costly. Gaseous mercury in the flue gas of power plants that burn low-rank coal is usually enriched in elemental Hg rather than the more readily captured oxidized mercury.4 Because lignite is the most prevalent form of coal found in the U.S. Gulf Coast region, the following objectives were addressed to evaluate the viability of removing mercury from lignite through selective mining prior to combustion: (1) Evaluate mercury contents as a function of depth to predict mercury distribution in the lignite seam based on core data. (2) Compare mercury concentrations with the mineral composition of the lignite seam to determine if mercury concentrations are a function of specific stratigraphic/mineral sublayers that are visually identifiable. (3) Develop geostatistical procedures to evaluate the spatial distribution of mercury across the proposed mining areas and incorporate mercury distribution data into geographic information systems (GIS). Experimental Section Site Background. A single coal seam in a 475 acre parcel (D1 study area) within the Oak Hill mining area in Rusk County, TX, was evaluated. The Oak Hill mining area (9780 acres total) lays on the West Gulf Coast Plain section of the Gulf Coastal Plain physiographic province and has a surface topography composed of gently sloping hills entrenched by dendritic drainage systems. A majority of this mining area is composed of the Wilcox Group, where minable lignite is located within a shallow groundwater system at depths varying from 10 to 150 ft below ground.5 The (4) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Fuel Process. Technol. 2003, 82, 89– 165. (5) U.S. Environmental Protection Agency (USEPA). Environmental impact statement: Martin lake D area lignite surface mine Henderson, Rusk County, Texas, EPA 906/9-83-003, 1983.
10.1021/ef800291b CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
3950 Energy & Fuels, Vol. 22, No. 6, 2008 thickness of the lignite ranges from approximately 3 to 10 ft based on electronic geophysical logs obtained during the study. Sampling Strategy. There were two sampling methods incorporated within this study to acquire an understanding of both the horizontal and vertical distribution of mercury for evaluating future potential coal treatment processes. The first, collectively called “regular core sampling” was conducted to determine if there were locations within the coal seam that could be readily identifiable as areas typically containing relatively higher concentrations of mercury for exclusion during the mining process. This sampling method involved the extraction of 28 cores that were sectioned into a total of 415 analytical samples. These samples were divided according to the unique lithotypes and mineral impurities macroscopically identified within the coal seam. In addition, the top 12 in. (30.50 cm) of each seam was compositely sampled every 6 in. (15.24 cm) for comparison to sulfur data previously acquired by TXU Energy. When recovery was possible, 1 ft of overburden and 1 ft of underburden of the lignite seam were homogenized separately and analyzed for mercury. The second sampling method was “detailed core sampling”. Using this sampling method, three fully intact cores were selected for sectioning every 0.10 ft (3.05 cm). This type of sampling was intended to provide a detailed analysis of Hg stratification in relation to depth and to reflect minute variations within the coal seam because of mineralogy. Sampling the coal seam at this vertical scale was performed to more precisely identify zones where relatively high Hg coal might be encountered and avoided during mining. All cores for both sampling methods were collected within the D1 study area within the Oak Hill Mine. By comparing the results from these two types of sampling approaches, it was possible to determine whether location (depth and/or horizontal position) or lignite mineralogy play a greater role in mercury distribution within the investigated seam. Longitude, latitude, elevation (depth), and site number were recorded in Core Lithology Logs for all samples and located using a Trimble GPS unit. A Century Geophysical Logging Unit using Cs137 was used to determine density, natural gamma, and resistivity of the geological subsurface layers. Thickness and depth were determined by density because resistivity and natural gamma did not reveal the location of the lignite seam. These data, combined with the analytical data, were used to generate a map to predict the geospatial distribution of mercury within the coal seam. Lignite Sample Preparation. Samples were thoroughly prewashed with tap water in a strainer to retain fragmented portions of the coal. The outer edges of core samples were scoured with a porcelain-glazed knife during the prewash to remove any potential contamination caused by the coring barrel. The core sections went through a final rinse with Nanopure water. Detailed cores were sectioned every 0.1 ft using a hack-saw and stainless-steel cleaver. These detailed sections were then sprayed with Nanopure to remove the coal dust collected during sawing. All frozen samples were initially ground with a stainless-steel grinding mill at 2500 rpm. A 50.0 g portion of each ground sample was passed through a 250 µm Teflon sieve using an alumina oxide mortar and pestle for final grinding.6 All grinding and sieve equipment were thoroughly cleaned using standard U.S. Geological Survey (USGS) procedures.7 ASTM Method Modification. A pilot study was conducted to determine mercury recovery, analytical precision, and accuracy using ASTM method 6414-018 prior to conducting analysis on the field samples. CVAAS is a very sensitive form of analysis but is limited by spectral interferences from residual unoxidized organic (6) American Society for Testing and Materials (ASTM). ASTM Designation D 2013-01: Standard practices for preparing coal samples for analysis. ASTM International, West Conshohocken, PA, 2001. (7) U.S. Geological Survey (USGS). Cleaning of equipment for water sampling. In National Field Manual for the Collection of Water Quality Data; USGS: Reston, VA, 1998. (8) American Society for Testing and Materials (ASTM). ASTM Designation D 6414-01: Standard test methods for total mercury in coal and coal combustion residues by acid extraction or wet oxidation/cold vapor atomic absorption. ASTM International, West Conshohocken, PA, 2007.
Paul et al. bonds and chloride.9 The original recommendation of 10% HCl acid calibration standards and 75:25 HCl/HNO3 acid digestion from ASTM 6414-01 negatively skewed the slope of the linear calibration, thereby positively skewing Hg concentrations when various coal ranks (bituminous, sub-bituminous, and lignite) were analyzed using a Perkin-Elmer AAnalyst 700 coupled with the FIAS 100 flow injection autosampler. This effect may explain why 120-150% recoveries of Hg were reported in lignite when using the original ASTM 6414-01 test method. To counteract the effect of Cl-, PerkinElmer’s recommended conditions for flow injection systems (1.1% SnCl2, ACS certified for trace Hg determination, Fischer Scientific; in 3% HCl, trace metal grade, J. T. Baker) were used for analysis, which resulted in improved instrumental sensitivity. However, further modification was required to achieve optimal sensitivity. An additional modification of the digestion acid concentrations was evaluated by altering ASTM’s recommended 6 mL of HCl/2 mL of HNO3 (Instra-analyzed, J. T. Baker) stepwise to 1.5 mL of HCl/6.5 mL of HNO3 for the digestion of lignite. Mercury recovery was optimized at >98% at a ratio of 1.5 mL of HCl/5.0 mL of HNO3 based on analysis of the sub-bituminous standard reference material (SRM) 2682b. A total of 12 blind samples were then analyzed to verify the method for various coal types. The tests of these blind samples were conducted by performing four analytical trials on each blind sample for the original ASTM method and four trials upon the blind samples using the modified method. Of the 12 blind samples identified by Canada Quality Associates International, 3 were identified as either subbituminous or lignite reference materials; NIST SRM 2682b (subbituminous), EPRI ES-5 (lignite), and CANSPEX 2003-1 (lignite). These 3 were used to evaluate recoverable mercury efficiency for the modified method. Accuracy of each method was assessed by dividing the average test concentration by the certified value to determine percent mercury recovery. The modification of ASTM 6414-01 resulted in average recoveries of 97, 108, and 100% for sub-bituminous NIST SRM 2682b and the two lignite standards (EPRI ES-5 and CANSPEX 2003-1), respectively. These recoveries are within certified values developed by Canada Quality Associates International for CVAAS after comparison to baseline isotope dilution-cold-vapor-inductively coupled plasma-mass spectroscopy values. In a comment from TXU Power, an independent consultant found the precision of all sets of dry basis mercury results reported by laboratories on lignite tested on behalf of TXU Power were within the maximum expected precision for ASTM 6414.10 This is most likely attributed to the use of dedicated flow injection mercury sampler/cold-vapor atomic absorption spectroscopy (FIMS-CVAAS), which uses gold amalgamation to volatilize mercury for cold-vapor analysis. The modification made to ASTM D 6414 addressed within our research was specific to FIAS-CVAAS, in which mercury vapor is eluted using a different chemical process than FIMS-CVAAS. Digestion/Extraction Procedure. Approximately 1.000 g of sample was digested in a 36 sample Environmental Express heating block at 80 °C with 10.00 mL of Nanopure water, 1.50 mL of concentrated hydrochloric acid, and 5.00 mL of concentrated nitric acid for 1 h. The digestion vessels were removed and allowed to cool to room temperature. Each vessel was carefully depressurized and diluted with 28.0 mL of Nanopure water. Afterward, 5.00 mL of 5% potassium permanganate solution (5.0 ( 0.01 g of KMnO4 ACS certified for trace Hg determination; Fisher Scientific, Fair Lawn, NJ, and diluted to 100.00 mL with Nanopure water) was added. The mixture was shaken vigorously and allowed to stand for 10 min before 0.50 mL of 12% hydroxylamine/sodium chloride solution was added and mixed. The hydroxylamine/sodium chloride solution was prepared by dissolving 12 ( 0.01 g of hydroxylamine sulfate (Ultrex; J. T. Baker, Fair Lawn, NJ) and 12 ( 0.01 g of sodium chloride (ACS certified for trace Hg determination; Fisher Scientific, (9) Long, S. E.; Kelly, W. R. Anal. Chem. 2002, 74, 1477–1483. (10) McCall, M. Comment from TXU power to EPA Docket OAR2002-0056, in response to the proposed mercury rule 69 FR 4652, Jan 30, 2004; pp 1-37.
Mercury in a Gulf Coast Lignite Mine Phillipsburg, NJ) in Nanopure water and diluting to 100 mL. If a light purple color persisted for longer than 1 min, an additional 0.5 mL of the solution was added and mixed. Analysis was completed with the Perkin-Elmer AAnalyst 700 using a sample volume of 500 µL and analyzed at 253.7 nm, with an argon flow rate of 75 mL/min, and the slit width was set to 0.7 low. Samples were calibrated using a six-point calibration curve from 0-10 µg of Hg/L. For quality control purposes, a control sample was prepared using NIST sub-bituminous coal (SRM 2682b) certified reference material (CRM) with all analyses. Lignite reference material CANSPEX 2003-1 was also used later in conjunction with SRM 2682b during the analyses as a qualitycontrol reference material. Results obtained from the latter reference materials were considered to be more reflective of actual Hg recovery from the analytical samples because both the tested samples and CANSPEX 20003-1 were lignites. Other Coal Analyses. Subsamples of ground and sieved coal were sent to Advanced Analytical Laboratory (AAL, Tyler, TX) for analysis of pyritic sulfur and total carbon (TC). Pyritic sulfur was analyzed by ASTM D 2492 using a Spectro ICP. The TC was analyzed by a direct combustion method, ASTM D 5373, using a Perkin-Elmer 2400 series II CHNS analyzer. All Hg, pyritic sulfur, and TC values are reported on a dry basis. Statistical and Geostatistical Analyses. Spearman’s correlation was used to evaluate the relationship between Hg and core depth. Multiple regression was used to analyze the relationship between Hg, pyritic sulfur, and TC. Pyritic sulfur and TC were not measured in over- and underburden samples; therefore, these samples were removed from the statistical analysis. Geostatistical analysis was conducted upon the spatial distribution of mercury based on the analysis of the regular cores. To prepare the data for this analysis, the variable number of sections for all of the cores were consolidated into six layers using weighted averages derived from sample volume and concentration. These layers were consolidated by first grouping adjacent sections of the regular cores that had the least variance. The exception to this was that layer one always represented the overburden and layer six always represented the underburden. Therefore, if the over- and/or underburden were not recovered during the coring process, these cores were excluded from layer one and/or layer six. Each layer was analyzed separately to determine the spatial distribution of mercury. A single database was created for each layer containing the weighted concentration, mid-elevation, latitude, longitude, and core ID. A map illustrating the change in mercury concentration in relation to elevation consists of up to 28 points representative of each core that belonged to that particular layer. Inverse distance weighting (IDW) was used to predict the horizontal spatial distribution of mercury to provide an exact deterministic interpolation of the data11 that could reflect the heterogeneity of the mercury distribution in the coal seam. A second-order function was selected, and a nearest-neighbor 12-point selection was used to produce the IDW model.
Results Mercury Stratification in Oak Hill Mining Area “D1” To selectively mine areas containing relatively lower Hg concentrations, either these concentrations must correlate with depth or the mineral/maceral impurities containing high mercury concentrations found within the coal seam need to be large enough to be easily identifiable and relatively aggregated. Although there are instances of high Hg values within the top or bottom foot of the coal seam, these concentrations fluctuate from one area to the next. Mercury concentration and depth (Figures 1 and 2) had a small significant correlation (Spearman correlation coefficient ) -0.158, p ) 0.0012), such that as depth increased (11) Johnston, K.; Ver Hoef, J. M.; Krivoruchko, K.; Lucas, N. ArcGIS 9: Using ArcGIS Geostatistical Analyst; Environmental Systems Research Institute (ESRI): Redlands, CA, 2003.
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Figure 1. Scatter plot of measured dry-basis mercury concentrations by core depth for 3 detailed (every 0.1 ft increment) and 28 regular (sectioned according to mineral impurity/lithotype composition) cores from Oak Hill mining area D1, Henderson, TX. One additional point at 7.15 ft with a Hg concentration of 2079 µg/kg dry is omitted.
mercury concentrations decreased when samples from both detailed and regular cores are included. Spearman correlation was used because Hg concentrations were not normally distributed. Hg distributions along depth profiles among detailed cores (Figure 2) differ at a maximum of 2.00 mg/kg. Some core sections had Hg concentrations exceeding 1.00 mg/kg; these particular samples contained an unusually high occurrence of pyritic minerals based on visual observations during the grinding process. In the detailed cores, Hg concentrations regularly fluctuated by several hundred µg/kg, often within a 0.10 ft difference in depth. This is a significant difference in mercury concentration in relation to depth because the average Hg concentration found within the detailed lignite cores is ∼180 µg/kg. The high fluctuations of Hg concentration demonstrated within the detailed cores suggest that Hg is not related to depth. When relating mercury concentration to depth in Figures 1 and 2, measured mercury distributions vary greatly. Figure 3 is the product of the geostatistical analysis, which reveals that the mercury distribution of each layer varies significantly from the other.12 These are very general models with high error because mercury distribution is heterogeneous even at a scale of 0.1 ft as Figure 2 illustrates. Figure 2 provides a better example of the distribution of mercury within the lignite seam because it represents the vertical distribution of mercury as measured at a small scale, which magnifies the heterogeneity of mercury within the coal seam. Because the detailed cores illustrated in Figure 2 represent the variation of mercury distribution every 0.10 ft in depth, imagine what the true spatial distribution of Figure 3 would look like if the seam was measured every 0.10 ft horizontally throughout as compared to the several hundred foot spacing between sampling points. Although Figure 3 represents the correlation of mercury concentration from homogenized samples in relation to lag distance (distance between each sample point), the overall heterogeneous distribution of mercury is still apparent. Correlation of Mercury to Other Analytes. Observations of high pyrite/marcasite (represented by pyritic sulfur percentage) resulting in relatively high Hg concentrations were fairly common throughout the study (Figure 4). Mercury had a large positive significant correlation to pyritic sulfur (Spearman ) 0.645, p < 0.0001) and a small negative significant correlation to TC (Spearman ) -0.239, p < 0.0017). (12) Paul, J. C. Distribution of mercury in an east Texas lignite seam. Stephen F. Austin State University, 2005; pp 1-129.
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concentration, mercury and pyritic sulfur data were logtransformed. The best fit for a stepwise multiple linear regression was log(Hg) ) 0.377(log pyritic sulfur) + 0.207(organic sulfur) - 1.674 (r2 ) 0.52, p < 0.0001). Discussion
Figure 2. Measured dry-basis mercury concentrations for detailed cores (a) 04-23, (b) 04-40, and (c) 04-57 collected from Oak Hill mining area D1 (Henderson, TX) scaled at 0.10 ft increments. Given depths represent the midpoint of each sample.
Mercury and pyritic sulfur were square-root-transformed to normalize the data, and then pyritic sulfur and TC were fitted into a stepwise multiple regression model in relation to the Hg concentration. The initial model of Hg and pyritic sulfur was significant ((Hg) ) 5.525((pyritic sulfur)) + 7.562, r2 ) 0.40, p < 0.0001). The addition of TC did not improve the model at a level of p < 0.150. Additional unpublished data (n ) 285) was provided by TXU on core samples taken in a separate study from the Oak Hill Mine. A Spearman correlation coefficient of 0.74 between mercury and pyritic sulfur along with a 0.48 correlation between mercury and organic sulfur was calculated according to additional data provided by TXU (both p < 0.0001). This compares well to the 0.645 Spearman correlation coefficient between the Hg measurements provided by this study with pyritic sulfur as measured by AAL. To use multiple linear regression to further evaluate the combined affect of the predictor variables (pyritic sulfur and organic sulfur) on mercury
A study by Guven and Lee13 found that east Texas lignites from the Sabine Uplift were mainly comprised of the minerals quartz, bassanite (possibly resulting from ashing of gypsum), kaolinite, and halloysite. Pyrite, illite, smectite, chlorite, barite, and feldspars were found in smaller fractions of the coal but increased in concentration with decreased depth. Pyrite and bassanite (ashed gypsum) are considered to be formed in situ. Gypsum can form as a precipitate after the weathering of pyrite. These minerals were authigenically formed during diagenesis in an acidic environment within the coal seam.13 Mercury was possibly incorporated within these minerals as they formed. Pyrite/marcasite may also be pseudomorphs of plant tissue that formed syngenetically during peat deposition.14 The mineral pyrite was focused on in this study because research conducted by the USGS and other authors indicate that mercury is commonly bound to pyrite in coal.15-18 Other forms of mercury recognized by the USGS within coal include organically bound, elemental, or bound within the matrix of sulfide and selenide minerals.19 To further understand the significance of this finding, the mineralogy of pyrite is reviewed to demonstrate how the diagenetic origin of pyrite may influence mercury stratification within the studied coal seam. Pyrite may be syngenetic or epigenetic in origin. Syngenetic pyrite may be found within coal as framboids or euhedral or anhedral crystals filling in both macro- and micropore spaces or replacing macerals. Pyrite may also accumulate to form massive pyrite formations.20 Pyrite has been reported within joints, cracks, root channels, voids, and nodules of lignite. These various forms of pyrite are largely a function of contrasting environments of deposition.14 Marcasite (associated with pyrite) was identified within the studied coal seam and has an orthorhombic crystal lattice composed of FeS2. Marcasite also has radiating, isolated, or varied crystal structures that occasionally aggregate together in massive form.20 This mineral forms in environments that are more acidic than pyrite and oxidizes more readily than pyrite because of its metastable properties.14 Pyrite framboids range in diameter from 1 to 100 µm and are spherical polycrystalline aggregates of pyrite. These structures may represent pyritized fungal spores, algal cells, bacteria, or inorganic precipitates of mineral solutions. Syngenetic pyrite within coal is potentially a product of sulfide substrates that precipitated from the reduction of dissolved iron (Fe3+) with H2S (a product from bacterial reduction) in a reductive environment during the deposition of peat.20 (13) Guven, N.; Lee, L. J. Characterization of mineral matter in east Texas lignites: Final report. TENRAC/EDF-103, Texas Energy and Natural Resources Advisory Council, 1983. (14) Doolittle, J. J. Sulfide oxidation in lignite overburden as influenced by calcium carbonate. Texas A&M University, College Station, TX, 1991. (15) Diehl, S. F.; Goldhaber, M. B.; Hatch, J. R. Int. J. Coal Geol. 2004, 59, 193. (16) Goodarzi, F. Fuel 2002, 81, 1199–1213. (17) Bool, L. E.; Helble, J. J. Energy Fuels 1995, 9, 880–887. (18) White, D. M.; Edwards, L. O.; Eklund, D. A.; Dubose, D.; Skinner, F. Correlation of coal properties with environmental control technology needs for sulfur and trace elements. U.S. Environmental Protection Agency (USEPA), 1984. (19) Tewalt, S. J.; Bragg, L. J.; Finkelman, R. B. Mercury in US CoalsAbundance, distribution, and modes of occurence. U.S. Geological Survey Fact Sheet FS-095-01, 2001. (20) Ward, C. R. Int. J. Coal Geol. 2002, 50, 135–168.
Mercury in a Gulf Coast Lignite Mine
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Figure 3. Comparison of dry-basis mercury distribution in relation to changes in depth and latitude/longitude: layer a represents mercury distributions in the overburden; layers b-e are each layers from the lignite seam that increase in depth, respectively; and layer f is the underburden. Changes in mercury concentration (µg/kg) are represented by the color pattern.
In this study, the pyrites were scattered sporadically throughout the coal seam in unpredictable patterns, reflective of the mercury distribution. Although mercury was largely associated with pyritic sulfur, it appears that the mercury within the coal is binding to other substrates as well. This was noticed in parts of the core that have low concentrations of pyritic sulfur in contrast to relatively high concentrations of mercury (Figure 4). Areas of the seam containing low concentrations of pyritic sulfur in relation to mercury concentration may be in part due to oxidation of the pyrite within the seam. This form of oxidation would reduce the concentration of pyritic sulfur, although the mercury could remain in situ. For instance, pyrite may be oxidized to amakinite (Fe(OH)2) or Fe(OH)3-, in which the mercury would remain in situ but would no longer be affiliated with pyrite.21 The oxidation of pyrite can occur geochemically over an extended period of time or can occur much faster with microbes serving as a catalyst.22 Further explanation may be provided by a recent study by Diehl et al.,15 who found mercury strongly bound to pyrite. However, they also noted that mercury was bound to differing degrees depending upon the form of pyrite present (euhedral, framboidal, syngenetic, etc.). Some of the mercury appeared to be bound to the organic matrix that was adjacent to the epigenetic sulfide veins, which penetrated the coal bed. Mercury was also correlated to a lesser degree with organic sulfur, for (21) Sammut, J.; White, I.; Melvilles, M. D. Marine and Freshwater Research 1996, 47, 669–684. (22) Barret, J.; Hughes, M. N.; Karavaiko, G. I.; Spencer, P. A. Metal Extraction by Bacterial Oxidation of Minerals; Ellis Horwood: New York, 1993; p 100.
which mercury has a known affinity.15 More specifically, mercury will bind readily to thiol, as well as other forms of reduced sulfur.15,23,24 Recent research reported by Goodarzi16 stated that mercury in some Canadian feed coals was mostly associated with pyrite. However, Goodarzi also reported that other researchers have observed mercury to be primarily bound by the organic fraction of the coal, followed by pyrite and carbonate in other coals. Carbonate, although present, is not abundant in east Texas soils and would not be expected to play a major role in mercury binding, consistent with our observation that TC had little impact when predicting the Hg concentration in our stepwise multiple regression. Research reported by Goodarzi upon the Canadian feed coals supports the data provided from the Oak Hill mining area and other previously reported studies.15,23-25 Statistical analysis from the D1 mining area infers that a majority of the mercury is bound to pyrite, with the remaining bound to organic compounds, including organic sulfur. The different distribution and modes of occurrence of mercury in coal are likely a function of varied origins. Because there are many forms of pyrite that may have formed under various processes, it is reasonable to find mercury scattered heterogeneously throughout the lignite seam. Furthermore, the variable environments of deposition/precipitation of pyrite would influence its affiliation to mercury. Accumulation (23) Xia, K.; Skyllberg, U.; Bleam, W. F.; Bloom, P. R.; Nater, E. A.; Helmke, P. A. EnViron. Sci. Technol. 1999, 33, 257–261. (24) Skyllberg, U.; Xia, K.; Bloom, P. R.; Nater, E. A.; Bleam, W. F. J. EnViron. Qual. 2000, 29, 855–865. (25) Crowley, S. S.; Warwick, P. D.; Ruppert, L. F.; Pontolillo, J. Int. J. Coal Geol. 1997, 34, 327–343.
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sorption of soluble ions from adjacent water sources onto the surfaces of peat particles, and precipitation/biogeochemical cycling during the coalification process.18 Whether or not the substrate from which the pyrite formed previously was capable of bioconcentrating mercury (as may be expected in framboidal pyrite that could have formed from methylating microbes) would also determine the degree of mercury concentration within pyrite.20 The natural fluctuations occurring within each of these processes supports our prevalent correlation of mercury and pyrite but also allows for the occasional outlier where elevated mercury is not found associated with pyrite. The mercury distribution in this study is a reflection of coal heterogeneity. The distribution of Hg reflects the heterogeneous mineral/lithotype (i.e., pyrite) distribution within the seam, which was not visually aggregated in a manner that would be applicable to selective mining. Coal is heterogeneous at numerous levels. On the chemical level, it is a mixture of organic and inorganic constituents. The textures and chemical complexes within coal differ because of variable organic/inorganic inputs during the diagenesis and coalification of lignite. This would include inorganic constituents of the parent plant material, reactions from other organic sources, such as microbes,22 and inorganic components transferred into the coal bed.26 The heterogeneity of the mercury distribution appears to be a function of the minerals present during diagenesis and lignification that complexed mercury within the coal. Mercury distribution in the lignite at the Oak Hill mining area is not a function of depth. Mercury is heterogeneously distributed both horizontally and vertically throughout the coal seam. This implies that selective mining would not be a viable precombustion alternative to reducing mercury emissions.
Figure 4. Relation of dry-basis mercury (µg/kg) to pyritic sulfur (%) content in lignite coal cores: (top) 04-24, (middle) 04-26, and (bottom) 04-31, collected from the Oak Hill mining area D1 (Henderson, TX). Mercury was strongly correlated to pyritic sulfur (Spearman ) 0.645, p < 0.0001).
of mercury within pyrite could be influenced by the bioaccumulative properties of the plants during growth prior to deposition, detrital materials carried into the peat-forming environment (including volcanic sources and atmospheric input),
Acknowledgment. We thank TXU Energy and the Environmental Steering Committee for the Environmental Research Fellowship that funded this project. Lou Janke of Canada Quality Assurance International provided the blind samples for the method development stages of the research. Additionally, we thank James Hoskins, Brian Yates, Allan Pringle, Ben McNally, Carle Schroeder, Dustin Barnes, Ashley Tarkington, and Jordon Stone for laboratory assistance. The Perkin-Elmer AAnalyst 700 was obtained by a grant from the TLL Temple Foundation. Several anonymous reviewers provided important suggestions for improvements to the original manuscript. EF800291B (26) Laskowski, J. S. Coal Flotation and Fine Coal Utilization; Elsevier Ltd.: New York, 2001.
