Environ. Sci. Technol. 2008, 42, 8303–8309
Natural Mercury Isotope Variation in Coal Deposits and Organic Soils A B I R B I S W A S , * ,† J O E L D . B L U M , † B R I D G E T A . B E R G Q U I S T , †,§ G E R A L D J . K E E L E R , †,‡ A N D Z H O U Q I N G X I E †,| Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, and Air Quality Laboratory, University of Michigan, 109 South Observatory, Ann Arbor, Michigan 48109
Received May 24, 2008. Revised manuscript received August 5, 2008. Accepted August 16, 2008.
There is a need to distinguish among sources of Hg to the atmosphere in order to more fully understand global Hg pollution. In this study we investigate whether coal deposits within the United States, China, and Russia-Kazakhstan, which are three of the five greatest coal-producing regions, have diagnostic Hg isotopic fingerprints that can be used to discriminate among Hg sources. We also investigate the Hg isotopic composition of modern organic soil horizons developed in areas distant from point sources of Hg in North America. Mercury stored in coal deposits displays a wide range of both mass dependent fractionation (MDF, δ202Hg) and mass independent fractionation (MIF, ∆201Hg). δ202Hg varies in coals by 3‰ and ∆201Hg varies by 0.9‰. Combining these two Hg isotope signals results in what may be a unique isotopic “fingerprint” for many coal deposits. Mass independent fractionation of mercury has been demonstrated to occur during photochemical reactions of mercury. This suggests that Hg found in most coal deposits was subjected to photochemical reduction near the Earth’s surface prior to deposition. The similarity in MDF and MIF of modern organic soils and coals from North America suggests that Hg deposition from coal may have imprinted an isotopic signature on soils. This research offers a new tool for characterizing mercury inputs from natural and anthropogenic sources to the atmosphere and provides new insights into the geochemistry of mercury in coal and soils.
Introduction Mercury (Hg) is a globally distributed toxic metal that displays complex biogeochemical cycling at the Earth’s surface, controlled largely by its speciation and redox state (1-3). Atmospheric mercury that is deposited to terrestrial and aquatic ecosystems can be converted to methylmercury (4), which is a potent neurotoxin (5) that is bioaccumulated in food webs (5, 6). Methylmercury concentrations are especially * Corresponding author now at Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, AZ, 85721; phone: (520) 626-2142; (520) 621-2672; e-mail:
[email protected]. † Department of Geological Sciences. ‡ Air Quality Laboratory. § Now at Department of Geology, University of Toronto, Toronto, ON, Canada,
[email protected]. | Now at Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, China,
[email protected]. 10.1021/es801444b CCC: $40.75
Published on Web 10/08/2008
2008 American Chemical Society
elevated in fish (7) and this has prompted fish consumption health advisories (5). An important goal of Hg pollution research is to identify and quantify Hg inputs to the environment from specific natural and anthropogenic sources and ultimately to food webs. Human activities have increased the amount of actively cycling Hg in the environment by approximately 3-fold, and while coal combustion is estimated to be one of the dominant sources, there are several other important anthropogenic (i.e., municipal waste burning) and natural (i.e., biomass burning, hydrothermal) sources (3). Identifying and quantifying the relative importance of different sources of Hg is challenging, especially since atmospheric Hg can be deposited far from its source. Previous studies of Hg cycling in the environment have relied on diagnostic suites of elements associated with natural and anthropogenic processes in order to identify specific sources (8). This study investigates the extent to which Hg stored in different coal deposits varies in its Hg isotopic composition, to assess the potential for Hg isotopic ratios to be used to trace Hg emissions from coal. The natural fractionation of stable isotopes in the environment has been well characterized for a number of light elements, including H, C, N, O, and S (9). Recent analytical advancements have allowed high-precision isotope measurements of many previously unexplored isotope systems (10) including Hg (11-16). Mercury has 7 stable isotopes (196, 198, 199, 200, 201, 202, and 204 amu) with a 4% relative mass difference, is redox-sensitive, forms covalent bonds, and has a volatile form; thus, Hg isotopic variations have the potential to aid in better understanding the sources and transformations of Hg in the environment. Massdependent fractionation (MDF) in natural hydrothermal ore samples has been observed to produce up to a 5‰ variation in δ202Hg values (11, 13-15). A few studies have recently observed mass-independent fractionation (MIF) of Hg isotopes in natural systems (11, 17, 18). MIF had previously only been observed for a few elements, most notably O and S, and the distinctive signature that results has been useful in a wide variety of fields including cosmochemistry, paleoclimatology, and biogeochemistry (19, 20).
Experimental Section All standard, reagent, and sample solutions were produced using double deionized water (DDI, 18 MΩ) and trace metal grade acids that were purged for >6 h with Hg-clean Ar to remove traces of Hg. All glassware was cleaned sequentially with 1.6 M HNO3 and 1% BrCl (21) prior to rinsing with DDI. Sample Collection and Preparation. Coal samples from formations in the United States, China, and Russia-Kazakhstan were acquired and analyzed for their Hg isotopic compositions in this study. Organic soil samples from locations across North America were collected and analyzed similarly. Details of coal and soil sample collection are described in the Supporting Information. Hg Concentration Analysis of Coal and Soil Samples. All coal and soil samples were analyzed for Hg concentrations by combustion and atomic absorption spectroscopy (wavelength ) 253.7 nm) using a Nippon Instruments MA-2000 Hg analyzer following a Hg thermal desorption technique (22, 23). The detection limit for this method was 0.25 ng Hg g-1 and the limit of quantification was 1 ng Hg g-1. Analyses of quality control standards Wako Pure Chemical Ltd. Hg standard solution (138-13661, lot YPH 8098) and NIST SRM 3133 agreed with certified values to within ( 5%. Replicates were run for 10% of the analyses and agreed to within ( 5%. VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Combustion Apparatus. We employed an offline combustion and trapping method to isolate and purify Hg from organic matrices to overcome difficulties in analytical precision and sample reproducibility encountered during previous studies of coal deposits (24, 25). Mercury stored in coal and soil samples was volatilized using a step heating combustion procedure in a custom-made linear two-stage quartz glass furnace and preconcentrated in an oxidizing solution of 1% KMnO4 and 1.8 M H2SO4 (26) prior to analysis for Hg isotopic ratios (Figure SI-S2). The apparatus and methods developed in this study were based on a previous study (16) that isolated Hg in ore samples from other elements known to cause interferences with cold vapor Hg generation (Se, Au, Ag; (16)). However, the method was modified to accommodate analysis of low Hg concentration (
