Mercury Isotopic Evidence for Multiple Mercury Sources in Coal from the Illinois Basin

Coal combustion is the largest source of anthropogenic mercury (Hg) emissions to the atmosphere and, thus, has vast environmental implications. Recent developments in Hg stable isotope geochemistry offer a new tool for tracing sources and chemical transformations of anthropogenic Hg in the environment. We present here the first isotopic study of mercury in organic and inorganic constituents of four Pennsylvanian-age coal seams in the Illinois Basin, one of the main coal-producing areas in the USA. We report mass dependent isotopic variations relative to the NIST 3133 standard as δ²⁰²Hg and mass independent fractionation as Δ¹⁹⁹Hg and Δ²⁰¹Hg values. The data for Illinois coals show a wide range of δ²⁰²Hg (−0.75 to −2.68‰), Δ²⁰¹Hg (0.04 to −0.22‰), and Δ¹⁹⁹Hg (0.02 to −0.23‰). In contrast, vein pyrite from two coal seams is isotopically unfractionated relative to NIST 3133. Collectively, these data suggest that isotopically distinct Hg sources contributed to the organic and inorganic fractions of Illinois coals. The Δ²⁰¹Hg/ Δ¹⁹⁹Hg ratio of Illinois coals is 1:1, consistent with isotopic fractionation by photochemical reduction of Hg²⁺ prior to deposition in coal-forming environments. The isotopic composition of Hg in pyrite is more likely derived from hydrothermal fluids that precipitated reduced sulfur in Illinois coal seams. These results demonstrate, for the first time, the potential of Hg isotopes to discriminate between syngenetic (depositional) and epigenetic (hydrothermal) sources of Hg in coal. Our findings may be useful in distinguishing among various geological processes controlling the distribution of Hg in coal and monitoring the fractions of Hg in emissions associated with organic versus inorganic components of coal.