MERCURY SPECIATION IN MINE WASTE AND LICHEN SAMPLES FROM A FORMER MINE SITE BY HERFD X-RAY ABSORPTION SPECTROSCOPY

Chemical speciation is a primary control on mercury (Hg) bioavailability, potential for biomagnification in the food chain, and transport. Knowledge of Hg speciation is critical for understanding its complex biogeochemical cycling, but Hg in environmental samples is particularly difficult for spectroscopic studies because it poses a health hazard at low bulk concentrations (10s–100s ppm). We used High Energy Resolution Fluorescence Detection (HERFD) X-ray absorption spectroscopy of Hg Lα1 emission to examine Hg speciation in mine waste samples and lichen (Evernia prunastri (L.) Ach. and others) collected from the former Sulphur Bank Mercury Mine (SBMM) at Clear Lake, California. This work is part of a larger study funded by the US Environmental Protection Agency to understand and quantify sources of Hg to the food web using geochemical, mineralogical, and isotopic characterizations of sediment, water, gas, and biota. HERFD spectra were collected on composited and size-fractioned (<63 µm) samples of ore, waste rock, and calcined tailings from SBMM with bulk Hg concentrations ~35–10,000 ppm. Lichen samples (dried and powdered) had lower bulk Hg concentrations ~2–36 ppm. Using a beam size of ~200 µm x 700 µm, spatial scans of samples showed that Hg concentrations were variable on a scale of ~10–100 µm. Areas for spectral analysis were selected to be representative of Hg concentration, although test scans demonstrated spatial heterogeneity within samples. In general, most samples of waste rock contained mixtures of cinnabar (α-HgS) and meta-cinnabar (β-HgS) with minor weathering products such as corderoite, schuetteite, and/or Hg-sulfate. Sample spectra from calcined tailings and a waste rock dam were more variable and included significant fractions of Hg bonded to organic (thiol, cysteine, or selenocysteine) ligands together with α- or β- HgS. Several samples of lichen collected from the area had spectra nearly identical to mine waste samples with lower Hg concentrations. Lichen had higher amounts of organically associated Hg, with some samples showing evidence for minor fractions of reduced or methylated Hg in addition to α- or β- HgS. These results together with broader study data suggest that biological and atmospheric cycling exert a significant control on Hg speciation and result in a more labile Hg fraction compared with recalcitrant Hg-sulfide in surface samples.