Combined XANES and EXAFS study on the bonding of methyl mercury to thiol groups in soil and aquatic organic matter

We combined x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy to determine the coordination chemistry of methyl mercury (CH 3 Hg) in natural organic matter from an organic soil (OS), potentially soluble organic matter extracted from the soil (PSOS) and in a humic stream draining the soil (SOS). The sum of concentrations of highly reduced organic S groups (Org-S RED ), such as thiol (RSH), sulfide (RSR) and disulphide (RSSR) was 39, 42 and 47% of total S in SOS, PSOS and OS, respectively. No traces of inorganic sulfides were detected. Hg L III -edge EXAFS analysis were determined on samples added CH 3 Hg to yield CH 3 Hg/Org-S RED ratios in the range 0.01-1.62. At low ratios Hg was associated to one C atom (the methyl-group) at an average distance of 2.03 ± 0.02 Å and to one S atom at an average distance of 2.34 ± 0.03 Å in the first coordination shell. At higher CH 3 Hg/Org-S RED ratios RSH groups were saturated and O and/or N atoms gradually took part in the bonding. Based on the assumption that RSH is the only S group accounting for the very strong complexation of CH 3 Hg, approximately 17% of total organic S consisted of RSH groups in the organic soil, 14% in potentially soluble organic matter extracted from soil and 9% in aquatic organic matter.