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

Abstract

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 (CH3Hg) 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-SRED), 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 LIII-edge EXAFS analysis were determined on samples added CH3Hg to yield CH3Hg/Org-SRED 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 CH3Hg/Org-SRED 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 CH3Hg, 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.