X-ray absorption studies of CH 3Hg +-binding sites in humic substances

Abstract

Methylmercury cation (CH 3Hg +) is known to have a strong affinity for organic matter in soil, sedimentary, and aquatic environments. The objective of this study is to determine the dominant ligands binding CH 3Hg + in humic acids by evaluating several CH 3Hg +-ligand complexation models, using mercury L III-edge extended X-ray absorption fine structure (EXAFS). The model CH 3Hg + binding ligands examined in this study include thiol (-SH), hydrogen polysulfide (-SSH or -SSSH), sulfide (-S-), disulfide (-SS-), carboxyl (-COOH), and amine (-NH 2). Based on the atomic geometry around mercury in each model structure, we distinguished CH 3Hg +-binding ligands in two different humic acids (soil and aquatic). We observed CH 3Hg + preferentially binds to thiol ligands. After saturating reactive thiol ligands, the majority of CH 3Hg + binds to carboxyl ligands rather than to amine or other reduced sulfur ligands than thiol. We found no evidence of significant CH 3Hg +-sulfide/disulfide or CH 3Hg +-hydrogen polysulfide complexes in any samples. When CH 3Hg + binds to a humic ligand, the C-Hg bond in CH 3Hg + remains intact. Some heavy atoms are proximately coordinated around the mercury atom in the sample containing the highest CH 3Hg + levels used in this study.