X-ray absorption studies of CH₃Hg⁺-binding sites in humic substances

Methylmercury cation (CH₃Hg⁺) 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 CH₃Hg⁺ in humic acids by evaluating several CH₃Hg⁺-ligand complexation models, using mercury L_III-edge extended X-ray absorption fine structure (EXAFS). The model CH₃Hg⁺ binding ligands examined in this study include thiol (-SH), hydrogen polysulfide (-SSH or -SSSH), sulfide (-S-), disulfide (-SS-), carboxyl (-COOH), and amine (-NH₂). Based on the atomic geometry around mercury in each model structure, we distinguished CH₃Hg⁺ -binding ligands in two different humic acids (soil and aquatic). We observed CH₃Hg⁺ preferentially binds to thiol ligands. After saturating reactive thiol ligands, the majority of CH₃Hg⁺ binds to carboxyl ligands rather than to amine or other reduced sulfur ligands than thiol. We found no evidence of significant CH₃Hg⁺-sulfide/disulfide or CH₃Hg⁺-hydrogen polysulfide complexes in any samples. When CH₃Hg⁺ binds to a humic ligand, the C-Hg bond in CH₃Hg⁺ remains intact. Some heavy atoms are proximately coordinated around the mercury atom in the sample containing the highest CH₃Hg⁺ levels used in this study.