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 LIII-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.
Bibliographical noteFunding Information:
We gratefully acknowledge the following grants that supported this research: USDA National Research Initiative (Soils and Soil Biology) Grant No. 99-00732, USDA National Research Initiative (Soils and Soil Biology) Grant No. 98-00802, Federal Hatch Project No. WIS04048, and DOE Environmental Management Science Project Grant No. DE-FG07-96ER. We thank Drs. Lisa Miller, Syed Khalid, Kaumudi Pandya, Michael Sansone and Larry Fareria for their assistance at X19A, X11A, and X10C at the NSLS, Brookhaven National Laboratory. We thank the reviewers and the editor Dr. Donald Sparks for their time and effort, and for helpful comments.