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http://dx.doi.org/10.7857/JSGE.2011.16.6.122

pH Dependence of CH3Hg+-binding Sites in Humic Acid: An X-ray Absorption Study  

Yoon, Soh-Joung (Mineral Resources Research Department, Korea Institute of Geoscience and Mineral Resources)
Bleam, William F. (Department of Soil Science, University of Wisconsin)
Publication Information
Journal of Soil and Groundwater Environment / v.16, no.6, 2011 , pp. 122-132 More about this Journal
Abstract
Mercury accumulates in biota mainly as methylmercury. In nature, methylmercury shows high affinity to organic matter and $CH_3Hg^+$-organic matter complexation affects the mobility and bioavailabiity of methylmercury. In this study, we examined the methylmercury binding sites in an aquatic humic acid as affected by the pH condition using Hg $L_{III}$-edge extended X-ray absorption fine structure (EXAFS). We evaluated methylmercury binding humic ligands using methylmercury-thiol, methylmerury-carboxyl, and methylmercury-amine complexation models. When $CH_3Hg^+$-to-humic reduced sulfur ratio is 0.3, we found that most of $CH_3Hg^+$ binds to thiol ligands at pH 5 and 7. At pH 7, however, some carboxyl or amine ligand contribution is observed, unlike at pH 5 where $CH_3Hg^+$ almost exclusively binds to thiol ligands. The carboxyl or amine ligand contribution may indicate that some types of thiol ligands in the natural organic matter have relatively low complexation constants or acid dissociation constants compared to those of some carboxyl or amine ligands. Analysis results indicate that ~0.2 fraction of methylmercury binds to amine or carboxyl ligands and ~0.8 to thiol ligands at pH 7.
Keywords
Methylmercury; EXAFS; Complexation Ligands; Humic acid; 1R105H;
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