An assessment of mercury-species-dependent binding with natural organic carbon

In contrast to many other metal/metalloid contaminants, mercury (Hg) displays a complex environmental speciation behaviour. Widespread, detectable environmental Hg species include divalent mercury (Hg2+), elemental mercury (Hgo) and monomethylmercury (CH3Hg+). Less frequently encountered compounds include dimethylmercury ((CH3)2Hg; more common in marine environments), monovalent mercury (Hg22+; a species that disproportionates via the following reaction: Hg22+ ⟺ Hg2+ + Hgo) and, more recently, detectable quantities of ethylmercury compounds (e.g., C2H5Hg+, C2H5HgC2H5, etc.) have been reported in environmental media. Environmental mercury is frequently observed to be associated with natural organic matter in aquatic environments. The present study assesses the potential binding of Hg2+, Hgo, CH3Hg+ and (CH3)2Hg with natural organic matter by hydrophobic and ionic mechanisms. The findings suggest: (1) although potential hydrophobic binding with natural organic matter will likely occur in the following sequence (CH3)2Hg > Hgo > CH3HgX (where X = OH− or Cl−), none of these species appear to be strongly hydrophobic and hence, hydrophobic processes cannot explain observed organic matter-mercury associations in low-solids-content aquatic systems; (2) hydrophobic binding mechanisms of the various mercury species are insufficient to explain observed environmental bioconcentration factors; (3) ionic mercury binding with carboxyl-type sites on natural organic carbon is not sufficiently energetic to explain observed Hg-organic matter associations in the environment; and (4) environmental mercury binding with organic matter probably occurs through ionic reactions with naturally occurring sulfhydryl binding sites.