| Abstract
| - A new method for the determination of the concentrationand conditional stability constant of dissolved organicmatter that binds mercury (Hg) has been developed usingan in vitro assay of reducible Hg. The technique is awet chemical analogue to electrochemical approachesnow in use for ligand studies of many other trace transitionmetals in natural waters. Ligand characteristics areobtained from additions of ionic Hg to buffered lake, riverwater, and seawater and determination of the wetchemically reducible fraction following equilibration of thespike. This approach is robust, as demonstrated by (i)analysis using three reducing agents of varying strengths,(ii) replicate analyses, (iii) comparison to well-characterizedcomplexing species (chloride and EDTA) using a competitiveion-exchange resin, and (iv) kinetic studies. Resultsindicate that Hg-complexing equivalents are present inthe dissolved phase (<0.2 μm) ranging from <1 to >60 nNconcentrations and with log conditional stability constants(log K‘) in the range of 21−24. Only one ligand classwas found in the natural waters analyzed. There was indirectevidence for a class of organic ligands that formedreducible complexes with Hg in freshwater. Such ligandcharacteristics indicate that the vast majority of ionic inorganicHg dissolved in freshwater and coastal saltwaters isassociated with organic complexes. Concentrations, affinities,and kinetics implicate multidentate chelation sites as theprincipal complexing moieties for Hg and discourage the useof humic carboxylic acids as a proxy for the ligands/functional groups.
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