Determination of the mercury complexation characteristics of dissolved organic matter in natural waters with "reducible Hg" titrations.
A new method for the determination of the concentration and conditional stability constant of dissolved organic matter that binds mercury (Hg) has been developed using an in vitro assay of reducible Hg. The technique is a wet chemical analogue to electrochemical approaches now in use for ligand studies of many other trace transition metals in natural waters. Ligand characteristics are obtained from additions of ionic Hg to buffered lake, river water, and seawater and determination of the wet chemically reducible fraction following equilibration of the spike. This approach is robust, as demonstrated by (i) analysis using three reducing agents of varying strengths, (ii) replicate analyses, (iii) comparison to well-characterized complexing species (chloride and EDTA) using a competitive ion-exchange resin, and (iv) kinetic studies. Results indicate that Hg-complexing equivalents are present in the dissolved phase (<0.2 microm) ranging from <1 to >60 nN concentrations and with log conditional stability constants (log K') in the range of 21-24. Only one ligand class was found in the natural waters analyzed. There was indirect evidence for a class of organic ligands that formed reducible complexes with Hg in freshwater. Such ligand characteristics indicate that the vast majority of ionic inorganic Hg dissolved in freshwater and coastal saltwaters is associated with organic complexes. Concentrations, affinities, and kinetics implicate multidentate chelation sites as the principal complexing moieties for Hg and discourage the use of humic carboxylic acids as a proxy for the ligands/functional groups.
Lamborg
Carl H
CH
Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, Connecticut 06340, USA. carl.lamborg@uconn.edu
Tseng
Chun-Mao
CM
Fitzgerald
William F
WF
Balcom
Prentiss H
PH
Hammerschmidt
Chad R
CR
D002614:Chelating Agents / Q000737:chemistry; D004492:Edetic Acid / Q000737:chemistry; D004563:Electrochemistry; D008024:Ligands; D008628:Mercury / Q000032:analysis* / Q000737:chemistry*; D008962:Models, Theoretical; D009930:Organic Chemicals; D012995:Solubility; D017186:Titrimetry; D014873:Water Pollutants / Q000032:analysis*
D002614:Chelating Agents; D008024:Ligands; D009930:Organic Chemicals; D014873:Water Pollutants; D004492:Edetic Acid; D008628:Mercury