Strong hg(II) complexation in municipal wastewater effluent and surface waters

The speciation of mercury(II) in the aquatic environment is greatly affected by the presence of ligands capable of forming extremely strong complexes with Hg(II). In this study, a novel competitive ligand exchange (CLE) technique was used to characterize Hg(II)-complexing ligands in samples collected from three municipal wastewater treatment plants, a eutrophic lake, a creek located downstream of an abandoned mercury mine, and a model water containing dissolved Suwannee River humic acid. These samples contained 3.3-15.9 mg/L dissolved organic carbon and were amended with 1.0-1.7 nM Hg(II) for CLE analysis. Results indicated that all samples contained labile Hg(II)-complexing ligands with conditional stability constants similar to those of reduced sulfur-containing ligands. Two wastewater effluent samples also contained approximately 0.5 nM of ligands that formed extremely strong Hg(II) complexes that did not dissociate in the presence of competing ligands. The conditional stability constant of these extremely strong or nonlabile complexes (i.e., (c)K(HgL)) were estimated to be greater than 10(30), for the reaction Hg(2+) + L' = HgL. The third wastewater sample and the eutrophic lake sample contained lower concentrations, 0.07-0.09 nM, of nonlabile Hg(II)-complexing ligands. The results suggested that these extremely strong Hg(ll)-complexing ligands should account for most of the dissolved Hg(II) species in municipal wastewater effluent and may dominate Hg(II) speciation in effluent-receiving waters.