Complexation of mercury(II) by reduced sulfur-containing ligands in municipal wastewater effluent and oxic surface waters.

摘要

The complexation of Hg(II) plays an important role in the biogeochemistry and toxicity of mercury in aquatic environments. The goal of this dissertation was to assess the complexation of Hg(II) in municipal wastewater effluent and oxic surface waters. A novel analytical tool was developed and applied to investigate Hg(II) speciation in systems that are aquatic sources of mercury and to understand the mechanisms that lead to strongly complexed Hg(II).; A competitive ligand exchange (CLE) method was developed that enabled the measurement of Hg(II) complexation without separation or pre-concentration of ligands from the aqueous samples. In this method, CLE titrations were conducted using two different competing ligands: glutathione (GSH) and diethydithiocarbamate (DEDC). Hg(II) complexes with the competing ligands, HgH2(GSH)2 2- and Hg(DEDC)2, were separated from the Hg(II) complexes with naturally-occurring ligands by C18 solid phase extraction (SPE).; This analytical tool was employed to compare the Hg(II)-complexing ligands in municipal wastewater effluent with ligands in other surface water environments. Effluent from three wastewater treatment plants contained strong Hg(II)-complexing ligands that were inert to the competing ligands, GSH and DEDC. These strong complexes were not detected in the two surface water sites. Conditional stability constants, cKHgL, for these strong Hg(II) complexes were estimated to be greater than 1030 (for Hg 2+ + L = HgL), indicating that the strong ligands were inorganic sulfide rather than reduced sulfur-containing ligands associated with natural organic matter. The concentration of the ligands was greater than 1 nM, and thus, should dominate the speciation of Hg(II) discharged by wastewater treatment plants.; Further studies to characterize the strong Hg(II)-complexing ligands in the wastewater effluent indicated that they were stable in the presence of oxygen and under conditions mimicking chlorine disinfection. In contrast, inorganic sulfide ligands in the form of bisulfide (HS-) and metal-sulfide complexes were unstable in the presence of oxygen and chlorine.; Additional laboratory studies indicated that complexation by metals prevented the oxidation of reduced sulfur-containing ligands. In addition, Hg(II) was observed to replace Zn and Fe in metal-sulfide complexes to form stable HgS complexes. However, this replacement occurred over days for aged Zn- and Fe-sulfides, which was consistent with the slow formation of strong Hg(II) complexes observed in experiments with wastewater effluent.; The results of this study suggested that mercury discharged by wastewater treatment plants is strongly complexed. Other surface water environments that have a source of sulfide may also contain strongly complexed Hg(II) because these strong complexes are not readily oxidized by O2. This research provides a better understanding of Hg(II) biogeochemistry that may be useful for improving wastewater treatment systems and for devising regulatory strategies that minimize the adverse effects of mercury on humans and the aquatic environment.