COMPLEXATION OF MERCURY(II) IN MUNICIPAL WASTEWATER: INSIGHTS FOR THE FATE OF MERCURY IN EFFLUENT-RECEIVING WATERS

This paper summarizes previous research investigating the chemical speciation of mercury (Hg) in municipal wastewater effluent. Effluent samples were collected from treatment plants in Northern California. The chemical speciation of Hg(II) in these samples was compared to model solutions containing known Hg-ligand composition. Competitive ligand exchange-C18 solid phase extraction was used to quantify the stabilities of Hg(II)-ligand complexes. The results indicated that the wastewater effluent contains ligands that are capable of forming very stable complexes with Hg(II). These stable Hg(II)-ligand complexes were most similar to Hg-sulfide species in the model solutions and were unlikely to consist of Hg-NOM complexes, which have weaker thermodynamic stabilities. Further treatment of the effluent and model Hg(II)-sulfide samples with hypochlorite indicates that these species would not easily oxidize during chlorination of wastewater. Results of this research suggest that municipal wastewater effluent contains Hg-sulfide complexes or colloids that could persist in the water column of effluentreceiving waters. The degradation of these sulfide-containing compounds by dissolved oxygen is likely to be slow (half-times of days or more). Thus, the relative proximity of ecosystems that are net producers of methylmercury will be critical in determining the relative bioavailability of wastewater-derived Hg.