Abstract-Silver, inorganic sulfide, and thiol compounds were measured in municipal wastewater effluent, receiving waters, and pore waters from an anoxic lake sediment in order to predict silver speciation in these systems. We found submicromolar concentrations of inorganic sulfide even in fully oxic surface water. This inorganic sulfide is likely to exist in the form of colloidal metal sulfides, which have been shown to be stable under oxidizing conditions for periods of several hours. Inorganic sulfide in both the wastewater effluent and receiving waters was found to be 200 to 300 times in excess of silver concentrations, whereas inorganic sulfide in the pore waters was 1,000 to 15,000 times in excess of silver concentrations. With sulfide in excess of silver, we predict silver sulfide complexes to dominate silver speciation. Thiols were present at low nanomolar levels in pore waters but were not detectable (Ͻ1 nM) in wastewater effluent or receiving waters. Thiols do not appear to be important to silver speciation in these freshwater systems. Partitioning of silver into particulate (Ͼ0.45 m), colloidal (10 kDa, 0.45 m), and dissolved (Ͻ10 kDa) size fractions showed that a significant proportion of silver is in the colloidal (30-35%) and dissolved phases (15-20%). Dissolved phase concentrations were relatively constant in the treatment plant effluent and receiving waters, suggesting that silver in the Ͻ10-kDa size fraction is strongly complexed by ligands that are not significantly affected by aggregation or sorption processes.

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Abstract-Dissolved silver (Ag) in the presence of amorphous iron sulfide (FeS) rapidly equilibrates to ultra-trace levels (ϳ5 ng/L), which agrees with equilibrium calculations based on Ag 2 S solubility. This finding, along with the authors' discovery that crystalline Ag 2 S (acanthite) precipitates rapidly under supersaturated conditions, suggests that a discrete Ag-S phase is formed. Furthermore, potential interferences (Cl Ϫ , Cu 2ϩ , and dissolved organic carbon) show no effect on the Ag-FeS system. Only the presence of organosulfur (thiol) ligands at 10 Ϫ3 to 10 Ϫ4 M result in increased dissolved silver. Agreement between calculations and measured values were good for 1.8 ϫ 10Ϫ4 M 3-mercaptopropanoic acid; however, calculations predicted higher dissolved silver than measured values for higher thiol levels, possibly due to Ag-thiol binding to FeS.

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