Date of Award

1-1999

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geochemistry

Supervisor

J.R. Kramer

Abstract

Recently, background silver concentrations in freshwater have been shown to be only 0.001-0.01 nanomolar. However, locations with significant sewage input have concentrations at least two orders of magnitude greater. Although geochemical processes appear to regulate background silver at picomolar levels, there is concern that point-discharges of silver may produce acute and/or chronic toxic responses in aquatic organisms. How silver speciates in these waters will largely determine its bioavailability and toxicity. This thesis project investigated silver(I) speciation in freshwater. Silver binds much more strongly to sulfide, S(-II), containing ligands compared with ligands containing nitrogen and oxygen. Determinations of silver-sulfide formation constants gave log K values of 11.0 to 13.0 compared with log K values of <6.0 for ligands containing nitrogen and oxygen. Thermodynamic calculations suggest that silver in freshwaters is over-saturated relative to Ag₂S(s) solubility (Ksp = 35.94, 25℃, μ = 0.0) and that predominant silver species in freshwater should be AgHS〫and Ag-thiolates. However, field studies at a sewage treatment plant, Dundas, Ontario and at an old mining camp, Cobalt, Ontario, demonstrated that silver is 90%-100% colloidally bound, which explains silver's apparent over-saturation in natural waters. Concentrations of aquo silver species (AgHS〫and Ag-thiolates) represent <10% of total silver in these waters. The field studies also demonstrated that inorganic sulfide is present in oxic waters at 10-200 nM (80-100% colloidally bound) while thiol concentrations are below 1 nM. Using a competitive ligand equilibration-solvent extraction technique we showed that silver is bound to the inorganic sulfide. This inorganic sulfide is likely present as metal sulfide clusters that are bound to natural organic matter (NOM). The complexation of sulfide by metals would explain its persistence even in oxic waters. In sediments silver likely forms Ag₂-S(s) as long as the silver to sulfide ratio is <1. A laboratory study demonstrated that silver reacts rapidly with sedimentary FeS and behaves thermodynamically like a distinct acanthite, Ag₂S(s), phase. Silver measurable in the porewaters is probably associated with colloidal metal sulfides, as was observed for the surface waters and sewage treatment plant effluents. In summary, the findings of this thesis research showed for the first time that sulfides are important to the speciation of silver not only in anoxic sediments but also in oxic surface waters and sewage treatment plant effluents. Based on these findings, we recommend that the SEM/AVS approach, developed as a sediment quality criterion, could also be used as a water quality criterion for silver. However, before adopting this approach, toxicity studies need to be carried out using well-characterized silver-sulfide species to verify that sulfide protects aquatic organisms from silver bioaccumulation and toxicity.

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