Comparison of direct and operational methods for probing metal bioavailability and speciation in aquatic systems

Direct and operational analytical methods were used to probe and establish relationships between metal speciation and bioavailability in controlled laboratory experiments and contaminated sediments. Bacterial biosensors responsive to intracellular Hg and Zn reported the bioavailable fraction of various dissolved and particulate metal phases. Square-Wave Voltammetry (SWV) detected the electrochemically labile fraction. X-ray Absorption Spectroscopy (XAS) and sequential extraction methods were used to probe metal distribution in particulate samples. Adherence to the Free-Ion Activity Model (FIAM) was examined by verifying if the bioavailable fraction of metal was predicted by the free-metal ion concentration.; An apparent violation of the FIAM was observed for the Hg-EDTA system. While the Hg-EDTA complex was not detected by SWV, the Hg biosensor reported the presence of intracellular Hg when exposed to Hg complexed by excess EDTA.; The electrolabile Zn fraction measured in sediment pore water was less than the bioavailable fraction, suggesting that some bioavailable complexes are not detectable by voltammetry. The bioavailable fraction of Zn determined for bulk sediments did not correlate with dissolved Zn, suggesting that particulate Zn phases contribute to the bioavailable fraction. Despite low solubility product constants, the concentration of Zn sulfide and Zn phosphate correlated positively with the bioavailable Zn measured in the sediment, agreeing with the controlled laboratory experiments.; Due to high microbial sulfate reduction in sediments from Lake DePue contaminated with Zn, Zn sulfide was expected to accumulate with depth; however, the concentration of Zn sulfide decreased with depth. Additional evidence suggests that Zn sulfide accumulating during the summer months was reoxidized during the winter months, causing sulfur to cycle between sulfate and sulfide within the upper sediments and never depositing permanently within the sediment column.; XAS confirmed that sequential extraction targets Zn phosphate during the step targeting carbonates and the reducing step targeting Fe and Mn oxides suggesting that traditional extraction methods may not accurately measure Zn speciation when Zn phosphates are present because the carbonate and reducible phases could be overestimated. A new sequential extraction method was validated by XAS and proved better at assessing metal distribution in anaerobic sediments.