Technetium reduction in sediments of a shallow aquifer exhibiting dissimilatory iron reduction potential

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Abstract

Pertechnetate ion [Tc(VII)O4−] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O2 (<1 mg L−1) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1–2.5 μmol (dry wt. sediment) g−1]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.

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