Linearity and reversibility of iodide adsorption on sediments from Hanford, Washington under water saturated conditions.

A series of adsorption and desorption experiments were completed to determine the linearity and reversibility of iodide adsorption onto sediment at the Hanford Site in southeastern Washington. Adsorption experiments conducted with Hanford formation sediment and groundwater spiked with dissolved (125)I (as an analog tracer for (129)I) indicated that iodide adsorption was very low (0.2 mL/g) at pH 7.5 and could be represented by a linear isotherm up to a total concentration of 100 mg/L dissolved iodide. The results of desorption experiments revealed that up to 60% of adsorbed iodide was readily desorbed after 14 days by iodide-free groundwater. Because iodide adsorption was considered to be partially reversible, even though small amount of initial iodide is retarded by adsorption at mineral-water interfaces, the weak adsorption affinity results in release of iodide when iodide-free pore waters and uncontaminated groundwaters contact the contaminated sediments in the vadose zone and aquifer systems.

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

Pertechnetate ion [Tc(VII)O(4) (-)] 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 O(2) (<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 micromol (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.

Effect of electron donor and solution chemistry on products of dissimilatory reduction of technetium by Shewanella putrefaciens.

To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separating or immobilizing (99)Tc in waste or groundwaters, the effects of electron donor and the presence of the bicarbonate ion on the rate and extent of pertechnetate ion [Tc(VII)O(4)(-)] enzymatic reduction by the subsurface metal-reducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction products were evaluated through a combination of high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and thermodynamic calculations. When H(2) served as the electron donor, dissolved Tc(VII) was rapidly reduced to amorphous Tc(IV) hydrous oxide, which was largely associated with the cell in unbuffered 0. 85% NaCl and with extracellular particulates (0.2 to 0.001 microm) in bicarbonate buffer. Cell-associated Tc was present principally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(IV) hydrous oxide the dominant solid-phase reduction product, but in bicarbonate systems much less Tc(IV) was associated directly with the cell and solid-phase Tc(IV) carbonate may have been present. In the presence of carbonate, soluble (<0.001 microm) electronegative, Tc(IV) carbonate complexes were also formed that exceeded Tc(VII)O(4)(-) in electrophoretic mobility. Thermodynamic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of E(h) and pH conditions typical of natural waters. Thus, carbonate complexes may represent an important pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubility Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.