The reduction kinetics of Fe(III)citrate, Fe(III)NTA, Co(III)EDTA-, U(VI)O(2) (2+), Cr(VI)O(4) (2-), and Tc(VII)O(4) (-) were studied in cultures of dissimilatory metal reducing bacteria (DMRB): Shewanella alga strain BrY, Shewanella putrefaciens strain CN32, Shewanella oneidensis strain MR-1, and Geobacter metallireducens strain GS-15. Reduction rates were metal specific with the following rate trend: Fe(III)citrate > or = Fe(III)NTA > Co(III)EDTA- >> UO(2)(2+) > CrO(4)(2-) > TcO(4)(-), except for CrO(4) (2-) when H(2) was used as electron donor. The metal reduction rates were also electron donor dependent with faster rates observed for H(2) than lactate- for all Shewanella species despite higher initial lactate (10 mM) than H2 (0.48 mM). The bioreduction of CrO(4) (2-) was anomalously slower compared to the other metals with H(2) as an electron donor relative to lactate and reduction ceased before all the CrO(4)(2-) had been reduced. Transmission electron microscopic (TEM) and energy-dispersive spectroscopic (EDS) analyses performed on selected solids at experiment termination found precipitates of reduced U and Tc in association with the outer cell membrane and in the periplasm of the bacteria. The kinetic rates of metal reduction were correlated with the precipitation of reduced metal phases and their causal relationship discussed. The experimental rate data were well described by a Monod kinetic expression with respect to the electron acceptor for all metals except CrO(4)(2-), for which the Monod model had to be modified to account for incomplete reduction. However, the Monod models became statistically over-parameterized, resulting in large uncertainties of their parameters. A first-order approximation to the Monod model also effectively described the experimental results, but the rate coefficients exhibited far less uncertainty. The more precise rate coefficients of the first-order model provided a better means than the Monod parameters, to quantitatively compare the reduction rates between metals, electron donors, and DMRB species.
Aqueous iodine removal via adsorption onto Fe oxides could provide an efficient remedial pathway for the vadose zone and groundwater contamination. We conducted a series of macroscopic batch experiments to determine the extent of the time-dependent iodate (IO3–) and iodide (I–) adsorption onto four Fe oxides (i.e., ferrihydrite, goethite, magnetite, and hematite) at different pH values and solution ionic strengths (IS). The results showed that the IO3– adsorption extent [in terms of the average distribution coefficient (Kd) after 2 days of reacting time] followed the order: ferrihydrite (927.5 mL/g) > goethite (84.9 mL/g) > magnetite (23.8 mL/g) > hematite (9.5 mL/g). However, the range of specific surface area (SSA)-normalized Kd values was narrow (2–4.6 mL/m2), suggesting SSA control over the adsorption extent. The adsorption extent was correlated negatively with both pH and IS, implying IO3– outer-sphere adsorption. The adsorption extent increased or decreased with time (up to ∼48%) after 200 days, at relatively high or low I concentration ranges, respectively, likely because of multiple geochemical reactions, including interparticle diffusion, mineral transformation, and I speciation changes. I– adsorption was insignificant for all Fe oxides. Because of its large SSA, ferrihydrite could be efficient at removing aqueous iodate, potentially decreasing the time of groundwater plume spreading.
