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 redu ction processes in separating or immobilizing Tc-99 in waste or groundwater s, 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 reducti on by the subsurface metal-reducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction product s were evaluated through a combination of high-resolution transmission elec tron microscopy, X-ray absorption spectroscopy, and thermodynamic calculati ons. When H-2 served as the electron donor, dissolved Tc(VII) was rapidly r educed 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 mu m) in bicarbonate buffer. Cell-associated Tc was present princ ipally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(I V) hydrous oxide the dominant solid-phase reduction product, but in bicarbo nate systems much less Tc(IV) was associated directly with the cell and sol id-phase Tc(IV) carbonate may have been present. in the presence of carbona te, soluble (<0.001 pm) electronegative, Tc(IV) carbonate complexes were al so formed that exceeded Tc(VII)O-4(-) in electrophoretic mobility. Thermody namic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of E-h and pH conditions t ypical of natural waters. Thus, carbonate complexes may represent an import ant pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubilit y Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.