Bacterial Reduction and Removal of Technetium from Solution

Abstract

A novel Phosphorlmager-based technique was used to screen microorganisms for the ability to reduce Tc(VII) (as the pertechnetate ion; TCO4 −), a by-product of the nuclear industry. The photosynthetic eubacteria (cyanobacteria) Coetosphaerium sp., Osciltatoria sp, Microcystis aeruginosa and Anabaena cylindrica were unable to reduce the radionuclide, despite reports in the literature to the contrary. Several anaerobic bacteria were however able to reduce Tc(VII). The facultative anaerobe, Shewanella putrefaciens reduced Tc(VII) to a range of low oxidation state Tc species but did not accumulate the radionuclide. Another facultative anaerobe, Escherichia coli, and the obligate anaerobe Geobacler metallireducens were able to reduce and accumulate substantial quantities of Tc, probably as a low valence insoluble oxide. The enzyme responsible for Tc reduction in E. coli was identified, using genetic and physiological approaches, as the hydrogenase III component of the formate hydrogen Iyase complex. Either hydrogen or formate were used as electron donors for the bioreduction. Resting cells of E. coli, immobilised in a flow through bioreactor, coupled formate oxidation to the reduction and accumulation of Tc, with no loss in reactor efficiency noted for up to 2 weeks (the duration of the experiments). An overview of the development of the bioprocess to laboratory scale is given, possible future developments highlighted and a comparison with competing technologies made.