Technitium-99, a fission product of uranium, is formed during nuclear reactions and has been released into the environment during weapon testing and the disposal of low- and intermediate-level wastes. As a result, 99Tc has been found in ground water at the sites where nuclear wastes have been reprocessed or stored and it remains a significant contaminant in effluents from nuclear fuel reprocessing plants. More information on this element can be found at the Web Elements Tc page. The dissimilatory Fe(III)-reducing bacterium, Geobacter sulfurreducens, reduces and precipitates Tc(VII) by two mechanisms. Washed cell suspensions of G. sulfurreducens couple the oxidation of hydrogen to enzymatic reduction of Tc(VII) to Tc(IV), leading to the precipitation of TcO2. This bacterium can also reduce Tc(VII) to Tc(IV) abiotically via an indirect, Fe(II)-mediated mechanism. Acetate supports the reduction of Fe(III), and the Fe(II) formed is able to transfer electrons abiotically to Tc(VII). This reaction is enhanced by the addition of low concentrations of the electron shuttle anthranquinone-2,6-disulfonate (AQDS) ([http://www.ncbi.nlm.nih.gov/pubmed/10966385|Lloyd et al., 2000]).