Reduction of actinides and fission products by Fe(III)-reducing bacteria

Microbial metabolism plays a pivotal role in controlling the solubility and mobility of radionuclides in waters contaminated by nuclear waste. The distribution and activity of dissimilatory Fe(III)-reducing bacteria are of particular importance because they can alter the solubility of radionuclides via direct enzymatic reduction or by indirect mechanisms catalyzed via a range of electron shuttling compounds. Using a combination of the techniques of microbiology, biochemistry, and molecular biology, we have characterized the mechanisms of electron transfer to key radionuclides by Fe(III)-reducing bacteria. The mechanisms of enzyme-mediated reduction of problematic actinides, principally U(VI) but including Pu(IV) and Np(V), are described in this review. In addition, the mechanisms by which the fission product Tc(VII) is reduced are also discussed. Direct enzymatic reductions of Tc(VII), catalyzed by microbial hydrogenases, are described along with indirect mechanisms catalyzed by microbially produced Fe(II). Finally, we describe new results that demonstrate the transfer of electrons from biogenic U(IV) to Tc(VII), leading to coprecipitation of Tc(IV) and U(IV), and opening the way for treatment of liquid wastes cocontaminated with both uranium and technetium in one step.