Biological reduction of uranium in groundwater and subsurface soil

Biological reduction of uranium is one of the techniques currently studied for in situ remediation of groundwater and subsurface soil. We investigated U(VI) reduction in groundwaters and soils of different origin to verify th e presence of bacteria capable of U(VI) reduction. The groundwaters origina ted from mill tailings sites with U concentrations as high as 50 mg/l, and from other sites where uranium is not a contaminant, but was added in the l aboratory to reach concentrations up to 11 mg/l. All waters contained nitra te and sulfate. After oxygen and nitrate reduction, U(VI) was reduced by su lfate-reducing bacteria, whose growth was stimulated by ethanol and trimeta phosphate. Uranium precipitated as hydrated uraninite (UO2. xH(2)O). In the course of reduction of U(VI), Mn(IV)and Fe(III) from the soil were reduced as well. During uraninite precipitation a comparatively large mass of iron sulfides formed and served as a redox buffer. If the excess of iron sulfid e is large enough, uraninite will not be oxidized by oxygenated groundwater . We show that bacteria capable of reducing U(VI) to U(IV) are ubiquitous i n nature. The uranium reducers are primarily sulfate reducers and are stimu lated by adding nutrients to the groundwater. (C) 2000 Elsevier Science B.V . All rights reserved.