Remediation of uranium-contaminated soils using uranium extractants and microbial uranium reduction

Previous studies in the authors' laboratory have indicated that enzymatic reduction of U(VI) to U(IV) by dissimilatory Fe(III)- or sulfate-reducing microorganisms can effectively precipitate uranium from solution. This microbial U(VI) reduction is considered to have an important influence on uranium geochemistry in a number of sedimentary environments. Furthermore, microbial U(VI) reduction has been shown to have the potential to remove dissolved uranium from a variety of contaminated waters. However, in contaminated soils, uranium is typically associated with a solid rather than an aqueous phase. Treating this contamination may be difficult because extractants which are strong enough to leach uranium from soils may also prevent subsequent reclamation of the uranium by such techniques as biosorption. A technique was developed in which uranium could be effectively leached from soils or sediments with a bicarbonate extractant under aerobic conditions. The soil extract was then treated with U(VI)-reducing microorganisms to precipitate the uranium. In this manner, uranium dispersed in a large volume of contaminated soil could readily be concentrated into a compact, pure precipitate. The effectiveness of this technique for treating various types of contaminated soils and the utilization of various extractants and bioreactor designs are discussed