Microbial Transformations of Actinides in TRansUranic and Mixed Wastes

A. J. Francis

Environmental Sciences Department

Brookhaven National Laboratory

Upton, New York 11973

The actinides in wastes and contaminated soils are present in various forms, such as elemental, oxide, coprecipitates, inorganic, and organic complexes, and naturally occurring minerals. Of the actinides Th, U, Pu, Am, and Np are of primary concern because of the potential for migration from the waste repositories and contamination of the environment. Microorganisms play a major role in the transformation of the actinides which affect the solubility, bioavailability, and mobility. Under appropriate conditions, dissolution or precipitation of actinides is brought about by the direct enzymatic or indirect actions of microorganisms. These include (i) oxidation-reduction reactions, (ii) changes in pH and Eh, (iii) chelation or production of specific sequestering agents, (iv) biosorption by biomass and biopolymers, (v) formation of stable minerals, and (vi) biodegradation of actinide-organic complexes. Fundamental information on the biotransformation of actinides under various microbial process conditions will be useful in developing appropriate remediation and waste management strategies as well as predicting the microbial impacts on the long-term performance of the waste repositories. Among the actinides transformation of uranium by microorganisms has been extensively studied. We have only limited information on the biotransformation of other actinides. Initial results on the dissolution, precipitation, and biosorption of inorganic and/or organic complexes of the ²³⁸U, ²³²Th, ²³⁹Pu, ²⁴³Am, and ²³⁷Np by pure and mixed cultures of bacteria isolated from wastes and from hypersaline environments will be presented. In addition the mechanisms of biotransformation of several forms of uranium i.e., ionic, inorganic- and organic-complexes, and mixed metal-organic complexes commonly present in wastes and contaminated sites under various microbial process conditions will be discussed.