THE USE OF MICROORGANISMS FOR THE REMEDIATION OF SOLUTIONS CONTAMINATED WITH ACTINIDE ELEMENTS, OTHER RADIONUCLIDES, AND ORGANIC CONTAMINANTS GENERATED BY NUCLEAR-FUEL CYCLE ACTIVITIES

Many heavy elements, including actinides, form insoluble precipitates with ligands such as inorganic phosphate (abbreviated Pi). This can be generated biochemically, e.g. using the activity of a phosphatase enz yme of a Citrobacter sp., which forms HPO42- in juxtaposition to nucle ation sites On the cell surface; insoluble metal phosphate promotes th e formation of large crystals of for example, HUO(2)PO(4.)4H(2)O, to l oads of several times the weight of the biomass. For use the biomass i s immobilized within a flow-through column. The metals can be removed efficiently from dilute solution since the continuous production of a high localized concentration of Pi allows the solubility product of th e metal phosphate to be exceeded, even in the presence of competing ch elating ligands (e.g. citrate). Application of this approach to the re moval of uranium, americium, plutonium and neptunium from acid mine dr ainage waters (U) and laboratory test solutions (Am, Pu, Np) is descri bed. The phosphate 'donor' molecule (phosphatase substrate) is an orga nophosphate, usually glycerol 2-phosphate. Tributyl phosphate has also been cleaved enzymatically to support the removal of uranium from sol ution by a new mixed culture. Some metal species such as technetium (V II), TcO4-, do not form insoluble phosphates. Here, the reductase acti vity of other microorganisms can be harnessed to the bioreduction of T c(VII) to insoluble species which are precipitated onto the biomass. S pecial problems can occur in plant decontamination, where soluble meta l-ligand complexes may be generated. A mixed microbial culture which g rows at the expense of ethylene diamine tetraacetic acid (EDTA) chelat ed to various metals is described for the first time. This has potenti al for the release of metals into available, unbound forms for further remediation. ln conclusion, there is scope for the use of micro-organ isms for the remediation of several types of wastes arising from activ ities associated with the nuclear fuel cycle. This technology is envis aged as a 'polishing' step, as an adjunct to more conventional physico -chemical treatments, with additional metal transformations, and enhan cement of metal availability that would be difficult to achieve by che mical means alone.