A serious effort has gone into finding biological means to reduce forms of more soluble uranium [U(VI)] compounds from aqueous waste streams, which appear at various stages of the nuclear fuel cycle. By exploiting the physiology of several different groups of microorganisms it has been shown that U(VI) can be adsorbed to microbial surfaces or biochemically converted to the very insoluble reduced [U(IV)] oxidation state, usually the uranite form (UO2). Several organisms have been reported to carry out reduction of U(VI) to U(IV) anaerobically. A nonviable preparation of a thermotolerant ethanol producing strain of Kluyveromyces marxianus was found to rapidly take up uranium(VI) with an efficiency of up to 150 mg U/g dry wt biomass, but with lower binding at lower pH values (Bustard and McHale 1997; Bustard et al. 1997). Likewise, a cross-linked immobilized residual biomass from distillery spent wash was found capable of sorbing over 200 mg U/g dry wt biomass (Bustard and McHale 1997). A proprietary method using processed granules of the fungus R. arrhizus proved successful for uranium ion immobilization (Brierley and Brierely 1993) at 50 mg U/g biomass dry weight, in batch uptake studies. Complete uranium removal was demonstrated for dilute uranium ore bioleaching solutions (< 300mg/l) with eluate concentrations after desorption approaching 5000 mgU/l.
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