BIODEGRADATION OF URANIUM-CITRATE COMPLEXES - IMPLICATIONS FOR EXTRACTION OF URANIUM FROM SOILS

Citrate is often used as a complexing agent to mobilize sorbed and pre cipitated uranium in both in situ and ex situ extraction of soils and nuclear reactor components. The biodegradability of U-citrate complexe s is an important control over the potential migration of residual ura nium after the extraction process is complete. In solutions buffered a t pH 6-7, limited biodegradation of citrate is observed within 10 days with initial U:citrate molar ratios ranging from 1.2 to 1:8; however, over 99% of the citrate is biodegraded rapidly at pH 8-9. The increas e of pH may have shifted the equilibrium speciation of uranium from (U O2-citrate)(2)(2-) to (UO2)(3)(OH)(7)(1-) and, consequently, raised th e bioavailability of citrate. At pH 6-7, a significant amount of urani um is also observed to associate with biomass, whereas only a negligib le amount is observed at pH 8-9. Our experimental results suggest that the residual concentration of uranium-citrate complexes left in the t reated soils can be reduced rapidly if the soil water pH is held betwe en 8 and 9 after the extraction processes.