Citrate is often used as a complexing agent to mobilize sorbed and precipitated uranium in both in situ and ex situ extraction of soils and nuclear reactor components. The biodegradability of U-citrate complexes is an important control over thepotential migration of residual uranium after the extraction process is complete. In solutions buffered at 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 99of the citrate is biodegraded rapidly at pH 8-9. The increase of pH may have shifted the equilibrium speciation of uranium from (UO{sub}2-citrate){sub}2{sup}2- to (UO{sub}2) {sub}3(OH) {sub}7{sup}1- and, consequently, raised the bioavailability ofcitrate. At pH 6-7, a significant amount of uranium is also observed to associate with biomass, whereas only a negligible amount is observed at pH 8-9. Our experimental results suggest that the residual concentration of uranium-citrate complexes left inthe treated soils can be reduced rapidly if the soil water pH is held between 8 and 9 after the extraction processes.
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