THE CHEMISTRY OF URANIUM IN EVAPORATION POND SEDIMENT IN THE SAN-JOAQUIN VALLEY, CALIFORNIA, USA, USING X-RAY-FLUORESCENCE AND XANES TECHNIQUES

Evaporation ponds in the San Joaquin Valley (SJV), CA, used for the di sposal of irrigation drainage waters, contain elevated levels of urani um. The ponds are filled periodically and support algae which upon eva poration become incorporated in the sediments as layers of decaying or ganic matter. This rich source of organic matter promotes reducing con ditions in the sediments. Our research was conducted to characterize o xidation/reduction reactions that affect soluble and sediment U(IV)/U( VI) concentrations in the SJV ponds. Studies were done to (I) determin e soluble U(VI)/U(IV) in waters in contact with a pond sediment subjec ted to changes in redox status, (2) observe U solid oxidation state as a reducing pond sediment underwent (in vitro) oxidation, and(3) deter mine U solid oxidation state with respect to depth in pond surface sed iment layers. Low pressure ion-exchange chromatography with an eluent of 0.125 M H2C2O4/0.25 M HNO3 was used for the separation of U(IV) and U(VI) oxidation states in the drainage waters. Soluble U(VI) and U(IV ) coexisted in sediment suspensions exposed to changes in redox potent ial (Eh) (-260 mV to +330 mV), and U(VI) was highly soluble in the oxi dized, surface pond sediments. X-ray near edge absorption spectroscopy (XANES) showed that the U solid phases were 25% U(IV) and 75% U(VI) a nd probably a mixed solid [U3O8(s)] in highly reducing pond sediments. Sediment U(IV) increased slightly with depth in the surface pond sedi ment layers suggesting a gradual reduction of U(VI) to U(IV) with time . Under oxidized conditions, this mixed oxidation-state solid was high ly soluble. Copyright (C) 1997 Elsevier Science Ltd