Chronospeciation of uranium released in soil during a long-term DU shell weathering experiment

Corrosion process was investigated of depleted uranium (DU) ammunition fragments buried for three years in aerobic soils continuously irrigated with water. The continuing corrosion process was triggered through formation of soluble uranyl oxyhydrate phases such as metaschoepite and becquerelite, which were identified by micro-Raman and X-ray diffraction spectroscopy. The soil was not amended by phosphates and, therefore, no uranyl phosphates were found as corrosion products on the DU surfaces by X-ray photoelectron spectroscopy. A speciation modelling at high temporal sequence (chronospeciation approach) indicated that the abundant Fe oxyhydroxides in the soil immobilized the U(IV) released through DU corrosion. During the first two years, therefore, only <10 mg of U(VI) was thus found in the leachates from the soil columns, even though >3 g of DU had been corroded. However, the degree of this immobilization was found to be controlled by the amount of dissolved inorganic and organic carbon (DIC and DOC) in the soil pore water providing for U(VI) complexation competing with surface complexation by the Fe hydroxides. The chronospeciation approach applied is useful to improve our understanding and ability to predict the long-term fate of U(VI) and the mechanisms controlling U(VI) mobility in soil contaminated with DU shells.