A method for determining the oxidation state of uranium in natural waters

Abstract

Uranium exhibits radically different geochemical properties in its two oxidation states. Hexavalent uranium is quite soluble, especially when complexed by carbonate ions in alkaline solutions. Tetravalent uranium, by contrast, is very insoluble, and it would be expected to be rapidly removed from natural waters either by precipitation of a mineral phase such as UO 2(s) or by adsorption to solid surfaces. It is important to know the oxidation state in which uranium will occur under specified conditions of Eh and pH as this relates both to processes forming economically valuable uranium deposits and to the mobility of uranium released to the environment during various stages of the nuclear fuel cycle. In this work, yield monitors in each oxidation state [ 232U(IV)], 236U(VI)] are added to water samples, and uranium oxidation states are separated by a NdF 3 coprecipitation in which only the U(IV) is carried. The precipitate is removed by filtration. Subsequently, TiCl 3 is added to reduce U(IV) and the NdF 3 coprecipitation is repeated. A similar method is widely used for separating oxidation states of plutonium. Results from a meromictic lake (Fayetteville Green Lake, Syracuse, New York, USA) and from an anoxic marine basin (Cariaco Trench, Venezuela) where total H 2S concentrations in the bottom waters reach 1 mM 1 −1 and 50 μM 1 −1 respectively show that in both cases the dissolved uranium is present in its oxidized form.