Sorption and reduction of uranium(VI) on iron corrosion products under reducing saline conditions

Pertinent to the behaviour of carbon steel based nuclear waste packages in saline geological environments, sorption and reduction of U(VI) on real container corrosion products has been studied in an Ar/8%H-2 flushed glove box. A carbon steel was corroded in MgCl2-brine at elevated temperatures. U(VI) was added and the redox states both of uranium and iron and their distribution among solid and liquid phases were investigated. The corrosion products initially consisted to more than 97% of hydrous Fe(II) oxides containing Cl- and Mg2+, but always some magnetite was present. In the course of the sorption step, the amount of magnetite increased. Reaction products buffer pH values of the system in a very narrow neutral range. Less than 1% of iron or uranium were found in colloidal state and already after one day, more than 98% of initially dissolved uranium was found associated with the immobile phases. A reciprocal relationship of solution concentrations of uranium with the nominal mass of magnetite was found. Behaviour of uranium species was rationalized in terms of Eh/pH diagrams. Reduction of hexavalent to tetravalent uranium was observed but to a much less pronounced extent than expected from thermodynamical considerations.