Effect of anions on the sorption of uranyl ions on hydrous oxides: Application of the surface hydrolysis model

Abstract

The sorption of uranyl ions on hydrous titanium oxide (HTiO), thorium oxide (HThO) and magnetite (MAG) has been studied in the presence of NO 3 −, Cl −,SO 4 2− and CO 3 2− as a function of pH and anion concentration at a total ionic strength of 0.1 mol dm −3. The distribution of surface hydroxyl groups in different forms and the surface pH have been evaluated from pH-titration curves in the presence of varying anion concentrations. An enhancement in sorption and a shift of the sorption isotherm to lower pH were observed on HTiO in the presence of 0.001 mol dm −3 SO 4 2− compared to 0.1 mol dm −3 ClO 4 −. In 0.1 mol dm −3 SO 4 2− the sorption isotherm is shifted to higher pH on all oxides. Chloride and nitrate ions do not significantly affect the uranyl sorption. At low pH and low SO 4 2− concentration, the enhanced uranyl sorption on HTiO could be explained in terms of the surface hydrolysis model by taking into account the formation of (UO 2) 2(OH) 2SO 4 in solution and dimer-like structures resembling this species on the oxide surface. At higher SO 4 2− concentrations and low pH, the sorption of uranium on HTiO involves the species UO 2(SO 4) 2 2− the oxide surface acting as an anion exchanger. In the presence of carbonate ions, the oxide surfaces act as cation exchangers, and uranyl sorption involves the breaking up of anionic carbonate complexes, a process which depends upon the surface acidity and capacity.