Towards an understanding of the sorption of U(VI) and Se(IV) on sodium bentonite

The sorption behaviour of uranium(VI) and selenium(IV) on a purified bentonite sample has been investigated and the results compared with those for sorption onto the unpurified material and goethite. Batch sorption data for U(VI) on purified bentonite show strong sorption in the pH range 5 to 8 (Rd>104 cm3 g−1). Outside this range the sorption decreases although the Rd remains above 103 cm3 g−1 across the pH range studied. U(VI) sorption was also found to decrease rapidly with increasing carbonate concentration. Thermodynamic modelling of the data indicates that sorption at low pH may be due to an ion-exchange process, whereas above pH 5 surface complexation dominates. The sorption of Se(IV) on purified bentonite was found to be significant only below pH 7. Sorption of Se(IV) on goethite showed a strong dependency on pH; with strong sorption below pH 8, and a sharp decrease in sorption above pH 8. Sorption of Se(IV) onto the smectite component of impure bentonite is therefore only likely to be significant at low pH and sorption is dominated by a goethite-like surface of an iron constituent mineral above pH 5.