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

The sorption behaviour of uranium(VI) and selenium(IV) on a purified benton ite sample has been investigated and the results compared with those for so rption 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 (R-d > 10(4) cm(3) g(-1)). Outside this range the sorption decreases although t he R-d remains above 10(3) cm(3) g(-1) across the pH range studied. U(VI) s orption was also found to decrease rapidly with increasing carbonate concen tration. Thermodynamic modelling of the data indicates that sorption at low pH may be due to an ion-exchange process, whereas above pH 5 surface compl exation dominates. The sorption of Se(IV) on purified bentonite was found t o be significant only below pH 7. Sorption of Se(IV) on goethite showed a s trong dependency on pH; with strong sorption below pH 8, and a sharp decrea se in sorption above pH 8. Sorption of Se(IV) onto the smectite component o f 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 mi neral above pH 5. (C) 1998 Published by Elsevier Science B.V. All rights re served.