SPECIATION OF URANIUM IN GORLEBEN GROUNDWATERS

The sorption behavior of the UO22+ ion onto humic colloids in one of the Gorleben groundwaters (Gohy-2227) is investigated at different CO? partial pressures from pH 1 to pH 10 by ultrafiltration and anion exchange. From pH 1 to pH 4 the sorption of the UO22+ ion increases with pH. Desorption occurs above pH 4.5 and is contingent upon the CO2 partial pressure. Based on the experimental results and literature data, the thermodynamic stability constant for the ternary complex of uranylhydroxohumate is found as log beta (UO2(OH)HA) = 14.7 +/- 0.5 Speciation of natural uranium in five Gorleben groundwaters is performed by ultrafiltration and the results are compared to the thermodynamic calculation for U(VI). A large discrepancy is found. Over 75% of the natural uranium is colloid-borne, while the calculation predicts less than 1% of U(VI) to be in humic colloids. This fact and the low groundwater Eh suggest that much of the uranium in the examined Gorleben groundwaters appears to be in the tetravalent oxidation state.