Distribution coefficients for the sorption of Th, U, Np, Pu, and Am on Opalinus Clay

The distribution coefficients (K-d) are determined for the sorption of Th(IV), U(VI), Np(V), Pu(III/IV/VI), and Am(III) in the system natural Opalinus Clay (OPA) from Mont Terri, Switzerland, and synthetic OPA pore water at pH 7.6 as a function of solid-to-liquid ratio and element concentration. A linear sorption behavior under the chosen experimental conditions was obtained. The K-d values strongly depend on the oxidation state of the actinide. The K-d values for the tri- and tetravalent actinides are in the range of 29-159 m(3)/kg. U and Np in the oxidation states six and five, respectively, are weakly sorbed on OPA with K-d values of 0.04-0.05 m(3)/kg. The K-d values for the redox-stable Am(III) and Th(IV) are in the same range as for Pu(III) and Pu(IV) which are redox sensitive. The K-d value of Pu(VI) (13 +/- 3 m(3)/kg) is about one order of magnitude lower than the K-d values of Pu(III) and Pu(IV), but still more than two orders of magnitude higher compared to the values obtained for U(VI) and Np(V). This discrepancy is attributed to the partial reduction of Pu(VI) to Pu(IV) during sorption.