The Uptake of Actinides by Hardened Cement Paste in High-Salinity Pore Water

The interaction of the actinides Pu(III), Am(III), Np(V), Np(VI), and U(VI) with hardened cement paste (HCP) prepared from ordinary Portland cement was investigated by batch experiments in a diluted caprock solution (I = 2.5 M) as a function of the solid-to-liquid (S/L) ratio (0.5-20.0 g L-1) and pH (10-13). Independent of the oxidation state of the actinides, strong sorption was observed with Rd values between 104 and 5 x 105 L kg-1. For the hexavalent actinides U(VI) and Np(VI), a decrease in sorption was observed with increasing pH, which could be due to the formation of the AnO2(OH)42- species. CE-ICP-MS measurements of the supernatant solution from the U(VI) batch sorption experiment at pH >= 10 indicate that UO2(OH)3- and UO2(OH)42- dominate the speciation. Pu LIII-edge XANES and EXAFS measurements showed oxidation of Pu(III) to Pu(IV) when interacting with HCP. Calcium silicate hydrate (C-S-H) phases effectively immobilize Pu(IV) by incorporating it into the CaO layer. This was observed in a C-S-H sample with C/S = 1.65 and HCP at pH 12.7. Compared to data published in the literature on the retention of actinides on HCP at low ionic strength, the influence of high ionic strength (I = 2.5 M) on the sorption behavior was insignificant.