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

Abstract

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 × 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.