| Summary: | 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<sup>−1</sup>) and pH (10–13). Independent of the oxidation state of the actinides, strong sorption was observed with <i>R<sub>d</sub></i> values between 10<sup>4</sup> and 5 × 10<sup>5</sup> L kg<sup>−1</sup>. 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 AnO<sub>2</sub>(OH)<sub>4</sub><sup>2−</sup> species. CE-ICP-MS measurements of the supernatant solution from the U(VI) batch sorption experiment at pH ≥ 10 indicate that UO<sub>2</sub>(OH)<sub>3</sub><sup>−</sup> and UO<sub>2</sub>(OH)<sub>4</sub><sup>2−</sup> dominate the speciation. Pu L<sub>III</sub>-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.
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