Sorption of Pu to the CILVA immobilization matrix in reducing and oxidizing conditions: Impact of ISA, Ca2+ and Cl- on Pu sorption values

In the current study, two additional variations to the CILVA system are investigated for their impact on Pu sorption: (i) the redox state of the system, and (ii) the concentration of chlorides ([Cl-]). To assess the impact of O2 ingress in the disposal system and as such its redox state, a screening study was designed and executed, with the oxidation state of the system being represented by an oxidized CILVA substrate and/or a decreasing concentration of S2-. These conditions showed that the redox state of the system seems to indeed have an impact on the Rd: Pu sorption is higher in oxidizing conditions when compared to the reducing conditions studied previously. To assess the impact of [Cl-], an SRM study was designed, comprising 23 experiment to complement the previous SRM study. In addition to a variation of [Ca2+] and [ISA] in the same range as the previous study, [Cl-] was varied as well. The results show there is no clear indication of a first order effect of [Cl-] on Pu sorption. Only at [ISA]>10E-3M, the [Cl-] affects Pu sorption. At these higher ISA concentrations, the exact effect chlorides have, is a function of the [Ca2+]. Indeed, there are clear indications of an interaction effect between the three factors [ISA], [Ca2+] and [Cl-]. At high [Ca2+] and high [ISA], a decrease in [Cl-] seems to further decrease Pu sorption (i.e. the interaction between [ISA] and [Ca2+] on one hand and [Cl-] on the other hand is positive, while the linear effects of [ISA] and [Ca2+] are negative). In contrast, at low [Ca2+] and high [ISA], an increase in [Cl-] seems to further decrease Pu sorption as well. It was hypothesized that these observed interactions are the result of different uptake processes of Cl-, Ca2+ and ISA, a.o. at the cement surface, with Cl- either binding to Ca2+ in the surface, or competing with ISA for the same sorption sites (or both).