Low leaching characteristics and encapsulation mechanism of Cs+ and Sr2+ from SAC matrix with radioactive IER

Abstract

Radioactive ion exchange resin is a problematic waste requiring proper immobilization to ensure that the resin's radioactive ions are stable for storage and ultimate disposal in water environments, such as seawater or groundwater. Sulphoaluminate cement hardened paste, with the dense structure, great retention capacity for ions, and resistance to SO42− and Cl−, is a potential cementation material to encapsulate 137Cs+ and 90Sr2+ in radioactive ion exchange resin. The leaching rates of Cs+ and Sr2+ from the sulfoaluminate cement matrix solidifying radioactive ion exchange resin and their immobilization characteristics of the predominant hydration products (ettringite; aluminum hydroxide gel) of sulfoaluminate cement were investigated in depth. The experimental results showed that the sulphoaluminate cement radwaste matrix had a better retention capacity for Cs+ and Sr2+ than the OPC radwaste matrix. The 42 days cumulative leaching fractions of Cs+ and Sr2+ were 6.21 × 10−2 and 5.77 × 10−3 cm, respectively, which could be described by the FRDM or DDIM model for Cs+, and the DDIM model for Sr2+. The retention of Cs+ and Sr2+ originated from the chemical absorption by ettringite, aluminum hydroxide gel, and the physical barrier. The ettringite and aluminum hydroxide gel's retention rates for Cs+ are above 70% and 40%, respectively, while those of Sr2+ are above 88% and 10%, respectively. In detail, Cs+ and Sr2+ could be absorbed by both ettringite and aluminum hydroxide gel. Unlike Cs+, Sr2+ could enter the ettringite's crystal lattice by replacing Ca2+ and entering the negative channel.