Engineering Stabilization of Cs-Enriched Ion Exchangers with Cement and Geopolymers

The Fukushima Daiichi Nuclear Power Plant accident resulted in a significant release of radioactive cesium, some of which is concentrated in the interim-stored melting fly-ash (MFA) produced by wastes collected from decontamination works. Final disposal scenarios consider further volume reduction of MFA into spent adsorbents, such as copper ferrocyanide (CuFeCN) or zeolite. This study investigates the long-term leaching behavior and cesium stabilization mechanisms of calcined CuFeCN and zeolite solidified with either blast furnace slag (BFS) cement or a metakaolin-based geopolymer (MKGP). Leaching experiments were conducted in ultra-pure water and synthetic seawater for up to one year. The long-term leaching rate ( LR ) of MKGP was one to two orders of magnitude higher in seawater than in ultra-pure water, whereas that of BFS cement remained on the same order in both leachates. The LR of MKGP-solidified zeolite was one to two orders of magnitude smaller than calcined CuFeCN forms, whereas that of BFS cement was only one order smaller, likely due to a lower Cs diffusion coefficient. Within a year, the MKGP-solidified calcined CuFeCN showed a reduction in immobilization ratio from 60.9% to 13.9%, from ultra-pure water to seawater, respectively. In contrast, zeolite solidified with MKGP exhibited superior Cs retention, as its sorption sites remained. EXAFS analysis showed Cs in calcined CuFeCN with MKGP solidification predominantly existed in the same state as Cs-mixed MKGP, indicating the matrix provides the major sorption sites. MKGP solidification of mobile-Cs wastes, such as calcined CuFeCN, requires optimization of the mixing ratio based on available immobilization sites.