Encapsulation of Sr-loaded titanate spent adsorbents in potassium aluminosilicate geopolymer

Titanate adsorbents have been used to remove Sr radioisotopes from contaminated water at the Fukushima Daiichi Nuclear Power Station site. This process leads to the generation of spent adsorbents, which require stabilization for long term storage and disposal. Geopolymers are candidates for matrices to achieve this consolidation. However, more data are needed to assess the leaching behavior of Sr from the adsorbents embedded in a geopolymer matrix. In this study, leaching experiments and observations of the Sr distribution of spent titanate adsorbent embedded in geopolymers, loaded with Sr at realistic concentrations were conducted. The experimental results illustrate that only 0.75% of the Sr was leached out from a K-geopolymer loaded with 30% (by solid weight) of spent adsorbent after 360 days of immersion in deionized water. From the observations of Sr distributions by electron and isotope microscopy, Sr remained in the titanate adsorbent and did not diffuse into the geopolymer matrix. Leaching of Sr (loaded at a similar concentration) from the K-geopolymer without the adsorbent was also limited, only 0.05% after 360 days of leaching. In conclusion, both titanate adsorbent in K-geopolymer matrix, and the K-geopolymer itself, offer an attractive potential for Sr immobilization.