Effects of borosilicate glass on pollucite crystallization and Cs+ immobilization in geopolymer materials

Since Fukushima nuclear power plant accident there have been many publications on immobilization of radioactive ¹³⁷Cs in the crystalline structure of pollucite (CsAlSi ₂O ₆ ). In this article, we report a novel method where Cs-polluted soil (metakaolin) and a specially formulated borosilicate glass power are sintered together to form a pollucite-based glass crystalline materials. Fine pollucite crystals are uniformly distributed in the matrix of glass phase. This is equivalent to pollucite being encapsulated by the borosilicate glass and theoretically this should enhance the chemical durability. Extensive characterizations of this material with differential scanning calorimetry (DSC), r-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and electronic differential system (EDS) reveal optimum glass content and that the formation temperatures of pollucite in the glass matrix was as low as 900 ℃, which is significantly lower than those used in forming Cs-pollucite by conventional sintering methods. Test showed that the Cs ⁺ leaching rate of sample with 20 wt% glass content was only 4.19×10 ^–4 g·m ^–2·d ^–1, lower than the sample without glass addition almost by an order of magnitude. This study not only presents an improved technique for ¹³⁷Cs immobilization but may also provide a methodology for immobilization of other potential radioactive species.