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[2901-14-11] Immobilization Process of Selenate in Cancrinite by Hydrothermal Method
Chairman：Jaeheon Lee （University of Arizona）, Mitsuaki Matsuoka (Kansai University)
Keywords:Immobilization of Selenate, Hydrothermal Method, Cancrinite, Sodalite
Anionic radionuclides are known to be hardly immobilized in most minerals and in addition they possess quite long half-life in the order of 105~107 years. In Fukushima Daiichi Nuclear Power Plant (FDNPP) anion exchange resin is utilized to immobilize these oxo-anions like selenate (79SeO42-)/selenite (79SeO32-) in Advanced Liquid Processing System. However, resins are not always suitable to store underground for long years, due to high biodegradability. The technologies for stable storage of anionic radionuclides are strongly desired to develop before decommissioning of the FDNPP reactor. It has been known that hydrocalumite and ettringite are typical anionic exchangers for selenite. However, these matrixes have low chemical stability under low pH conditions. In this study, cancrinite and sodalite were proposed as selenate matrixes, where both matrixes are stable in the wider range of pH.
The hydrothermal reaction has been performed to study on co-precipitation of selenate and selenite with cancrinite (Na(6+2y+z)(AlSiO4)6(X)y(OH)z.nH2O and/or sodalite (Na(6+2y+z)(AlSiO4)6(X)y(OH)z.nH2O. At the beginning of the reaction, zeolite (NaAlSiO4.nH2O) was formed as the dominant phase in accompany with hydrosodalite as the minor phase. Selenate was incorporated in hydrosodalite, not in the zeolite. As the time progressed, the hydrosodalite was transformed into cancrinite keeping selenate. It was revealed by phase quantification using WPPF method that after hydrosodalite was consumed by transformation into cancrinite, zeolite started to be transformed into cancrinite probably through hydrosodalite as an intermediate. While the molar ratio of Al/Si was mostly constant during the phase transformation, the molar ratios of Se/Al and Se/Si always increased over time. This indicates cancrinite accommodates selenate much more than hydrosodalite. This observation was also supported by FTIR and SEM images. The maximum sorption capacity of selenate was recorded as 0.518 mmol-Se/g. Meanwhile, selenite was not effectively co-precipitated with either hydrosodalite or cancrinite. This method is selective immobilization of selenate over selenite.
The hydrothermal reaction has been performed to study on co-precipitation of selenate and selenite with cancrinite (Na(6+2y+z)(AlSiO4)6(X)y(OH)z.nH2O and/or sodalite (Na(6+2y+z)(AlSiO4)6(X)y(OH)z.nH2O. At the beginning of the reaction, zeolite (NaAlSiO4.nH2O) was formed as the dominant phase in accompany with hydrosodalite as the minor phase. Selenate was incorporated in hydrosodalite, not in the zeolite. As the time progressed, the hydrosodalite was transformed into cancrinite keeping selenate. It was revealed by phase quantification using WPPF method that after hydrosodalite was consumed by transformation into cancrinite, zeolite started to be transformed into cancrinite probably through hydrosodalite as an intermediate. While the molar ratio of Al/Si was mostly constant during the phase transformation, the molar ratios of Se/Al and Se/Si always increased over time. This indicates cancrinite accommodates selenate much more than hydrosodalite. This observation was also supported by FTIR and SEM images. The maximum sorption capacity of selenate was recorded as 0.518 mmol-Se/g. Meanwhile, selenite was not effectively co-precipitated with either hydrosodalite or cancrinite. This method is selective immobilization of selenate over selenite.
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