Stability relationships between solid cesium aluminosilicates in aqueous solutions at 200 °C

Abstract

Interconversion reactions among the solids SiO 2 (quartz and fused), γ-AlOOH (boehmite), CsAlSiO 4 , CsAlSi 2 O 6 (pollucite), and CsAlSi 5 O 12 , in contact with aqueous solutions at 200 °C, have been investigated. Results support earlier suggestions that CsAlSi 5 O 12 is not stable with respect to CsAlSi 2 O 6 + SiO 2 . CsAlSiO 4 is only stable, if at all, when the activity product of dissolved Cs + and OH − , with respect to unit molality standard states, exceeds about 10 −2 . CsAlSi 2 O 6 is stable with respect to boehmite and quartz when the cesium hydroxide activity product exceeds 10 −7.8±1.5 . The significance of these results to the possible immobilization of radioactive cesium as a solid aluminosilicate is discussed. Based on a comparison with published estimates of enthalpies of formation, the following revised values are suggested:[Formula: see text]Keywords: cesium aluminosilicates, enthalpy of formation, hydrothermal reactions, pollucite, thermodynamic stability.