Energetics of Salt-Bearing Sodalites, Na₈Al₆Si₆O₂₄X₂(X = SO₄, ReO₄, Cl, I): A Treatment Option for Pertechnetate-Enriched Nuclear Waste Streams

An alternative option for treating anion-enriched reprocessed nuclear waste streams is to immobilize technetium-99 (99Tc, β = 293.7 keV, t1/2 = 2.1 × 105 years) and other anions in micro- and mesoporous materials. Here we determine the thermodynamic stability of anion bearing sodalites, Na8Al6Si6O24X2 (X = SO4, ReO4, Cl, I), to improve our understanding of the driving forces that control framework assembly using high temperature oxide melt solution calorimetry. Raman and FTIR spectroscopy illustrate a strong dependence for vibrational features on anion size and enabled the development of a linear model that predicted the vibrational features for numerous anion bearing sodalites to within ±20 cm-1 (i.e., OH, F, Br, ClO4, NO3, and MnO4). The largest negative enthalpy of formation from elements and the lack of structural water demonstrate that the perrhenate sodalite (Na8Al6Si6O24[ReO4]2), a chemical analogue for pertechnetate sodalite (Na8Al6Si6O24[TcO4]2), is more thermodynamically stable than all other anion bearing sodalites evaluated. The enthalpies of the reaction between nepheline and the sodium salt, which provides the guest anion species, was negative only for the ReO4 and NO3 bearing sodalites. We report for the first time the enthalpy of the ion exchange reactions for different anion bearing sodalites relative to the perrhenate sodalite, which is a key step in gaining the ability to tune sodalite material properties and structure during treatment and the immobilization of 99Tc in the presence of competing anions.