Thermal effect on the leachability of extraframework Co²⁺ in zeolite X

A partially Co²⁺-exchanged zeolite X was thermally treated to simulate the effect of decay heat on the leachability of extraframework Co²⁺. To have a mechanistic insight into thermal effect, X-ray diffraction, scanning electron microscopy, ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy, and Co K-edge X-ray absorption spectroscopy were employed with leaching tests. Although thermal treatment at ≤ 600 °C did not lead to the collapse of zeolite framework, it removed H₂O molecules from the coordination shell of extraframework Co²⁺, which in turn changed its coordination structure in a way to strengthen the interaction between Co²⁺ and the lattice oxygens. In leaching tests, the sample treated at higher temperature for a longer period showed less remobilized Co²⁺ by forming a Co(OH)₂-like surface precipitate and a Co hydrotalcite-like phase. Notably, the formation of the latter phase indicated the abstraction of the framework Al, the extent of which increased with the treatment temperature and duration. Two mechanisms, the concurrent extraction of Al with Co²⁺ remobilization and the hydrolysis-promoted Al abstraction, were proposed to account for thermally promoted dealumination. This study suggests that the exposure of Co²⁺-exchanged zeolite X to decay heat lessen the risk of extraframework Co²⁺ to be reintroduced into groundwater.