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, Al-27 magic angle spinning nuclear magnetic resonance spectroscopy, and Co Kedge X-ray absorption spectroscopy were employed with leaching tests. Although thermal treatment at <= 600 degrees 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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

In this study, thermal treatment of partially Co2+-exchanged zeolite X was found to alter the coordination structure of Co2+ and reduce the risk of re-mobilization. High temperature treatment resulted in the formation of surface precipitates and hydrotalcite-like phases, as well as the extraction of framework Al.