Adsorption of Cs Ions in Hydroxy-Al Interlayered Clay Minerals and the Aging Mechanism

A decrease in the desorption rate of Cs⁺ from natural sediment was observed with increasing Cs⁺ sorption time. This aging effect poses a serious issue as it hinders the removal of radioactive cesium ions from natural sediments. In this study, adsorption and desorption experiments and molecular simulations were conducted on artificially weathered hydroxy-Al-interlayered clay minerals to elucidate the mechanism underlying this aging effect. The adsorption selectivity of Cs⁺ was independent of the hydroxy-Al concentration; however, the desorption rate from the low-concentration hydroxy-Al phlogopite was significantly lower than that from the high-concentration samples. This difference can be attributed to the presence of collapsed and wedge zones in the interlayer of low-concentration hydroxy-Al. During aging tests for Cs adsorption, a 5-fold coordination of Al was observed in its nuclear magnetic resonance spectrum. Molecular dynamics simulations revealed that Cs⁺ in the wedge zone was highly mobile owing to its weak interactions with the basal plane. Cs⁺ fixation was observed near the edges of the electrically neutral hydroxy-Al sheets, within the collapsed zone, and within the hydroxy-Al sheets. The proposed aging mechanism of Cs⁺ in hydroxy-Al-interlayered clay minerals involves two steps: (1) Cs⁺ penetrates the interlayer space, which is expanded by the presence of hydroxy-Al segments, and (2a) it gradually migrates into the collapsed zone or (2b) into the inner hydroxy-Al layers. The structure of the (2b) model can explain the presence of 5-fold coordination of Al, and the stability of Cs⁺ in the hydroxy-Al sheets was evaluated using density functional theory calculations. These findings can contribute to the development of an efficient desorption method for Cs⁺ from natural sediments and the design of materials capable of removing or immobilizing Cs⁺ from aqueous solutions.