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

TL;DR

This study investigates how cesium ions bind to clay minerals over time, revealing a mechanism that could help in removing radioactive cesium from sediments.

Contribution

The paper proposes a two-step aging mechanism for Cs+ in hydroxy-Al-interlayered clay minerals, supported by experimental and simulation data.

Findings

Cs+ desorption rate decreases with longer sorption time due to aging effects.

Cs+ fixation occurs in collapsed zones and hydroxy-Al sheets, explained by a two-step mechanism.

Molecular simulations and NMR data support the presence of 5-fold coordinated Al in aged samples.

Abstract

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.

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