（2025） 

A research team from Kyoto Prefectural University and the Japan Atomic Energy Agency elucidated this stabilization mechanism at the atomic scale. 

 

Figure 1. A natural mica mineral present in soil 

This specimen represents a relatively large crystal; most soil-borne micas are only a few micrometers in size.

Mica is a naturally occurring mineral that plays an important role in limiting the uptake of radioactive cesium by crops after the Fukushima Daiichi Nuclear Power Plant accident. In its normal state, the interlayer space of mica is bridged by dehydrated potassium ions (K⁺), creating a compact structure with a basal spacing of about 1.0 nm. Cesium cannot enter this tightly closed configuration. 

Through weathering, however, K⁺ is released and replaced by hydrated cations, slightly widening the interlayer spacing. At the boundary between collapsed and expanded layers, frayed edge sites (FES) form—tiny wedge-shaped regions that serve as energetically favorable sites for cesium fixation. 

 

Figure 2. Structural model of mica layers 

The sheets are mainly composed of silicon (Si), aluminum (Al), and oxygen (O). Partial substitution of Si⁴⁺ by Al³⁺ generates a net negative charge, which is balanced by interlayer cations such as K⁺ that bridge adjacent sheets. 

The research team constructed computational models of mica with systematically varied interlayer distances to determine the range in which cesium becomes most stable. They also conducted adsorption experiments in which interlayer K⁺ was replaced by the slightly larger Rb⁺, verifying the computational results. 

 

Figure 3. Expansion of the interlayer region in the mica model

The basal spacing (d₀) is approximately 1.0 nm; the sheets were expanded stepwise by Δd to simulate weathering-induced swelling. 

 The results demonstrated that cesium achieves its highest stability when the interlayer expands by roughly 0.2 nm beyond the collapsed state. In contrast, substitution of K⁺ with Rb⁺ significantly reduced cesium stability—meaning cesium was less strongly retained. These findings were consistent between experiment and simulation.

Crucially, the computational analysis showed that this reduction in stability arises exclusively from K⁺→Rb⁺ substitution in the wedge-shaped part of the FES, whereas substitution in other interlayer regions had little effect. 

Figure 4. Schematic diagram (left) of ion exchange at the mica sheet edge (K⁺→Cs⁺ or Rb⁺→Cs⁺) and relationship (right) between interlayer expansion and cesium stability. Lower ΔΔG values indicate stronger binding. 

 Further confirmation showed that replacing interlayer K⁺ with the slightly larger Rb⁺ leads to a measurable decrease in cesium stability. Importantly, this effect is localized: only the wedge-shaped part of the FES is affected by ionic substitution, while the fully collapsed regions remain largely unchanged. 

These results provide microscopic structural insights into how natural micaceous minerals immobilize radioactive cesium.

The findings are expected to contribute to the future design of functional materials capable of effectively capturing cesium and related radionuclides in contaminated environments.