Following the accident at the Fukushima Daiichi Nuclear Power Plant, much of the released radioactive cesium (Cs) has remained in the surface layer of soils. This is thought to be because clay minerals in soil adsorbed the cesium. The adsorption mechanism is very complex, however, and its details were not fully understood. 

In this study, the nanoscale behavior of adsorption (on the order of one billionth of a meter) was clarified by combining experiments with supercomputer-based first‑principles (DFT) simulations.

 • Structures of clay minerals and adsorption sites 

Clay minerals in soil have a sandwich‑like layered structure. 

Ions such as potassium (K) and sodium (Na) usually reside between the layers (the interlayer). When a solution containing Cs enters, Cs replaces interlayer K or Na and becomes adsorbed by the clay mineral. 

 

Figure 1. Structure of a clay mineral and multiple adsorption sites 

 

There are three types of adsorption sites (Fig. 1): an expanded (swelled) interlayer (a widened, water‑containing interlayer), a collapsed interlayer (a narrowed, less‑hydrated interlayer), and the frayed edge site (FES) (an opened portion at the end of a layer). 

It has been considered that Cs adsorbs to the FES at low concentration and to the collapsed interlayer at high concentration. However, the structure at intermediate concentrations and how the dominant adsorption site transitions with concentration had remained unclear. 

• Methods 

We systematically investigated samples over a wide concentration range—from very low to high—by combining the following techniques:  

• Extended X‑ray absorption fine structure (EXAFS) measurements:  

           obtain nanoscale information such as the distance from Cs to neighboring atoms.

•  First‑principles (DFT) simulations using a supercomputer:  

          compare with the experimental results to test the plausibility of multiple adsorption models. 

• High‑energy‑resolution fluorescence‑detection X‑ray absorption near‑edge structure (HERFD–XANES):  

         evaluate the bonding nature (ionic vs covalent) of Cs. 

• How a collapsed interlayer forms: comparison of two scenarios 

The adsorption environment of Cs changes with concentration. At low concentration, adsorption first occurs at the **FES**; as concentration increases, adsorption spreads to adjacent sites; with further increase, **collapsed interlayers** become more prevalent. Two hypotheses (scenarios) for how a collapsed interlayer forms were examined (Fig. 2): 

Scenario 1: Cs adsorbs to a slightly widened region around the FES, which then collapses. 

        • Interlayer spacing is only slightly expanded. 

        • Cs sits near the middle of the interlayer. 

        • The Cs–O distance increases compared with the fully collapsed state. 

Scenario 2: Cs adsorbs to one side of an expanded interlayer and then the interlayer collapses.

        • Interlayer spacing is largely expanded. 

        • Cs is displaced toward one layer. 

        • The Cs–O distance becomes shorter. 

Figure 2. Two candidate scenarios  

To identify which scenario is more realistic, we performed first‑principles (DFT) simulations with a supercomputer and observed local structures with EXAFS. The key difference is the interlayer spacing at the adsorption site. The DFT results show that when the interlayer spacing is only slightly opened, Cs remains near the center; when the spacing is greatly opened, Cs shifts toward one side (Fig. 3). 

 

Figure 3. First‑principles structural models and calculated results 

EXAFS further revealed that the Cs–O distance tends to increase with concentration, supporting Scenario 1 as the more realistic pathway (Fig. 4). 

Figure 4. Cs–O distance from EXAFS 

We also used HERFD–XANES, which provides much sharper spectra than conventional XANES, enabling an evaluation of bonding characteristics. When the peak position shifts to higher energy, ionic character increases; when it shifts to lower energy, covalent character increases (Figs. 5 and 6). For Cs adsorbed on clay minerals, the peak positions indicate predominantly ionic bonding. First‑principles calculations yielded consistent results. 

 

Figure 5. Conventional XANES vs HERFD–XANES 

 

Figure 6. Peak centroid energy in HERFD–XANES spectra 

Overall, the study reveals that Cs, despite being bound ionically, is strongly adsorbed because of the nanoscale structure formed by the clay mineral layers.  

This finding enables more accurate predictions of Cs behavior in the environment, and it is expected to inform safer strategies for managing radioactive waste involving Cs and clay minerals.