This study elucidated the location where Cs atoms resided at the solid–water interface in smectite hydrates that composed bentonite using EXAFS spectroscopy. Samples were prepared by adding 4ml of a 0.1M CsCl solution to 2g dry weight of suspended KunipiaF bentonite at pH 3.9, 7.1, and 9.8. The Cs K-edge absorption was measured in transmission mode at a synchrotron, Spring-8, Japan, using a Si(311) monochromator at room temperature for the wet paste and air-dried compacted samples. Data were analyzed with the program TECHXAS. The structural parameters were determined by a two-oxygen-shell fit using the ab initio program FEFF6 and ATOMS code. EXAFS analyses have suggested that for air-dried samples the first shell consisted of about 5O at a distance of 3.19–3.20 Å, and the second shell consisted of about 7O at a distance of 3.59–3.60 Å. For the wet paste, the first shell was observed at a distance of 3.15–3.16 Å with the coordination number of about 7O. The second shell consisted of about 6O at a distance of 3.59–3.60 Å. Debye-waller factors of the first oxygen shells were 0.021 and 0.04 Å2 for the air-dried and wet paste samples, respectively. For the second oxygen shells they were 0.039 and 0.052 Å2, respectively. Based on coordination numbers and Debye-waller factors, it concluded that the adsorption sites of Cs+ on smectite hydrates were positioned near the edge of the oxygen hexagonal cavity of the siloxane sheets on basal surface, and appeared between two aluminol sheets at fractured sections, assuming that the first shells were formed by oxygen of inter-layer water, and that the second shells consisted of oxygen forming clay minerals. Furthermore, in the wet paste Cs ions were considered to form incomplete inner-sphere-like complexes and to roam freely in inter-layer water in smectite.

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