Adsorption site for Cs ⁺, Ba ²⁺ and Sr ²⁺ in bentonite by means of EXAFS spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to estimate adsorption sites for ions Cs ⁺, Ba ²⁺ and Sr ²⁺ in bentonite. Samples of EXAFS experiments were prepared using a 0.1M CsCl, BaCl ₂ and SrCl ₂ solution. The K-edge absorptions were measured in transmission mode for the wet pastes and for air-dried compacted samples at pH 10.0 at room temperature. Data analysis was done by a two oxygen-shell fit around adsorbed ions. The used phase shifts and amplitude functions were obtained from the sulfate compounds using ATOMS code and the ab initio program FEFF6. The derived structural parameters revealed that in the wet pastes adsorbed atoms resided in the vicinity of the edge of a oxygen hexagonal cavities, and they were wrapped up by the water molecules if adsorbed atoms were over the surface of clay minerals. However, adsorbed atoms were seemed to fluctuate intensely, to roam in interlayer water, and to show a large mobility as large values of Deby-Waller factors were obtained. In airdried compacted samples adsorbed atoms were sandwiched between two clay minerals. The side of adsorbed atoms was surrounded by water. If adsorbed atoms resided in fractured sections in edge-to-edge contact, they were kept between two fractured sections of aluminol sheets in both the air-dried and the wet past samples. In the case of edge-to-face contact adsorbed atoms were sandwiched between a fractured section of aluminol sheets and an edge of oxygen hexagonal cavities on the mineral surface, regardless of water content.