Cs⁺ Speciation on Soil Particles by TOF-SIMS Imaging

Soil particles exposed to CsI solutions were analyzed by imaging time-of-flight secondary ion mass spectrometry and also by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). The results showed that Cs⁺ could be detected and imaged on the surface of the soil particles readily at concentrations down to 160 ppm, which corresponds to 0.04 monolayer. Imaging revealed that most of the soil surface consisted of aluminosilicate material. However, some of the surface was more quartzic in composition, primarily SiO₂ with little Al. It was observed that adsorbed Cs⁺ was associated with the presence of Al on the surface of the soil particles. In contrast, in high SiO₂ areas of the soil particle where little Al was observed, little adsorbed Cs⁺ was observed on the surface of the soil particle. Using EDS, Cs⁺ was observed only in the most concentrated Cs⁺-soil system, and Cs⁺ was clearly correlated with the presence of Al and I. These results are interpreted in terms of multiple layers of CsI forming over areas of the soil surface that contain substantial Al. These observations are consistent with the hypothesis that the insertion of Al into the SiO₂ lattice results in the formation of anionic sites, which are then capable of binding cations.