Nuclear magnetic resonance study of Cs adsorption onto clay minerals

Abstract

The release of radioactive cesium into the environment in the aftermath of disasters such as the Fukushima Daiichi disaster poses a great health risk, particularly since cesium easily spreads in nature. In this context, we perform solid-state nuclear magnetic resonance (NMR) experiments to study Cs+ ions adsorbed by clay minerals to analyze their local structure. The NMR spectra show two kinds of peaks corresponding to the clays (illite and kaolinite) after immersion in CsCl aqueous solution; the peak at −30 ppm is assigned to Cs+ on the clay surface while that at −100 ppm is assigned to Cs+ in the silicate sheet in the clay crystal. This result is consistent with the fact that Cs+ with smaller coordination number yields a small field shift in the NMR spectra. Moreover, after immersion in KCl aqueous solution, these peaks disappear in the NMR spectra, thereby indicating that our assignment is reasonable. This is because Cs+ on the clay surface and in the silicate sheet is easily subject to ion exchange by K+. We believe that our findings will contribute to a better understanding of the pathway through which Cs transfers from the soil to plants and also to the recovery of the agriculture in Fukushima.