An EXAFS study on the effects of natural organic matter and the expandability of clay minerals on cesium adsorption and mobility

Abstract

The relationship between cesium (Cs) adsorption on clay minerals with various expandabilities and Cs mobility in environment was investigated using sequential extraction, batch adsorption, X-ray diffraction (XRD), generalized adsorption model (GAM), and Cs LIII-edge extended X-ray absorption fine structure (EXAFS) analyses with molecular simulations using the density functional theory (DFT). In particular, the difference between the affinities of illite (non-expansion) and vermiculite (intermediate expansion) for Cs and the effect of humic acid (HA) addition on the Cs/clay mineral system were highlighted in this study. These two factors affect Cs mobility and bioavailability in surface soil and sediments.The batch adsorption results showed that Cs adsorption was inhibited to some extent in the ternary clay+HA+Cs system because of (i) the blocked access of Cs to the frayed edge site (FES) and type II site [inner–sphere (IS) complex in GAM] by HA, and (ii) the reduced availability of the interlayer site in vermiculite. EXAFS analysis further confirmed that the adsorbed Cs in clay minerals was drastically changed by the sequential addition of HA. In addition, the dominant IS complex in the illite+Cs and illite+Cs+HA systems (in which HA was added after Cs adsorption on illite) can be converted to the outer–sphere (OS) complex largely in the illite+HA+Cs system (in which HA was added prior to Cs adsorption). These results are consistent with the sequential extraction and GAM results.The IS complex of dehydrated Cs+ mainly formed at the FES and interlayer site on illite (non-expansion) without resulting in any illite structural changes. However, on vermiculite (intermediate expansion), the dehydrated Cs+ can be adsorbed as an IS complex associated with the siloxane group of the di-trigonal cavity in the tetrahedral SiO4 sheet. This adsorption is accompanied by collapse of the layer, which can be easily coated by HA molecules to prevent Cs fixation. However, a nearly complete OS complex was observed at the planar site of montmorillonite (large expansion). These processes were confirmed by sequential extraction, batch adsorption, XRD, and EXAFS, which clearly showed that Cs mobility in soil highly depends on clay mineral expandability, natural organic matter (NOM), and the coupling of both effects. The atomic-scale information given by EXAFS is consistent with the distribution data from adsorption experiments, GAM, sequential extraction, and DFT. These results can be used as a basis for a clearer understanding of Cs behavior in natural systems.