Nature of the sites involved in the process of cesium desorption from vermiculite.

Three particle size fractions of sodium-saturated vermiculite (10-20, 1-2 and 0.1-0.2 μm), differing only in their ratios of external-to-total sorption sites, were used to probe the nature of the sites involved in desorption of cesium ions. The sorption was investigated for initial aqueous concentrations of cesium ranging from 5.6×10(-4) to 1.3×10(-2) mol/L, and the cesium desorption was probed by exchange with ammonium ions. The results showed that (1) the amounts of desorbed cesium were strongly dependent on the particle size for a given initial aqueous cesium concentration and (2) the amounts of desorbed cations (Na(+) and Cs(+)) strongly decreased with increasing initial cesium aqueous concentration, irrespective of the particle size investigated. Quantitative analysis of these results suggested that cesium ions sorbed on external (edge+basal) sorption sites can be desorbed by ammonium ions. As a contrast, most of cesium ions sorbed on interlayer sites remain fixed due to the collapse of the structure under aqueous conditions. This study provides important information, such as the nature of the sites involved in the exchange process, when the thermodynamic formalism is considered to describe the ion-exchange process involving cesium and high-charge swelling clay minerals in polluted soil environments.