Experimental and modelling studies of caesium sorption on illite

A natur al illite (illite du Puy) was purified and converted to the homo-ionic Na form. The conditioned Na-illite was characterised in terms of its mineralogy, chemical inventory, and surface properties. The structural formula was determined from EDS analyses (SEM/TEM) and bulk chemistry. A cation exchange capacity of 127 mEq/kg was determined by the Na isotope dilution method at neutral pH. The sorption of Cs was measured as a function of NaClO4 background electrolyte concentration (1.0, 0.1 and 0.01 M), Ca concentration and pH in the range approximate to 3 to approximate to 10. Before obtaining these measurements the kinetics of Cs uptake were determined at initial concentrations of 2 X 10(-8) M and 7 X 10(-5) M, representing the extremes of the range investigated, and was found to be concentration dependent. The supernatant solutions after: centrifugation were analysed for major cations in all of the sorption tests. A two-site cation exchange model was developed to describe the sorption of Cs over the whole range of experimental conditions. The two-site types were termed frayed edge sites, FES (high affinity/low capacity) and type II sites (low affinity/high capacity). At low NaClO4 concentrations, Cs sorption decreased at pH values less th;ln neutral. This was interpreted in terms of competitive effects from H, and K released by the partial dissolution of illite, which cannot be avoided at low and high pH values. Selectivity coefficient values for Cs-Na, Cs-K, K-Na, and H-Na exchange equilibria on the FES sites, and Cs-Na exchange on the type II sites are given for illite together with the corresponding site capacities. Copyright (C) 1999 Elsevier Science Ltd.