Behavior of Cs in Grimsel granodiorite: sorption on main minerals and crushed rock

Abstract

In this study the sorption of cesium was investigated on four different minerals; quartz, plagioclase, potassium feldspar and biotite as well as granodiorite obtained from the Grimsel test site in Switzerland. The experiments were conducted in the presence of the weakly saline Grimsel groundwater simulant by determining the distribution coefficients using batch sorption experiments and PHREEQC-modelling across a large concentration range. In addition, the purity of the minerals was measured by XRD and the specific surface areas by BET method using krypton. The distribution coefficients of cesium were largest on biotite (0.304 ± 0.005 m³/kg in 10^–8 M). Furthermore, the sorption of cesium on quartz was found to be negligibly small in all investigated concentrations and the sorption of cesium on potassium feldspar and plagioclase showed similar behavior against a concentration isotherm with distribution coefficients of 0.0368 ± 0.0004 m³/kg and 0.18 ± 0.04 m³/kg in 10^–8 M. Finally, cesium sorption behavior on crushed granodiorite followed the trend of one of its most abundant mineral, plagioclase with distribution coefficient values of 0.107 ± 0.003 m³/kg in 10^–8 M. At low concentrations (< 1.0 · 10^–6 M) cesium was sorbed on the frayed edge sites of biotite and once these sites are fully occupied cesium sorbs additionally to the Type II and Planar sites. As a consequence, the sorption of cesium on biotite is decreased at concentrations > 1.0 · 10^–6 M. Secondly cesium sorption on potassium feldspar and plagioclase showed similar non-linear behavior with varying concentration. The results were used to assist the interpretation of cesium diffusion process in the 2.5 year in-situ experiment carried out in the underground laboratory at Grimsel test site in Switzerland (2007–2009).