Kinetics and irreversibility of cesium and uranium sorption onto bentonite colloids in a deep granitic environment

Abstract

Sorption of radionuclides onto a stable colloidal phase may significantly enhance their transport in groundwater. A key point, to be analysed to assess the relevance of colloids in the safety of a deep geological radioactive waste repository, is the irreversibility of the colloid/radionuclide bond. In this work, sorption and desorption kinetics of cesium and uranium(VI) onto bentonite colloids in a granitic reduced environment was studied by means of batch experiments, carried out in anoxic conditions under N 2 atmosphere. Sorption kinetics was followed during 18 weeks, and sorption isotherms were also carried out to get additional information on sorption mechanisms. The water used in all the experiments was an alkaline, low ionic strength (pH=9.5 and I=1×10 −3M) granitic groundwater from the NAGRA's Grimsel Test Site (GTS), Switzerland, which also presents reduced Eh (−200 mV). In this water, bentonite colloids were shown to be stable during several months. Both cesium and uranium presented a nonlinear sorption behaviour in the range of concentration investigated. In kinetic experiments, the measured log Kd for Cs ([Cs]=1×10 −7 M) was 3.94±0.15, and this value did not show significant variations with time. However, the adsorption of cesium on bentonite colloids involves two reactions, a rapid exchange on planar sites (hours) and a slower component (days) in which cesium diffuses to less available but highly selective sites. This slow process, that can be evidenced only when very low tracer concentrations are used (<1×10 −9 M), is most probably responsible for the fixation of a fraction of the sorbed cesium, and for the partial sorption irreversibility shown in desorption tests. Kd values measured after several desorption experiments increased significantly with the age of the sorption complex. For example, for the sample with 1-day contact time, the second desorption Kd was ∼8600 ml/g whereas the 5 and 8 weeks contact time samples showed second desorption Kd ∼15 000 and ∼30 000 ml/g, respectively. The measured log Kd for U ([U(VI)]=4×10 −7 M) varied from 2.91 to 3.21 (±0.15) during 18 weeks of the kinetic experiment. The main variation of Kd values took place in the first 4 weeks, and then a very slow increasing trend was observed, which could be probably attributed to a partial reduction of U(VI) to U(IV). In desorption tests with uranium, desorption K ds were independent on the initial contact time. Nevertheless, a certain sorption/desorption hysteresis was observed, which is most probably due to the contribution of surface complexation reactions, at the edge sites of clay colloids, to uranium sorption. Hence, U sorption is not completely reversible.