Influence of boom clay organic matter on the adsorption of Eu3+ by illite - geochemical modelling using the component additivity approach

The solid-liquid distribution of europium (Eu) between an adsorptive surface and a solution phase containing a competitive colloid is the result of a delicate balance between several individual chemical reactions. In this study, adsorption isotherms of Eu in presence of dissolved Boom Clay natural organic matter were experimentally determined under conditions relevant for a geological repository (trace Eu concentrations, anoxic conditions, ∝ 0.014 mol l-1 NaHCO3 background electrolyte). It was found that both the concentration and size distribution (or operational cut-off used to discriminate between ''mobile'' and ''immobile'' colloids) of natural organic matter has a strong influence on the observed solid-liquid distribution. The experimental data were subsequently modelled using a component additive approach with two well-established sorption/interaction models: the 2 SPNE SC/CE model for describing Eu adsorption on illite, and Humic Ion-Binding Model VI for describing Eu complexation to natural organic matter. Model parameters were gathered from dedicated measurements in batch systems containing only Eu and the interacting phase under study, under similar conditions as in the ternary isotherm experiments. Mutual interactions between illite and natural organic matter were studied and quantified. Under the experimental conditions of this study, it was found that these interactions were only of minor importance. The two models were subsequently combined to blind predict the Eu solid-liquid distribution in the ternary batch experiments. Within an error margin of 0.5 log Kd units, the additivity approach succeeded well in predicting Eu uptake in all experimental systems studied. A sensitivity analysis was performed to select the most important model parameters influencing the Eu uptake, and the robustness of the model. This study has shown that the component additivity approach for describing and predicting uptake of trivalent lanthanides/actinides under Boom Clay conditions, is promising, and may help in unraveling the complex behaviour of these radionuclides witnessed in migration experiments. (orig.)