Influence of Boom Clay organic matter on the adsorption of Eu3+ by illite – geochemical modelling using the component additivity approach

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) . 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. 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 logKd 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.