Eu solubility / Organic matter / Boom Clay water / Geochemical modeling Summary. The influence of natural organic matter (NOM) on the speciation and solubility of europium (Eu) was studied under geochemical conditions representative for the Boom Clay. Different organic matter types were used, and analysis was performed both after 0.45 µm microfiltration and after 30 000 MWCO ultrafiltration to distinguish between larger colloids (assumed to be immobile under in situ conditions) and small dissolved species.Equilibrium was approached from undersaturation starting from synthesised Eu(OH) 3 (s), which, during the experiment, transformed into EuOHCO 3 (s), in agreement with thermodynamic considerations. In the absence of NOM, the Eu solution concentrations after 0.45 µm filtration exceeded the thermodynamic solubility of EuOHCO 3 (s) by several orders of magnitude, indicating the presence of inorganic Eu colloids. In the presence of NOM, the Eu solubility increased with increasing NOM concentration as was expected, but, surprisingly, was dependent on the operational size cut-off: at an identical NOM concentration in the filtrate, the Eu solution concentration after 0.45 µm filtration was consistently higher compared to the Eu concentration after 30 000 MWCO filtration. This latter observation necessitates detailed knowledge concerning the pore size cut-off of Boom Clay under in situ conditions in order to use the correct Eu-NOM complexation constant and/or maximum solubility in transport calculations. At higher NOM concentrations (TOC > 30 mg/L) the Eu solubility after 0.45 µm filtration was seemingly independent of the NOM concentration.In contrast, after 30 000 MWCO ultrafiltration, the Eu solution increased linearly with increasing DOC, from the expected thermodynamic solubility (∼ 5 × 10 −7 mol L −1 ) at 0 mg L −1 DOC to ∼ 3 × 10 −5 mol L −1 at 80 mg L −1 DOC. All of the data sets were modelled using the Nagra/PSI database [1] for solubility, hydrolysis and inorganic aqueous complexation reactions, and fitted organic complexation reactions between Eu 3+ and NOM functional groups. Both a free ligand approach (with electrostatic correction) and the humic ion-binding model VI [2], which was for the first time successfully introduced into Phreeqc geochemical code, were tested and provided equally good fits to the data.

Europium / Sorption modelling / Natural organic matter / Boom Clay / Complexation / Component additivitySummary. 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 NaHCO 3 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 K d 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.