Montmorillonite colloids: II. Colloidal size dependency on radionuclide adsorption

•Radionuclide batch adsorption onto different sized montmorillonite colloids.•Geochemical modelling was used to simulate the experimental results.•U(VI), Np(V) and Tc(VII) did not adsorb to the montmorillonite colloids.•Strong adsorption of Th(IV) and Pu(IV) was found independently of the colloidal size.•For Th and Pu average KD values can be used for future reactive transport modelling.

Bentonite is a strong radionuclide (RN) adsorbent. As a consequence, it is proposed as one of the engineered safety barriers in many nuclear waste disposal concepts in granite formations. Despite the many beneficial effects of bentonite, in contact with groundwater of low ionic strength montmorillonite colloids may be released from the bentonite buffer and transported towards the biosphere carrying the RNs bound to it. During the transport of colloids in bedrock fractures, size separation of clay colloids may occur, which may further affect RN mobility. In this work, RN adsorption (Th(IV), U(VI), Np(V), Tc(VII) and Pu(IV)) onto size fractionated montmorillonite colloids was studied in a synthetic, carbonated groundwater. Fractionation was done by simple settling procedures and sequential centrifugation. We combined batch adsorption experiments and geochemical modelling for the adsorption studies. U(VI), Np(V) and Tc(VII) did not adsorb to montmorillonite in the synthetic groundwater. Adsorption of Th(IV) and Pu(IV) is strong but, within experimental uncertainties, not significantly affected by the fractionation process. Montmorillonite colloids obtained by fractionation of the raw clay material but in the presence of organic matter during the initial separation step present significantly reduced uptake of Th and Pu. Based on the results, implementation of an “average log KD” (i.e. average distribution coefficients) for all colloidal sizes in reactive transport modelling codes would be acceptable.