Montmorillonite colloids: II. Colloidal size dependency on radionuclide adsorption

Abstract

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 K-D" (i.e. average distribution coefficients) for all colloidal sizes in reactive transport modelling codes would be acceptable. (C) 2016 Elsevier B.V. All rights reserved.

Keywords

Montmorillonite colloids; Particle size effect; Adsorption; Radionuclides; Nuclear waste disposal

Published in

Applied Clay Science
2016, Volume: 123, pages: 292-303 Publisher: ELSEVIER SCIENCE BV

UKÄ Subject classification

Physical Chemistry
Geochemistry


Publication identifier

DOI: https://doi.org/10.1016/j.clay.2016.01.017

Permanent link to this page (URI)

https://res.slu.se/id/publ/84456