DFT Study of Sorption of Palladium onto Montmorillonite in a Saline Environment

Abstract

Palladium-107 is one of the radionuclides of interest for the safety assessment of deep geological disposal of used nuclear fuel. Bentonite clay is used in the engineered barrier system of a deep geological repository for used nuclear fuel because of its low permeability, large swelling capacity, retention properties and thermal stability. With montmorillonite being the main component of the bentonite clay, this study focuses on using the Density Functional Theory (DFT) simulation to explore the sorption mechanism of palladium on montmorillonite in a Na-Ca-Cl saline environment. The method of substituting Se(IV) species with Pd species and phlogopite edge surfaces with the basal and edge surfaces of montmorillonite is justified with benchmark simulation results. The presence of PdCl+, PdCl2, PdCl3-, and PdCl42- complexes in the Na-Ca-Cl saline environment is confirmed by the calculated negative free energy. The most favourable palladium chloride species for sorption reactions onto montmorillonite in the saline environment is PdCl42-, followed by PdCl3- , PdCl2 and PdCl+.