Diffusion behavior of Np(V) coexisting as multispecies in compacted Na-montmorillonite

The diffusion of multivalent radionuclides in compacted bentonite is strongly affected by aqueous speciation, which complicates the elucidation of diffusion behavior. In this study, through-diffusion experiments were conducted on 237Np(V) at controlled carbonate concentrations (5 × 10−4–5 × 10−1 mol L−1) to investigate multispecies diffusion. The results show a clear dependence of 237Np(V) diffusion on carbonate concentration, with cation-dominated diffusion (NpO2+) at low carbonate concentrations and anion-dominated diffusion (NpO2CO3− and NpO2(CO3)23−) at high carbonate concentrations. The experimental effective diffusion coefficient (De) showed reasonable agreement with the values calculated using the weighted average of species-specific De derived from the integrated sorption and diffusion (ISD) model and the relative abundance of coexisting aqueous Np(V) species. Furthermore, PHREEQC transport simulations using the species-specific De derived from the ISD model reproduced some features of the experimental observations. These findings suggest that multispecies diffusion can be quantitatively evaluated based on the relative abundances of individual species and species-specific De, contributing to the parameterization of multivalent radionuclide diffusion under geochemical conditions relevant to geological disposal.