Diffusion and sorption of neptunium(V) in compacted montmorillonite: effects of carbonate and salinity

Abstract

Diffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (D_e) and distribution coefficients (K_d) of ²³⁷Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m⁻³ were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO₃). D_e values for carbonate-free conditions were of the order of 10⁻¹⁰–10⁻¹¹ m² s⁻¹ and decreased as salinity increased, and those for carbonate conditions were of the order of 10⁻¹¹–10⁻¹² m² s⁻¹ and showed the opposite dependence. Diffusion-derived K_d values for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency of D_e and K_d qualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO₂⁺ and NpO₂CO₃⁻ for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies of D_e for salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.