Reactive transport modelling of neptunium migration in Opalinus Clay

One-dimensional simulations of Np diffusion through Opalinus Clay (OPA) were conducted. The experimental data were based on two laboratory diffusion experiments. Both experiments used the same setup for two different drill core samples and exhibited differences in the measured Np concentration profiles. Previous studies showed that the Fe(II)-bearing mineral phases in the OPA lead to a partial reduction of the initially used Np(V) to Np(IV). Diffusion and sorption were the governing processes. For the simulation of diffusive transport, both experimentally determined effective diffusion coefficients (single-component) and a species-specific multi-component (MC) diffusion approach were used. Sorption processes were integrated in the reactive transport simulations using surface complexation models for Np(V) and Np(IV) on illite and montmorillonite. Three scenarios were simulated that increased in terms of their geochemical process complexity. Scenario 1 only considered surface complexation of Np(V) on various illite quantities. Redox reactions via pyrite dissolution and oxidation together with surface complexation of Np(IV) and Np(V) on illite and montmorillonite were added to the system in scenario 2. In scenario 3, redox reactions were simulated via Fe associated with the clay minerals. Clay mineral quantity had only a minor effect on the Np distribution in the cores. Instead, the Np(IV)/Np(V) ratio was essential for the migration lengths. The ratio was controlled by the inherent redox potential of the core sample. Consequently, the difference between the experiments could not be attributed to variations in the clay mineral composition of the used core samples, but rather to differences in the initially inherent redox conditions and accessibility of Fe. The simulation results showed that measurements of core mineralogy, composition of boundary solutions as well as determination of Np oxidation states along the concentration profile are essential to capture the entire geochemical picture of future laboratory investigations of redox-active radionuclides. • Process-based quantification of two Np concentration profiles in Opalinus Clay. • Models must include diffusion and sorption of both Np(IV) and Np(V) to reproduce the profiles. • Reactive transport simulations revealed: inherent redox potential is more crucial than clay content.