Heterogeneous adsorption of actinides on host rocks at the pore scale: Application of an improved surface complexation model

In Germany, Opalinus clay strata are considered as a potential host rock for the storage of high-level nuclear waste. The adsorption efficiency of actinides in the host rock at the pore scale is essential for better understanding and prediction of actinide retention at the continuum scale, i.e., core scale and above. Surface complexation models (SCM) are a powerful tool for describing adsorption processes of actinides onto mineral surfaces. At the pore scale, the surface energy is a key constraint, which is modified by multiple parameters, e.g., crystallographic orientation, crystal defects, and nanotopography [1, 2]. Current studies on SCM [3] utilize simple retention coefficients to characterize the fluid-solid interactions without considering the effect of surface energy.

In this study, calcite and phyllosilicates are considered as two important mineral types in the Opalinus clay rock. We propose an improved SCM that implements crystal surface energy to simulate Eu (III) and U(IV) adsorption processes onto the surfaces of reference crystals. The reactions and parameters describing adsorption are modified based on the experimental results, which focus on the adsorption efficiency and its dependence on surface energy at the pore scale. The preliminary results provide the quantitative insights into the actinide retention variability in the host rock at the pore scale, which contribute to a comprehensive understanding at the core scale and above.