Diffusion and retention of Co2+ and Zn2+ in compacted homocationic forms of illite: Role of the electrical double layer

Surface diffusion of cationic species has frequently been postulated to explain the results of diffusion studies in compacted clay minerals and clay rocks. However, the underlying mechanism of this process is not well understood, and the factors controlling the diffusive flux are not yet satisfactorily quantified. In this study, the role of ion-specific molecular interactions in the electric double layer formed at the clay mineral-fluid interface is investigated, particularly their effect on the diffusive transport of ⁵⁷Co²⁺ and ⁶⁵Zn²⁺ tracers. To this end, in-diffusion experiments were conducted at different background electrolyte concentrations in compacted illite with Li⁺, Na⁺, K⁺ and Cs⁺ forms. The alkali cations in this series have decreasing hydration enthalpy (${\Delta H}_{hyd}$) and an increasing effective hydrated ion radius. The diffusion data were interpreted using the “two site protolysis non electrostatic surface complexation/electrical double layer” (2SPNE SC/EDL) model. The diffusion and sorption behaviour of ⁵⁷Co²⁺ and ⁶⁵Zn²⁺ in various background electrolyte concentrations and homoionic forms of illite was compared in terms of the effective diffusion coefficient D_e and the sorption distribution coefficient R_d. The extent of surface diffusion was assessed via surface diffusion ratio (φ). The results suggest that ${\Delta H}_{hyd}$ of ions is a critical factor controlling surface charge neutralisation, and consequently the distribution of the mobile species between the diffuse layer near the mineral surface and the bulk-like water in macroscopic pores. Although ⁶⁵Zn²⁺ has higher R_d values compared to ⁵⁷Co²⁺, the surface diffusion phenomenon is equally relevant for both tracers studied in this work. For the 0.03 M and 0.1 M background electrolytes, φ follows the order $Li\approx Na>K>Cs$, while in 0.5 M electrolyte solution the contribution of surface diffusion is negligible in most of the homocationic forms.