Activation energies of the self-diffusion of HTO, 22Na+ and 36Cl? in a highly compacted argillaceous rock (Opalinus Clay)

The temperature dependence of the self-diffusion of HTO, ²²Na⁺ and ³⁶Cl^? in Opalinus Clay (OPA) was studied using a through-diffusion technique, in which the temperature was gradually increased in the steady state phase of the diffusion. The measurements were done on samples from two different geological locations. The dependence of the effective diffusion coefficient on temperature was found to be of an Arrhenius type in the temperature range between 0 and 70 °C. A slight difference between the two locations could be observed. The average value of the activation energy of the self-diffusion of HTO in OPA was 21.1 ± 1.6 kJ mol^?1, and 21.0 ± 3.5 and 19.4 ± 1.5 kJ mol^?1 for ²²Na⁺ and ³⁶Cl^?, respectively. The measured values for HTO are slightly higher than the values found for the bulk liquid water (HTO: 18.8 ± 0.4 kJ mol^?1). This indicates that the structure of the confined water in OPA might be slightly different from that of bulk liquid water. Also for Na⁺ and Cl^?, slightly higher values than in bulk liquid water (Na⁺: 18.4 kJ mol^?1; Cl^?: 17.4 kJ mol^?1) were observed.The Stokes–Einstein relationship, based on the temperature dependency of the viscosity of bulk water, could not be used to describe the temperature dependence of the diffusion of HTO in OPA. This additionally indicates the slightly different structure of the pore water in OPA.