Salt effects on Re(VII) and Se(IV) diffusion in bentonite

Abstract

The through-diffusion method was employed to investigate the effect of the salt concentration on the diffusion behaviour of Re(VII) and Se(IV) in compacted bentonite. The accessible porosity, εacc, of Re(VII) increased from 0.25 to 0.38 with an increase of the ionic strength from 0.1 to 1.0mol/L. The dependency of the accessible porosity εacc on the salt concentration could be described approximately by a first-order decay exponential function. The interlayer porosity of GMZ bentonite was 21% of the total porosity at a bulk dry density of 1600kg/m3 at 12°C. The De values of Re(VII) and Se(IV) increased from 2.6×10−11 to 6.1×10−11m2/s and from 1.6×10−11 to 4.7×10−11m2/s in GMZ bentonite with an increase of the ionic strength from 0.1 to 1.0mol/L, respectively. In the case of Se(IV), the adsorption showed a different ionic strength dependency in GMZ bentonite and montmorillonite. The ionic strength has no significant effect on the adsorption (Kd=1.8×10−4m3/kg) on GMZ bentonite, indicating the formation of inner-sphere surface complexes. In the case of synthetic montmorillonite, the Kd values increased with increasing ionic strength. This can be explained by the fact that the electric potential of surface sites becomes less negative with increasing ionic strength resulting in an increase in the adsorption of Se(IV) at a pH above the point of zero salt effect (PZSE). The De values of both Re(VII) and Se(IV) increased with increasing salt concentration. Both anions are totally excluded from the interlayer space and partially from the diffuse double layer. Ionic strength alters the effective diffusion coefficient of anions by controlling the thickness of the diffuse double layer. Anion diffusion data from the literature (effective and apparent diffusion coefficients) were satisfactorily evaluated by Archie's relation.