Re(VII) diffusion in bentonite: Effect of organic compounds, pH and temperature

In order to understand the diffusion properties of anionic radionuclides (RN) in bentonite in the presence of organic compounds with chelating and reducing properties, the effect of humic acid (HA), citrate, oxalate and formate on Re(VII) diffusion was investigated by a through-diffusion method under different pH (pH   7.0 and 10.0) and temperature conditions (10–65   °C). When the molar ratio of oxalate to Re(VII) was higher than 1:1, the accumulated mass and flux were observed to decrease drastically as a function of time. It can be explained by photocatalytic impact of Fe(III) on the reduction of Re(VII) to insoluble Re(IV) by oxalate. Thus, the diffusion of Re(VII) was also observed to decrease in the presence of minor citrate, oxalate and formate. Effective diffusion coefficient (D e ) values decreased from 8.7   ×   10 −   11   m 2 /s to (5.1–5.9)   ×   10 −   11   m 2 /s. In contrast, HA had no significant impact on Re(VII) diffusion. The D e values of 1.54   ×   10 −   11 and 6.5   ×   10 −   12   m 2 /s were obtained at pH   7.0 and 10.0, respectively. HA thus neither has an effect on the speciation of Re(VII) nor does it change the charge of the surface. The latter is probably due to the fact that HA preferentially interacts with the aluminol sites of the montmorillonite edges without changing the zeta potential of the surface. The dependence of D e of Re(VII) on temperature in the absence and presence of citrate or formate was described by the Arrhenius equation . The activation energy, E a , was in the range of (20.7–25.1) kJ/mol, demonstrating that Re(VII) diffusion followed a pore water diffusion model .