Diffusion behavior of Se(IV) and Re(VII) in GMZ bentonite

The diffusion behavior of HSeO3− and ReO4− was investigated in Gaomiaozi (GMZ) bentonite by the through-diffusion method. The effective diffusion coefficient De, porosity ε and rock capacity factor α were measured in order to evaluate the impact of porosity on anion diffusion in compacted bentonite. The De values of HSeO3− and ReO4− were (4.5–54) × 10− 12 and (3.0–53) × 10− 12 m2/s at dry densities from 1300 to 1800 kg/m3, respectively. HSeO3− sorbed on GMZ bentonite with a distribution coefficient Kd in the range of (1.4–1.8) × 10− 4 m3/kg, whereas ReO4− showed little sorption. Moreover, the diffusion behavior of HSeO3- and ReO4- was similar to that of SeO32-and 99TcO4-. Since the minimum average pore diameter at a bulk dry density below 1800 kg/m3 is much larger than that of anions, the constrictivity δ could be neglected. The De or the apparent diffusion coefficient Da was related to porosity ε by Archieʹs law and by a pore diffusion model with linear and logarithmic relations between τ and ε. The best fit was obtained by Archieʹs relation.