Experimental and modeling study of the diffusion path of Ce(III)-EDTA in compacted bentonite

The impact of the diffuse double-layer on the diffusion of radionuclide anions remains unclear, impeding the application of Archie's law for adsorptive anions. Through-diffusion experiments were carried out to measure the effective diffusion coefficient and distribution coefficient of Ce(III) in the presence of EDTA. A multi-porosity model, in combination with Archie's law, was used to estimate the effective diffusion coefficient. The predominant species, CeEDTA−, exhibited an effective diffusion coefficient of (0.88–1.92) × 10−11 m2/s and the distribution coefficient of (0.81–1.18) × 10−3 m3/kg at the compacted dry density of 1300–1800 kg/m3. The accessible porosity, the diffuse double-layer porosity, and the interlayer porosity were analyzed using the multi-porosity model, based on the characterization of bentonite via Scanning electron microscopy with energy dispersive spectrometry, X-ray diffractometer, and Nitrogen-Brunner-Emmett-Teller method. Due to the low external surface area of Zhisin bentonite (27.7–35.3 m2/g), the impact of the diffuse double-layer was insignificant on the total and accessible porosities. Additionally, the diffuse double-layer porosity had insignificant impact on the effective diffusion coefficient of CeEDTA− in compacted bentonite, as its value was one order magnitude lower than that of the interlayer porosity and the accessible porosity.