Adsorption and Diffusion Behavior of Re(Ⅶ) in Cu-Bentonite

This work aims to evaluate the potential of copper-modified bentonite(Cu-bentonite) as a barrier material for deep geological nuclear waste repositories. It specifically focuses on its capability to adsorb and impede the migration of rhenium(Re(Ⅶ)), which serves as an experiment alternative for the long-lived fission product technetium-99(⁹⁹Tc). Comprehending the adsorption behavior and diffusion mechanisms of Re(Ⅶ) in modified bentonites is crucial for assessing the long-term safety of repositories. The chemical composition and surface characteristics of bentonite were analyzed using scanning electron microscope-energy dispersive spectroscopy(SEM-EDS), X-ray fluorescence spectroscopy(XRF), multi-angle particle size and high-sensitivity zeta potential analyzer, and X-ray photoelectron spectroscopy(XPS). First, batch experiments were conducted to investigate the adsorption behavior of Re(Ⅶ) on Cu-bentonite. Then, the diffusion behavior of Re(Ⅶ) through compacted Cu-bentonite samples was studied using the through-diffusion method. To establish a baseline and evaluate the modification effect, the diffusion behavior of Re(Ⅶ) was also studied in various unmodified bentonites: Gaomiaozi bentonite(GMZ), Na-bentonite, Ca-bentonite, and Anji bentonite(from Zhejiang province, China). This study examined the influence of key parameters, such as temperature on adsorption and compaction density on diffusion, on the performance metrics. Adsorption studies reveal that the adsorption capacity and distribution coefficient(K_d) of Re(Ⅶ) on Cu-bentonite decrease with increasing temperature. Diffusion experiments demonstrate that both the effective diffusion coefficient(D_e) and the apparent diffusion coefficient(D_a) of Re(Ⅶ) decrease significantly as the compaction density of the compacted Cu-bentonite increases from 1 300 kg/m³ to 1 800 kg/m³. Specifically, D_e droppes from about 2.15×10⁻¹¹ m²/s to 0.55×10⁻¹¹ m²/s, and D_a decreases from about 9.35×10⁻¹¹ m²/s to 5.00×10⁻¹¹ m²/s. Concurrently, the rock capacity factor(α) decreases from about 0.23 to 0.11 with increasing compaction density. This decreasing trend of diffusion coefficients with increasing compaction density is consistent and observes not only in Cu-bentonite but also in all the tested unmodified bentonites(GMZ, Na-, Ca-, Anji). Analysis indicates that the reduction in diffusion coefficients is primarily attributed to changes in the interlayer pore structure and diffusion pathways resulting from increased compaction density. Crucially, under identical density conditions, the mobility of Re(Ⅶ) in Cu-bentonite remains consistently low. Copper modification significantly enhances the capability of bentonite to retard Re(Ⅶ) migration. The increased adsorption capacity at higher temperatures and the significant reduction in diffusion rates associated with higher compaction densities are favorable properties for barrier materials. The dominant mechanism for reduced diffusion appears to be the alteration of the pore structure limiting diffusion pathways as density increases.