Influence of Copper and Copper Oxides on Adsorption and Diffusion Behavior of Se(Ⅳ/Ⅵ) in Tamusu Clayrock

The Tamusu clayrock of Inner Mongolia is a significant candidate host rock for geological disposal of high-level radioactive waste in China. ⁷⁹Se(Ⅳ) and ⁷⁹Se(Ⅵ) are key concerns in the long-term safety assessment of repositories due to their relatively high migration rates in the natural environment. Copper-iron-based alloys represent one of the important candidate materials for high-level waste storage containers. Copper may exist in various oxidation states(Cu(0), Cu(Ⅰ), or Cu(Ⅱ)) in complex geological environments, and its interfacial reactions can significantly affect the speciation and migration capacity of selenium. This study systematically investigated the influence of different copper species(Cu(0), Cu(Ⅰ), and Cu(Ⅱ)) on the adsorption and diffusion behavior of Se(Ⅳ) and Se(Ⅵ) in Tamusu clayrock under simulated groundwater conditions, employing both static batch experiments and through-diffusion column tests. Key transport parameters were determined, including equilibrium adsorption capacity(q_e), distribution coefficient(K_d), apparent diffusion coefficient(D_a), and effective diffusion coefficient(D_e). Results demonstrate that the retardation efficacy of different copper species follows the order: Cu(Ⅰ)+clay＞Cu(0)+clay＞Cu(Ⅱ)+clay＞clay. The introduction of Cu(Ⅰ) significantly enhances the clay’s selenium adsorption capacity, while Cu(Ⅱ) exhibites a comparatively weaker effect. The underlying mechanisms include: (1)Reduction: Cu(Ⅰ) partially reduces Se(Ⅳ) to elemental Se(0); (2)Ligand exchange and complexation: clay coordinates with Cu(Ⅰ) to form surface active sites( \equiv Si—O—Cu), which undergo ligand exchange with aqueous \mathrmSeO_3^2- or \mathrmSeO_4^2- to form ternary surface complexes; (3)Charge neutralization: Cu(Ⅰ) partially neutralizes the negative surface charge of the clay, mitigating electrostatic repulsion against anions.