Stability of Beishan Granite Fault Gouge Material Colloids and Their Adsorption of Pu(Ⅴ)

Colloids are heterogeneous systems consisting of finely dispersed phase suspended in a continuous phase, with particle sizes ranging from 1 nm to 1 000 nm. In the near-field environment of the deep geological disposal repository, colloid generation occurs through several pathways including hydrolysis of multivalent radionuclides in solidified waste forms, the dissolution of corrosion products in packaging containers, and the geochemical alteration at the interfaces of buffer/backfill materials. In the far-field environment, colloids are primarily formed through the dissolution of fracture-filling materials in the host rock, resulting in the generation of silicate and carbonate colloids. In China, granite is selected as the host rock type for the deep geological disposal repository. Due to tectonic activities, some granite regions develop extensive fracture networks and fragmentation zones, which are known as granite fault gouge. Under the long-term erosion of groundwater, the fault gouge material can dissolve and form fault gouge material colloids. The colloidal particle size of the fault gouge material colloids is small and the surface is negatively charged. Therefore, it easily adsorbs radionuclides and facilitates their migration. The stability of granite fault gouge material colloids under different environmental conditions and the adsorption kinetics of Pu(Ⅴ) on fault gouge material colloids were investigated in this paper by aggregation kinetics experiments and batch adsorption experiments, respectively. The mass concentration of granite fault gouge material colloids is 437 mg/L, which can be stable for 30 d in pure water system. The results of the aggregation kinetics experiments indicate that pH has a minor influence on the stability of the granite fault gouge material colloids, whereas the type and concentration of cations significantly affect colloidal stability. Two kinetics processes of aggregation of granite fault gouge material colloids are consistent with the DLVO (Derjaguin Landan Verwey Overbeek Theory) theory, including the reaction-limited aggregation stage and the diffusion-limited aggregation stage. The critical coagulation concentrations (CCC) of the colloids in NaCl and CaCl₂ solution systems are determined to be 5.01×10⁻³ mol/L and 4.90×10⁻⁴ mol/L, respectively, demonstrating that Ca²⁺ induces markedly more pronounced aggregation of the fault gouge material colloids compared to Na⁺. The adsorption kinetics experiments reveal that the granite fault gouge material colloids exhibit strong adsorption affinity for Pu(Ⅴ), reaching adsorption equilibrium within approximately 40 h. The adsorption process of Pu(Ⅴ) on the granite fault gouge material colloids follows pseudo-second-order kinetics. This process is dependent on the concentrations of plutonium and colloids and may involve other interactions such as ion exchange and surface complexation.