The surrounding rock of a repository is the last barrier to block the migration of radionuclides, and there is a potential risk of co-migration of mineral colloid and nuclides in the fissure.

This study aims to investigate the main components of the surrounding rock mineral (SRM) colloid and its adsorption mechanisms for radionuclide Cs⁺, as well as to assess the co-migration risk of mineral colloid and Cs⁺.

Firstly, SRM colloid was prepared from rock samples collected from a cavern disposal repository, and its chemical composition and microscopic structure were characterized using X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) techniques. Then, batch adsorption experiments were conducted to investigate the effects of contact time, temperature, initial Cs⁺ concentration, pH value, and coexisting ions on Cs⁺ adsorption by SRM colloid. Finally, adsorption kinetics, thermodynamics, and isotherm models were used to analyze the adsorption mechanism.

The characterization results show that SRM colloids are mainly composed of silicate minerals including biotite, K-feldspar, and albite. The adsorption experiments reveal that the saturated adsorption capacity of SRM colloid for Cs⁺ is 47.13 mg·g^-1, and the adsorption process follows pseudo-second-order kinetics (R²=0.999). Under acidic conditions (pH<6) and in the presence of high concentrations of K⁺ and Ca²⁺, colloid stability decreases and Cs⁺ adsorption is significantly inhibited. Conversely, in alkaline environments (pH>6), colloid stability improves and Cs⁺ adsorption increases to a maximum of 50.7 mg·g^-1 at pH value of 8. Interestingly, Mg²⁺ shows a promoting effect on Cs⁺ adsorption as its concentration increases, with adsorption capacity reaching 53.1 mg·g^-1 at 1 mmol·L^-1 Mg²⁺.

The adsorption of Cs⁺ by SRM colloid is a monolayer, irreversible chemical process that fits well with the Langmuir isotherm model (R²=0.942). Mg²⁺ promotes Cs⁺ adsorption by altering the microstructure of SRM colloid and increasing the interlayer spacing. Experimental results demonstrate that SRM colloid can facilitate Cs⁺ co-migration, but this risk can be mitigated by modifying the pH of backfill materials to acidic conditions or by using calcium-based materials instead of magnesium-based materials during repository construction.