Distribution of plutonium in a ternary system of water-colloid-bentonite: Implications for radionuclide migration

Bentonite is widely used as an engineering barrier material in deep geological repositories for radionuclide plutonium. Understanding its adsorption behavior in groundwater environments is critical for repository safety assessments. However, the distribution dynamics of plutonium in the water-colloid-bentonite three-phase system remain poorly understood. This study investigates the distribution behavior of plutonium in this system through batch experiments, using sodium bentonite as the solid-phase matrix and evaluating the effects of pH and ionic strength on colloidal adsorption stability. Results show that plutonium predominantly adsorbs onto the solid phase (65-78%) and colloids, with the 200-450 nm colloidal fraction being the primary adsorption site. At adsorption equilibrium, the partition coefficients (K_s+c/d and K_s/d) ranged from 3 × 10⁴ to 5 × 10⁴ mL/g, exceeding K_s/d+c by 1-3 orders of magnitude, while K_c/d ranged from 10⁴ to 10⁶ mL/g, indicating strong competition between colloidal and solid phases for plutonium adsorption. Compared to ionic strength, pH exerted a more pronounced influence on the plutonium distribution coefficients. As pH increased, especially under alkaline conditions (pH ≥ 9), colloidal stability was enhanced, leading to a sharp rise in colloidal content, with the proportion of plutonium associated with the colloidal phase reaching up to 44%. Alkaline conditions favored colloid stabilization but hindered plutonium retention, and high-salinity groundwater reduced plutonium partition coefficients by over an order of magnitude. These findings elucidate the mechanisms governing plutonium adsorption in groundwater systems, providing a scientific foundation for optimizing high-level radioactive waste disposal strategies and evaluating deep geological repository sites.