Insights into the formation of Pu(IV) pseudo-colloids on mineral colloid surfaces: Experimental and thermodynamic...

Plutonium released into the environment through nuclear activities poses significant ecological risks due to its high toxicity and long-term radiological hazards. Understanding Pu transport in groundwater is critical for contamination remediation, yet existing models often overlook competitive adsorption of Pu between mobile colloids and immobile phases (e.g., rock grains). In this study, competitive partitioning experiments of Pu(IV) in a three-phase system (water-granitic colloid-granite grain) were conducted under anaerobic conditions at pH 8.15. Thermodynamic calculations were performed to investigate the surface complexation of Pu (in different valence states) with hydroxyl groups (SOH) on granitic colloids. Experimental results demonstrate that increasing granitic colloid concentrations (5.1-774.4 mg/L) significantly enhanced Pu(IV) partitioning into the colloidal phase, with the colloidal fraction (F_col) rising from 33.36% to 87.50%, while the immobile Pu(IV) fraction (F_im) decreased from 59.50% to 12.11%. Thermodynamic modeling identifies ≡SOPuOH₃ (84.7%) and ≡SOPuOH₄^- (15.3%) as the dominant Pu(IV) surface complexes, with their concentrations positively correlated to colloid levels. These two surface complexes formed via Pu^4 + complexation with SOH on colloid surfaces, confirmed by stable species ratios (5.5:1) across colloid concentrations. Notably, Pu(IV) pseudo-colloids, rather than intrinsic-colloids from hydrolysis, govern Pu(IV) mobility at trace concentrations (2.48 × 10^-10 mol/L in this study). These findings highlight the critical role of natural colloids in enhancing Pu transport by outcompeting immobile granite surfaces, emphasizing the necessity of incorporating colloid competition into predictive models for accurate environmental risk assessment.