Mechanistic insights into redox-driven formation, transformation and stability of Fe-HA-Cd nanocolloids at particle-water interfaces.

Natural organic matter (NOM) colloids are frequently encountered at the anoxic-oxic interface in subsurface environments. Their surface-rich functional groups and redox capacity exert a significant influence on the fate and transport of Fe and Cd in aquatic systems. The present study demonstrated that stable Fe-HA-Cd colloids formed in both anoxic and oxic environments, with hydrodynamic diameters stabilized at 97.4-134.5 nm at an HA concentration of 64.3 mg C/L. The incorporation of Fe promoted the formation of Cd colloids on the surface of HA to a certain extent. However, the high concentration of Fe(II) (C/Fe <22.4) and Fe(III) (C/Fe<7.0) in both anoxic and oxic conditions inhibited the formation of Cd colloid by competitive adsorption and co-precipitation, respectively. Furthermore, the redox effect in the oxic transformation of Fe(II)-HA-Cd(II) colloid led to the release of truly dissolved Cd from colloidal particles to the water. The aggregation kinetics and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory demonstrated that Fe-HA-Cd colloids reduced particle stability compared to HA-Cd(II) colloids. Additionally, the depolymerization behavior of Fe-HA-Cd colloids during aggregation exhibited variability under different conditions, particularly with regard to the time-dependent size effect. This study offers detailed data on the formation, oxidative transformations, and stability of Fe-HA-Cd colloids in anoxic-oxic environments rich in organic matter. The findings provide valuable insights into Cd partitioning and environmental behavior between particulate and dissolved states, essential for understanding Cd pollution and advancing effective remediation strategies.