Colloidal phenomena and aggregation mechanisms of cerium oxide nanoparticles in aqueous systems: effects of monovalent and divalent cations, and Suwanee River humic and fulvic acids

This study examines the interfacial interactions of cerium oxide nanoparticles (CeO₂ NPs) with average particle size below 25 nm in aqueous environment in the presence of monovalent cations (Na⁺) and divalent cations (Ca²⁺) and humic substances (humic acid (HA) and fulvic acid (FA)). Characterization of the dynamic growth of nanoaggregate-CeO₂-complexes formed, and assessesment of the colloidal stability of these nanoaggregate-CeO₂-complexes in such aqueous environments were performed in order to determine possible risks to their environmental surroundings. Hydrodynamic diameter and zeta potential of CeO₂ NPs were measured over time in these aqueous environments. Results indicated that Ca²⁺ was more efficient than Na⁺ in enhancing the homoaggregation of CeO₂ NPs. The critical coagulation concentration (CCC) of Na⁺ (30 mM) was twice as large as the CCC of Ca²⁺ (15 mM) at pH 5 (pH below pH value of point of zero charge, pH_PZC). The heteroaggregation of CeO₂ NPs and HA was enhanced at higher Na⁺ concentrations (> 100 mM NaCl) due to electrostatic attraction and at higher Ca²⁺ concentrations (> 1 mM CaCl₂) due to bridging effect. Furthermore, HA was more efficient than FA in enhancing CeO₂ NPs heteroaggregation in the presence of Ca²⁺, and FA was more efficient than HA in enhancing CeO₂ NPs heteroaggregation in the presence of Na⁺.