Colloid Systems and Interfaces Stability of Cerium Oxide Nanoparticles in Aqueous Environments: Effects of pH, Ionic Composition, and Suwanee River Humic and Fulvic Acids

This study investigates the colloid systems and interfaces stability of cerium oxide nanoparticles in aqueous environments as a function of pH, monovalent cations (Na⁺) and divalent cations (Ca²⁺), and humic substances (humic acid (HA) and fulvic acid (FA)). Results show that the solution chemistry affected the colloidal stability and aggregation kinetics of CeO₂ NPs. The pH point of zero charge (pH_PZC) of CeO₂ NPs was measured at pH 10.2 with diameter of CeO₂ NPs aggregates of ∼1,700 nm. The effects of Na⁺ and Ca²⁺ and HA and FA on the magnitudes and rates of aggregation were pH-dependent. In addition, when salts were present in the aqueous systems, although the CeO₂ NPs were stable at pH < pH_PZC (expect for 1 mM of NaCl/CaCl₂) and pH > pH_PZC (except for 0.5 mM CaCl₂), the aggregation was enhanced at pH = pH_PZC, with the diameter of CeO₂ NPs in the ∼1,300–3,600 nm range. HA also stabilized CeO₂ NPs under pH > pH_PZC with an enhanced aggregation of pH = pH_PZC with the diameter of CeO₂ NPs in the ∼1,500–1,900 nm range, and in the presence of 0 and 1 mM of NaCl/CaCl₂ at pH < pH_PZC. At three pH levels (8.2, 10.2, and 12.2) and under all different electrolyte concentrations (0–1 mM of NaCl or CaCl₂), FA (0.14 mg/L) exhibited a greater degree of efficiency in stabilizing CeO₂ NPs than HA (5 mg/L), with CeO₂ NPs aggregates growing at low rates and resulting in diameter of ∼95–115 nm.