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

Abstract

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 (pHPZC) 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 < pHPZC (expect for 1 mM of NaCl/CaCl2) and pH > pHPZC (except for 0.5 mM CaCl2), the aggregation was enhanced at pH = pHPZC, with the diameter of CeO2 NPs in the ∼1,300–3,600 nm range. HA also stabilized CeO2 NPs under pH > pHPZC with an enhanced aggregation of pH = pHPZC 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 < pHPZC. 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.