Colloidal stabilization of CeO₂ nanomaterials with polyacrylic acid, polyvinyl alcohol or natural organic matter

Engineered nanomaterials (ENM) such as nano-sized cerium dioxide (CeO₂) are increasingly applied. Meanwhile, concerns on their environmental fate are rising. Understanding the fate of ENM within and between environmental compartments such as surface water and groundwater is crucial for the protection of drinking water resources. Therefore, the colloidal stability of CeO₂ ENM (2 mg L^-1) was assessed with various surface coatings featuring different physico-chemical properties such as weakly anionic polyvinyl alcohol (PVA), strongly anionic polyacrylic acid (PAA) or complex natural organic matter (NOM) at various water compositions in batch experiments (pH 2-12, ionic strength 0-5 mM KCl or CaCl₂). While uncoated CeO₂ ENM aggregate in the range of pH 4-8 in 1 mM KCl solution, the results show that PAA, PVA and NOM surface coatings stabilize CeO₂-ENM at neutral and alkaline pH in 1 mM KCl solution. Stabilization by PAA and NOM is associated with strongly negative zeta potentials below -20 mV, suggesting electrostatic repulsion as stabilization mechanism. No aggregation was detected up to 5 mM KCl for PAA- and NOM-coated CeO₂ ENM. In contrast, CaCl₂ induced aggregation at >2.2 mM CaCl₂ for PAA and NOM-coated CeO₂ ENM respectively. PVA-coated ENM showed zeta potentials of -15 mV to -5 mV in the presence of 0-5 mM ionic strength, suggesting steric effects as stabilization mechanism. The hydrodynamic diameter of PVA-coated ENM was larger compared to PAA and NOM at low ionic strength, but the size did not increase with ionic strength of the suspensions. The effect of ionic strength and counter ion valency (pH 7) on the colloidal stability of ENM depends on the prevailing stabilization mechanism of the organic coating. NOM can be similarly effective in colloidal stabilization of CeO₂-ENM as PAA. Our results suggest natural Ca-rich waters will lead to ENM agglomeration even of coated CeO₂-ENM.