Co-effects of UV/H2O2 and natural organic matter on the surface chemistry of cerium oxide nanoparticles

Abstract

The widespread industrial applications of cerium oxide (CeO₂) nanoparticles (NPs) have increased their likelihood of entering natural and engineered aqueous environments. This study investigates the surface chemistry changes of CeO₂ NPs at pH 5.4 in the presence of both UV/H₂O₂ and natural organic matter (NOM). These conditions are relevant to advanced oxidation processes (AOPs). The results indicated that NOM stabilized CeO₂ NPs in solution through surface complexation between the COO⁻ functional groups of NOM and the CeO₂ surfaces, reversing the zeta potential of CeO₂ from 39.5 ± 2.7 mV to −38.3 ± 1.8 mV. UV/H₂O₂ treatment reduced the colloidal stability of CeO₂ NPs, increased the percentage of Ce³⁺ on the surface from 17.8% to 28.3%, and lowered the zeta potential to close to neutral (3.8 ± 3.4 mV). With UV/H₂O₂ and NOM together, NOM coated on CeO₂ NPs acted as a protective layer, making the direct reactions between reactive oxygen species (ROS) and CeO₂ and their impacts on the colloidal stability insignificant in a short reaction period. During the UV/H₂O₂ treatment, the adsorption of superoxide radicals (O₂˙⁻) dominated in neutralizing the surface charge of CeO₂, leading to decreased electrostatic repulsive forces between nanoparticles and a higher extent of sedimentation. These new findings provide important implications for understanding the colloidal stability, sedimentation, and surface chemical properties of CeO₂ NPs in aqueous systems where NOM and ROS are present.