Arsenite oxyanions affect CeO2 nanoparticle dissolution and colloidal stability

Abstract

While highly reactive cerium oxide nanoparticles (CeO₂ NPs) are widely used in industry, their transport in aquatic systems is not well understood. To fill this knowledge gap, the interactions of CeO₂ NPs with arsenite (As³⁺), a toxic metalloid and potential co-present contaminant, were investigated with respect to CeO₂ NP colloidal stability, dissolution, and surface redox reactions. Arsenite showed distinctive effects at different concentrations, with a high As³⁺ concentration (10⁻⁴ M) inducing 90% of CeO₂ NPs to settle from solution after 8 hours, while lower As³⁺ concentrations (10⁻⁵ or 10⁻⁶ M) led to only 20% of CeO₂ NPs settling. The dissolution of NPs was most significant in the 10⁻⁵ M As³⁺ system owing to a lesser extent of aggregation, exposing more CeO₂ surface for dissolution. In the three As³⁺ concentration systems, >97% of aqueous arsenic remained as As³⁺ over 6 hours. On the NP surface, adsorbed As^III was oxidized to As^V, resulting in 58–70% of the adsorbed arsenic remaining as As^III. Simultaneously Ce^IV was reduced to Ce^III, increasing Ce^III on the CeO₂ NP surface from 17% (without arsenite) to 21–25% (with arsenite). Further mechanistic analyses revealed that the adsorption of arsenite was the main contributor to neutralizing the CeO₂ NP surface potential, enhancing particle sedimentation. These findings suggest that the fate and transport of CeO₂ NPs in our experimental systems are strongly affected by arsenite concentration and its adsorption on NPs. The results also highlight the importance of the interplay between NP aggregation, oxidation, and dissolution in predicting the behaviors of CeO₂ NPs and associated toxic elements in aquatic systems.