Redox chemistry of CeO2 nanoparticles in aquatic systems containing Cr(vi)(aq) and Fe2+ ions

Abstract

CeO₂ nanoparticles (NPs) are extensively used in industrial applications owing to their high redox-catalytic activities and, as a result, may appear in aquatic environments where other redox-active species may coexist. To better predict the fate and transport of these nanomaterials, a comprehensive, mechanistic understanding of the physicochemical behaviors and transformation of CeO₂ NPs in complex, redox-active aqueous systems is needed. In this study, we investigated redox reactions of CeO₂ NPs with Fe²⁺ and Cr(VI) (i.e., model redox-sensitive species) at pH 5. We found that the coexistence of 0.1 mM Fe²⁺ and 1 (or 5) μM Cr(VI)(aq) promoted formation of Fe(III) (hydr)oxides and increased CeO₂ NP colloidal stability. Specifically, without Cr(VI), Ce³⁺(aq) was rapidly released from the CeO₂/Fe²⁺ redox reaction, while the subsequent oxidation of Fe²⁺ to Fe³⁺ and formation of Fe(III) (hydr)oxides was slow. However, when Fe²⁺ and Cr(VI) coexist with CeO₂ NPs, the dissolution of CeO₂ NPs was slower than without Cr(VI), and Fe(III) (hydr)oxide precipitation on and near CeO₂ NPs significantly increased. The fast formation of Fe(III) (hydr)oxides can be attributed to facilitated Fe³⁺ hydrolysis by Cr(VI)(aq). Consequently, these new hybrid Fe(III)–CeO₂ NPs (i.e., CeO₂ NPs coated with Fe(III) solid phases) formed during redox-induced surface transformations exhibited a higher hydrophilicity, a more positive surface charge, and a greater colloidal stability compared to CeO₂ NPs in systems without Cr(VI). These findings reveal unexplored changes in surface chemistry and mobility of CeO₂ nanomaterials in complex redox-active aqueous systems.