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