Redox chemistry of CeO nanoparticles in aquatic systems containing Cr()(aq) and Fe ions

• Published in: Environmental science. Nano 2019-07, Vol.6 (7), p.2269-228
• Main Authors: Ray, Jessica R, Wu, Xuanhao, Neil, Chelsea W, Jung, Haesung, Li, Zhichao, Jun, Young-Shin
• Format: Article
• ISSN: 2051-8153; 2051-8161
• DOI: 10.1039/c9en00201d

CeO 2 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 CeO 2 NPs in complex, redox-active aqueous systems is needed. In this study, we investigated redox reactions of CeO 2 NPs with Fe 2+ and Cr( vi ) ( i.e. , model redox-sensitive species) at pH 5. We found that the coexistence of 0.1 mM Fe 2+ and 1 (or 5) μM Cr( vi )(aq) promoted formation of Fe( iii ) (hydr)oxides and increased CeO 2 NP colloidal stability. Specifically, without Cr( vi ), Ce 3+ (aq) was rapidly released from the CeO 2 /Fe 2+ redox reaction, while the subsequent oxidation of Fe 2+ to Fe 3+ and formation of Fe( iii ) (hydr)oxides was slow. However, when Fe 2+ and Cr( vi ) coexist with CeO 2 NPs, the dissolution of CeO 2 NPs was slower than without Cr( vi ), and Fe( iii ) (hydr)oxide precipitation on and near CeO 2 NPs significantly increased. The fast formation of Fe( iii ) (hydr)oxides can be attributed to facilitated Fe 3+ hydrolysis by Cr( vi )(aq). Consequently, these new hybrid Fe( iii )-CeO 2 NPs ( i.e. , CeO 2 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 CeO 2 NPs in systems without Cr( vi ). These findings reveal unexplored changes in surface chemistry and mobility of CeO 2 nanomaterials in complex redox-active aqueous systems. CeO 2 nanoparticles are extensively used in industrial applications owing to their high redox-catalytic activities and, as a result, may appear in aquatic environments where they undergo significant surface chemistry transformation with other redox-active species.