Sunlight Switches Natural Organic Matter (NOM)-Mediated Redox Pathways and Suppresses CeO₂ Nanoparticle Dissolution

The environmental transformation of engineered CeO₂ nanoparticles is strongly mediated by natural organic matter (NOM); however, the interplay between NOM and light in regulating their fate remains unclear. Here, we systematically investigated the dissolution behavior of CeO₂ in the presence of NOM under dark and illuminated conditions. Results revealed that NOM-induced Ce³⁺ release was significant in the dark but suppressed under visible light, which is a striking phenomenon validated in both aqueous and soil systems. Spectroscopic and microscopic analyses reveal that in the dark, the carbonyl-rich aliphatic components of NOM preferentially coordinate with Ce(III) at the oxygen vacancy sites of CeO₂, promoting continuous ligand-driven Ce³⁺ release. Under visible light, however, the dominant interfacial redox pathway shifted. We found that the photoactivation of quinone chromophores within macromolecular NOM diverts interfacial electron transfer away from surface Ce(IV) reduction toward dissolved O₂, initiating a cascade of reactive oxygen species (ROS) formation that degrades NOM and stabilizes the CeO₂ surface against dissolution. In contrast to previously reported light-enhanced dissolution of metal oxides by NOM, this work reveals that the effect of NOM photochemistry is fundamentally governed by the intrinsic dissolution pathway of the oxide and thus provides a conceptual basis for predicting when illumination will enhance or suppress metal release in natural environments.