Interplay of Structural Properties and Redox Behavior in CeO2 Nanoparticles: Impact on Reactivity and Bioavailability

The environmental redox transformation of CeO₂ is crucial for evaluating its ecological risk and understanding the geochemical cycling of cerium (Ce). In this study, we examined the effects of crystallinity on CeO₂ dissolution and monitored the structural evolution during redox transformations. The reductive dissolution and reoxidation behavior of CeO₂ (100 mg/L) was examined in the presence of 200 μM citrate. Our findings indicate that ligand-induced dissolution is more pronounced in CeO₂ with lower crystallinity under both dark and light conditions. This dependence is related to the intensive ligand complexation at oxygen vacancy sites, resulting in a higher complexation of Ce(III) and more efficient photoelectron generation for Ce(IV) reduction. During cyclic dissolution-reprecipitation, CeO₂ notably transformed into an amorphous phase, progressively decreasing the crystallinity of the nanoparticles. Consequently, the dissolution fraction of well-crystallized CeO₂ increased significantly from 1.2% in the first cycle to 11.4% in the third cycle, suggesting a transition to structures with higher interfacial reactivity. Similar transformation and dissolution behavior was observed in redox oscillations in a soil environment. Additionally, hydroponic exposure experiments with Arabidopsis thaliana, treated with 100 mg/L CeO₂ for 7 days, demonstrated increased Ce uptake by roots post-transformation, with a higher proportion of CePO₄ detected within the plants. This comprehensive study not only provides vital mechanistic insights into the transformation processes of CeO₂ but also aids in assessing the ecological risks associated with engineered CeO₂ nanomaterials.