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.