Synergistic Effects of Organic Ligands and Visible Light on the Reductive Dissolution of CeO2 Nanoparticles: Mechanisms and Implications for the Transformation in Plant Surroundings

Ceriumoxide (CeO2) nanoparticles are one of the mostimportant engineered nanomaterials with demonstrated applicationsin industry. Although numerous studies have reported the plant uptakeof CeO2, its fate and transformation pathways and mechanismsin plant-related conditions are still not well understood. This studyinvestigated the stability of CeO2 in the presence of organicligands (maleic and citric acid) and light irradiation. For the firsttime, we found that organic ligands and visible light had a synergisticeffect on the reductive dissolution of CeO2 with up to30% Ce releases after 3 days, which is the highest release reportedso far under environmental conditions. Moreover, the photoinduceddissolution of CeO2 in the presence of citrate was muchhigher than that in maleate, which are adsorbed on the surface ofCeO(2) through inner-sphere and outer-sphere complexation,respectively. A novel ligand-dependent photodissolution mechanismwas proposed and highlighted: upon electron-hole separationunder light irradiation, the inner-sphere complexed citrate is morecapable of consuming the hole, prolonging the life of electrons forthe reduction of Ce(IV) to Ce(III). Finally, reoxidation of Ce(III)by oxygen was observed and discussed. This comprehensive work advancesour knowledge of the fate and transformation of CeO2 inplant surroundings. Visiblelight and organic ligands synergistically causethe reductive dissolution of CeO2, and the extent of photopromotionrelies on the interaction between CeO2 and ligands.