Cerium oxide nanoparticles transformation at the root-soil interface of barley ( Hordeum vulgare L.).

The transformation of cerium oxide nanoparticles (CeO ₂ -NPs) in soil and its role in plant uptake is a critical knowledge gap in the literature. This study investigated the reduction and speciation of CeO ₂ -NPs in barley ( Hordeum vulgare L.) cultivated in soil amended with 250 mg CeO ₂ -NPs kg ^-1 soil. Synchrotron micro-X-ray fluorescence (μXRF) was employed for spatial localization and speciation of CeO ₂ -NPs in thin sections of intact roots at the soil-root interface. Results revealed that Ce was largely localized in soil and at the root surface in nanoparticulate form (84-89%). However, a few hot spots on root surfaces revealed highly significant reduction (55-98%) of CeO ₂ -NPs [Ce(IV)] to Ce(III) species. Interestingly, only roots in close proximity to hot spots showed Ce uptake which was largely CeO ₂ (89-91%) with very little amount Ce(III) (9-10%). These results suggest that the reduction of CeO ₂ -NPs to Ce(III) is needed to facilitate uptake of Ce.