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

The transformation of cerium oxide nanoparticles (CeO2-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 CeO2-NPs in barley (Hordeum vulgare L.) cultivated in soil amended with 250 mg CeO2-NPs per kg soil. Synchrotron micro-X-ray fluorescence (XRF) was employed for spatial localization and speciation of CeO2-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 (84-90%). However, a few hot spots on root surfaces revealed highly significant reduction (56-98%) of CeO2-NPs [Ce(iv)] to Ce(iii) species. Interestingly, only roots in close proximity to hot spots showed Ce uptake which was largely CeO2 (90-91%) with very little amount of Ce(iii) (9-10%). These results suggest that the reduction of CeO2-NPs to Ce(iii) is needed to facilitate uptake of Ce. Future studies should investigate the reducing agents (e.g. exudates, microbes) involved in CeO2-NPs in barley roots.