Dissolution and transformation of cerium oxide nanoparticles in plant growth media

From environmental modeling of engineered nanomaterial (ENM) release, it is clear that ENMs will enter soils, where they interact with soil compounds as well as plant roots. We analyzed three different size groups of cerium dioxide nanoparticles (CeO₂-NPs) in respect to chemical changes in the most common plant growth medium, Hoagland solution. We created a simple environmental model using liquid dispersions of 9-, 23-, and 64-nm-uncoated CeO₂-NPs. We found that CeO₂-NPs release dissolved Ce when the pH of the medium is below 4.6 and in the presence of strong chelating agents even at pH of 8. In addition, we found that in reaction with Fe²⁺-ions, equimolar amounts of Ce were released from NPs. We could elucidate the involvement of the CeO₂-NPs surface redox cycle between Ce³⁺ and Ce⁴⁺ to explain particle transformation. The chemical transformation of CeO₂-NPs was summarized in four probable reactions: dissolution, surface reduction, complexation, and precipitation on the NP surface. The results show that CeO₂-NPs are clearly not insoluble as often stated but can release significant amounts of Ce depending on the composition of the surrounding medium.