Geochemical cycle of exogenetic CeO2 nanoparticles in agricultural soil: Chemical transformation and re-distribution

Abstract

Investigating the geochemical behavior of CeO₂ nanoparticles (NPs) is important for understanding the environmental fate and risk of the exogenetic Ce in soil. However, systematic explorations of the interaction between critical soil components and CeO₂ NPs are lacking. In this report, a theoretical geochemical cycle of exogenetic CeO₂ NPs and CeCl₃ involving their response to soil organic matter (SOM) and Fe/Mn oxides were demonstrated by microcosm experiments. The Ce(IV) in CeO₂ NPs shows a slight reduction of (3.0–6.7%) after the treatment in humic acid. The reduction product was verified to be Ce(III)-oxalate. The transformation on chemical speciation stimulated the re-distribution of Ce in soil, with its main host phase changing from a residue state (94.68–96.50%) to an Fe/Mn oxide binding state (75.56–79.42%). Different from the SOM, Mn oxides (pyrolusite and nsutite) had an opposite redox effect on Ce compounds, greatly oxidizing the Ce(III) to Ce(IV) (95.2–100%). Simulated studies indicate that Mn oxide or ambient O₂ played a key role in the oxidation process. We speculate that disparate redox effects by soil components cause the speciation transformation and cyclic distribution and of Ce species in soil, potentially affecting the correlative mineral formation and metal bioavailability.