Description
Understanding the interactions between trace metals and organic matter (OM) is crucial due to its influence on metal speciation, mobility, toxicity, and bioavailability. Cerium (Ce) is of particular interest due to its industrial use, natural occurrence and unique redox chemistry. It exists in both the +III and +IV oxidation states, enabling redox cycling in surface environments. However, the mechanisms by which OM influences the redox behavior of Ce remains poorly understood. In this study, we combined advanced spectroscopic and microscopic analyses with thermodynamic modeling to elucidate the influence of OM on Ce redox processes over a pH range of 4–10 under oxic (ambient atmosphere) and anoxic (N2 anaerobic chamber) conditions. The results showed a strong pH dependent Ce oxidation in the presence of OM and O2, with negligible Ce(IV) at pH 5 and nearly complete oxidation at pH 10. Dissolved oxygen (O₂) was identified as the primary oxidant, as almost no Ce(IV) was detected under N2-atmosphere. At low [Ce], complexation of Ce(III) by OM was identified as key mechanism limiting its oxidation to Ce(IV) whereas, at high [Ce] or the absence of NOM, oxidation was driven by hydrolysis and precipitation of CeO₂ nanoparticles. OM exerted a broader control on Ce speciation by (i) constraining the crystal growth of these nanoparticles to ~2nm crystallites and (ii) accumulating Ce(III) within a ~1nm-thick layer around CeO₂ nanoparticles, tentatively attributed to the formation of CeO2-OM-Ce(III) ternary complexes. Overall, these findings provide fundamental insights into the impact of OM on cerium fate in natural environments.
KEYWORDS. Cerium, Organic matter, Redox, CeO2 nanoparticles
| Speaker information | PhD 2nd year |
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