Investigations of Co(II) to Co(III) oxidation mechanisms at Fe_3-δO₄ nanoparticle surfaces

Understanding and predicting the interaction mechanisms between Co and spinel iron oxides (Fe_3-δO₄), including magnetite, maghemite and their solid solutions, is of major interest in both the environmental and industrial contexts. While the surface complexation of Co²⁺, its polymerization and the surface precipitation of Fe_3-δO₄@CoO have been well documented, the oxidation of Co²⁺ to Co³⁺ and interaction mechanisms between Co³⁺ and Fe_3-δO₄ nanoparticles in the presence of O₂ have attracted less attention. In this study, experimental and modeling results, combined with XAS and XMCD analyses at the L_2,3-edges, as well as TEM and XRD analysis, enabled the differentiation of several Co species according to the Co concentration, at pH 8 under atmospheric O₂. At the lowest Co concentrations investigated, Co²⁺ prevailed as surface complexes, incorporated into Fe_3-δO₄ nanoparticles, or as Co(OH)₂-like surface precipitates. Increasing the Co concentration led to the formation of an additional Co₃O₄-like phase on the surface of Fe_3-δO₄ nanoparticles. Quick-XAS measurements at the Co K-edge allowed the kinetics of Co sorption and oxidation on Fe_3-δO₄ nanoparticles to be followed, supporting the equilibrium observations. These results provide a more detailed understanding of mechanisms for Co sorption onto Fe_3-δO₄ nanoparticles under oxic conditions, which offers an alternative environmentally friendly route for the synthesis of Co-doped Fe_3-δO₄ nanoparticles. In addition, this enables understanding of the potential interactions between Co and Fe_3-δO₄ nanoparticles in environmental systems.