Hausmannite (Mn₃O₄) is an important sorbent for retaining ionic molecules and mediating redox reactions in reducing soil and sediment environments. Its metastable properties, ability for metal substitution, photocatalytic activity, and self-assembly are major reasons for the widespread industrial application of hausmannite, but such effects have not yet been fully characterized. In this study, the effects of metal substitution on the self-assembly, transformation, and morphological and colloidal properties of hausmannite nanoparticles were examined. Metal-substituted hausmannite with 2% of Co, Cu, Ni^, and Zn was prepared by hydrothermal synthesis. Samples were collected at various time points during the synthesis procedure and washed for further incubation. Batch incubations were conducted to study the transformation of the metal-substituted hausmannite at pH 4 and 7 under oxic conditions, using transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The results showed that the self-assembly during synthesis was inhibited in the Co, Ni^, and Zn treatments, while the self-assembly from nanosphere to octahedral particles was observed in the control and Cu treatment based on the TEM and bimodal Rietveld refinement of XRD data. Agglomeration of the nanospheres by Ostwald ripening was observed during the synthesis procedure, but not in the incubation experiments. The morphology and crystal structure observed from TEM and XRD results were significantly different between the treatments, which indicates that the metal substitution determines not only the morphology and the colloidal properties but also affects the crystal structure. After incubation under oxic conditions at pH 7, the control was completely transformed into manganite while no transformation was observed in the metal-substituted hausmannite. In addition, the transformation into an amorphous structure was identified at pH 4 in all samples. Further investigation using X-ray absorption and X-ray photoelectron spectroscopy will enable us to interpret the oxidation states, local structure, and chemical properties of amorphous structures. The results of this work deepen our understanding of Mn (hydr)oxides transformation and surface properties upon environmental redox perturbation.

Junho Han

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