Fabrication of manganese ferrite (MnFe₂O₄) microsphere-coated magnetic biochar composite for antimonate sequestration: Characterization, adsorption behavior, and mechanistic understanding

A hierarchically structured manganese ferrite microsphere-coated magnetic biochar (BC) composite (MS-MnFe₂O₄/MBC) was synthesized by a facile solvothermal process to remove Sb(V) from aqueous solutions. In addition to conventional analytical techniques (e.g., XRD, SEM, TEM, XPS), X-ray absorption spectroscopy (XAS) measurements, including X-ray absorption near edge structure and extended X-ray absorption fine structure, were employed to characterize the synthesized MS-MnFe₂O₄/MBC and to explore its mechanism of interaction with Sb(V) in aqueous solutions. Hierarchically structured single-phase MnFe₂O₄ microspheres composed of numerous nanocrystallites (6.5–7 nm) were successfully fabricated on the BC surface. The XAS analysis results indicated that MnFe₂O₄ microspheres consisted of Mn²⁺ and Fe³⁺ atoms and possessed a partially inverse spinel structure with an inversion degree of 0.4. After adsorption, the combined results of X-ray photoelectron spectroscopy and XAS analyses demonstrated that the valence state of the adsorbed Sb species was Sb(V), and that its interaction with MS-MnFe₂O₄/MBC was attributed to the inner-sphere surface complexation through bidentate mononuclear edge-sharing and bidentate binuclear corner-sharing. The high potential of the adsorbent for Sb(V) removal was demonstrated in two actual water matrices, viz., tap water and river water. Overall, the findings indicate that MS-MnFe₂O₄/MBC has excellent practical applicability for Sb(V) removal from contaminated water.