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Substitution-mediated enhanced adsorption of low concentration As(V) from water by mesoporous MnxFe3−xO4 microspheres

Yili Xie,Chen Tian,Weiyi Chen,Can Wu,Zhangbin Liu,Ping Ning,Hong Deng,Zhang Lin

Manganese ferrite (MnxFe3−xO4) microspheres with a spinel structure are very effective adsorbents for arsenic (As) removal. In this study, highly ordered mesoporous MnxFe3−xO4 has been synthesized by a facile and convenient solvothermal method. By comparing the different doping amounts of manganese (Mn), the results show that Mn1.8Fe1.2O4 has a relatively higher efficiency for arsenate (As(V)) removal due to the modified surface complexes caused by the optimum Mn substitution. Rietveld XRD refinement reveals that Mn1.8Fe1.2O4 microspheres possess the highest distribution ratio of Mn/Fe situated between the octahedral (Oh) and the tetrahedral (Td) sites of the unit cell. The as-prepared Mn1.8Fe1.2O4 microspheres with MnO6Oh sites in Fdm symmetry exhibit a promising ability for toxic As(V) adsorption, which is mainly attributed to the increase of coordinated complexes. Extended X-ray absorption fine structure (EXAFS) analysis indicates that the coordination structures of As(V) on Mn1.8Fe1.2O4 are present as bidentate binuclear complexes with a regular distance of As–Fe = 3.41 Å. Apart from this, As(V) can also bind with Mn-doped sites through the monodentate coordination mode at a distance of As–Mn = 3.51 Å with a preferred coordination number (CN) of 4.4. This work discloses the correlation between the superior As(V) adsorption ability and MnO6Oh sites in Mn1.8Fe1.2O4 and provides an emerging and promising method to enhance the adsorption capacity of As via modifying Mn doping sites based on the pollutant structure to achieve synergistic adsorption effects.