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

Abstract

Manganese ferrite (Mn_xFe_3−xO₄) microspheres with a spinel structure are very effective adsorbents for arsenic (As) removal. In this study, highly ordered mesoporous Mn_xFe_3−xO₄ has been synthesized by a facile and convenient solvothermal method. By comparing the different doping amounts of manganese (Mn), the results show that Mn_1.8Fe_1.2O₄ 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 Mn_1.8Fe_1.2O₄ microspheres possess the highest distribution ratio of Mn/Fe situated between the octahedral (O_h) and the tetrahedral (T_d) sites of the unit cell. The as-prepared Mn_1.8Fe_1.2O₄ microspheres with MnO₆O_h 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 Mn_1.8Fe_1.2O₄ 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 MnO₆O_h sites in Mn_1.8Fe_1.2O₄ 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.