Defective magnesium ferrite nano-platelets for the adsorption of As(V): The role of surface hydroxyl groups.

In this work, magnesium ferrite (MgFeO) nano-platelets with rich defects and abundant surface hydroxyl groups were synthesized, and used for the removal of low concentration As(V) in aqueous solution. Results from scanning electron microscopy (SEM) showed that the as-synthesized MgFeO nano-platelets were consisted of many individual nanospheres. Rietveld refinement of X-ray diffraction (XRD) data indicated that the Mg ions substituted the Fe ions at both the octahedral and the tetrahedral sites of the crystal structure. Batch adsorption experiment showed that the equilibrium concentration of As(V) could be reduced down to 4.9 μg·L when the initial concentration of As(V) is 1 mg·L, which complied with the drinking water standard of WHO (10 μg·L). The adsorption capacity of synthesized MgFeO towards As(V) was higher than commonly used iron oxide adsorbents (FeO, γ-FeO and α-FeO). Mechanistic studies proved that the superior adsorption capacity was attributed to: (1) increased amount of surface hydroxyl groups that resulted from the surface defects. (2) formation of tridentate hexanuclear surface complexes instead of bidentate binuclear complexes, and (3) formation of excess Mg-OH surface hydroxyl groups and As-Mg monodentate mononuclear surface complexes. This work disclosed the correlation of the superior As(V) adsorption ability with the surface hydroxyl groups in defective MgFeO, and propose MgFeO as a potential candidate for the remediation of As-contaminated water.