Removal mechanism of selenite by Fe₃O₄-precipitated mesoporous magnetic carbon microspheres

Highlights: • MCMSs were prepared via green hydrothermal carbonization and coprecipitation. • MCMSs displayed effective removal of Se(IV) from wastewater. • Se(IV) formed inner-sphere complexes with MCMSs and was reduced to insoluble Se⁰. • MCMSs can be easily separated and recycled by an external magnetic field. - Abstract: A mesoporous composite of magnetic carbon microspheres (MCMSs) was synthesized via introducing Fe₃O₄ nanoscale particles to the surface of carbon microspheres (CMSs) by coprecipitation. Scanning electron microscopy and transmission electron microscopy showed the Fe₃O₄ nanoscale particles were dispersedly immobilized on the surface of CMSs. The MCMSs demonstrated effective removal of selenite (Se(IV)) from wastewater. MCMSs showed the regular pattern where the lower pH value, the lower residual Se(IV) concentration. The coexisting sulfate, nitrate, chloride, carbonate, and silicate had no significant effect on Se(IV) removal, whereas phosphate hindered the removal of Se(IV) by competing with Se(IV) and formed inner–sphere complexes with Fe₃O₄ on the surface of MCMSs. Through X–ray photoelectron spectroscopy analysis, Se(IV) can not only form inner–sphere complexes with MCMSs, but also be reduced to insoluble elemental selenium (Se⁰) by Fe₃O₄ which was oxidized and formed γ–Fe₂O₃. Moreover, the superparamagnetic MCMSs can be easily separated from solution by means of an external magnetic field. The high removal efficiency for Se(IV) and rapid separability of MCMSs made them promising materials for the application in the practice.