Mechanism analysis of MnFeO/FeS for removal of Cr(VI) from aqueous phase.

By using NaS as a sulfur source, sulfur-doped MnFeO was prepared using one-step solvent thermal method and utilized to remove hexavalent chromium. The materials were characterized through scanning electron microscopy, transmission electron microscopy, high-resolution TEM, X-ray diffraction, Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller method, zeta potential test, vibrating sample magnetometry, and X-ray photoelectron spectroscopy. When the pH was 3, the adsorption capacity of MnFeO/FeS to Cr(VI) was 43.36 mg/g, which was roughly five times that of MnFeO (8.90 mg/g). MnFeO/FeS and MnFeO fitted the Freundlich and pseudo-second-order kinetic models well. The electrochemical test analysis results showed that MnFeO/FeS had a faster MnFeO electron transfer rate and higher electron transfer capacity than MnFeO, and thus promoted the reduction of Cr(VI) to Cr(III). This finding could be attributed to the lower electronegativity of the sulfur element than the oxygen element. In addition, the formation of additional FeS through sulfur doping improved the Cr(VI) removal ability of the prepared materials. The XPS and desorption results showed that more than 80% of the adsorbed Cr(VI) were reduced to Cr(III), which indicated that reduction was an important mechanism for Cr(VI) removal. This study verified that sulfur-doped manganese ferrite can be utilized in the high-efficiency removal of Cr(VI).