Removal behavior and mechanism of Co(II) on the surface of Fe–Mn binary oxide adsorbent

• Fe–Mn binary oxide was synthesized using simultaneous oxidation and coprecipitation.
• It has high removal efficiency toward Co(II).
• The thermodynamic parameters of Co(II) sorption on Fe–Mn binary oxide was calculated.
• The mechanism of Co(II) on the surfaces of Fe–Mn binary oxide was studied.
• Mn was reduced when Co(II) was oxidized to Co(III), while Fe showed no difference.

An amorphous Fe–Mn binary oxide adsorbent was developed using simultaneous oxidation and coprecipitation, and characterized by Brunauer–Emmett–Teller analysis, X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The removal behavior of the prepared absorbent for cobalt (II) was studied. Results showed that iron and manganese in adsorbent existed mainly in the +III and +IV oxidation states, respectively. The adsorbent exhibited the best depletion of Co(II) at pH 6–8, and the best removal ability at an ionic strength of 0.01 mmol L−1. The Freundlich isotherm model fitted the Co(II) adsorption data best. The FTIR spectra of the absorbent before and after Co(II) adsorption showed that the surface hydroxyl groups, Mn–OH and Fe–OH, existed at the surface and formed Mn–O–Co or Fe–O–Co. The iron oxide was the main adsorbent, and the manganese oxide also showed adsorption and oxidation ability. The XPS spectra confirmed that the Mn in the absorbent was reduced whereas the Fe showed no difference after reaction with Co(II). The XPS measurements of cobalt adsorbed on the adsorbent indicate that Co(II) has been oxidized to Co(III).

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