Comparative study on arsenate removal mechanism of MgO and MgO/TiO2 composites: FTIR and XPS analysis

The adsorption of arsenate (As(V)) ions onto magnesium oxide (MgO) and magnesium oxide/titanium oxide (MgO/TiO2) adsorbents was investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and zeta potential analysis to reveal the molecular mechanism. Water and carbon dioxide molecules in air changed the surface structures of MgO and TiO2, which resulted in Mg(OH)(2) formation and surface carbonate-like species. On the surface of MgO, unidentate carbonate, bicarbonate and bidentate carbonate were formed, which did not contribute to As(V) removal. The formed Mg(OH)(2) directly reacted with As(V) to form magnesium arsenate, which was confirmed by XRD and FTIR. On the TiO2 surface monodentate carbonate, bicarbonate and CO2- species were confirmed by FTIR. Among them, the surface monodentate carbonate species participated in the ion exchange process towards As(V) ions, which was determined by FTIR and quantified by calculation from the XPS C 1s spectra. The occurrence of FTIR bands at 844 cm(-1) (As-O bond) and 808 cm(-1) (As-OTi bond) suggested that the protonated bidentate binuclear complex, (TiO)(2)AsO(OH) was possibly the dominant species on the TiO2 surface.