Arsenate Adsorption on an Fe−Ce Bimetal Oxide Adsorbent: Role of Surface Properties

An Fe−Ce bimetal adsorbent was investigated with X-ray powder diffraction (XRD), transmission electron micrograph (TEM), Fourier transform infrared spectra (FTIR), and X-ray photoelectron spectroscopy (XPS) methods for a better understanding of the effect of surface properties on arsenate (As(V)) adsorption. In the adsorption test, the bimetal oxide adsorbent showed a significantly higher As(V) adsorption capacity than the referenced Ce and Fe oxides (CeO2 and Fe3O4) prepared by the same procedure and some other arsenate adsorbents reported recently. XRD measurement of the adsorbent demonstrated that the phase of magnetite (Fe3O4) disappears gradually with the increasing dosage of Ce4+ ions until reaching a molar ratio of Ce4+ to Fe3+ and Fe2+ of 0.08:0.2:0.1 (Fe−Ce08 refers to the adsorbent prepared at this ratio), and the phase of CeO2 begins to appear following a further increase of the Ce dose. Combined with the results of TEM observation, it was assumed that a solid solution of Fe−Ce is formed following the disappearance of the magnetite phase. Occurrence of a characteristic surface hydroxyl group (M−OH, metal surface hydroxyl, 1126 cm-1), which showed the highest band intensity in the solid solution state, was confirmed on the bimetal oxide adsorbent by FTIR. Quantificational calculation from the XPS narrow scan results of O(1s) spectra also indicated that the formation of the bimetal Fe−Ce08 was composed of more hydroxyl (30.8%) than was the formation of CeO2 and Fe3O4 (12.6% and 19.6%). The results of adsorption tests on Fe−Ce08 at different As(V) concentrations indicated that both the integral area of the As−O band at 836 cm-1 and the As(V) adsorption capacity increased almost linearly with the decrease of the integral area of M−OH bands at 1126 cm-1, proving that the adsorption of As(V) by Fe−Ce08 is mainly realized through the mechanism of quantitative ligand exchange. The atomic ratio of Fe on Fe−Ce08 decreased from 20.1% to 7.7% with the increase of the As atom ratio from 0 to 16% after As(V) adsorption, suggesting that As(V) adsorption might be realized through the replacement of the M−OH group of Fe (Fe−OH) with arsenate. The well splitting of three υ3 bands at As−O band (836 cm-1) of FTIR and the hydroxyl ratio (1.7) of Fe−Ce08 calculated from the XPS results suggested that the diprotonated monodentate complex (SOAsO(OH)2) is possibly dominant on the surface of Fe−Ce08.