Adsorptive removal of phosphate by a nanostructured Fe-Al-Mn trimetal oxide adsorbent

A nanostructured Fe-Al-Mn trimetal oxide adsorbent for phosphate removal with a Fe:Al:Mn molar ratio of 3:3:1 was prepared using simultaneous oxidation and coprecipitation method. The adsorbent was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscope (SEM) with an EDAX (energy dispersive analysis of X-ray). The results indicated that the Fe-Al-Mn trimetal oxide was amorphous and nanostructured. Zeta potential measurements showed that it possessed a high isoelectric point (around 9). The phosphate removal gradually decreased with the increasing of pH (4-10.5). The adsorption isotherms on the adsorbent were described by Langmuir, Freundlich and Temkin models at pH 6.8 and followed the fitting order: Freundlich > Temkin > Langmuir. At 25 degrees C, the maximum adsorption capacity for the adsorbent was about 48.3 mg/g, which was higher than their reported single component oxide. The kinetic data were described better by the pseudo-second-order adsorption kinetic rate model. Thermodynamic analyses indicated that the phosphate adsorption on the Fe-Al-Mn trimetal oxide was endothermic and spontaneous in nature. The sequence of coexisting anions studied competing with phosphate was SiO32- > HCO3- > SO42-. The results of zeta potential, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that electrostatic attraction and replacement of surface hydroxyl groups (M - OH) by phosphate via the formation of inner-sphere complex were the main adsorption mechanism. The Fe-Al-Mn trimetal oxide with good specific affinity towards phosphate was a promising adsorbent for phosphate removal from natural waters and municipal wastewaters. (C) 2012 Elsevier B.V. All rights reserved.