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

Abstract

Abstract 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 °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.