The behavior of phosphate adsorption and its reactions on the surfaces of Fe–Mn oxide adsorbent

Abstract

Abstract A Fe–Mn oxide adsorbent (FMO) with the Fe/Mn molar ratio of 5:1 was combined to remove phosphate in aqueous solution. An apparently irregular surface and heterogeneous porous structure were found in the SEM images of FMO. XPS spectra showed that manganese and iron existed mainly in the +IV and +III oxidation states, respectively. Batch experiments were carried out to investigate the effect of contact time, initial phosphate concentration, temperature, pH and coexisting anions on phosphate removal. Kinetic data of phosphate adsorption onto FMO followed the pseudo-second-order kinetic model, indicating that the phosphate adsorption process was chemisorption. The equilibrium data were analyzed by both Freundlich and Langmuir isotherm models and the Langmuir isotherm model fitted better with the maximum adsorption capacity of 18.4 mg/g at 308 K and pH 7.0. Thermodynamic parameters demonstrated that the phosphate adsorption process was nonspontaneous and endothermic. The phosphate adsorption was critically dependent upon pH value with the maximum adsorption amounts occurring under acidic conditions and decreased with the increase of solution pH. Additionally, the terminal pH increased under acidic conditions while decreased under alkaline conditions. Coexisting anions of nitrate, silicate, sulfate and carbonate had no negative effect on the adsorption of phosphate, inferring the inner-sphere surface complexes were involved in the phosphate adsorption process. Phosphate adsorption by FMO was achieved by electrostatic attraction and replacement of hydroxyl groups. FMO, with large adsorption capability, is a promising attractive adsorbent for phosphate removal from water body.