The role of Mn oxide doping in phosphate removal by Al-based bimetal oxides: adsorption behaviors and mechanisms

This study investigated the behaviors and mechanisms of phosphate adsorbed onto manganese (Mn) oxide-doped aluminum (Al) oxide (MODAO). The isotherm results demonstrated that the maximum amount of phosphorus (P) adsorbed onto MODAO was 59.8 mg/g at T = 298 K (pH 6.0). This value was nearly twice the amount of singular AlOOH and could increase with rising temperatures. The kinetic results illustrated that most of the P was adsorbed onto MODAO within 5 h, which was shorter than the equilibrium time of phosphate adsorption onto AlOOH. The Elovich model effectively described the adsorption kinetic data of MODAO because of its heterogeneous surface. The optimal solution pH for phosphate removal was approximately 5.0 because of electrostatic interaction effects. Meanwhile, the decrease in P uptake with increasing ion strength suggested that phosphate adsorption occurred through an outer-sphere complex. Phosphates would compete for adsorption sites on the surface of MODAO in the presence of fluoride ion or sulfate. In addition, the spectroscopic analysis results of Fourier transform infrared spectroscopy and X-ray photoemission spectroscopy indicated that removal mechanisms of phosphate primarily include adhesion to surface hydroxyl groups and ligand exchange.