Synthesis and behavior of a magnetic composite based on crushed autoclaved aerated concrete for low-concentration phosphate removal

In this work, a novel magnetic composite using crushed autoclaved aerated concrete (MCAAC) as a carrier was prepared and applied to low-concentration (1 mg/L) phosphate removal. The structural and physical properties of MCAAC were characterized by X-ray fluorescence, X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, and saturation magnetization. Relevant mechanisms for phosphate removal under different conditions were also investigated. The adsorption and precipitation were studied separately to evaluate their roles in phosphate removal. The influence factors, including pH, calcium ion concentration, contact time, and Fe3O4 load, were studied and optimized. Results showed that MCAAC had been successfully synthesized and easily separated from the reaction mixture by a portable magnet. The phosphate removal rate by MCAAC reached 93.99% in the first 5 min and achieved reaction equilibrium (99.88%) in 120 min. The removal mechanism for low-concentration phosphate by MCAAC included a fast and large removal representing precipitation, then a slow and long removal due to adsorption. With the addition of phosphate, the pH and calcium ion concentration decreased in the suspension of MCAAC and hydroxyapatite (Ca-5(OH)(PO4)(3)) formed on the surface of MCAAC, suggesting calcium phosphate precipitation may be a major mechanism in phosphate removal. Besides, comparison of the relative contribution of the adsorption and precipitation to the total removal of phosphate revealed that adsorption accounted for 20%-40%. The adsorption process was fitted to Langmuir model and pseudo-second-order kinetic model. All results demonstrated that MCAAC could be used as a promising material for low-concentration phosphate removal.