A solid waste, crashed autoclaved aerated concrete, as a crystalline nucleus for the removal of low concentration of phosphate

In this study, crashed autoclaved aerated concrete (CAAC) was recycled as a solid waste for the removal of low concentration of phosphate (1 mg/L). The effects of dissolved calcium (Ca2+) and pH on phosphate removal were investigated. The materials including calcium chloride (CaCl2) and calcium carbonate (CaCO3) were selected to investigate the removal mechanism of low concentration of phosphate by CAAC. X-ray fluorescence, X-ray diffraction patterns, FT-IR, SEM, and EDS analyses were used to characterize the surface shape, contained elements, and internal structure of CAAC. Results showed that the removal rate of phosphate was increased according to the increase in pH. The CAAC showed poor effect on phosphate removal at acidic condition, but had great removal efficiency in the pH range of 10–12 due to the formation of hydroxyapatite (HAP, Ca5(OH)(PO4)3) corresponding to characterization. Dissolved calcium and insoluble calcium compounds in CAAC can combine with the phosphate and hydroxyl groups to form HAP in alkaline condition and subsequently remove the phosphate. SEM images exhibited the surface of CAAC including some pieces of irregular crystal. After phosphate removal, its surface was covered with a layer of agglomerations confirmed to be HAP according to characterization. To avoid adjusting the pH of solution to strong alkalinity, CAAC was modified by sodium hydroxide solution (NaOH) to increase its activity. Modified products showed a high efficiency for phosphate removal (98.67%). Collectively, the present results demonstrated that the employment of crystallization using CAAC could be a promising technology for the advanced treatment of phosphate from low concentration in municipal secondary effluent.