Influencing Factors and Product Analysis of Synthetic Tobermorite-seeded Crystallization of Calcium Phosphate for Phosphorus Removal and Recovery

摘要

With synthetic wastewater, lab-scale batch experiments were carried out to investigate the feasibility of synthetic tobermorite, a hydrothermally synthetic byproduct accompanied with the preparation of potassium salts from potassium feldspar, seeded crystallization of calcium phosphate for phosphorus removal and recovery. The products obtained were characterized by using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) techniques, powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR). The results show that the residual phosphorus (P) in aquatic phase after 24 h can meet the Chinese P discharge criteria of 0.5 mg·L-1 under the condition of the optimum pH value (about 8), Ca/P molar ratio (Ca/P=2:1) and seed dosage (2.4 g·L-1). Higher Ca and P initial concentration are favorable to crystallization of calcium phosphate. The co-existing citric acid, CO3 2- and Mg2+ can affect P removal efficiency. Individual NH4+does not show significant effect on P recovery, but once combined with Mg2+, more significant effect than individual Mg2+ was observed. Effects of temperature on the phosphorus removal show that the removal process of phosphorus by synthetic tobermorite is endothermic at 22-40 °C. The correlation of temperature with reaction order was obtained. The synthetic tobermorite can be used for more than 18 times and the main recovered product is hydroxyapatite (HAP) with lower crystallization degree. P recovery result from synthetic potato wastewater shows the synthetic tobermorite seed can be used for P removal and recovery from hard waters.