Research on simultaneous removal and recovery of phosphorus from sewage by tobermorite induced crystallization

A synthetic tobermorite crystal was obtained by hydrothermal method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR). Taking the simulated anaerobic digestion liquid as the treatment object, the performance of synthetic tobermorite-induced crystallization in removing phosphorus from wastewater was analyzed, and the effect of the seed crystal on CO₃ ²⁻ reduction was studied. The results showed that the purity of synthetic tobermorite was high, resulting in higher pH and the release of a large amount of Ca²⁺ in the water. After adding 2 g/L seed crystals into ultra-pure water and tap water, respectively, pH of the solutions could reach 11.7 and 9.1, with the concentration of Ca²⁺ reaching 43 and 30 mg/L, respectively. The Interface-controlled crystal growth model well described the removal process of phosphorus by synthetic tobermorite-induced calcium phosphate crystallization, and the reaction conformed to the second-order equation (n=2). The presence of CO₃ ²⁻ could inhibit the performance and rate of phosphorus removal in the crystallization reaction of calcium phosphate. However, it was found that the phosphorus removal rate and reaction rate were improved quickly after adding synthetic tobermorite seed, which had a certain eliminate with the interference of CO₃ ²⁻ in the precipitation reaction of calcium phosphate. Under the condition of maximum CO₃ ²⁻ concentration, the phosphorus removal rate increased significantly from 4.6% to 76.7%, and the rate constant k increased from 0.76 L/(mol·min) to 67.74 L/(mol·min). Moreover, after the synthetic tobermorite was reused for four times, the phosphorus removal efficiency was still higher than that without adding synthetic tobermorite, thus showing its good reusability.