Effects of Commonly Occurring Metal Ions on Hydroxyapatite Crystallization for Phosphorus Recovery from Wastewater

Hydroxyapatite crystallization for phosphorus recovery and removal from wastewater has attracted considerable attention for its potential economic and environmental benefits because hydroxyapatite can used as an effective compound fertilizer containing phosphorus (P) for industrial and agricultural applications. As hydroxyapatite is obtained through precipitation and crystallization from wastewater, it is important to evaluate the roles of metal ions commonly found in wastewater during the hydroxyapatite crystallization process. Batch crystallization experiments were conducted to investigate the influence of Mg2+, Fe3+, Cu2+, and Zn2+ on P removal efficiency, and crystallized products were characterized using scanning electron microscopy as well as energy dispersive spectroscopy and X-ray diffraction. The presence of Mg2+ improved the phosphorus removal rate, but hydroxyapatite crystalline purity was reduced due to the co-precipitation of struvite and hydroxyapatite. Fe3+ and Cu2+ did not significantly affect the crystalline structure of hydroxyapatite because the two metal ions easily formed hydroxyl metal compounds with low solubility in alkaline solution, which is rarely involved directly in the hydroxyapatite crystallization process. There was strong background interference from Zn2+ on the hydroxyapatite X-ray diffraction spectra, indicating that the crystallized products comprised a mixture of several amorphous substances. A comprehensive understanding of the effects of metal ions on hydroxyapatite crystallization will help improve the quality of hydroxyapatite products recovered from wastewater.