Abstract
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.
Highlights
Phosphorus (P) is a limited and non-renewable mineral resource that makes a major contribution to agricultural and industrial development [1], and the main element for causing water eutrophication [2,3]
Madsen [24] investigated the inhibition behavior of Cu2+ and Zn2+ on crystal growth and the morphology of brushite (CaHPO4 ·2H2 O), and the results showed that both two metal ions demonstrated strong inhibitory effects on the crystallization of brushite
Yan and Shih [21] quantitatively evaluated the effects of Ca2+ and Fe3+ on struvite crystallization, and the results indicated that both Ca2+ and Fe3+
Summary
Phosphorus (P) is a limited and non-renewable mineral resource that makes a major contribution to agricultural and industrial development [1], and the main element for causing water eutrophication [2,3]. The HAP crystallization process includes nucleation and crystal growth, which are affected by physicochemical factors such as pH, supersaturation, Ca/P, temperature, mixing, and impurity ions. The P crystallization process includes nucleation and crystal growth processes affected by various factors such as the pH, supersaturation ratio, temperature, and mixing, and by the existence of foreign ions [21]. We explored the optimal initial pH and Ca/P ratio for HAP crystallization, and four metal ions (Mg2+ , Fe3+ , Cu2+ , and Zn2+ ) common in municipal sewage were selected to observe their effects on HAP crystallization at different concentrations. The results of this study will improve the current understanding of the HAP crystallization mechanism and provide a promising approach for the effective recovery of quality HAP products from wastewater as a renewable nutrient resource
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