Selective precipitation of phosphate from semiconductor wastewater

Selective precipitation of phosphate from fluoride-containing wastewater was studied using magnesium salts. Wastewater sampled from a semiconductor manufacture was found to contain 936 mg/L of fluoride (F-), 640 mg/L of sulfate (SO 4 2-), 118 mg/L of phosphate (PO 4 3-), and 26.72 mg/L ammonium (NH 4+). Magnesium chloride (MgCl₂) was more effective than magnesium oxide (MgO) in inducing precipitation reactions between magnesium and phosphate. Effects of both molar ratio ([Mg2+]: [PO 4 3-]) and pH were examined and experimental results were compared with those from equilibrium modeling by PHREEQC. A total of 41.72% of phosphate was removed and recovered when at molar ratio of 3:2 and pH of 10. Precipitation of phosphate increased with increasing molar ratio and 66.19% of phosphate was recovered when molar ratio was 3:1. Precipitation of phosphate increased as pH changed from 8 to 10. However, it decreased as pH increased from 10 to 12, probably because of competition between phosphate and hydroxyl (OH-) ions. Solid precipitates were characterized by field emission scanning electron microscope with energy dispersive spectrometer and X-ray diffraction. In accordance with theoretical modeling, it was found that the precipitate in pH range of 8-10 was predominantly bobierrite [Mg ₃ (PO₄) ₂.8 H₂ O] and some amorphous precipitates. However, brucite [Mg (OH)₂] precipitate would start to form when pH became higher than 10. Results showed that MgCl₂ can be a selective precipitation reagent for phosphate removal from semiconductor wastewater since it does not form precipitate with fluoride, sulfate, and ammonium.