Recovery of phosphate and ammonium nitrogen as struvite from aqueous solutions using a magnesium-air cell system

The addition of alkaline and magnesium sources during the recovery of NH₄⁺ and PO₄^3- in the form of struvite using the traditional struvite precipitation method increases the production cost. To solve this problem, a magnesium-air cell (MAC) system was used herein to recover NH₄⁺ and PO₄^3- as struvite from wastewater using a magnesium strip (Mg²⁺) and the oxygen adsorbed on the surface of a titanium plate (OH^-) as the anode and cathode, respectively. Experimental parameters (i.e. initial solution pH, temperature, NH₄⁺/PO₄^3- molar ratio, NH₄⁺ and PO₄^3- initial concentrations and stirring intensity) were found to affect the removal rate of NH₄⁺ and PO₄^3-. The presence of Ca²⁺ decreased the struvite purity. At Ca²⁺/PO₄^3- ratios of 0:1 and 0.5:1, the purity of the obtained struvite after 6 h was 93.8% and 58.9%, respectively. Struvite with a purity of 95.7%, electricity with an average output power of 2.53 mW, and an energy density of 1.05 W/m² were obtained when the proposed system was used to recover NH₄⁺ and PO₄^3- from an actual supernatant of domestic sludge anaerobic digestion. Scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analyses showed that the obtained struvite exhibited almost the same physicochemical properties as commercial struvite. Thus, the MAC system can be regarded as an effective method for recovering NH₄⁺ and PO₄^3- in the form of struvite from wastewater.