X-Ray Diffraction Analysis of the Effect of the NH4+: PO4³- Molar Ratio on the Formation of Crystallized Struvite and Recovery Rate of Phosphorus

Phosphorus and nitrogen are directly responsible for the eutrophication of water bodies, and thus their removal from wastewater is obligatory, although this is not performed in many cases. However, when it is considered that global phosphorus resources will be exhausted in the near future, phosphorus recovery from wastewater will become a more viable process. Crystallization of phosphorus and nitrogen in the form of struvite (MgNH₄PO₄-6H₂O), a slow-releasing and valuable fertilizer, is one feasible technique for this purpose. In this study, the effect of different NH₄ ⁺: PO₄ ^3- molar ratios on the formation of crystallized struvite and recovery rate of phosphorus was estimated by X-ray diffraction (XRD). By maintaining the same conditions (PO₄ ^3-P concentration 60 mgL^-1, molar ratio Mg²⁺:PO₄ ^3- of 1:1, and reaction pH 10.0), the results showed the precipitate was mainly composed of Mg₃(PO₄)2-22H₂O when the NH⁴⁺:PO₄ ^3- molar ratios were 1:1, 1.2:1 or 1.6:1. However, the precipitate became a mixture of Mg₃(PO₄)2-22H₂O and MgNH₄PO₄-6H₂O when the NH₄ ⁺:PO₄ ^3- molar ratio was 2:1. Finally, the precipitate was mainly MgNH₄PO₄-6H₂O when the NH₄ ⁺:PO₄ ^3- molar ratios were 3:1 and 6:1. Overall, the recovery rate of phosphorus increases from 51.6% to 84.6% with an increase of the NH₄ ⁺:PO₄ ^3- molar ratio from 1:1 to 6:1.