A 31P NMR and TG/DSC-FTIR Investigation of the Influence of Initial pH on Phosphorus Recovery as Struvite

Phosphorus can be reclaimed from nutrient-rich sources as the mineral struvite (MgNH 4 PO 4 ·6H 2 O) for reuse as fertilizer. This study determines the impact of initial pH (pH i ) from 8 to 11 on the fraction of precipitated struvite from a MgCl 2 –(NH 4 ) 2 HPO 4 –NaCl–H 2 O system. The rate of P removal from solution increases with pH i and maximizes at pH i 10. Scanning electron microscopy (SEM) of recovered precipitates shows changes in morphology and decreasing particle size with increasing pH i . 31 P nuclear magnetic resonance spectroscopy (NMR) confirms that struvite constitutes 96–99% of the phosphate at pH i 8–10, with newberyite (MgHPO 4 ·3H 2 O) as a minor crystalline phase. At pH i 11, 60% of the solid is struvite, with 22% of the phosphate contained in an amorphous phase and 18% as sodium phosphate. Thermogravimetric analysis (TG) reveals a correlation in the mass loss from the solids with the percentage of struvite detected. Coupling Fourier transform infrared spectroscopy (FT-IR) with TG indicates that the molar concentration of evolved H 2 O(g) and NH 3 (g) is influenced by the adsorption of NH 4 + (aq) at pH i 8–10 and by the low percentage of struvite at pH i 11. Overall, results indicate that both the amount of P recovered and the fraction of struvite are optimized at pH i 10. These findings can be used as a starting point in the selection of a suitable pH i for struvite recovery from nutrient-rich wastes.