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 (MgNH4PO4.6H(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 MgCl2-(NH4)(2)HPO4-NaCl-H2O system. The rate of P removal from solution increases with pH; 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). P-31 nuclear magnetic resonance spectroscopy (NMR) confirms that struvite constitutes 96-99% of the phosphate at pH(i) 8-10, with newberyite (MgHPO4.3H(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 H2O(g) and NH3(g) is influenced by the adsorption of NH4+(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, for struvite recovery from nutrient-rich wastes.