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

摘要

Phosphorus can be reclaimed from nutrient-rich sources as the mineral struvite (MgNH4PO4·6H2O) for reuse as fertilizer. This study determines the impact of initial pH (pHi) from 8 to 11 on the fraction of precipitated struvite from a MgCl2-(NH4)2HPO4-NaCl-H2O system. The rate of P removal from solution increases with pHi and maximizes at pHi 10. Scanning electron microscopy (SEM) of recovered precipitates shows changes in morphology and decreasing particle size with increasing pHi. 31P nuclear magnetic resonance spectroscopy (NMR) confirms that struvite constitutes 96-99% of the phosphate at pHi 8-10, with newberyite (MgHPO4·3H2O) as a minor crystalline phase. At pHi 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 pHi 8-10 and by the low percentage of struvite at pHi 11. Overall, results indicate that both the amount of P recovered and the fraction of struvite are optimized at pHi 10. These findings can be used as a starting point in the selection of a suitable pHi for struvite recovery from nutrient-rich wastes.