A 31P NMR and TG/DSC-FTIR Investigation of the Influence of Initial pH on Phosphorus Recovery as Struvite
2014
Ma, Ning | Rouff, Ashaki A. | Phillips, Brian L.
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.
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