Transformation Processes and Kinetics in the Magnesium Phosphate Mineral System

Phosphorous is an important nutrient for all living beings. Together with nitrogen, it is a crucial element for the global agriculture, yet it is a limited natural resource. The mineral struvite (MgNH4PO4·6H2O) occurs in guano deposits, wastewater pipelines, and pathogenic urinary stones. It´s recovery and reutilization as a slow release fertilizer has become a common process in wastewater treatment. However, it´s instability under atmospheric conditions leads to transformation to other phosphate phases, a process that is still poorly understood.

We investigated this transformation by reacting synthetic, μm- and mm-sized struvite crystals at different temperatures in open and closed systems. We monitored the transformation of struvite with time using ex-situ X-ray diffraction (XRD) and processing with Rietveld refinement. Complementary, transformations of mm-sized crystals were investigated by imaging with optical and scanning electron microscopy and by analysis with Raman spectroscopy.

XRD analysis shows that struvite transforms to newberyite (Mg(PO3OH)·3H2O) at room temperature over several weeks by losing water and ammonium. Optical microscopy reveals newberyite as opaque, white or dark brown patches, inside crystals and at crystal sides. At elevated temperatures (37 and 60 °C), struvite predominantly transforms to dittmarite (MgNH4PO4·H2O) with newberyite as minor product, indicating a temperature-driven loss of water that is faster than ammonium loss. The presence of newberyite and dittmarite was confirmed by Raman spectroscopy. Transformation rates of both the open and closed system were evaluated. These findings are important to understand long-term processes that may influence the properties of wastewater-derived struvite as slow-release fertilizer.