Spatially Resolved Product Speciation during Struvite Synthesis from Magnesite (MgCO3) Particles in Ammonium (NH4+) and Phosphate (PO43–) Aqueous Solutions

While population growth necessitates a significant increase in crop production, stringent environmental regulations require that it be done using sustainable nutrient sources. Nutrients in the form of NH4+ and PO43– are recovered from wastewater streams via precipitation, using water-soluble magnesium ions to form sustainable, slow-release fertilizer, struvite (MgNH4PO4·6H2O). However, the magnesium needed is mainly incorporated in the crystal lattices of very low-solubility minerals. This work utilizes a combination of powder X-ray diffraction (pXRD) and ex situ Raman and energy-dispersive X-ray spectroscopies combined with ion chromatography to characterize transformation products of low-solubility MgCO3 particles in NH4+- and PO43–- containing aqueous solutions with and without Ca2+ ions present. Although pXRD showed struvite as the predominant solid product for the molar ratio [Mg2+/NH4+/PO43–] of [0.2:1:1] and higher, ex situ Raman spectra evidenced formation of a dypingite-like phase along with stru...