Spatially Resolved Product Speciation during Struvite Synthesis from Magnesite (MgCO3) Particles in Ammonium (NH4 +) and Phosphate (PO4 3–) Aqueous Solutions

• Published in: Journal of physical chemistry. C 2019-04, Vol.123 (14), p.8908-8922
• Main Authors: Lu, Baoying, Kiani, Daniyal, Taifan, William, Barauskas, Dovydas, Honer, Kenneth, Zhang, Lihua, Baltrusaitis, Jonas
• Format: Article
• Language: English
• Subjects: MATERIALS SCIENCE
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b12252

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 PO4 3– 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 PO4 3–- containing aqueous solutions with and without Ca2+ ions present. Although pXRD showed struvite as the predominant solid product for the molar ratio [Mg2+/NH4 +/PO4 3–] of [0.2:1:1] and higher, ex situ Raman spectra evidenced formation of a dypingite-like phase along with struvite. Single-crystal Raman spectroscopy in combination with scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy showed Ca2+ incorporation into the structure of struvite crystals as submicron crystallites at the Ca2+/Mg2+ ratio of 0.2, from both CaCO3 and CaCl2 and at the Ca2+/Mg2+ ratio of 1, in the case of CaCO3. Moreover, distinct solid product speciation was observed when Ca2+ was present in aqueous solutions when using CaCl2; for example, hydroxyapatite was observed for Ca2+/Mg2+ = 1 when CaCl2 was used. The results reported here unravel the effect of the physicochemical solution parameters, such as concentration of MgCO3, pH, Ca2+ concentration, and solubility of Ca-containing precursors, on the formation of struvite crystals. This shows that recovery of nutrients containing N and P from wastewater streams is possible in the form of a slow-release fertilizer (struvite) using low-solubility, abundant magnesium-containing minerals.