Spectroscopic and Microscopic Identification of the Reaction Products and Intermediates during the Struvite (MgNH4PO4·6H2O) Formation from Magnesium Oxide (MgO) and Magnesium Carbonate (MgCO3) Microparticles

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• 作者：Erica Kirinovic ; Amanda R. Leichtfuss ; Criztel Navizaga ; Hanyu Zhang ; Jennifer D. Schuttlefield Christus ; Jonas Baltrusaitis
• 刊名：ACS Sustainable Chemistry & Engineering
• 出版年：2017
• 出版时间：February 6, 2017
• 年：2017
• 卷：5
• 期：2
• 页码：1567-1577
• 全文大小：730K
• ISSN：2168-0485

文摘

One of the key global challenges forthcoming will be maintaining a clean, useable natural water supply. Anthropogenic wastewater is an unavoidable result of population growth and societal development; therefore, the treatment of wastewater is of the utmost importance. The mineral struvite (magnesium ammonium phosphate hexahydrate, MgNH₄PO₄·6H₂O) is a crystalline material that occurs naturally in decomposing organic materials and been observed in sludge derived from the anaerobic digestion of animal farming liquid wastes and treated wastewater sludge. The accumulation of struvite on pipe walls and equipment surfaces has plagued the wastewater treatment industry though the formation of struvite prior to the treatment process could potentially provide a pathway for the sustainable recovery of the major nutrients nitrogen (N) and phosphorus (P). Current methods of nutrient removal from wastewater are mostly based on insoluble Fe, Al, and Ca salt formation followed by landfill disposal without returning them to the environment. Struvite is one of the most promising chemical platforms for recovering nutrients, which previously was done using expensive water-soluble magnesium salts. Our objectives were to examine the potential of low solubility, naturally abundant magnesium inorganic materials (MgO and MgCO₃) for the utilization of nutrient recovery from wastewater via time-resolved ex situ XRD, ATR-FTIR and Raman analyses, and SEM measurements to identify reactive intermediates and use spectroscopic data for kinetics analysis. Our data suggest that a common reactive intermediate between homo- and heterogeneously nucleated struvite exists that is due to the amorphous magnesium hydroxide structural units. The presence of low coordination O_4C–H, O_3C–H, and O_1C–H surface hydroxyl groups, associated with the surface steps, edges, and kinks, is proposed to enhance struvite formation, thus an increase in their abundance and stabilization are suggested for the preparation of MgO and MgCO₃ before the struvite recovery. Two different crystal morphologies (needle and rhomboidal) were observed for reactions with 600 and 4000 ppm of (NH₄)₂HPO₄, which were proposed to form due to the kinetic control of the reaction at higher concentrations. Finally, Raman spectroscopy was used to measure the relative kinetics of struvite formation utilizing the relative populations of the magnesium carbonate and struvite, as obtained from the area under the 950 and 1125 cm⁻¹, respectively, peaks providing a spectroscopic method to monitor reactive solid magnesium source conversion into struvite.