XRD-Rietveld analysis as a tool for monitoring struvite analog precipitation from wastewater: P, Mg, N and K recovery for fertilizer production

Abstract

The objective of this study was to select between four proposed synthesis conditions, which one would be the most propitious for struvite precipitation from wastewater and, on such product, detail its crystallochemical aspects using the Rietveld refinement method to determine the chemical formula and chemistry analyses by Wavelength Dispersive X-ray Fluorescence (WDXRF) for validation. The struvite synthesis was investigated for pH 10 and 12 and phosphorus concentrations of 100 and 300 mg/L under the same room temperature (RT) condition and a magnesium to phosphorus to nitrogen (Mg: P: N) molar ratio of 1: 1: 2. Struvite was successfully obtained under a moderately alkaline pH (10). No struvite precipitation occurred under the same temperature and Mg: P: N molar ratio for a highly alkaline pH (12). Under RT condition, 300 mg/L P, a Mg: P: N molar ratio of 1: 1: 2, and a pH of 10, a product with two struvite phases was obtained: an 75% struvite analog ([K0.34(NH4)0.66]MgPO4.6H2O) and 25% struvite (NH4MgPO4.6H2O). Last, the Rietveld tool is useful for monitoring struvite production from wastewaters, since the chemical ratio (stoichiometry of the phase) is what will define its classification in: struvite, struvite analog, K-struvite or mixed. In addition, it will help in the classification of the fertilizer product (chemical content of nutrients).

Keywords

• Struvite
• Struvite analog
• Rietveld refinement
• X-ray diffraction – XRD