A kinetic and structural analysis of the effects of Ca- and Fe ions on struvite crystal growth

Abstract

Developing an optimal waste treatment strategy for phosphate recovery requires a thorough understanding of the kinetics and mineralization processes of struvite (MgNH4PO4·6H2O) growth. In the present study, struvite was grown in MAP (magnesium, ammonium, and phosphate) solutions with a fixed Ca/Mg ratio (1:1) and ferric dosing. Furthermore, the kinetic rate constant of struvite growth in relation to pH change was determined, and precipitates formed from the solution were examined using XRD (X-ray diffraction) and SEM (scanning electron microscopy) methods. At pH values of 7.5 and 9, calcium and ferric ions appear to have significantly controlled struvite production toward the formation of calcium and iron phosphates. The lowest (1 ppm) and highest (372 ppm) ferric ions dosed throughout the study formed precipitates with a significant struvite content when the pH solution was 9. The presence of Ca2+ and Fe3+ changed the morphology of the rod-like struvite, resulting in irregularly shaped crystals. Managing the smallest ferric ions in wastewater can improve phosphorus recovery for high-quality fertilizer products. The use of ferric-calcium complex as a precipitant may also be advantageous in terms of phosphorus recycling.