Revealing and quantifying the effect of cattiite coprecipitation on the purity of K-struvite in aqueous solution

Abstract

To investigate the effect of cattiite (Mg3(PO4)2·22 H2O) coprecipitate with K-struvite on the purity of K-struvite, K-struvite crystallization in aqueous solution was performed at different pHs and the initial Mg2+ concentrations. The coprecipitation was quantified by Simultaneous Thermal Analyzer, Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy as well as Inductively Coupled Plasma Mass Spectrometer. Genetic algorithm-back propagation artificial neural network model (GA-BPANN) was also innovatively employed to simulate and predict K-struvite purity. The results revealed that cattiite coprecipitated with K-struvite rather than bobierrite (Mg3(PO4)2·8 H2O) in the solution. The low initial Mg2+ concentration and high pH were more conducive to K-struvite crystallization, thereby inhibiting the cattiite co-precipitation with K-struvite. Specifically, K-struvite crystals with the purity of 93.88% were achieved at pH 11 and initial Mg2+ concentration of 3 mmol/L, whereas cattiite crystals with the purity of 98.99% were observed at pH 9 and the initial Mg2+ concentration of 50 mmol/L. Notably, K-struvite and cattiite crystals have obvious differences in morphology and particle size, separately exhibiting small rod-like and large plate-like structures. Interestingly, GA-BPANN accurately simulated and predicted K-struvite purity at various pHs and initial Mg2+ concentrations. The findings herein provide a theoretical foundation for obtaining high-purity K-struvite from waste streams at industrial scale, which also provide a reference for exploring the coprecipitation of multiple minerals in waste streams.