Quantitative, morphological, and structural analysis of Ni incorporated with struvite during precipitation

Struvite precipitation is a promising strategy for the simultaneous recovery of nitrogen and phosphorus from waste streams. However, waste streams typically contain high amounts of metal contaminants, including Ni, which can be easily sequestered by struvite, but the behavior of Ni during struvite precipitation remains unclear. Thus, this study investigates the influence of Ni concentrations on struvite precipitation. The quantitative X-ray diffraction (QXRD) results revealed that the purity of struvite decreased from 96.6 to 41.1% with the Ni concentrations increased from 0.1-100 mg·L^-1. At lower Ni concentrations of 0.1-1 mg·L^-1, scanning electron microscopy (SEM) showed a roughened surface of struvite crystal, and this was combined with X-ray absorption near edge structure (XANES) data that indicated a stack of Ni-OH and Ni-PO₄ on struvite surface. At Ni concentrations of 10-25 mg·L^-1, Ni primarily crystalized as Ni-struvite (NiNH₄PO₄·6H₂O), as detected by QXRD. At higher Ni concentrations of 25-100 mg·L^-1, the co-precipitation of amorphous Ni phosphate(s) (e.g., Ni₃(PO₄)₂) and Ni hydroxide (e.g., Ni(OH)₂) was identified by XANES. Specifically, the X-ray photoelectron spectroscopy (XPS) analysis detected the formation of amorphous Mg hydroxide(s) and phosphate(s) at Ni of 25-100 mg·L^-1. The overall results revealed that Ni formed Ni-OH and Ni-PO₄ on struvite surface at 0.1-1 mg·L^-1, whereas Ni precipitated as separated phases (e.g. Ni-struvite, Ni hydroxide and phosphate) at 10-100 mg·L^-1. The existence of Ni disturbed the crystal growth of struvite and promoted the formation of Ni-struvite, amorphous products during struvite formation.