Iron-dependent formation and structure-property relationship of struvite precipitated from simulated wastewater

Wastewater-derived struvite (MgNH₄PO₄·6H₂O) microcrystals could replace a significant fraction of traditional nitrogen and phosphorus fertilizers, supporting nutrient recycling and sustainability while mitigating environmental impacts such as water eutrophication and atmospheric pollution. However, phosphorus recovery is often hindered by iron interference, as iron (Fe) can strongly bind phosphate to form insoluble iron phosphate minerals, such as vivianite, through pH, redox, and organic matter dependent processes. These Fe-containing anthropogenic solids represent an unexplored source of anthropogenic iron flux into the environment. This study investigates the influence of Fe concentration on struvite formation using magnesium carbonate (MgCO₃) as a naturally abundant magnesium (Mg) source in simulated wastewater containing up to 500 ppm of Fe³⁺. Iron-containing struvite (Fe-struvite) microcrystals were successfully synthesized and characterized using complementary structural and spectroscopic techniques. Up to 150 ppm Fe³⁺ precursor concentration yielded well-defined crystalline Fe-struvite, whereas higher levels resulted in amorphous phases. Low Fe³⁺ levels (up to 10 ppm) caused a lattice contraction in struvite due to limited magnesium substitution. Raman micro-spectroscopy revealed that low Fe³⁺ concentrations (<10 ppm) allowed coexistence of hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) with struvite, whereas at >150 ppm Fe³⁺, only amorphous phosphate was present. Ion chromatography showed equilibrium aqueous phosphate (PO₄^3-) concentrations decreasing from 235 ppm to 38 ppm with increasing Fe³⁺, while ammonium removal from solution was inhibited. X-ray fluorescence analysis indicated that at >150 ppm Fe³⁺, an iron-rich amorphous phase with Fe : P ≈ 1 : 1.75 was produced. The findings indicate that recovered Fe-struvite can serve as an anthropogenic iron source to the environment.