Gas–Liquid Reactions to Synthesize Positively Charged Fe₃O₄ Nanoparticles on Polyurethane Sponge for Stable and Recyclable Adsorbents for the Removal of Phosphate from Water

The application of most current phosphate adsorbents is limited by their high cost, low removal capacity, difficulty of recovery, and short lifetime. In this study, we developed a gas–liquid reaction assisted by a coordination method to prepare highly positively charged ferroferric oxide (Fe3O4) nanoparticles loaded on polyurethane sponge. It was found that the gas–liquid reaction drastically decreases the size and increases the loading capacity of Fe3O4 nanoparticles as compared with the conventional liquid method. Further, the use of trimethylamine vapor induced the coordination of Fe3+, facilitated the formation of free Cl ions, and inhibited the hydrolysis of Fe–Cl bonds, thus greatly decreasing the amount of hydroxyl groups and increasing the surface positive charge on Fe3O4 nanoparticles. As a result, the Fe3O4 nanoparticles in this study have a saturated PO43– adsorption capacity of 229.8 mg·g–1, which was appreciably higher than that of conventional Fe3O4 adsorbents (57.8 mg·g–1). Our study further revealed that the introduction of a thin layer of polyurethane coating on the surface of Fe3O4 nanoparticle-composited adsorbents could drastically improve their stability while preserving the adsorption capacity under the impact of water (500 rpm stirring for 72 h). The composited adsorbents also preserve the adsorption capacity after recycling three times. Finally, the adsorption experiment on real river wastewater indicated that the composited adsorbents enable the decrease of phosphate concentration from 0.6 to 0.02 ppm, reflecting the application potential for relieving phosphate pollution in neutral waters