Study on the efficacy of micron zero-valent iron on phosphate removal and its mechanism

In order to study the effect and its mechanism of phosphate removal by micron zero-valent iron (mZVI), some important water chemistry parameters which affected the removal efficiency of phosphate were investigated firstly. Then, the phosphate removal efficiency by mZVI and the dynamics distribution of phosphate in the corrosion products of mZVI including magnetic iron oxide and suspended iron oxide were studied, and the changes of physical and chemical parameters (pH, DO, ORP) during the reaction process were monitored and the reaction process in the system further analyzed. Finally, the reaction products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS) and other characterization methods. The results showed that lower pH could significantly increase the reactivity of mZVI and accelerate the removal of phosphate. The increase of ionic intensity could accelerate the reaction rate of mZVI. ${\mathrm{AsO}}_4^{3-}$ and ${\mathrm{SiO}}_3^{2-}$ had a significantly inhibitory effect on the phosphate removal, while ${\mathrm{NO}}_3^{-}$ and ${\mathrm{CO}}_3^{2-}$ were the opposite and ${\mathrm{SO}}_4^{2-}$ had little effect on it. The content of the removed phosphate in the magnetic solid was equivalent to that in the suspended solid. SEM analysis of the reaction product showed that the morphology of the solid surface under the phosphate and non-phosphate conditions was different. XPS analysis indicated that Fe²⁺ and Fe³⁺ were mainly produced on the phosphate removal by mZVI. The 1,10-phenanthroline masking experiment evidenced that the combination of phosphate and Fe²⁺was the main way to remove phosphate by mZVI. The XRD results showed that phosphate precipitated with Fe²⁺ as H₂FeP₂O₇, which proved that precipitation existed during phosphate removal by mZVI.