Thermal Stability and Decomposition Products of P-Doped Ferrihydrite.

This work aimed to determine the effect of various amounts of P admixtures in synthetic ferrihydrite on its thermal stability, transformation processes, and the properties of the products, at a broad range of temperatures up to 1000 °C. A detailed study was conducted using a series of synthetic ferrihydrites FeHO·4HO doped with phosphates at P/Fe molar ratios of 0.2, 0.5, and 1.0. Ferrihydrite was synthesized by a reaction of Fe(SO) with 1 M KOH at room temperature in the presence of KHPO at pH 8.2. The products of the synthesis and the products of heating were characterized at various stages of transformation by using differential thermal analysis accompanied with X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. Coprecipitation of P with ferrihydrite results in the formation of P-doped 2-line ferrihydrite. A high P content reduces crystallinity. Phosphate significantly inhibits the thermal transformation processes. The temperature of thermal transformation increases from below 550 to 710-750 °C. Formation of intermediate maghemite and Fe-phosphates, is observed. The product of heating up to 1000 °C contains hematite associated with rodolicoite FePO and grattarolaite FePO. Higher P content greatly increases the thermal stability and transformation temperature of rodolicoite as well.