Thermal Stability and Decomposition Products of P-Doped Ferrihydrite.

Gabriela Pieczara,Adam Gaweł,Olaf Borkiewicz,Grzegorz Rzepa,Maciej Manecki

doi:10.3390/ma13184113

Gabriela Pieczara, Adam Gaweł + Show 3 more

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https://doi.org/10.3390/ma13184113

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Abstract

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 Fe5HO8·4H2O doped with phosphates at P/Fe molar ratios of 0.2, 0.5, and 1.0. Ferrihydrite was synthesized by a reaction of Fe2(SO4)3 with 1 M KOH at room temperature in the presence of K2HPO4 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 FePO4 and grattarolaite Fe3PO7. Higher P content greatly increases the thermal stability and transformation temperature of rodolicoite as well.

Highlights

Ferrihydrite (Fe5 HO8 ·4H2 O) is a widely known hydrous ferric oxyhydroxide that is widespread in both terrestrial and aquatic systems [1]
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 small amount of potassium K was detected in all samples containing phosphate, increasing with increasing P content (Figure S1). This contamination comes from K2 HPO4, which was the source of PO4 in the synthesis

Summary

Introduction

Ferrihydrite (Fe5 HO8 ·4H2 O) is a widely known hydrous ferric oxyhydroxide that is widespread in both terrestrial and aquatic systems [1] It is formed in near-surface environments during the oxidation reaction of Fe(II) to Fe(III), as a nearly amorphous nanomineral with a high surface area [2,3,4,5], and a high concentration of active hydroxyl sites [6]. Natural ferrihydrite always contains a variety of impurities that affect its chemical and physical properties [12,13,14,15,16,17,18,19] Some impurities such as phosphate are important nutrients, while other impurities may be unwanted compounds, toxic metals and metalloids (e.g., Cd, Pb, As). In the presence of phosphate, the solubility of ferrihydrite decreases [25,26]

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