Mineralogy and reactivity of amorphous Si-ferrihydrites

Abstract

Si-containing ferrihydrate is an amorphous Fe oxide that exists in natural environments. The objectives of this study were to evaluate mineralogy and surface composition and the reactivity of Si-containing ferrihydrites for phosphate. Ferrihydrite samples with 0 to 1 Si/Fe molar ratios were synthesized in the laboratory by the reaction of Fe 2(SO 4) 3 with NaOH to an equilibrium pH of 8.2 in the presence of Na 2SiO 3. Samples were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS). Electron diffraction, XRD, IR, and XPS indicated that samples with Si/Fe molar ratios ≥ 0.10 had Si incorporated into the structure. For samples containing Si/Fe molar ratios ≤ 0.05, Si was adsorbed on the oxide surface predominantly as silicate monomers and possibly as small units of polymerized silica linked at only one or two corners of the OSiO tetrahedra. Phosphate adsorption by samples containing Si/Fe molar ratios ≤ 0.05 was predominantly due to replacement of surface-OH groups. The IR study indicated that the phosphate anions were adsorbed on the oxide surface by a binuclear-bridging complex. For ferrihydrite samples containing Si/Fe molar ratios ≥ 0.10, phosphate ions replaced silicate ions held by a monodentate ligang-binding mechanism and/or as small units of polymerized silica. For each of the phosphate-treated ferrihydrite samples, the P (2p) XPS peak was observed at 133.3 eV, which indicated that HPO 4 2− was the dominant species adsorbed on the oxide surface.