The Influence of [Fe(II)], [Si], and pH on the formation of lepidocrocite and ferrihydrite during oxidation of aqueous FeCl2 solutions

Abstract

Lepidocrocite was synthesized from the oxidation of FeCl₂ solutions with and without soluble Si and at various Fe(II) concentrations and pH. Pure well crystallized lepidocrocite was only obtained if CO₂ was excluded (to prevent goethite formation) and if pH was kept between 5 and 7 (to prevent maghemite and ferrihydrite formation). Si in solution prevented crystallization of lepidocrocite with increasing Si/Fe atomic ratio (varied between 0 and 0·156) and at the same ratio with decreasing pH. Most of the Si added is probably included in the lepidocrocite and ferrihydrite structure (up to 4·4% Si) and these Si contents are linearly correlated with the initial Si/Fe ratio. No indication from XRD and i.r. spectroscopy was found for changes in the lepidocrocite structure due to Si uptake. The ferrihydrite containing 4·35% Si showed a broad Si-O-stretching vibration which in contrast to pure silica (1080 cm⁻¹) is shifted to lower frequencies (940 cm⁻¹) indicating some Si-O-Fe bonds.