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

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.