The influence of sucrose and glycerol on the formation and transformation of iron oxides — The implication for soil formation

Abstract

The influence of sucrose and glycerol on the laboratory transformation of synthetic lepidocrocite and on the synthesis of Fe(III) oxide phases by Fe 2+ oxidation in solution was studied by powder X-ray diffraction, transmission electron microscopy and by measuring hydrolysis rates. The morphologies of samples were compared with those of similar Fe oxides sampled from organic rich pedogenic environments. The transformation of lepidocrocite to goethite in M KOH at 80°C is inhibited by a sufficient concentration of sucrose or glycerol. Dissolution of the serrated-ended laths of lepidocrocite occurs and the dissolved Fe is recrystallised on the surfaces of the undissolved lepidocrocite, rather than forming goethite as expected in the absence of organic compounds. The influence of sucrose and glycerol on the synthesis of lepidocrocite during the controlled oxidation of Fe(II) chloride solutions at pH 7 depends on the nature of the organic additive and its concentration. The presence of sucrose at 0.06 M concentration retarded lepidocrocite formation and caused ferrihydrite to form. Glycerol at high concentrations (0.15 M) also caused ferrihydrite formation even though its presence almost doubled the reaction time. However, at lower concentrations of glycerol, goethite in association with ferrihydrite were the preferred products. Four natural Fe oxide samples from organic-rich pedogenic environments (placic horizons and surface water precipitates) were examined and found to exhibit morphological features similar to those produced by synthesis in the presence of sucrose and glycerol. This indicates that simple organic compounds in the soil influence the formation of ferrihydrite associated with goethite and poorly crystalline lepidocrocite.