At a low temperature, lepidocrocite-to-goethite transformation occurred in the presence of Fe(ii) ions, whereas lepidocrocite was stable in the absence of Fe(ii) ions.

This paper described the preparation of low crystalline lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) by air oxidation of Fe(OH)2 under irradiation by visible light in the presence of trace ethylenediaminetetraacetic acid (EDTA). The oxidation rate could be significantly accelerated by visible light in the presence of EDTA. This provided an alternative approach to the synthesis of low crystalline γ-FeOOH. A possible oxidation mechanism of iron(II) was proposed, which involved the formation of an FeIIEDTA complex under light irradiation to produce O2 •-/HO2 •, leading to the acceleration of the oxidation rate. The phase transformation from low crystalline FeOOH to hematite (α-Fe2O3) was also investigated. In contrast with the high crystalline FeOOH, the low crystalline γ-FeOOH and α-FeOOH converted easily to α-Fe2O3. The α-Fe2O3 nanoparticles exhibited uniform shape and a narrow size distribution.

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