Phase transformations in the ferrihydrite/cysteine system

Abstract

In the presence of cysteine, ferrihydrite transforms into haematite, goethite or lepidocrocite at pH 6–8. Which compound forms depends on the cysteine-iron ratio and the nature of the buffering medium. In the absence of bicarbonate ions (which suppress lepidocrocite formation), the transformation product changed from haematite and lepidocrocite, to lepidocrocite and goethite and finally to goethite as the cystein-iron ratio rose from 0.1 → 0.4. Interaction of cysteine with ferrihydrite leads to partial reduction of the interfacial ferric sites. Goethite and lepidocrocite formed from the partly reduced ferrihydrite by a dissolution/reprecipitation process, whereas haematite formed by a solid-state mechanism at a cysteine-iron ratio which was too low to permit extensive dissolution of partly reduced ferrihydrite to take place. Conversion of ferrihydrite to lepidocrocite under the influence of cysteine represents a new transformation route for the system. Transmission electron microscopy showed that the lepidocrocite thus formed consisted of well-developed laths or plates. Reducing organic ligands such as cysteamine, glutathione and penicillamine did not induce the formation of lepidocrocite.