Characterization of Formation of Ferrous and Ferric Oxides in Aqueous Solution from a Multidisciplinary Viewpoint

This article reviews investigations on the formation of inhomogeneous ferrous (FeII) and ferric (FeIII) oxides in aqueous solution. These globally abundant materials are important in extensive fields of research including the corrosion of steels, mineralogy, environmental remediation, and biochemistry. Iron oxides are polycations consisting of FeII and FeIII ions bridged by hydroxyl groups and oxyanions. Solution conditions such as pH, temperature, the presence of foreign ions, and the presence of oxygen markedly influence the composition and structure of iron oxides and their transformation into various forms. Redox reactions of iron oxides are of great importance in controlling the extent of conversion between FeII and FeIII forms and the resulting composition of isolated species. Electrochemical potential versus pH (E-pH) diagrams are particularly useful in considering the details of iron oxide speciation as a function of solution conditions. Analytical techniques such as Mössbauer spectroscopy, X-ray diffraction, and electron microscopy are used to characterize the structure and morphology of iron-oxide samples. The involvement of mixedvalent (FeII/FeIII) species known as green rusts (GRs) in the formation of common iron oxides is described in terms of the oxidation level, pH, and foreign ion content of solutions. The terrestrial origin of iron oxides and their possible role in the abiogenesis of organisms also is described. Despite their inhomogeneity, the varied forms of iron oxides are important components of many natural and industrial processes.