Transformation of Hydroxycarbonate Green Rust (GR1(CO32-)) into Ferric (oxyhydr)Oxides: The Role of Silicate.

Green rusts (GRs) commonly occur in the environment and affect the redox processes in suboxic environments, such as paddy soils, ground-water, sediments and iron corrosion products. The transformation of GR1(CO₃^2-) was conducted in the presence of silicate (Si) and followed by combining solution analysis and product characterizations by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). Results indicate that Si significantly affects the transformation of GR1(CO₃^2-) through adsorption on the mineral surfaces. The transformation time and the type of transformation products and their crystallinity and morphology all depend on the Fe/Si molar ratio. The transformation can be promoted in the presence of low Si concentration as compared to the control without Si. The products are goethite for Fe/Si ratios of 48 - 12, but the crystallinity and particle size of goethite decrease and its morphology changes from acicular (absence of Si) to plate-like or isodimensional particles with decreasing Fe/Si, comparable to those of natural goethite samples commonly found in soils. At Fe/Si = 3, the products are platelets of EX-GR1 with some goethite and ferrihydrite. The possible pathway of oxidative GR1(CO₃²⁻) transformation in the presence low concentrations of Si (Fe/Si ≥ 12) and in the absence of Si could be denoted as GR1(CO₃²⁻) → amorphous γ-Fe^IIIOOH-like phase → α-Fe^IIIOOH via a dissolution-oxidation-precipitation mechanism. In addition, the transformation of amorphous γ-Fe^IIIOOH-like phase to α-Fe^IIIOOH is rapidly catalyzed by Fe²⁺ released during oxidative transformation of GR1(CO₃²⁻) into amorphous Fe^IIIOOH. Clearly, the oxidative transformation of green rust to various crystalline iron (oxyhydr)oxides depends on the type and concentration (Fe/anion molar ratio) of co-existing silicate and other anions. Therefore, it is suggested that the prominent occurrence of natural goethite in soils in the form of plate-like or isodimensional particles will be, at least partly, related to the ubiquitous presence of silicates in soil environments.