Effect of Cd(II) on the ripening of ferrihydrite in alkaline media

Abstract

To acquire a better understanding of the influence exerted by the presence of Cd + during the process of transforming ferrihydrite to goethite, the morphological and structural changes of several samples obtained by the addition of Cd + to a suspension of nascent goethite were explored, and their chemical reactivity in acid media assessed. The samples (series Gi) were obtained by adding, at different times during the synthesis process, Cd 2+ ions to ferrihydrite (Fe5HO 8 .4H 2 O) formed in alkaline media. The suspensions were aged for 5 days at 70°C, and the amorphous materials were extracted using a HCl solution (series G HCl-i ). The X-ray diffraction (XRD) patterns showed that a goethite-like phase was formed, and chemical analyses indicated that the Cd content, x Cd , increased with the earlier addition of the Cd + ions to the Fe oxyhydroxide suspension. Lattice parameters and cell volume, obtained by the Rietveld simulation of XRD data, indicated an enlargement of the cell parameters of goethite in line with the Cd-for-Fe substitution. In order to determine the influence of oxalate ions on the non-extracted solids, a second set of samples was also prepared that was kept in contact with an ammonium oxalate solution for 4 h (series G ox-i ). The dissolution behavior of two series of Cdgoethites and of a third series, obtained from coprecipitation of Fe + and Cd + ions in alkaline media, was observed. Kinetics measurements in 4 M HCl showed that the initial dissolution rate of samples G ox-i decreased with aging time, while the opposite effect was observed for series G HCl-i . Dissolution-time curves were well described by the Kabai equation, and activation energies were calculated using the Arrhenius equation. The results indicate that the presence of Cd during the crystallization process of goethite leads to the formation of a Cd goethite with modified morphology, structural parameters, and chemical reactivity.