Characterization of Ni-Containing Goethites by Mössbauer Spectroscopy and Other Techniques

Abstract

A set of goethite samples from the oxidized level of Vermelho Ni-lateritic deposit was investigated by TEM, XRD, TGA-DTA, FTIR and Mossbauer spectroscopy and compared to a series of synthetic goethite samples, with similar Ni content. Natural Ni-containing goethite presents poor crystallinity and does not incorporate more than about 4 mol% Ni. Natural goethite particles also contain significant amounts of Al and Cr (1.6 and 0.7 mol%, in average, respectively). The wavenumbers of the OH deformation vibration bands δ-OH (in-plane vibration) and lengthening γ-OH (out of plane vibration) are coarsely negatively correlated with Ni content. The weight loss during the thermal analysis of synthetic samples shows a good linear relationship with Ni content, pointing to a proton uptake in order to compensate a possible charge imbalance due to a Ni2+ for Fe3+ substitution. The Mossbauer spectra of Ni-goethite present one sextet whose parameters are in agreement with those usually reported for goethite. The magnitude of the hyperfine field slightly decreases with increasing Ni, while the other parameters remain unchanged. These variations may be explained by Ni incorporation, but also by differences in the crystallinity. The effect of the low Ni concentration in our samples was a weak to medium correlation of the analytical signals with the Ni content in goethite, but when considered together, the results indicate that isomorphic substitution is occurring.