Synthesis and characterization of mixed Fe(III):Al hydroxides and oxides with potential application as pigments obtained from the mixture of goethite and boehmite gels

Abstract

This work proposes a methodology modification on the sol-gel based process reported in the literature to obtain mixed hydroxides and oxides of Fe³⁺ and Al³⁺. The method was based on the mixture of the hydroxide gels precursors obtained under different conditions of pH and Fe:Al ratios. The materials were characterized by X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), energy dispersive spectroscopy, and in relation to the optical properties by colorimetric technique. In general, the mixed hydroxides had higher crystallinity when mixing the gels at a Fe/Fe+Al molar ratio of 0.6. It was observed the influence of the Al content added in the structuring of the oxide phase, where the isomorphic substitutions of Fe³⁺ by Al³⁺ occurred; these substitutions were identified by XRD and FTIR. The characteristics observed by these techniques reflected in the color properties of the obtained materials, where different colorations were observed depending on the crystalline structure and the aluminum content in the materials. The materials obtained by the methodology employed are low cost and possible to be applied as pigments in the paint industry.

Keywords:
mixed hydroxides; mixed oxides; colorimetry; ceramic pigments