We obtained natural red soil (RS), rich in iron, from the region of Palotina in the state of Paraná, Brazil. The RS sample was purified by suspension in water and sieved to remove plant particulates. It was then treated thermally at 800°C to remove organic volatiles; this sample was called RS800. The samples were characterized using X-ray diffraction, X-ray fluorescence, infrared and electronic spectroscopy, ζ-potential analysis and scanning electron microscopy. Colorimetric studies were performed according to the CIEL*a*b* system. Tests have shown that RS800 has the ability to remove methylene blue (MB) dye from wastewater. Thus, it was used as an adsorbent at various temperatures (25, 35, 45°C). According to the Langmuir model, the maximum adsorption capacity (qmax) was 23.256 mg g–1 (25°C). Unexpectedly, increasing temperature reduced qmax to 21.659 mg g–1 at 35°C and to 21.186 mg g–1 at 45°C. Therefore, RS800 must be used at room temperature (25°C), making its application in large-scale wastewater treatment feasible. After using RS800 as an adsorbent, the solids were filtered, dried, pulverized, and used as hybrid pigments in commercial white paints. Pigmented paints were used to paint a plaster specimen and colorimetric measurement was performed. These paints were tested for colour stability in acidic and alkaline environments. The results indicate that RS800 is efficient in the treatment of water contaminated with cationic dyes and can be reused as a hybrid pigment.

Artificially coloured smectites (smectite pigments) were prepared via the sorption of anionic dyes on smectite saturated with ferric ions (Fe-smectite). Fe-smectite has a surface charge and is capable of decolourizing aqueous solutions containing single-component anionic dyes (Amaranth, Brilliant blue FCF and Tartrazine) or multi-component dyes (dye mixtures). Kinetic and equilibrium models were used to describe the sorption of individual dyes, whereas the decolourization of the multi-component system was studied by monitoring the reduction in the intensity of the absorption bands in the visible light region. The Langmuir–Freundlich dual-site model presented the best fit to the experimental data, and the sorption kinetics followed the pseudo-second order model. The smectite pigments were dispersed in colourless paint (10%, w/w), acting as organic–inorganic hybrid pigments. Colorimetric measurements of the powdered smectite pigments dispersed in colourless real-estate paint showed chemical compatibility without the need for solvents as dispersants. These properties allow the application of coloured smectites as pigments in a sustainable circular economy.