Natural Portuguese clayey materials and derived TiO2-containing composites used for decolouring methylene blue (MB) and orange II (OII) solutions

The use of some Portuguese clay materials as alternative low-cost adsorbents, as well as substrates to immobilize titania for decolouring methylene blue (MB) and orange II (OII) dye solutions, was studied. For MB adsorption studies, the experimental data were mathematically fitted using Langmuir and Freundlich adsorption models. The clayey materials showed distinct adsorption behaviour. The dye removal was high with the use of the bentonite-rich sample collected on the Benevila region (AV), the other illitic–kaolinitic (OV) and kaolinitic (BA) presented lower efficiency. The studied clays were not capable to adsorb OII dye molecules, due to their anionic character. The removal of OII was improved by using photocatalysis, testing new clay/titania composites prepared by mixing distinct proportions of each component. Decolouring of OII solutions reached acceptable levels (between 94–80%), but the materials are less efficient than pure commercial TiO2 powder. Concerning MB solutions, the combined effect of adsorption and photocatalysis resulted in very satisfactory performances, even for mixtures containing relatively low content of TiO2 catalyst. By testing titania gel impregnated clays, high OII decolouration efficiency (up to 99%) is obtained in short time (2 h). However, the decolouration of MB solutions by these materials was much slower (20 h) although the final colour attenuation reached 96.5%.