A systematic study for the removal of anionic dyes by sepiolites modified with a homologous series of trimethylammonium-surfactants from single and binary component solutions

Trimethylammonium surfactants have been extensively used for modification of sepiolite to improve its sorption ability for anionic dye removal. Here, a detailed systematic sorption study is presented to choose the best surfactant for modification having the highest removal efficiency for different types of anionic dyes belonging to anthraquinone (Lanaset-Green-B), azo (Lanaset-Red-2B) and complex (Lanaset-Red-G) subclasses. Adsorption kinetics and equilibria of the dyes onto the pristine and decyl-, dodecyl-, teradecyl- and hexadecyl-tetramethylammonium sepiolites were investigated in single and binary component systems. Multilayered arrangements have been proposed for surfactants on the sepiolite based on x-ray diffraction and diffuse-reflectance-Fourier-transform spectroscopy analyses. The arrangements and the size of the surfactants comparing to dyes play important role in the adsorption process. The proposed dye-surfactant interactions are consistent with all kinetics and equilibrium results. The tetradecylammonium modified sepiolite has the best capacity for all dyes (similar to 0.32 mmol/g) at the first-stage of isotherms and is independent of the molecular structure of dyes. This correlated to the numbers of free head groups of surfactant oriented upward. The highest capacity is 0.50 mmol/g for Lanaset-Red-2B at the second stage of the isotherms on the dodecylammonium modified sepiolite which is attributed to dye-dye interactions.