Removal of cationic dyes from a synthetic effluent using a calcium surfactant

The present study aimed to evaluate the removal efficiency of two types of cationic dyes of different classes – methylene blue (MB) and malachite green (MG) – from a synthetic effluent using a calcium surfactant (CaSF) originated from used frying soybean oil. The Fourier Transform Infrared Spectroscopy (FTIR) spectra showed that the functional groups present on the surface of CaSF can form surface complexes or bonds with the dye molecules and, consequently, promote their adsorption. The adsorption kinetics studies indicated that the equilibrium point of the process is reached in 90 min for both dyes. Equilibrium studies indicated that the adsorption isotherm models that best fit MB and MG were the Langmuir and the Dubinin–Radushkevich models, respectively. The maximum adsorption capacities of MB and MG, according to the Langmuir model, were 199 and 123 mg·g−1, respectively. In the sight of the high MB and MG removal efficiency (84 and 100%, respectively), the use of CaSF is an excellent alternative for the treatment of effluents contaminated by cationic dyes. The adsorption–desorption cycle studies showed that CaSF maintains a good dye removal efficiency for up to three cycles.