Simultaneous removal of anionic and cationic dyes on quaternary mixtures by adsorption onto banana, orange and pomegranate peels

Abstract

Disposal of untreated wastewater released by industries using dyes, causes serious damage to aquatic life and to the ecosystem in general. Both physical and chemical methods have been used for the removal of these dyes from wastewater; among them, adsorption has been found to be a very efficient technique. In this study, the ability to simultaneously remove cationic (MB) and anionic dyes (RR120, RB5 and RBBR) from a quaternary mixture solution by using zero-cost materials as banana, orange and pomegranate peels, was investigated. The optimal removal rates were achieved at pH 2.0 and a dose of 6.0 g/L. The comparative results of natural adsorbents on dye removal, separately and simultaneously in solution showed that MB adsorption on banana and orange peels was augmented from 89 to 96 and 43 to 87%, respectively, in the presence of anionic dyes (synergistic effect). On the other hand, a competitive effect was determined, mainly between anionic dyes, resulting in a decrease in their respective percentage. Banana peels were selected as the adsorbent material for further study in a quaternary dye mixture containing all dyes. Parameters affecting dye adsorption such as pH solution, initial concentration of dyes, contact time and temperature were examined. Based on the experimental results, two mechanisms for interactions were proposed: (a) the cationic MB molecules are connected to the negatively charged anionic dyes or (b) electrostatic attractions occurred among cationic MB molecules and negatively charged anionic dyes in the solution enhancing the adsorption efficiency. Kinetic experiments showed that equilibrium for anionic dyes reached at around 180 min (3 h), and for MB was reached much faster, at 30 min; 87% of 20 mg/L was found to be removed in just 5 min. Langmuir isotherm model and pseudo-second-order kinetic model were best fitted to the experimental results. According to thermodynamics the spontaneous nature of their adsorption was confirmed.