Adsorption of organic dyes by HDPy+-modified clay: Effect of molecular structure on the adsorption

The effect of the molecular structure of three dyes such as methyl orange (MO), indigo carmine (IC) and phenol red (PR) on the surface of Tunisian HDPy+-modified clay has been studied. Batch sorption experiments were carried to evaluate the influence of pH, contact time, initial dye concentration, and temperature. All studies showed a difference in the adsorption capacity suggesting that the retention of dyes was influenced by structure and functional groups of dyes. The pH results suggested that the adsorption of MO dye on HDPy+-clay was pH-independent and the IC and PR adsorption capacity decreased slightly along with the increasing solution pH of 6.0. These results provided that dye adsorption onto modified clay should be mainly controlled only by hydrophobic interaction mechanism for MO dye, along with a considerable contribution of the anionic exchange mechanism for IC and PR dyes. The adsorption onto HDPy+-clay attained equilibrium quickly within 30 min for IC, PR and 60 min for MO. Four adsorption kinetics equations (pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich) were employed to evaluate experimental kinetics data. The adsorption kinetic followed the pseudo-second order kinetic model. The monolayer adsorption capacity of the adsorbent, predicted by the Langmuir model, was 227.27, 326.40, and 344.82 mg/g for MO, IC, and RP, respectively. The adsorption isotherms suggeted that HDPy+-clay could be employed as low-cost material for the removal of dyes from wastewater. Thermodynamic parameters indicated the feasibility and spontaneity of the adsorption process. Its process was exothermic in nature for MO dye and endothermic in nature for IC and PR dye. (C) 2019 Elsevier B.V. All rights reserved.