Removal of C.I. Basic Green 4 (Malachite Green) from aqueous solutions by adsorption using cyclodextrin-based adsorbent: Kinetic and equilibrium studies

Batch sorption experiments were carried out for the removal of C.I. Basic Green 4 (Malachite Green), a cationic dye from its aqueous solution using cyclodextrin-based material (CD/CMC material) as adsorbent. The operating variables studied were adsorbent mass, particle size, agitation speed, solution pH, contact time and initial dye concentration. Adsorption experiments indicated that the adsorption capacity was dependent of operating variables and the process was strongly pH-dependent. Kinetic measurements showed that the process was uniform and rapid. Sorption of dye reached equilibrium in 120 min. In order to investigate the mechanism of sorption, adsorption data were modeled using the pseudo-first-order and pseudo-second-order kinetic equations, and intraparticle diffusion model. On the basis of the non-linear chi-square test, it was found that the adsorption kinetics followed a pseudo-second-order model for the dye concentration range studied in the present work, suggesting that the rate-limiting step may be chemisorption. Equilibrium isotherm was analyzed using the Langmuir and the Freundlich isotherms. The characteristic parameters for each model have been determined. The Langmuir model yields a much better fit than the Freundlich model. The maximum sorption capacity was 91.9 mg/g at 25 °C and the negative value of free energy change indicated the spontaneous nature of adsorption.