Dynamic adsorption of Ni(II) and Cd(II) ions from water using 8-hydroxyquinoline ligand immobilized PVDF membrane: Isotherms, thermodynamics and kinetics

摘要

In this study, 8-hydroxyquinoline (8-HQ) as a multi-dentate chelating agent was immobilized on the surface of PVDF microfiltration membrane via solvent evaporation method. The resultant membranes were applied for adsorptive elimination of nickel and cadmium ions from water. Dynamic adsorption tests were performed on the membranes at various ligand concentrations, pH values, contact times, temperatures and initial metal concentrations to optimize the adsorption condition. SEM-EDX analysis was performed for membrane surface characterization. Adsorption capacity of modified PVDF membranes was enlarged with increasing 8-HQ concentration from 0.5 to 2 wt.%; while, with further increase (2.5 wt.%), no considerable improvement was achieved. Adsorption was enhanced at higher pH values as well as lower temperatures. Compared to Langmuir and Freundlich isotherms, three-parameter Sips model could superiorly describe the adsorption equilibria. Moreover, adsorption of the ions on the membrane was favorable, exothermic (△H~0 < 0), and chemically natured (chemisorption) in light of thermal analyses results. The results also indicated that Cd(II) is more favorably adsorbed by the membrane compared to Ni(II). Finally, adsorption kinetic was examined using pseudo-first-order and pseudo-second-order models, with the latter suggesting superior accordance. Besides, intra-particle diffusion model detected that boundary layer resistance and solid-phase diffusion are promising rate-controlling mechanisms before and after equilibrium, respectively. Sequential adsorption/desorption experiments confirmed that the ligand modified membranes can be readily regenerated and reused.