Enhanced adsorption of anionic dyes by surface fluorination of zinc oxide: A straightforward method for numerical solving of the ideal adsorbed solution theory (IAST)

Abstract

This paper describes an experimental and modeling analysis of methyl blue (MB) and acid orange 7 (AO7) adsorption from aqueous solutions in single and binary systems on surface fluorinated ZnO as a novel, low-cost and facile synthesized adsorbent. The adsorbents were characterized by FT-IR, XPS, and zeta potential analyzer. In the single-component adsorption tests, four adsorption isotherm models, including the Freundlich, Langmuir, BET and Dubinin–Radushkevich (D–R) were investigated. The equilibrium data for the adsorption of MB demonstrated the best fit to the Freundlich model, while AO7 adsorption was best represented by the BET model. The experimental data from bisolute adsorption of anionic dyes at relatively high concentration were analyzed by incorporating the data derived from the single-component isotherms into the ideal adsorbed solution theory (IAST). A straightforward numerical solution for IAST is proposed in this study and the predicted results are compared with the conventional solutions. For the bisolute system, incorporating single-component isotherms based on “best-fit” criteria, i.e. Freundlich for MB and BET for AO7, into the IAST gave the best prediction. The proposed method is applicable to any pair of single-component isotherms, regardless of the complexity and number of parameters. Three models, namely, pseudo-first-order, pseudo-second-order and intra-particle diffusion were employed to explain the adsorption kinetics. The adsorption of target dyes on fluorinated ZnO could be well described by the pseudo-second-order kinetic model. Maximum values of the rate constant, k2, obtained from the MB and AO7 adsorption data, were 0.110 and 0.078 (g mg−1min−1), respectively. Intra-particle diffusion model showed that the adsorption rates were not exclusively controlled by the diffusion step.