Single and binary adsorption of azo and anthraquinone dyes by chitosan-based hydrogel: Selectivity factor and Box-Behnken process design

Abstract

Adsorption of a binary mixture of anionic dyes onto chitosan-based hydrogel (SAH) was investigated. Erichrome black T (EBT) and reactive blue 2 (RB2) were used as a model azo and anthraquinone dyes respectively. Adsorption potential of SAH was investigated under laboratory conditions as a function of initial concentration (50–400 mg/L), temperate (25–55 °C) and solution pH. In a single dye system, adsorption trend follows the Langmuir and pseudo second-order kinetics models. The overall rates of EBT and RB2 were noted to be controlled by film, pore and particle diffusions throughout the adsorption period. For binary dye-system, selectivity factor (A) was introduced into the extended Langmuir model, and the equilibrium data fitted well with the modified isotherm model (R2 = 0.96–0.99). The values of ΔG° (−17.012 to −22.473 kJ/mol) of the binary dye system are within the ranges of physisorption mechanism. Box-Behnken model was utilized to estimate the competitive effects of the variables in binary dye system, and the experimental data indicates higher affinity for EBT (520.21 mg/g) compared with RB2 (407.05 mg/g). The reusability of SAH has proven that the adsorbent is durable and cost-effective.