Copper and chromium(VI) removal by chitosan derivatives—Equilibrium and kinetic studies

Chitosan sorbents, cross-linked and grafted with amido or carboxyl groups, were prepared and their sorption properties for Cu(II) and Cr(VI) uptake were studied. Equilibrium sorption experiments were carried out at different pH values and initial ion concentrations. The equilibrium data were successfully fitted to the Langmuir–Freundlich (L–F) isotherm. The calculated maximum sorption capacity of the carboxyl-grafted sorbent for Cu(II) was found to be 318 mg/g at pH 6, while the respective capacity for Cr(VI) uptake onto the amido-grafted sorbent was found to be 935 mg/g at pH 4. Thermodynamic parameters of the sorption process such as ΔG0, ΔH0, and ΔS0 were also calculated. The experimental kinetic data were successfully fitted to a novel phenomenological diffusion-reaction model (DIFRE), which combines: (i) mass transfer of the metal ions from the bulk solution on the sorbent surface; (ii) diffusion of the ions through the swollen polymer particle; and (iii) instantaneous local chelation (for cations) or electrostatic attraction (for anions) on the amino groups of the polymer. The regeneration of sorbents was affirmed in four sequential cycles of sorption–desorption experiments, without significant loss in sorption capacity.