Adsorptive removal of Congo red from aqueous solution using a chitosan-g-poly (acrylic acid-co-itaconic acid) hydrogel: equilibrium, kinetics and thermodynamics

Abstract

Textile effluent carrying Congo red (CR) is hard to treat, because the dye is highly soluble, chemically stable and toxic at low concentration. Here a hydrogel was prepared by graft copolymerising acrylic acid (AA) and itaconic acid (IA) onto chitosan, giving CH-g-poly(AA-co-IA), with potassium persulfate as initiator and N,N′-methylenebisacrylamide as crosslinker. It was examined by FTIR, XRD and FESEM, then tested on CR in batch mode. Uptake was strongly pH dependent: removal peaked at pH 3 (91.6 %) and fell to 14.2 % at pH 10, placing the protonated amine groups of the chitosan backbone at the centre of the binding step. A dose of 2 g L−1 removed more than 90 % of the dye from a 50 mg L−1 solution, and equilibrium was reached at about 210 min. Kinetic data were described best by the pseudo-second-order equation (R2 = 0.9974 by non-linear fitting, 0.9991 in linear form); the intraparticle diffusion plot was multilinear, with a fast diffusion-controlled stage followed by a much slower approach to equilibrium. Equilibrium data followed the Freundlich model (R2 = 0.9981, n = 2.20) more closely than the Langmuir model, which indicates an energetically heterogeneous surface; the Langmuir capacity was 100.7 mg g−1 at 298 K. The mean free energy from the Dubinin–Radushkevich model was 11.5 kJ mol−1. Thermodynamic parameters (ΔH° = +22.92 kJ mol−1, ΔS° = +88.74 J mol−1 K−1, ΔG° = −2.61 to −6.14 kJ mol−1) describe a spontaneous, endothermic and entropy-driven process.