Removal of anionic and cationic dyes using porous chitosan/carboxymethyl cellulose-PEG hydrogels: Optimization,...

Chitosan (CS)/carboxymethyl cellulose (CMC) porous hydrogels chemically crosslinked by epichlorohydrin were synthesized using polyethylene glycol (PEG) as a pore-forming agent for anionic (Congo red, CR) and cationic (methylene blue, MB) dyes removal from aqueous solutions. The swelling ratio of hydrogels prepared with 2 % CS and 2 % CMC (CS₂/CMC₂) exhibited optimal performance at different pHs. The addition of PEG into hydrogels (denoted as CS₂/CMC₂-PEG_1.25) exhibited a significantly higher adsorption for CR and MB, increasing from 117.83 to 159.12 mg/g and 110.2 to 136 mg/g, respectively. The comprehensive analyses of Fourier transform infrared spectroscopy, thermalgravimetric study and scanning electron microscopy showed that CS₂/CMC₂-PEG_1.25 hydrogels became more porous with no significant changes in intermolecular and intramolecular interactions, compared with CS₂/CMC₂ hydrogels. The adsorption process for CR and MB conformed to the pseudo-second-order and pseudo-first-order kinetics models, respectively. The results of adsorption isotherm for CR followed both Freundlich and Langmuir models with the maximum adsorption capacities of 1053.88 mg/g, whereas the isotherm for MB fitted the Langmuir model better with the maximum adsorption capacities of 331.72 mg/g. The thermodynamic study results proved that the CR and MB adsorption by hydrogels was spontaneous, but the CR adsorption was endothermic and the MB adsorption was exothermic.