Supramolecular self-assembling hydrogel film based on a polymer blend of chitosan/partially hydrolyzed polyacrylamide for removing cationic dye from water

The supramolecular self-assembling hydrogel film was designed using a polymer blend of chitosan and partially hydrolyzed polyacrylamide (CS/PHPA), which was constructed with a controllable sol-gel transition, using a green one-pot external salt-assisted sol-gel transition and aqueous solution casting method that does not employ any toxic chemical crosslinking agents. The structure of hydrogel film was confirmed by zeta potential, FT-IR, XRD, SEM, EDX, and TG/DTGA. The swelling properties of gels were well investigated and the results revealed a rapid swelling rate. Notably, the composite hydrogel film exhibited remarkable thermo- and pH-induced sol-gel transitions. The zeta potential of supramolecular CS/PHPA hydrogel was −51.5 mV (pH 7.0), which provided a good adsorption performance for the cationic dye methyl blue (MB). The adsorption process obeyed the pseudo-second-order kinetic model and the maximum adsorbed amount for MB was high up to 282 mg/g. The adsorption equilibrium data correlated well with the Langmuir model, suggesting that the MB was removed through the monolayer and physical adsorption mechanism. After five adsorption/desorption cycles using a mixture of (HCl:1-Butanol) as eluent, the dye adsorption efficiency decreased from 96.45 to 79.80%. The possible adsorption mechanism is believed to be the non-covalent interactions (the electrostatic interactions together with hydrogen bonding between the N atom of MB and the H atom of functional groups of the hydrogel. Overall, the "green" gelation process provides a new opportunity for the rational design of self-assembling physical hydrogels as adsorbents for cationic and anionic dye removal.