Synthesis, Swelling Characteristics, and Dye Adsorption Mechanism of a New Stimuli-Responsive Cationic Hydrogel

In this study, a new multi-stimulus cationic copolymer hydrogel was developed by bulk photo-polymerization of N-acryloyl-N’-propyl piperazine (AcrNPP) and poly (ethylene glycol) methacrylate (PEGMA) with ethylene glycol dimethacrylate (EGDMA) as crosslinker. The effect of multiple external stimuli, concentration of monomers, and equilibrium swelling of the hydrogels was studied in detail. The gels swelled in acidic solutions due to protonation of tertiary amine in the piperazine ring, and de-swelled in basic solutions. The presence of PEGMA in the hydrogel enhanced the swelling and imparted water-responsive property, leading to disintegration at high concentration of 49.85%. The hydrogel was evaluated as an adsorbent for the removal of an anionic dye, Congo red (CR) from water. The dye uptake capacity of the hydrogel increased with increase in the initial dye concentration. Interestingly, the swelling ratio of the hydrogel decreased at high dye concentration due to the formation of additional physical crosslinks within the hydrogel matrix. The dye uptake capacity of the hydrogel decreased with increasing temperature due to the negative-temperature responsive property of the hydrogel. The time-dependent adsorption data was fitted with seven kinetic models. Pseudo second-order model best described the kinetics of adsorption process, and the adsorption of CR onto the hydrogel could be very well described with phase-boundary controlled models. The adsorption was a multistep process with surface adsorption followed by intraparticle diffusion.