Ionic Liquid Cross-Linked High-Absorbent Polymer Hydrogels: Kinetics of Swelling and Dye Adsorption

The use of polymer gels for the removal of toxic chemicalsfromwastewater is an important area in terms of both academic and industrialresearch. This work presents a simple approach to the fabricationof chemically cross-linked cationic hydrogel adsorbents using designedionic liquid-based cross-linkers and their successful use in the removalof organic dyes. Two different ionic liquid cross-linkers, [VIm-4VBC][Cl](ILA)/[DMAEMA-4VBC][Cl] (ILB), are synthesized by the simple nucleophilicsubstitution reaction of 4-vinylbenzyl chloride (4VBC) separatelywith 1-vinylimidazole (VIm) and 2-(dimethylamino)ethyl methacrylate(DMAEMA). Cross-linked poly(acrylamide) (CPAam) and poly(2-hydroxyethylmethacrylate) (CPHEMA) hydrogels are then prepared from the correspondingmonomers and as-synthesized cross-linkers (ILA and ILB) by free radicalpolymerization in the presence of a redox initiator combining ammoniumpersulfate (APS) and N,N,N & PRIME;,N & PRIME;-tetra-methyl-ethylene-diamine(TEMED). The dried CPAam and CPHEMA xerogels exhibit macroporous morphologyand high thermal stability. The hydrogel samples exhibit high swellingbehavior, and the diffusion of water molecules into the hydrogelsfollows pseudo-Fickian kinetics. The cationic cross-linking sitesin the hydrogel networks allow preferable binding with anionic dyes,and these dye uptake capacities are determined using different modelanionic dyes via UV-vis spectroscopy. The dye adsorption ontothese hydrogels follows a pseudo-second-order kinetic model. The adsorptionmechanism is also analyzed by employing intraparticle diffusion andBoyd kinetic models. The relationship between the maximum equilibriumadsorption capacity (q (m)) of the hydrogelsfor eosin B (EB) dye and the equilibrium EB concentration can be betterdescribed by Langmuir and Freundlich isotherm models, and the estimated q (m) using the Langmuir isotherm can reach morethan 100 mg g(-1). The cross-linked hydrogels canbe easily regenerated and have a recycling efficiency of >80% forup to three consecutive dye adsorption-desorption cycles, whichis promising for their use in wastewater treatment.