Sodium alginate-and chitosan-based hydrogels with different network charges for selective removal of cationic and anionic dyes from water

Abstract

The grafting of chitosan (CH) and sodium alginate (SA) biopolymers with glycidyl methacrylate (GMA) and acrylamide (AAm) monomers, com-bined with graphene oxide (GO), led to the formation of bio-based hydrogels. These hydrogels, named CH-GO-hydrogel (GO/CH-g-poly (AAm-co-GMA)), CH-GO-hydrogel (CH-g-poly (AAm-co-GMA)), SA-GO-hydrogel (GO/SA-g-poly(AAm-co-GMA)), and SA-hydrogel (SA-g-poly(AAm-co-GMA)), were tested as selective dye adsorbents. While the chitosan-based hydrogels exhibited positive zeta potential values ranging from þ27.5 to þ0.1 mV, alginate-based samples had negative values between −10.4 to−41.7 mV in pH conditions from 3.0 to 9.0. Adding GO nano-fillers reduced the swelling capacity of both hydrogels, with water absorption (WA) values for SA-GO-hydrogel and SA-hydrogel recorded at 10.1 and 22.2 g/g, respectively. The ability of these materials to adsorb dyes, specifically crystal violet (cationic) and Congo red (anionic), was confirmed. Factors such as adsorbent dosage, initial pH, dye concentration, shaking time, and temperature were analyzed to determine dye adsorption capacity. Interestingly, the pristine hydrogels, free of GO, performed better than their nanocomposite counter-parts. Adsorption capacities (qm) for crystal violet and Congo red with SA-hydrogel, SA-GO-hydrogel, CH-hydrogel, and CH-GO-hydrogel was 909.1, 714.3, 454.5, and 400.0 mg/g, respectively.