Influence of Genipin and Multi-walled Carbon Nanotubes on the Dye Capture Response of CS/PVA Hybrid Hydrogels

Multifunctional hybrid hydrogels based on chitosan (CS) and poly(vinyl alcohol) (PVA), chemically crosslinked with genipin (GEN), and filled with multi-walled carbon nanotubes (MWCNTs) were synthesized and characterized to be used as dye adsorbents in polluted water. Solutions (10(-5) M) of methyl orange (MO) and acid blue 113 (AB) were tested. Each dye solution was analyzed in initial acid and basic pH conditions (pH 6.02 +/- 0.2 and 10.0 +/- 0.4). The physicochemical, mechanical properties, dye removal efficiency, adsorption kinetics, and possible hybrid hydrogels' adsorption mechanism were examined. Incorporating GEN enhanced the polymeric network structure and the mechanical stability of CS/PVA-based hydrogels; their elastic modulus increased to ninefold even in aqueous acid media. The CS/PVA/GEN and CS/PVA/GEN/MWCNTs hydrogels presented removal efficiencies up to 96.8% and 93.2% (0.30 and 0.36 mg/g) for MO and 90-100% (0.70-0.77 mg/g) for AB. Regardless of the initial pH value of the dye solutions, all of them presented a final value of pH similar to 5, which implies that electrostatic attractions can occur during the adsorption process. Experimental data were successfully fitted to pseudo-firstorder kinetics. Thus, physisorption processes acted during the removal of MO and AB. FT-IR spectroscopy demonstrated that the hydrogel's amino (-NH2) and hydroxyl (-OH) groups are mainly involved in adsorption by electrostatic interactions, hydrogen bonding, n-pi stacking interactions, and pi-pi interactions. We demonstrated that using GEN and MWCNTs in the CS/PVA blend offers a useful approach for getting resistant, efficient, and low-cost adsorbent materials to successfully deal with water pollution caused by anionic organic dyes as MO and AB, among others.