Efficient removal anionic dyes in water using cationic crosslinked poly (N-vinyl imidazole) nanocomposite hydrogel

The escalating release of carcinogenic azo dyes into aquatic environments necessitates the urgent development of efficient adsorbents. This study addresses this challenge by synthesizing two poly(N-vinyl imidazole)-based nanocomposite hydrogels, VMG (non-ionic crosslinking) and VDG (cationic crosslinking via 3, 3′ -divinyl-1, 1′ (1, 6-hexanediyl) di-imidazolium dibromide), incorporating 4.0 wt% of N-doped graphene quantum dots (NGQDs) to potentially enhance adsorption capacity. Characterization was performed using FTIR, XRD, SEM–EDS, BET, TEM, zeta potential (ZP), and swelling tests. VDG was selected for anionic dye adsorption studies due to its higher swelling and porous structure. ZP measurements of the adsorbent indicated that the ZP value was influenced not only by the solution pH but also by the presence of NGQDs in the nanocomposite. BET results indicated that the resulting VDG exhibited a high surface area of 245.02 m²/g. Batch experiments demonstrated highly efficient removal of model anionic dyes, Congo red (CR) and Methyl orange (MO), achieving maximum adsorption capacities of 454.54 and 400.0 mg/g at pH 7.0- and 60-min contact time, conditions likely favoring electrostatic interactions. The adsorption isotherm and kinetic data best fit the Langmuir for both CR and MO, pseudo-first order model for CR and Elovich model for MO, suggesting monolayer adsorption and a predominantly chemisorption-controlled process. The thermodynamic data indicated that dyes adsorption onto the VDG was endothermic and spontaneous. These findings highlight the VDG nanocomposite as a promising and potentially high-capacity adsorbent for the effective removal of anionic dyes from wastewater.

Graphical Abstract