Hydrogel functionalization of spent tea leaves with 2-acrylamido-2-methyl-1-propanesulfonic acid and acrylamide for enhanced adsorption of cationic dyes

Herein, we report a crosslinked (cl) hydrogel of spent tea leaves (STLs) with two monomers, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) and acrylamide (Am), via free radical graft copolymerization using potassium persulfate and N, N′-Methylenebisacrylamide as the initiator and cross-linker, respectively. The as-synthesized hydrogel, STLs-cl-Poly(AMPSA-co-Am), was characterized using FTIR, FESEM-EDX, TGA, BET, zeta potential, and surface charge analysis. The STLs-cl-Poly(AMPSA-co-Am) hydrogel was evaluated for the adsorption of cationic dyes, namely, methylene blue (MB), and crystal violet (CV) from their aqueous solutions using various parameters, such as time (0–90 min), temperature (25–50 °C), pH (2.0–10.0), and concentration (25–400 ppm). The adsorption process was analyzed using kinetic models, namely pseudo-first-order, pseudo-second-order (PSO), and Elovich, and isotherm models, such as Langmuir, Freundlich, and Temkin. The maximum removal efficiencies for MB and CV were 90.50 ± 0.63% and 85.62 ± 0.59%, respectively. The adsorption process was best described by PSO and Langmuir model, confirming monolayer adsorption, with maximum adsorption capacities of 375.53 ± 6.33 and 344.05 ± 4.49 mg g−1 for MB and CV, respectively. The STLs-cl-Poly(AMPSA-co-Am) hydrogel showed high reusability for 8 cycles with a cumulative adsorption capacity of 682.76 mg g−1 for MB and 622.43 mg g−1 for CV. Thus, a novel STLs-based hydrogel was developed via free-radical graft copolymerization, in which STLs function simultaneously as a precursor and bio-filler, providing ample functional groups and a porous structure that enhance interactions with cationic dyes. This dual functionality enables effective biomass waste valorization for cost-effective, reusable, and environmentally sustainable dye removal from aqueous solutions.