Double network hydrogel based on cellulose nanofibers and acrylic terpolymer for enhanced methylene blue removal:...

Methylene blue (MB) in wastewater poses serious environmental and health concerns, necessitating the development of efficient and sustainable removal strategies. This study reports the synthesis of a novel CNF-reinforced hydrogel via free-radical polymerization of acrylic acid (AA), acrylamide (AM), and 2-hydroxyethyl methacrylate (HEMA) for MB removal from aqueous solutions. The incorporation of modified cellulose nanofibers (CNF) enhanced the adsorption efficiency of the hydrogel by 18.5%. The synthesized hydrogel was characterized using FT-IR, FE-SEM/EDS, TGA/DSC, XRD, XPS, and nitrogen adsorption-desorption analyses. Adsorption experiments showed that the process followed the pseudo-second-order kinetic model and the Langmuir isotherm, with a maximum adsorption capacity of 526.32 mg/g. Thermodynamic analysis indicated that the adsorption process was spontaneous and endothermic, predominantly governed by physisorption interactions. Zeta potential measurements (-57.2 mV at pH 7) confirmed the negatively charged surface of the hydrogel, enabling strong electrostatic interactions with cationic MB molecules, while hydrogen bonding further contributed to adsorption. Batch experiments demonstrated high removal efficiencies of 87-98% (RSDs: 3.48-6.16%) for MB in textile dyeing and urban wastewater under optimal conditions (30 mg adsorbent dose, pH 7, 20 min contact time, 10 mg/L initial concentration, 25 °C). The hydrogel exhibited a high swelling ratio of approximately 995%, indicating a three-dimensional network with abundant hydrophilic functional groups. Furthermore, the hydrogel retained 86% of its initial removal efficiency after eight successive adsorption/desorption cycles, demonstrating good reusability. This work presents the first report on the synthesis of CNF/p(AA-AM-HEMA) double-network (DN) hydrogel and highlights its potential for practical application in complex industrial wastewater treatment.