Magnetically retrievable 3D biopolymer sponges for enhanced removal and recycling of organic dyes

Synthetic dyes possess high toxicity, chemical stability, and resistance to biodegradation, posing severe environmental hazards. Therefore, their efficient removal from wastewater is essential. In this study, biodegradable and magnetically retrievable hybrid biopolymer sponges were fabricated from a gelatin–chitosan (Gel–Chi) matrix via freeze-drying and reinforced with Fe₃O₄ magnetic nanoparticles (MNPs) and pectin-coated Fe₃O₄ magnetic nanocomposites (MNCs). The resulting porous Gel–Chi–MNP (PS) and Gel–Chi–MNC (CS) sponges exhibited hierarchical porosity, tunable surface charge, and robust mechanical stability. Adsorption studies using methylene blue (MB, cationic) and methyl orange (MO, anionic) revealed maximum adsorption capacities of 312.4 mg g⁻¹ for MB and 276.8 mg g⁻¹ for MO under optimized conditions. Adsorption isotherms followed both Langmuir and Freundlich models, confirming the coexistence of monolayer and heterogeneous multilayer adsorption, while kinetic analysis indicated a hybrid mechanism involving physisorption and chemisorption. Notably, CS sponges displayed superior swelling capacity, higher reusability, and improved structural integrity compared with PS sponges, owing to the pectin coating that enhanced hydrophilicity and electrostatic interactions. The sponges retained >85 % of their dye removal efficiency after five successive adsorption–desorption cycles, demonstrating excellent regeneration potential. Furthermore, cost analysis and preliminary tests with metal ions validated their economic viability and versatility for practical wastewater treatment. Thereby, the Gel–Chi–MNC sponges represent sustainable, scalable, and environmentally friendly adsorbents for efficient dye removal and recycling.