Magnetic amino-functionalized hyper-crosslinked resin as a reusable adsorbent for methylene blue

Contamination of water resources by organic dyes is a major environmental issue associated with industrial activities. Methylene blue (MB), a widely used cationic aromatic dye, is particularly problematic due to its high chemical stability and toxicity, highlighting the need for efficient and cost-effective treatment strategies. In this study, a magnetic amino-functionalized hyper-crosslinked resin (Fe₃O₄@XDV-NH₂) was synthesized and evaluated as an efficient adsorbent for MB removal from aqueous solutions. The material was prepared via post-synthetic amino functionalization of a hyper-crosslinked polymer followed by in situ deposition of Fe₃O₄ nanoparticles, combining high surface area, permanent porosity, and chemical stability with easy magnetic separation. Structural and magnetic characterizations confirmed the uniform incorporation of Fe₃O₄ without significant loss of porosity. Amino functionalization enhanced hydrophilicity and introduced polar active sites, promoting strong interactions with MB. Batch adsorption experiments revealed a maximum adsorption capacity of 79.60 mg/g at 298 K and pH 9 and 0.06 g L^-1 adsorbent dosage. Adsorption kinetics followed the pseudo-second-order model (R² = 0.976). Equilibrium data were best fitted by the Langmuir isotherm model (R² = 0.996), indicating monolayer adsorption with a maximum theoretical capacity of 183.48 mg/g. Thermodynamic analysis showed that MB adsorption was spontaneous and exothermic. High removal efficiencies were maintained in complex matrices such as tap water (94.11%) and synthetic wastewater (78.41%). Moreover, the adsorbent retained approximately 89% of its initial capacity after six adsorption-desorption cycles, demonstrating good reusability. These results indicate that Fe₃O₄@XDV-NH₂ is a promising and sustainable adsorbent for dye-contaminated wastewater treatment.