Highly selective and reusable nanoadsorbent based on Fe₃O₄-embedded sodium alginate-based hydrogel for cationic dye...

This study aims to develop a stable and efficient magnetic nanocomposite hydrogel (MNCH) for selective removal of methylene blue (MB) and crystal violet (CV). MNCHs with different Fe₃O₄ contents (0-9 wt%) were synthesized following graft co-polymerization method using sodium alginate, acrylamide, itaconic acid, ammonium persulfate and N,N-methylene bisacrylamide. Among them, MNCH₅, with 5 wt% Fe₃O₄, showed highest removal efficiency (>95 %). Optimal dye removal occurred at pH 10, with 40 min for CV and 60 min for MB using 30 mg dose. MNCH was characterized using various techniques, with X-ray diffraction (XRD) revealing crystallite size of 30.5 nm, and Brunauer-Emmett-Teller (BET) indicating surface area of 59.80 m² g^-1. Adsorption kinetics followed fractal pseudo-first-order and fractal Vermeulen diffusion models, reflecting MNCH's heterogeneous nature as suggested by fractal exponent (h) ranging 0.38-0.44, significantly deviating from zero. Langmuir-Freundlich isotherm accurately described the process, demonstrating MNCH's superior affinity for MB (4216.69 mg g^-1) over CV (3730.17 mg g^-1). Thermodynamics of MB adsorption was exothermic as suggested by negative ΔH value, while CV adsorption was endothermic. Density functional theory confirmed stronger interaction between MNCH and MB (E_ads = -49.29 kcal mol^-1) compared to CV (E_ads = -41.30 kcal mol^-1). These findings underscore MNCH's excellent adsorption capacity, making it promising for removing dyes.