Highly selective and reusable nanoadsorbent based on Fe₃O₄-embedded sodium alginate-based hydrogel for cationic dye adsorption: Adsorption interpretation using multiscale modeling

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 m(2) 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 Delta H 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.