Insight into the Water Absorption and Triphenylmethane Dyes Adsorption Performance of a Novel Magnetic Sodium Alginate/β-CD-<i>g</i>-Poly(AA-<i>co</i>-AMPS)/Fe₃O₄ Hydrogel Composite

Dyes are colorant impurities found in various water bodies that can interfere with the normal functioning of an aquatic ecosystem when present in excess. In this study, magnetic Fe3O4 was synthesized via a simple coprecipitation method and incorporated into a polymer matrix composed of sodium alginate, β-cyclodextrin, and poly(acrylic acid-co-2-acrylamido-2-methyl-1-propanesulfonic acid) to develop a magnetic composite through in situ polymerization. The influence of different feed compositions during hydrogel synthesis on the swelling behavior and cationic dye adsorption capacity of the composite was investigated. The investigation focused on the correlation between swelling behavior and dye adsorption capacity, revealing a negligible, slightly negative correlation between water uptake and dye removal efficiency. The magnetic hydrogel composite, despite having a low specific surface area (5.48 m2 g–1), demonstrated maximum adsorption capacities (qm) ranging from 1965 to 2434 mg g–1 at 30 °C for the studied cationic dyes (crystal violet, malachite green, and basic fuchsin), with equilibrium achieved within 120 min. Furthermore, the adsorbent maintained significant adsorption performance in the presence of inorganic salts, retaining over 400 mg g–1 capacity even at a 0.1 mol L–1 ionic strength. After 15 sequential regeneration–reuse cycles, the adsorbent preserved its adsorption capacity, achieving qe values of 992–1043 mg g–1, confirming its long-term applicability. These findings demonstrate the high effectiveness and reusability of the magnetic composite for the adsorption of cationic triphenylmethane dyes.