Fe3O4@TMSPM/poly(AMPS) composite hydrogel for methylene blue removal from water: Insights into metachromasy

In this work, a Fe 3 O 4 @TMSPM/poly(AMPS) composite hydrogel was prepared, and its performance in the removal of methylene blue (MB) from aqueous solutions was evaluated. The material was synthesized by incorporating Fe 3 O 4 nanoparticles functionalized with 3-(trimethoxysilyl)propyl methacrylate (TMSPM) into a poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (poly(AMPS)) polymeric matrix and characterized by FTIR, XRD, TGA, and VSM, confirming its composite structure. The UV–Vis spectra evidenced the metachromasy effect of MB when it comes into contact with the Fe 3 O 4 @TMSPM/poly(AMPS) hydrogel. Therefore, adsorption studies were carried out considering the monomeric form (Sng-MB) at 664 nm and the aggregated form (Agg-MB) at 572 nm of the dye. The effect of pH (2–10), adsorbent dose (5–50 mg), ionic strength (NaCl and Na 2 SO 4 solutions at 0.01, 0.05, and 0.10 M), contact time (1–24 h), initial concentration (50–250 mg/L), and temperature (25–45 °C) was evaluated, showing that the process is highly efficient over a wide range of experimental conditions, with removal efficiencies above 98% even at high initial concentrations (up to 250 mg/L). The hydrogel also exhibited good performance under simulated dyeing conditions. The kinetic analysis showed differentiated behaviors for both species: the adsorption of Sng-MB fitted better to the pseudo–second–order model, whereas that of Agg–MB predominantly followed a pseudo–first–order model, suggesting different interaction mechanisms. The thermodynamic study indicated that the adsorption process is spontaneous and endothermic for both forms of MB, being more favorable for Sng–MB. Likewise, the hydrogel exhibited excellent stability and reusability, maintaining its efficiency after 10 consecutive adsorption–desorption cycles. Overall, these results provide new insights into hydrogels and the supramolecular organization of MB during the adsorption process.