Removal of methylene blue by acrylic polymer adsorbents loaded with magnetic iron manganese oxides: Synthesis, characterization, and adsorption mechanisms.

Dyes pose significant risks for aquatic environments and biological health in general owing to their non-biodegradable nature, carcinogenicity, and toxicity. The effective treatment of dye wastewater has become an important research topic. In this study, acrylic polymers (AP) loaded with magnetic iron manganese oxides (MIMO) (AP/MIMO) were prepared and used for the first time for the adsorption of methylene blue (MB). Carbon in AP/MIMO exists predominantly in the C-H and C-C forms, with its content reaching 50.7%. Oxygen and nitrogen in AP/MIMO exist mainly in the -CO- and -N-C forms, with contents of up to 41.5% and 73.3%, respectively. MB removal by AP/MIMO was consistent with the pseudo-second-order kinetic model (R = 0.99), equilibrium was achieved within 20 min, and the highest MB capacity of 2611.23 mg g was predicted by the Langmuir isotherm model (R = 0.91-0.94). AP/MIMO exhibited excellent MB adsorption performance in the pH range of 4-10, with a removal efficiency higher than 99.0% (MB = 100 mL 1000 mg L; AP/MIMO = 50 mg). Thermodynamic indicators, such as positive entropy (ΔS; 98.30 J⋅mol⋅K), negative Gibbs free energy (ΔG; -29.40, -28.50, and -27.50 KJ⋅mol), and positive enthalpy (ΔH; 2.30 KJ⋅mol), demonstrated that MB removal by AP/MIMO was autonomous, favorable, and endothermic. In addition, the integration of experimental results and theoretical calculations verified that electrostatic interactions were the primary mechanism for MB adsorption at carboxyl sites on AP/MIMO. The total interaction energy between AP and MB was -310.43 kJ⋅mol, and the electrostatic effect had a decisive contribution to the MB adsorption, with a value of up to -341.06 kJ⋅mol. AP and MB were most likely bound by -COO and S atoms. Overall, AP/MIMO exhibits high adsorption capacity and shows potential as a high-performance magnetic polymer for MB removal.