Removal of Congo red by core − shell magnetic chitosan submicrospheres: characterization and adsorption studies

Core-shell Fe₃O₄@poly(acrylic acid)/chitosan (Fe₃O₄@PAA/CS) submicrospheres were synthesized through the polymerization of acrylic acid in CS solution, using uniformly sized magnetite colloid nanocrystal clusters (MCNCs) as the core materials. The obtained submicrospheres were characterized using scanning electron microscopy, transmission electron microscopy, dynamic light scattering, Fourier-transform infrared, thermo-gravimetric, vibrating sample magnetometer, and X-ray diffraction analyses. The results confirmed that the submicrospheres with the Fe₃O₄ nano-core located in the central region and encapsulated by a CS shell exhibited superparamagnetic behavior. The removal efficiency of Congo red (CR) dye by magnetic submicrospheres was determined by investigating several factors, including pH, adsorbent dose, contact time, and dye concentrations. Over 97.4% of CR (90 mg L^-1) was removed at a dosage above 1.2 g L^-1. The maximum adsorption capacity obtained from the Langmuir isotherm model for CR was 143 mg g^-1 at 290 K. Adsorption kinetics and isotherm data were well described by the pseudo‑second‑order and Langmuir models, respectively. Furthermore, the submicrospheres were successfully regenerated and, subsequently, reused for four adsorption-desorption cycles without any noticeable loss of stability. The exceptional removal performance of magnetic submicrospheres on CR renders it a highly appealing adsorbent for the treatment of dye-containing wastewaters.