Unveiling the correlation of Fe₃O₄ fractions upon the adsorption behavior of sulfamethoxazole on magnetic activated carbon

Magnetic particles (MPs) assisted powdered activated carbon (PAC) is a promising composite material for adsorption removal of micropollutants. The fractional amount of Fe₃O₄ impacts the balance between adsorption capacity and magnetic property of magnetic activated carbons (MPACs), and therefore it affects the extent of sulfamethoxazole (SMX) removal. Here, five MPACs with different mass ratios of Fe₃O₄: PAC (1:1, 1:2, 1:4, 1:6, and 1:8) were prepared using a hydrothermal method and characterized by various spectroscopic methods. The spherical shaped MPs were monolayerly deposited on PAC with fewer pores blocked when the mass ratio of Fe₃O₄ was comparatively low (≤ 20%). MPAC6 (14.3 wt% of Fe₃O₄) had the best overall performance, with good Langmuir adsorption capacities for SMX (173.0 mg g⁻¹) and excellent magnetic properties (9.0 emu g⁻¹). Corresponding adsorption kinetics fitted well with the pseudo second-order kinetic model. The negative ΔG⁰ (−25.6 to −27.2 KJ mol⁻¹) and ΔH⁰ (−9.14 KJ mol⁻¹), and positive ΔS⁰ (0.55 KJ mol⁻¹ K⁻¹) properties indicated the spontaneous and exothermic nature of the adsorption process accompanied by an increase in entropy. The strong cation-assisted electron donor-acceptor and hydrophobic interactions were contributed to a high extent of SMX removal in the pH range of 2–4. Formation of negative charge-assisted H-bonds was responsible for the adsorption of hydrophilic SMX⁻ on negatively charged MPAC6 in alkaline solution. Desorption and regeneration experiments showed SMX removal was still 92.3% in the 5th cycle. These findings give valuable insights into the interactions between SMX and MPACs and guide for choosing sustainable magnetic adsorbents for environmental applications.