Adsorption efficiency and mechanism of diclofenac sodium in water using magnetic activated carbon

To effectively remove non-biodegradable diclofenac sodium (DS) from water and address the challenges of difficult solid-liquid separation and regeneration of adsorbents, this study employed Mn_0.6Zn_0.4Fe₂O₄ (MZF) magnetic nanoparticles as the magnetic core, and dopamine (DA) and powdered activated carbon (PAC) were utilized to synthesize the core-shell structured magnetic activated carbon Mn_0.6Zn_0.4Fe₂O₄@PDA-PAC (MZF@PDA-PAC) through a step-by-step deposition method. The texture properties, morphology, and magnetic properties of MZF@PDA-PAC were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), hysteresis loop measurements, and other techniques. The adsorption isotherm, kinetics, and thermodynamics of DS adsorption by MZF@PDA-PAC in water were investigated. The mechanism of DS adsorption by MZF@PDA-PAC in water was analyzed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results indicated that MZF@PDA-PAC exhibited an irregular spherical shape, with a specific surface area of 752.7 m²·g⁻¹, a mesoporous structure, excellent dispersibility in water, and outstanding magnetic separation performance. The maximum equilibrium adsorption capacity of DS adsorbed by MZF@PDA-PAC in water was 114.17 mg·g⁻¹. The DS-loaded MZF@PDA-PAC could be regenerated through in-situ oxidation using a 0.15 g∙L⁻¹ potassium peroxymonosulfate solution as the regenerant. After six adsorption and in-situ oxidation regeneration cycles, the equilibrium adsorption capacity of DS adsorbed by MZF@PDA-PAC remained at 75.6 mg·g⁻¹. The DS adsorption by MZF@PDA-PAC could be described by the Langmuir isotherm and the pseudo-second-order kinetic model, and it is a spontaneous, exothermic process accompanied by a decrease in system entropy. The DS adsorption by MZF@PDA-PAC in water was primarily dominated by physical adsorption, with the adsorption rate controlled by surface adsorption, intra-particle diffusion, and chemical adsorption. The adsorption mechanisms might include π-π conjugation, intermolecular hydrogen bonding, and electrostatic interactions. MZF@PDA-PAC exhibited advantages such as high capacity, the ability for in-situ oxidation regeneration, and high stability, demonstrating broad application prospects in the removal of refractory organic pollutants.