Abstract
<title>Abstract</title> <p> Carboxylated magnetic polymer microspheres integrate polymer-colloid interfacial functionality with magnetic separability, making them attractive for selective dye removal from water. Herein, monodisperse Fe <sub>3</sub> O <sub>4</sub> magnetic particles (MPs) were synthesized by a solvothermal method, sequentially modified with SiO <sub>2</sub> and methacrylate groups, and then coated with poly(acrylic acid)-based shells through reflux precipitation polymerization of acrylic acid and ethylene glycol dimethacrylate. The acrylic acid feed amount was varied to regulate shell content, surface charge, carboxyl density, colloidal size, and adsorption performance. Fourier-transform infrared spectroscopy, thermogravimetric analysis, vibrating-sample magnetometry, N <sub>2</sub> adsorption-desorption analysis, zeta potential measurements, potentiometric titration, scanning electron microscopy, and dynamic light scattering confirmed successful polymer coating and tunable surface properties. Among the samples, CMPs-4 showed the highest carboxyl density (3.36 mmol·g <sup>− 1</sup> ) and a strongly negative zeta potential, providing abundant binding sites for cationic dyes. CMPs-4 rapidly adsorbed methylene blue (MB) and methyl violet (MV), reaching near-equilibrium within 10 min. The adsorption kinetics followed the pseudo-second-order model, and the Langmuir-fitted maximum capacities were 322.58 mg·g <sup>− 1</sup> for MB and 653.59 mg·g <sup>− 1</sup> for MV. Mixed-dye experiments demonstrated preferential adsorption of cationic dyes over anionic dyes. The adsorbent retained more than 80% removal efficiency after five adsorption-desorption cycles and remained magnetically recoverable. Electrostatic attraction between surface carboxylate groups and cationic dye molecules, supplemented by hydrogen bonding, dominated the adsorption mechanism. </p>