Carboxylated magnetic polymer microspheres with tunable poly(acrylic acid) shells for charge-selective adsorption of cationic dyes

Magnetically recoverable polymer adsorbents have attracted considerable attention for dye wastewater treatment, however, the relationship between polymer-shell loading, accessible functional-group density, and charge-selective adsorption remains insufficiently understood. Herein, a series of carboxyl-functionalized magnetic polymer microspheres (CMPs) with tunable polymer-shell contents were prepared by precipitation polymerization using Fe 3 O 4 microspheres as magnetic cores and acrylic acid/ethylene glycol dimethacrylate as shell-forming monomers. Structural characterization demonstrated that increasing the acrylic acid feed progressively increased the polymer-shell thickness and surface carboxyl density, suggesting reduced accessibility because of increased swelling and decreased surface area. Consequently, CMPs-4 exhibited the optimal balance between functional-group density and accessibility. Under the optimized conditions, CMPs-4 showed excellent charge selectivity, exhibiting negligible adsorption toward the anionic dyes methyl orange and tartrazine, while achieving adsorption capacities of 250 mg·g −1 for methylene blue and 507 mg·g −1 for methyl violet. The adsorption followed a pseudo-second-order kinetic model and was well described by the Langmuir isotherm, indicating monolayer adsorption on homogeneous active sites. Salt-enhanced adsorption was attributed to the combined effects of polymer-shell contraction and dye aggregation under high ionic strength. FT-IR, XPS, zeta-potential measurements, and elemental mapping confirmed that adsorption was mainly governed by electrostatic interactions between surface carboxyl groups and cationic dyes, accompanied by hydrophobic and π-π interactions. Furthermore, CMPs exhibited excellent magnetic recoverability, regeneration stability, low Fe leaching, and effective dye removal from real textile wastewater. This work establishes a clear structure–property relationship between polymer-shell architecture and adsorption performance, providing a rational strategy for designing recyclable magnetic adsorbents for selective dye removal.