Design of Nanostructured Sulfonated Polymeric Nanoparticles for Sustainable Cationic Dye Removal from Water

The persistent discharge of cationic dyes into aquatic systems necessitates advanced adsorbents with precisely tunable interfacial properties and high removal efficiency. Herein, we report for the first time the synthesis of composition-controlled sulfonated polymeric nanoparticles (NPs) based on polystyrene (PSt) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) via a surfactant-free precipitation polymerization approach. Our findings showed that the NPs exhibited well-defined composition-dependent evolution in physicochemical properties, with hydrodynamic size decreasing from 1224 to 327 nm and surface charge rising from −36.1 to −51.0 mV with increasing PAMPS content. Furthermore, adsorption performance toward methylene blue (MB) and crystal violet (CV) demonstrated strong dependence on surface charge density, with removal efficiencies of 97–98% at low initial dye concentrations (10–20 mg L−1) and still above 82–87% at a higher initial concentration (100 mg L−1). At low initial dye concentrations (10–20 mg L−1), the most highly sulfonated nanoparticles (NP-PSt/AMPS-50) reach equilibrium capacities of approximately 9.25–971 mg g−1, while at 100 mg L−1, the capacities increase to about 82–86 mg g−1 for both MB and CV. Notably, the adsorption capacity (qe) increases systematically with the sulfonation degree, reflecting enhanced ion-exchange capacity and accessibility of surface-exposed –SO3− functionalities. Rapid uptake behavior is observed, with >60–70% removal achieved within 15 min and equilibrium established within 100–120 min. Importantly, the enhanced adsorption performance of NPs can be attributed to their self-organized core–shell-like architecture. Considering this structure, hydrophobic PSt-rich domains form the particle interior, while PAMPS segments are localized at particle–water interface, creating a sulfonate-enriched surface layer. This enhances active-site accessibility and electrostatic interactions with cationic dyes. The composition-dependent evolution of sulfonate functional groups, as evidenced by FTIR spectroscopy, along with the systematic decrease in hydrodynamic size and increase in zeta potential magnitude with increasing AMPS content, collectively indicate the surface localization of charged PAMPS segments. Overall, our findings provide a mechanistic framework for the rational design of charge-regulated polymeric nano adsorbents and highlight the potential of PSt/PAMPS NPs as scalable and sustainable materials for cationic dye removal in wastewater treatment systems.