Removal of Rhodamine B from aqueous solutions by hollow polydopamine microspheres: preparation, performance, and...

Water contamination, particularly from organic dyes such as Rhodamine B (RhB), has become a global environmental issue due to the toxicity, non-biodegradability, and carcinogenic properties of these substances. This study investigates the utilization of hollow polydopamine microspheres (H-PDA) as an efficient adsorbent for removing RhB from water. The H-PDA microspheres were synthesized via a template approach, involving coating of polystyrene (PS) microspheres with polydopamine (PDA) and then removing the PS core to create hollow structures. These prepared H-PDA microspheres demonstrated a hollow structure, and exceptional adsorption capabilities for positively charged dyes, particularly RhB. The maximum adsorption capacity of H-PDA was 244.6 mg/g at 25 °C under optimal conditions, achieving equilibrium within 90 min. The adsorption kinetics followed a linear pseudo-second-order model (R² = 0.9994), and Weber's intra-particle diffusion analysis indicated that the adsorption process occurred in multiple stages. Thermodynamic analyses revealed that the adsorption process was endothermic, spontaneous, and feasible, with a positive enthalpy change (ΔH⁰ = 27.2 kJ/mol). A combined approach of density functional theory (DFT) calculations and experimental data was used to elucidate the adsorption mechanism, revealing that it is primarily attributed to strong electrostatic interactions, hydrogen bonds, and π-π interactions. This study highlights the potential of H-PDA as a highly efficient and selective adsorbent for wastewater treatment, with significant implications for the removal of harmful organic dyes from contaminated water sources.