Hollow Sea Urchin-Shaped Polypyrrole Nanomaterial for Efficient Adsorption of Methylene Blue and Congo Red Dyes: A Comparative Study

In this study, hollow sea urchin-shaped nanostructured polypyrrole powder was synthesized to investigate its adsorption efficiency for two types of dyes: a cationic dye (methylene blue, MB) and an anionic dye (congo red, CR) in aqueous solutions. The polypyrrole (PPy) powder was synthesized in an acidic medium using a core-shell redox mechanism, with nanostructured MnO 2 powder employed as both the oxidizing agent and sacrificial nanostructured template. The morphology and structure of both the MnO 2 powder-based reactant and the resulting polypyrrole powder were characterized using FEG-SEM, TEM, EDX, and XRD techniques. These characterization techniques clearly revealed the formation of hollow and open microparticles of polypyrrole, with the presence of nanotubes on their surface. The pH PZC of PPy was experimentally determined to be 3.26. Subsequently, adsorption studies were performed to evaluate the effect of pH, temperature, kinetics, and isotherms on the adsorption capacity of PPy. The adsorption of congo red was favoured under acidic pH conditions, whereas methylene blue removal was favoured under basic pH conditions. Both adsorption processes exhibited adherence to the Langmuir isotherm and displayed pseudo-second-order kinetics. Thermodynamic analysis revealed the spontaneity of the adsorption processes (Δ G ° < 0) and their endothermic nature (Δ H ° > 0). Additionally, the reusability study demonstrated successful regeneration of PPy for up to four cycles. These findings underscore the potential of nanostructured polypyrrole as an effective adsorbent for the efficient removal of both cationic and anionic dyes, presenting a promising approach for environmentally friendly and clean processes.