PANI-mediated wastewater treatment: mechanistic insights into heavy metal remediation pathways

The present study explores the catalytic adsorptive efficiency of emeraldine salt-phase polyaniline (PANI) nanoparticles for the selective removal of three environmentally hazardous heavy metal ions, Cr(VI), Mn(VII), and Cu(II), from aqueous systems. Synthesized via oxidative polymerization in 2 M HCl and characterized through XRD, FTIR, DSC, SEM, TEM, EDX, zeta potential, UV-vis spectroscopy, and particle size analysis, the PANI nanoparticles possessed good physicochemical properties propitious to surface-mediated interactions and redox transformations. Time-resolved batch adsorption experiments were performed under identical ambient conditions without pH adjustment, with a view to simulating realistic scenarios pertinent to wastewater. The results manifested that there existed strong co-relation among wastewater remediation efficiency, distinct valency of metal ions and their removal mechanisms where Cr(VI) underwent redox reduction to Cr(III) followed by complexation with nitrogen sites of PANI; Mn(VII) was reduced to insoluble MnO₂, which precipitated without forming any polymer-metal complex; and Cu(II) formed weak chelation, its monitoring being done using a PAN indicator on account of its low intrinsic absorbance. Pseudo-first-order kinetic modeling revealed rapid uptake for Cr and Mn within the initial 5 min, while Cu followed a slower adsorption profile. This paper consequently points out that the electronic structure, redox potential, and charge density of the metal ions control their interaction with PANI and positions PANI as a versatile redox-active organic material that preferentially display better results while targeting high-valent metal ions in heterogeneous aqueous environments. These results provide basic insight into the design of intelligent polymeric systems for efficient and sustainable wastewater treatment.