Highly efficient removal of Pb from aqueous solution using polyaniline-cobalt composite nanorods: Kinetics, isotherm and mechanistic investigation.

Nanosized cobalt (Co) particles exhibit unique chemical, magnetic, electronic, and catalytic properties. Like nanoscale metallic iron, nanostructured Co and its composite nanostructures also show significant potential for the removal of toxic metal cations from water and wastewater. To explore this potential, composite nanorods (CNRs) of nanosized Co immobilized polyaniline (PANI) nanorods (NRs) matrix (PANI-Co CNRs) were synthesized and effectively applied for the treatment of lead ions (Pb⁺), serving as a model for heavy metal pollutants in water bodies. Physico-chemical characterization of PANI-Co CNRs revealed that weak ferromagnetic Co nanoparticles (NPs) were effectively deposited onto the surface of the PANI NRs. The enhanced surface properties and superior reactivity of PANI-Co CNRs resulted in greater Pb removal efficiency compared to their individual components. The adsorption kinetics were notably rapid, with the time required to reach equilibrium varying between 60 and 150 min for initial concentrations ranging from 50 to 150 mg/L, all at a pH of 5.0. The isotherm data revealed an impressive Pb adsorption capacity of 1130 mg/g at 25 °C, as determined using the non-linear Langmuir model. Exothermic and spontaneous Pb removal process was deduced from the thermodynamic investigations. Among co-contaminating metal ions, only Cu ions significantly affected the Pb removal performance of the PANI-Co CNRs, implying its possible applications in decontaminating industrial effluent laden with various metal ions. Mechanistic investigation revealed that the treatment process primarily involves the adsorption and precipitation of Pb onto the surface of PANI-Co CNRs, followed by its subsequent reduction to form metallic Pb (Pb).