Enhanced adsorptive degradation of Congo red in aqueous solutions using polyaniline/Fe0 composite nanofibers

• Polyaniline/Fe0 CNFs were applied firstly for adsorptive degradation of Congo red.
• Polyaniline/Fe0 CNFs showed enhanced removal efficiency than their constituents.
• Congo red degradation by polyaniline/Fe0 CNFs followed pseudo-first-order kinetics.
• Polyaniline/Fe0 CNFs retained original degradation efficiency up to 5th cycle.
• Possible degradation mechanism in polyaniline/Fe0 CNFs systems was proposed.

In this study, a simple approach was described for the fabrication of composite nanofibers (CNFs) of polyaniline/Fe0 (PANI/Fe0) using a template-free method for the reductive degradation and removal of Congo red (CR) from aqueous solutions. The PANI/Fe0 CNFs were prepared via rapid mixing polymerization of aniline monomers with Fe(III) chloride as an oxidant, followed by reduction of polymerization by products (Fe(II)/Fe(III)) as the Fe precursor. The PANI/Fe0 CNFs were characterized by Field Emission-Scanning Electron Microscopy (FE-SEM), High Resolution-Transmission Electron Microscopy (HR-TEM), Brunauer–Emmett–Teller (BET) method, X-ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS) and Vibrating Sample Magnetometry (VSM). These CNFs exhibited enhanced performance relevant to the adsorptive degradation/decolourization of CR, compared to PANI NFs and Fe0 nanoparticle counterparts. Batch experiments with a minimum dosage (1 g/L) of PANI/Fe0 CNFs showed complete degradation of 50 mg/L CR after 5 min of reaction. The CR degradation efficiency increased with decrease in initial concentration and solution pH, whereas it decreased with decrease in dosage of the CNFs. The CR degradation rate followed a pseudo-first-order kinetic model. Identification of the CR degradation products using liquid chromatography–mass spectrometry (LC–MS) revealed that the degradation mechanism proceeds through reductive cleavage of the azo linkage, resulting in the formation of 4-aminonaphthalenesulfonate ions and surface-adsorbed aromatic species, all being adsorbed on the CNFs surface at higher dosage. Six consecutive CR removal experiments using the same CNFs demonstrated that the CNFs retained the original CR removal efficiency up to 5th cycle, confirming their high recycling ability. Finally, the CNFs could be separated from the degradation fluid by exploiting external magnetic field.

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