Efficient removal of Congo red from aqueous solutions by adsorption onto interconnected polypyrrole-polyaniline nanofibres

Polypyrrole-polyaniline nanofibres (PPy-PANI NFs), with interconnected structures were used for the removal of Congo red (CR) from aqueous solutions. The NFs were synthesised without template via coupling of propagating PPy+ and PANI+ free radicals by in situ simultaneous polymerisation of Py and ANI monomers in the presence of a common oxidant, FeCl3. The as-prepared NFs were characterised by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) to confirm the presence of both PPy and PANI polymeric moieties. Interconnected and nanostructured fibres were observed by Field Emission-Scanning Electron Microscopic (FE-SEM) and High Resolution-Transmission Electron Microscopic (HR-TEM) images. The effects of various operational parameters including solution pH, contact time, initial concentration and temperature on removal of CR using PPy-PANI NFs were investigated in batch adsorption mode. Adsorption results demonstrated that the CR removal efficiency increased with decrease in solution pH. The adsorption kinetic data followed the pseudo-second-order equation whereas the equilibrium data fitted well to Langmuir model. The Langmuir maximum adsorption capacities (Qmax) were found to be 222.22 mg/g and 270.27 mg/g at 25 °C and 35 °C, respectively. This indicates that the synthesised NFs showed better adsorption capacity of CR compared to many other adsorbents. Experiment with real textile wastewater spiked with CR showed 99.5% colour removal by the use of 0.1 g NFs. Finally, four consecutive adsorption-desorption experiments were conducted to test the reusability of the adsorbent and it revealed that the NFs retained the original capacity up to the third adsorption cycle. These results imply that the NFs are found to be an effective adsorbent for the removal of CR from water as a result of their interconnected nanostructure.