UV-irradiated carbon nanotubes synthesized from fly ash for adsorption of congo red dyes in aqueous solution

Abstract

In this work, carbon nanotubes (CNTs) were synthesized from fly ash and used as adsorbent for the removal of congo red (CR) dye from aqueous solution. The as-synthesized CNTs were exposed to UV radiation for improving the adsorption of dye. The produced nanotubes were characterized by the scanning electron microscopy, transmission electron microscope, X-ray photoelectron spectroscopy, and Raman and FTIR spectroscopies. UV irradiation was found to be an effective tool for enhancing intensity of the observed functional groups and could induce extra defects on the sidewalls of the CNTs. The obtained results show that the adsorption of CR increases from 9.1 to 26.5 mg g−1 with increasing the UV irradiation time from 0 to 30 min and maximum adsorption was found to be at pH 4.5. The adsorption efficiency of the UV-irradiated CNTs was decreased from 99.14 to 53.5% by increasing the initial dye concentration from 5 to 50 mg L−1. Langmuir and Freundlich models were fitted to the adsorption equilibrium data and the results reveled that the adsorption of CR onto the CNTs followed the Langmuir isotherm and the maximum monolayer adsorption capacity was found to be 25.61 mg g−1. The results suggest that the synthesized CNTs, which contain naturally exist active groups, besides those induced by the UV irradiation, might be useful for the removal of CR and other water pollutants.