Synergistic interactions between multi-walled carbon nanotubes and toxic hexavalent chromium

Abstract

In this paper, the synergistic interactions between as-received multi-walled carbon nanotubes (MWNTs) and toxic hexavalent chromium (Cr(VI)) in solutions of different pH were investigated that aimed to functionalize the nanotubes and remove the toxic Cr(VI) by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy and transmission electron microscopy (TEM). The effects of pH value and MWNT concentration (dose) on Cr(VI) removal from polluted water with different initial Cr(VI) concentrations were investigated. Results revealed that MWNTs could be used for complete Cr(VI) removal through the reduction of Cr(VI) to Cr(III) in the polluted water with an initial Cr(VI) concentration ranging from 200 to 1000 μg L⁻¹ during half an hour treatment in a pH = 1.0 solution. The Cr(VI) solutions with different pH values had different effects on MWNTs. For pH = 1.0 Cr(VI) solution with a concentration of 1000 μg L⁻¹, the carboxyl and ether functional groups were found to form on the MWNT surface after 5 to 30 min Cr(VI) treatment and the carboxylate groups were formed as the treatment period increased to 60 min. The kinetics in different pH Cr(VI) solutions were derived. The redox kinetics in the pH = 1.0 solution was described by the pseudo-first-order behavior with respect to Cr(VI) and the typical value of the pseudo-first-order rate constant was calculated to be 0.05786 min⁻¹. In the pH = 7.0 solution, the adsorption kinetics rather than redox reaction dominated the Cr(VI) removal by the calculation and was explained by the pseudo-second-order model with a rate constant of 0.865 g mg⁻¹ min⁻¹.