Effective adsorption of chromium(vi)/Cr(iii) from aqueous solution using ionic liquid functionalized multiwalled carbon nanotubes as a super sorbent

Abstract

We report an interesting interaction between oxidized multiwalled carbon nanotubes (oxi-MWCNTs), tetra n-heptylammonium bromide (ionic liquid) and total chromium ((Cr(VI)/Cr(III)) in this study. The interaction between the IL and oxidized MWCNTs primarily involves electrostatic affinity between the quaternary ammonium cation, and surface carboxyl and hydroxyl groups in oxi-MWCNTs. The IL-oxi-MWCNT adsorbent acts as a host in welcoming the incoming guest hydrochromate anions and several interesting interactions such as cation–π interactions, electrostatic interactions as well as anion–π interactions could be conceptualized in this process. The abundant oxygen-containing functional groups on the surfaces of oxi-MWCNTs play an important role in Cr(VI)/Cr(III) sorption. Characterization of the adsorbent was performed using various characterization techniques such as cross polarization magic angle spinning nuclear magnetic resonance (¹³CPMAS-NMR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), powder-X-ray diffraction (Powder-XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy analysis (TEM), Brunauer–Emmett–Teller (BET) isotherm studies, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The capability of inductively coupled plasma mass spectrometry (ICP-MS) for Cr(VI) adsorption was extensively studied under different optimal parameters and the maximum adsorption capacity was found to be 85.83 mg g⁻¹ from a nonlinear Langmuir isotherm model. A kinetic study confirms a pseudo second order model and the process could be upgraded by column studies to a sample volume of 2000 mL. Effective regeneration of the adsorbent could be accomplished with sodium hydroxide and the potential of this novel adsorbent has been examined in the removal of Cr(VI)/Cr(III) from aqueous solutions.