Removal of hexavalent chromium via an adsorption coupled reduction mechanism using olive oil derived carbon nano-onions

Abstract

Carbon nano-onions (CNOs) were obtained in gram-scale by a simple flame pyrolysis method using olive oil. The olive oil carbon nano-onions (OCNOs) obtained had a quasi-spherical chain like onion structure with an average diameter of 46 ± 5.4 nm. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis were employed to determine the existence of surface functional groups (−OH, −COOH) on the OCNOs. The OCNOs were applied as adsorbents for the elimination of Cr(VI) ions from aqueous solutions using batch experiments. The ideal conditions for the sequestration of Cr(VI) ensued at pH = 2, T =25 °C, m =0.05 g, t =720 min, and CO =10 mg/L. The XPS data for Cr after adsorption on the OCNOs revealed that Cr(III) ions were produced in the reaction, consistent with the adsorption coupled reduction mechanism. The sequestration of Cr(VI) proceeded via monolayer surface coverage (Langmuir isotherm), with the kinetic information in line with the pseudo-second-order model. The thermodynamic features showed that the removal progression was viable, exothermic, and involved an associative mechanism. After the adsorbent regeneration, the removal of Cr(VI) ions was improved owing to the −OH ions added by the NaOH (regeneration solution). The CNOs produced by flame pyrolysis gave similar Cr(VI) removal data to that observed using more sophisticated CNO synthesis methods.