From waste cooking oil to oxygen-rich onion-like nanocarbons for the removal of hexavalent chromium from aqueous solutions

Themba D. Ntuli; Ludwe L. Sikeyi; Thomas H. Mongwe; Orlette Mkhari; Neil J. Coville; Edward N. Nxumalo; Manoko S. Maubane-Nkadimeng

doi:10.17159/sajs.2023/14006

Authors

Themba D. Ntuli DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0003-0863-5599
Ludwe L. Sikeyi DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0003-3917-4983
Thomas H. Mongwe DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0002-5578-512X
Orlette Mkhari DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0002-9580-2596
Neil J. Coville DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0001-5370-1386
Edward N. Nxumalo Institute for Nanotechnology and Water Sustainability, College of Science, Engineering and Technology, University of South Africa, Johannesburg, South Africa https://orcid.org/0000-0001-8225-2401
Manoko S. Maubane-Nkadimeng 1.DSI-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa; 2.Microscopy and Microanalysis Unit, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0001-9463-3033

DOI:

https://doi.org/10.17159/sajs.2023/14006

Keywords:

waste cooking oil, hexavalent chromium, onion-like nanocarbons, pyrolysis

Abstract

Vegetable cooking oil is used in domestic and commercial kitchens owing to its ability to modify and enhance the taste of the food through the frying process. However, as the oil is used through several frying cycles, it changes colour to dark brown and acquires an unpleasant smell. At this point, the waste oil is usually discarded, thereby finding its way into freshwater streams due to poor disposal and thus becoming an environmental pollutant. To provide an alternative, ‘green’ route to waste oil disposal, herein we report on the metal-free synthesis of onion-like nanocarbons (OLNCs) made from waste cooking oil via flame pyrolysis. The OLNCs were then applied in the removal of hexavalent chromium ions from aqueous solutions. The as-synthesised OLNCs were found to have similar properties (size, quasi-spherical shape etc.) to those synthesised from pure cooking oils. The Fourier-transform infrared spectroscopy data showed that the OLNCs contained C-O-type moieties which were attributed to the oxygenation process that took place during the cooking process. The OLNCs from waste oil were applied as an adsorbent for Cr(VI) and showed optimal removal conditions at pH = 2, t = 360 min, Co = 10 mg/L and Q⁰_max = 47.62 mg/g, superior to data obtained from OLNCs prepared from pristine cooking oil. The results showed that the OLNCs derived from the waste cooking oil were effective in the removal of hexavalent chromium. Overall, this study shows how to repurpose an environmental pollutant (waste cooking oil) as an effective adsorbent for pollutant (Cr(VI)) removal.

Significance:

Waste cooking oil outperformed olive oil as a starting material for the production of OLNCs for the removal of toxic Cr(VI) from water.
The superior performance of the OLNCs from waste cooking oil was attributed to the higher oxygen content found on their surface and acquired through the cooking process.
Not only are the OLNCs produced from waste cooking oil effective in the removal of Cr(VI), but they can be used multiple times before replacement, which makes them sustainable.