Synthesis of onion-like nanocarbons from cooking oil for the adsorptive and photocatalytic removal of Cr(VI) and methyl orange from aqueous solutions
Date
2024
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Abstract
Rapid industrialization and urbanization have coincided with increased concentrations of organic and inorganic pollutants in wastewater. These include trace metal ions and dyes such as Cr(VI) ions and methyl orange, respectively. Such pollutants tend to bioaccumulate in living organisms, resulting in toxic effects for humans and biota alike and they should thus, be removed from water streams. Two of the common and widely applied technologies for the removal of pollutants from wastewater are adsorption and photocatalysis, typically using carbon materials. This is because carbonaceous materials can serve as adsorbents and/or support materials for semiconductor nanoparticles.
This study explores the use of onion-like nanocarbons (OLNCs) as the material of interest for the removal of pollutants in water. The synthesis of OLNCs was achieved by the flame pyrolysis of vegetable cooking oil (olive oil) and waste cooking oil. First, the OLNCs synthesized from olive oil and waste cooking oil were used as adsorbents for the removal of Cr (VI). It was found that the optimum condition for the removal of Cr(VI) ions took place at pH = 2 and CO = 10 mg/L. The values found were t = 720 min and Q0max = 26.53 mg/g for the olive oil OLNCs and t = 360 min and Q0max = 47.62 mg/g for the waste cooking oil OLNCs. The improved removal of Cr(VI) ions by the OLNCs from waste cooking oil was attributed to increased surface oxygen moieties as a result of the oxygenation that took place during the cooking process. The removal mechanism for both adsorbents was consistent with the adsorption coupled reduction removal pathway as indicated by the presence of Cr(III) on the surface of the adsorbent and the presence of Cr(VI) ions in the solution. The experimental data followed the pseudo-second-order (PSO) model and the Langmuir adsorption isotherm. The OLNCs from waste cooking oil demonstrated higher adsorption efficiency than the OLNCs from olive oil, an indication of how waste material could be converted into a useful material.
Oxygen functional groups play a role in the surface adherence of chromium ions. Therefore, H2O2 and KOH were used as oxidizing agents for the OLNCs from olive oil. The OLNCs were chemically modified with 10% and 25% of H2O2 and 0.1 M and 0.2M of KOH respectively to improve their adsorption efficiency. Generally, the H2O2-treated adsorbents showed higher adsorption efficiency than the KOH-treated adsorbents. The Q0max values of the chemically modified adsorbents were significantly higher than iv that of the unmodified OLNCs from olive oil (26.53 mg/g) but relatively lower than those of waste cooking oil (47.62 mg/g). The Freundlich adsorption isotherm fitted both adsorbents indicating heterogeneous surface coverage of the surface. The PSO kinetic model was followed by both adsorbents predicting the possible chemisorption mechanism.
TiO2 is a well know and widely used photocatalyst, and to further explore the effect of OLNCs in photocatalysis, nanocomposites of OLCNs and TiO2 nanoparticles (TiO2/OLNCs) were prepared via hydrothermal synthesis. This was achieved by varying the mass of the OLNCs (10, 20, 30, and 50 mg) added to 100 mg of TiO2. The materials were labelled as TC-10, TC-20, TC-30, and TC-50 corresponding to the OLNCs mass. The nanocomposites were then used as photocatalysts in the photocatalytic degradation of methyl orange (MO). It was found that the photocatalytic degradation rate of MO using the TiO2/OLNCs was more rapid than TiO2 alone in this order TC-10 > TC-20 > TC-30 >TC-50.
The waste cooking oil OLNCs were also used as a support material for iron oxide nanoparticles to form iron oxide/OLNCs nanocomposite. The mass of the OLNCs was varied as follows (25, 50, 100, and 170 mg) the materials were labelled FeC-25, FeC50, FeC-100, and FeC-170. The nanocomposites were then used as photocatalysts for the photocatalytic reduction of Cr(VI) ions to Cr(III) ions. The reduction of Cr(VI) ions to Cr(III) ions was rapid, taking place in the first 15 min. This was followed by subsequent adsorption of the Cr(III) ions through the carbonyl groups of the nanocomposite.
Overall the results showed that the flame pyrolysis method was successfully used to convert vegetable cooking oil to carbon nanomaterials (OLNCs). The OLNCs showed promise in the removal of Cr(VI) and (MO). The results are comparable with other carbon nanomaterials, an indication that the as-synthesized OLNCs have potential applications in adsorption, reduction and photocatalysis.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2023
Keywords
Photocatalytic, Onion-like nanocarbons, Vegetable cooking oil