Synthesis and Application of Multifunctional Iron Oxide-Graphene Oxide Nanocomposite Materials for Sulfur Pollutant Removal

Abstract

Sulfur pollution mainly comes from the utilization of fossil fuels, which contain sulfur compounds such as thiol, sulfide, and thiophene. Their combustion releases in the atmosphere sulfur dioxide (SO₂), a toxic gas harmful to both the environment and human health. Petroleum refineries also generate wastewater with reactive and odorous sulfide compounds. To address these issues, this work proposes the synthesis of a multifunctional composite material consisting of graphene oxide (GO)-iron oxide. This material is proposed to be used as an adsorbent for the capture of gaseous SO₂ at room temperature and as a catalyst for the oxidation of organic sulfide compounds to sulfone and sulfoxide, applicable in fuel desulfurization or wastewater treatment processes. The properties and SO₂ capture capacities of GO-iron oxide material, obtained from the wet deposition of iron oxide nanoparticles on GO, were evaluated by studying the impact of different nanoparticle synthesis preparations. Three different methods were used: a polyol method and two co-precipitation methods, using either sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH) as reducing agents. The deposition of iron nanoparticles on the GO surface was confirmed through transmission electron microscopy (TEM), showing dispersed and round-shaped nanoparticles of less than 10 nm. In all cases, iron was mostly found in the Fe³⁺ oxidation state. However, the final iron content in the GO-iron oxide materials after wet deposition was different for each nanoparticle synthesis method, as indicated by Inductively coupled plasma atomic emission spectroscopy (ICP-OES) analysis. The breakthrough curves and the capacity calculations showed that the material containing nanoparticles from NH₄OH reduction had the highest capture capacity (3.1 mg SO₂/g_sorbent), five times greater than pristine GO. However, the SO₂ capture capacities based on iron content were respectively 114.1, 207.0, and 207.2 mg SO₂/g_Fe for GOFe₂O₃-polyol, GOFe₂O₃-NaOH, and GOFe₂O₃-NH₄, suggesting that both the type and concentration of nanoparticles influence the capacity. Additionally, based on X-ray photoelectron spectroscopy (XPS) analysis, it was suggested that the adsorbed SO₂ might interact with iron oxide or directly to the surface of GO. The regeneration tests indicated incomplete desorption at 100°C, with capture capacity decreasing after the first cycle and stabilizing after the second cycle. The catalytic activity of GO-iron oxide was also tested at 50°C using thioanisole as organic sulfide. In this case, iron magnetite nanoparticles were synthesized using co-precipitation and deposited on GO through wet deposition as well. The addition of magnetite was confirmed through TEM, Fourier-transform infrared spectroscopy (FTIR), UV absorption, and X-ray powder diffraction (XRD). GO-Fe₂O₃ presented interesting catalytic activity with the oxidation of thioanisole to heavy sulfone compounds with 64.2% conversion. The addition of H₂O₂ increases the conversion to 92.8%. The photocatalytic activity of the material was accessed using UV, sunlight, and visible light in the absence of H₂O₂. UV light promotes the highest conversion with 89.6% followed by sunlight (77.7%) and visible light (75.2%). These results suggest that graphene oxide-iron oxide has potential for SO₂ adsorption and catalytic oxidation of organic sulfide compounds. However, further optimization of the system and material is necessary for a more comprehensive evaluation. For instance, multiple regeneration cycles in the case of SO₂ capture have to be done to ensure the material's capacity stability over time. Additional analyses, such as XRD and high-resolution transmission electron microscopy (HR-TEM), are required to confirm the exclusive presence of Fe₂O₃. Increasing the iron concentration could facilitate the use of XPS to identify the interaction of SO₂ with the material. Finally, studying the effect of the nanoparticle size in the case of organic sulfide oxidation could enhance the photocatalytic activity of the GO-Fe₂O₃ material under low-energy light sources like visible light.