Versatile Application of Iron Oxide Nanoparticles in Environmental Challenges: From Trace Sulfur Dioxide Adsorption to Waste Glycerol Valorisation

Abstract

Nanomaterials and nanoparticles are now largely investigated for various applications due to their unique properties. Iron oxide nanoparticles are increasingly popular due to their non- toxicity, affordability and versatility as well as iron abundance. Their potential magnetism is a valuable property in the field of heterogeneous catalysis as it allows for an easy recovery of the catalyst. In this project, iron oxide nanoparticles were designed and synthesized using green approaches as well as more traditional coprecipitation approaches and investigated for environmental applications. The nanoparticles were used for the adsorption of trace concentration of SO2 and revalorisation of waste glycerol produced from the biodiesel production. Low concentrations of SO2 are present in industrial gaseous effluents even after flue gas desulfurization. Trace SO2 capture is challenging, and even low concentrations (ppm levels) of SO2 can deactivate or poison catalysts used in processes aiming to revalorise the desulfurized gas. In this work, cellulose-based adsorbents modified with iron oxide nanoparticles are proposed as a solution to remove trace SO2. Cellulose is selected as it is sustainable, abundant, and innocuous, has a high surface area and contains hydroxyl groups on its surface which facilitate nanoparticles deposition. The iron oxide nanoparticles were obtained from a plant-based reduction process using green tea extract. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to assess the nanoparticle deposition and characterize the adsorbents. It was observed that the deposition process was more effective on microcrystalline cellulose than on nanocrystalline cellulose. The amount of reducing agent used had an impact on the chemical speciation of the iron oxide deposited on the cellulose and the degree of particle agglomeration. Breakthrough capture experiments were conducted at room temperature using an inlet stream containing 25 ppm SO2 in argon. The impact on the adsorption capacity of the adsorbents synthesis parameters, such as the type of cellulose used, the deposition time, the iron loading on the adsorbent and the amount of reducing agent used for the deposition, were evaluated. Microcrystalline cellulose modified with iron oxide nanoparticles showed a significantly higher affinity for the SO2 than modified nanocrystalline cellulose, or both types of pristine cellulose. The optimal iron oxide nanoparticle deposition time on the microcrystalline cellulose was observed to be 72 hours. The adsorption capacity was directly related to the iron content in the adsorbent and increased from 0.017 mgSO2/gadsorbent to 0.45 mgSO2/gadsorbent when the iron content was increased from 0.16 wt% to 2.71 wt%. The chemical speciation of the iron also had a significant impact on the adsorption with Fe3O4 nanoparticles performing the best. Future work will focus on including CO2, O2 and H2O in the inlet gaseous stream and performing other modification to cellulose to increase its adsorption. Otherwise, biodiesel production has increased significantly over recent years to provide an alternative fuel. 10% w/w of glycerol is produced as a byproduct from the biodiesel production, and this has led to a saturation of the glycerol market, making it a waste material. Glycerol can however be valorized into platform chemicals through various reactions such as oxidation. We therefore propose a Fe3O4-based catalyst to perform this oxidation in both a Fenton-like system and a photocatalytic system to produce dihydroxyacetone which has a higher economical value. The Fe3O4 nanoparticles were obtained from two different methods: coprecipitation and steel revalorisation, and further compared to FeCl3 catalytic system. Both were doped with Ag nanoparticles using NaBH4 or green tea extract as reducing agents. The catalysts were characterized by dynamic light scattering (DLS), XRD, TEM and their band gap was measured using diffuse reflectance spectroscopy (DRS) and the Tauc method. Both types of Fe3O4 showed a nearly identical size distribution, but more nanoparticle agglomeration was observed on the revalorised Fe3O4. The Ag doping was confirmed by XRD and the bandgaps of all the doped catalysts were smaller than their non-doped counterparts. The catalysts were compared to a traditional Fenton oxidation of glycerol and the reaction was monitored by 1H NMR. Parameters such as the hydrogen peroxide to glycerol ratio, the doping, the type of Fe3O4 and the mass of catalyst were investigated. All the Fe3O4 based catalysts showed a higher dihydroxyacetone selectivity than the FeCl3, but a lower conversion. Dihydroxyacetone selectivity as high as 94% was measured when using Fe3O4 nanoparticles doped with Ag by using NaBH4. However, the conversions remained between 6% and 19% for the Fe3O4 catalysts. In the photocatalytic system, the intensity of the light, the pH and the temperature were investigated. Future work will focus on improving the conversion in the Fenton-like system, transitioning to only revalorisation Fe3O4 and performing more photocatalytic studies