Practical viability of naturally-occurring Ilmenite as a catalyst in advanced oxidation processes

Abstract

The demand for clean, quality, portable water is ever-increasing due to the global demand. One way to address this demand is through water reuse. Water becomes contaminated through many anthropogenic processes, and thus newer technologies are required to purify the water to acceptable standards. One of the main issues in wastewater is the presence of highly stable organic compounds that are resistant to conventional wastewater treatment. Advanced Oxidation Processes (AOPs) are relatively successful at degrading such compounds. One of the most commonly used AOP is the Fenton’s reagent, which uses iron salts and hydrogen peroxide to generate highly oxidising free radicals. However, this method has limitations as it tends to generate secondary waste streams with high iron concentrations. This work aims to investigate naturally occurring ilmenite (FeTiO3) as a viable catalyst for Fenton-like reactions to combat the limitations of the original reaction. Methyl Orange (MO) was chosen as a model compound in this study due to its high stability and resistance to conventional degradation methods. The catalyst was characterised by X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM/SEM-EDS) and Brunauer-Emmett-Teller (BET) surface area. The catalyst contained ilmenite predominately at

83.53 %, Hematite (Fe2O3) at 14.99 % and rutile at 1.2 %. These results are further supported by the SEM-EDS and XRF results. Degradation studies were carried out in the absence and presence of irradiation in a batch reactor. Under initial operating conditions, complete decolourisation was observed following 90 min of reaction time. The catalyst was recovered following 24 hours of reaction time and reanalysed in terms of XRD. The recovered material had similar XRD spectra as the original catalyst. This indicates that the material can be regarded as a catalyst. The process was optimised for pH, irradiation, catalyst loading and H2O2 dosage. An optimal pH of 2.5 under the presence of UVB irradiation led to the fastest rate of degradation. Iron leaching tests were done simultaneously with the pH optimisation. It was observed that at a pH of 5.0, no degradation or iron leaching occurred. This indicated that iron in solution was necessary for the reaction to occur. Thus, the reaction behaves more like a homogeneous system than a heterogenous system. Although iron leaching does occur, the XRD make-up of the catalyst did not change. Iron leaching was found to reach levels of 1.0 mg/L at a pH of 2.5. The optimal catalyst loading was 2000 mg/L; this loading led to the fastest degradation rate. It was also observed that an increase beyond 2000 mg/L did not affect the rate of degradation. Iron leaching tests were also conducted simultaneously with catalyst loading. At the optimum catalyst loading, an iron leaching concentration of 2.3 mg/L was found. Iron leaching kinetics were best predicted using the Pseudo-Second-Order (PSO) kinetics model. Finally, the optimum H2O2 dose was 1.0 mM. At doses exceeding 1.0 mM, the reaction becomes inhibited and thus, the degradation rate decreases. Under optimal conditions, complete decolourisation was achieved following 45 minutes of reaction time. The degree of mineralisation of the MO was determined through Total Organic Carbon (TOC) analysis. Experiments were carried out in cycles with stepwise addition of H2O2 every 60 minutes for five cycles. TOC decreased rapidly following the first cycle with a TOC of 62.95 %; thereafter, the TOC continually decreased more gradually with a final TOC of 40 % after five cycles. The presence of TOC in the sample indicates that intermediate organic compounds were still present in the solution. However, it was confirmed that the original pollutant, Methyl Orange, had been completely degraded by the reaction through Liquid Chromatography-Mass Spectrometry analysis (LC-MS). The intermediates remaining in the solution were hypothesised based on literature.

The practical viability of using ilmenite as a catalyst was assessed in terms of iron leaching content based on the South African Water Quality Guidelines for different water use purposes. Although the use of ilmenite led to minimal iron leaching (3.5 mg/L), the amount was still unsuitable for irrigation purposes because it could to rusting with the iron in solution becoming oxidised to form precipitates on metal surfaces; however, the iron content remains suitable for domestic usage as it is unlikely to cause any health effects at these concentrations for drinking and also poses no toxicity level for plants. This study proves that ilmenite can be an effective catalyst in Fenton-like processes to degrade organic, aromatic compounds with minimal iron leaching. Future work should investigate the degradation intermediates of methyl orange in this system and evaluate suitable iron removal methods.