Abstract
Iron oxides are suitable catalysts for the heterogeneous electro-Fenton (HEF) remediation of wastewater due to their high stability, availability at low costs, low toxicity and high catalytic performance. The HEF process attracts immense attention in this regard due to its ability to mitigate recalcitrant organic pollutants in wastewater, for example, pharmaceutical wastes.
This study explored the synthesis, characterisation and application of iron-based HEF catalyst nanoparticles for the degradation of recalcitrant pharmaceutical pollutants in wastewater. It focused mainly on the improvement of the catalytic performances of the HEF catalysts by inducing desirable catalytic properties like high stability, improved affinity (interaction) with the cathode for enhanced immobilisation, reduced nanoparticle size and surface area. In addition to catalyst improvements, this study sought to improve the degradation of wastewater pharmaceutical pollutants through the coupling of the HEF processes with contemporary advanced oxidation processes (AOPs). This gave rise to solar photo coupled HEF systems. In such case, this work explored the synthesis of photoanodes with enhanced photosensitivity to improve the utilization of solar irradiation for the efficient degradation of pharmaceutical pollutants through the synergistic effects of the coupled systems.
In all the HEF investigations done in this work, the synthesised nanoparticles were immobilised on the carbonaceous cathodes, with the anode(s) placed at the centre of the reaction cell. All the synthesised nanoparticle catalysts were characterised using FTIR, XRD, SEM and TEM. Evaluations of percentage degradation and mineralisation were done using UV-Vis spectroscopy and total organic carbon (TOC) measurements respectively.
To improve the immobilisation of magnetite nanoparticles (MNPs) on the carbon felt (CF) cathode, firstly, we explored the novel improvisation of magnets, applied round the outside of the reaction cell, to exert a pulling effect (attraction) on the MNPs immobilised on the CF cathode placed covering the entire interior surface of the reaction cell. This led to improved immobilisation of the nanoparticles, reduced their falling off the cathode, reduced MNPs agglomeration and therefore led to enhanced catalyst activity and stability in the HEF degradation of aspirin. An improvement of 23% aspirin
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degradation was recorded when magnets were used compared to the negative control without magnets. Furthermore, the magnet enhanced immobilisation of magnetite induced high catalyst stability at a wider pH (2.0 – 12.0) and showed good recyclization and reusability up to 10 cycles.
In another study, the application of bimetallic catalysts in HEF processes was investigated by applying synthetic ilmenite (FeTiO3) nanoparticles as a HEF catalyst for the degradation of tetracycline in wastewater. In this work, the co-catalysis of Ti in augmenting the ferrous catalysed HEF reaction was observed. Ilmenite showed superior HEF catalysis than the mono metallic iron oxides haematite (α-Fe3O4) and goethite (FeO(OH)) nanoparticles. Ilmenite owes its enhanced HEF catalysis than the common mono metallic iron oxides to the presence of Ti in its crystal structure, which plays a very significant co-catalysis effect. To further improve the HEF performance of ilmenite, it was doped with ferrous ions and a slight improvement was noticed in its activity. Furthermore, ilmenite showed high stability at wider operational pHs, low Fe/Ti metal solubility and high reusability. Due to the co-catalysis effects of Ti, ilmenite nanoparticles demonstrated better HEF catalysis in the degradation of tetracycline in wastewater than the mono metallic iron oxides such as goethite and haematite.
Having successfully applied magnets in the enhanced magnetite nanoparticle immobilisation at the cathode, we further applied this technique in the coupled heterogeneous solar photo electro Fenton (SPEF) and the photo electrocatalytic (PEC) oxidation using a La doped bismuth ferrite (La-BFO) photoanode in the degradation of acetyl salicylic acid (aspirin). The La-BFO photoanode was observed to have a better performance than the undoped BFO photoanode. Both the PEC, denoted in this chapter as La-BFO/CF (light), and the coupled La-BFO/SPEF systems showed improved aspirin degradation activity in response to solar irradiation. Finally, it was observed that the coupling of AOPs, i.e., the Fenton, photocatalysis, PEC, EF and the solar PEC/SPEF (La-BFO/SPEF) oxidation processes led to the enhanced aspirin degradation. Therefore, it can be concluded that the doping of BFO photoanode with La enhances its solar photo- (i) catalytic oxidation and (ii) electro-Fenton degradation of tetracycline in wastewater.
Finally, further explorations on magnetite nanoparticles were done to fabricate its HEF catalysis activities by coating it with polyethylene glycol (PEG). Various concentrations of PEG were introduced on the magnetite nanoparticle surfaces and applied on the HEF degradation of sulfamethoxazole (SMX). The magnetite: PEG ratio of 1:1 produced the optimum HEF performance. It was further observed that the PEG-coated magnetite
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nanoparticles had better HEF performance than the uncoated ones. In the same vein, the PEG-coated nanoparticles showed higher reusability, up to 89% (SMX) degradation after 10 HEF cycles, contrary to the 72% degradation obtained when the uncoated magnetite nanoparticles were applied for the same number of HEF cycles. PEG induced desired properties like increased surface area (reduced nanoparticles size), reduced agglomeration, improved magnetic properties, improved stability and improved water dispersibility. Further experiments showed negligible compatibility differences between the thus-fabricated magnetite nanoparticles and two carbon-based cathodes, i.e., carbon felt, and graphite felt. It can therefore be concluded that the coating of MNPs with PEG enhances their HEF catalytic activity towards the degradation of SMX in wastewater.