3D-Printed photoelectrocatalytic system for advanced water treatment

Gustavo Corte Tedesco

Photoelectrocatalysis (PEC) is a sustainable, versatile and powerful advanced oxidation process (AOP) for environmental remediation. The PEC technology can be used for the treatment of recalcitrant organic contaminants from water and wastewater via direct or indirect oxidation mechanisms. At the current stage, PEC technology is mainly found in lab-scale applications. Development of highly efficient photoelectrodes and optimised photoreactor designs are necessary to evolve the technology readiness level towards real-scale applications. This research aimed to design an optimised photoelectrode with enhanced mass and photon transfer characteristics, which was undertaken via use of additive manufacturing technology (i.e. 3D printing). The research also focused on the identification and adoption of optimal characteristics of photo and electrochemical reactors for building a lab-scale PEC reactor set-up. The work began with the comparison of Ti/TiO2 electrodes produced from titanium foil substrate and commercial pure titanium powder, which was 3D-printed as Ti substrate. A range of characterisation techniques showed that the electrodes prepared from the different substrates had almost identical photo-electrocharacteristics. The electrodes also exhibited identical performance for PEC treatment of methylene blue with 39+3% and 38+1% removal after 120 minutes for the cp-Ti and Ti-foil electrodes, respectively. Benzothiazole was further used as organic contaminant surrogate for comparison with other AOPs and testing of the cp-Ti electrode. The highest degradation achieved was 98+2% after 120 minutes of PEC treatment. The next stage of the work involved investigation of the effects of polishing of the electrode substrate prior to anodisation. Besides changes in the morphology of the semiconductor surface, which led to smoother titanium nanotube arrays, the polished and non-polished electrodes also showed essentially identical photo-electrocharacteristics. The polished and non-polished electrodes were used for degradation of a range of recalcitrant organophosphate ester flame retardants (OPFRs) and to evaluate the effects of different water matrices. No significant differences were observed for removal of OPFRs from the use of the polished vs non-polished electrodes. However, water matrix did effect performance of both electrodes. The best performance of removal of OPFRs was obtained for aqueous solution (ultrapure water) with added electrolytes. Tap water matrices with no electrolyte addition affected the removal of the compounds, lower removals were observed in these experiments. Also, the presence of additional organic contaminants, such as acetone was found to inhibit degradation of the target compound. This interference affect was also confirmed for benzothiazole degradation experiments. Individual OPFR degradation levels in ultrapure water were 74%+9, 37%+10, 33%+9, 31%+11 and 3%+5 for triphenyl phosphate, tris(butyl) phosphate, tris(isobutyl) phosphate, tris(2-butoxyethyl) phosphate and tris(2-chloroisopropyl) phosphate, respectively. The results up to this point confirmed a 3D-printed Ti/TiO2 electrode could perform the same as an electrode produced from a titanium foil and did not require polishing during preparation. The next phase of the project was the design and manufacturing of an innovative threedimensional electrode. The resulting three-dimensional design incorporated several characteristics favourable for PEC systems, such as highly reactive surface area-tovolume- ratio, and light capturing properties. When combined into the PEC set-up this novel electrode provided optimal design and operational characteristics such as enhanced mass transfer and light absorption properties. The three-dimensional electrode was initially tested in a beaker set-up against a flat electrode and provided superior performance for the treatment of two recalcitrant organic contaminants: methylene blue and benzothiazole. For methylene blue, the degradation rate improved from 0.0131 to 0.0281 min-1 by upgrading the PEC system with the three-dimensional electrode. For the PEC treatment of benzothiazole, the degradation rate improved from 0.0330 to 0.0454 min-1, for the flat vs the three-dimensional electrode, respectively. A comparison of the electrical energy per order with other AOPs was carried out and the proposed PEC reactor with three-dimensional electrode showed high competitiveness, which was also attributed to the use of UVA-LED as light source. Then, the work moved towards its last stage in which the three-dimensional electrode was combined into a flow cell PEC reactor for investigation of treatment performance when compared to a flat electrode. It was proven that the innovative high-profile electrode provided superior capabilities for the treatment of benzothiazole when compared to the conventional flat electrode. For example, when operating at a flow rate of 80 L h-1, the high-profile electrode outperformed the flat electrode (87+3% vs 66+4% benzothiazole degradation, respectively, after 120 minutes). Also, during hydrodynamic investigation and computational fluid dynamics (CFD) simulation, with different flow rates. The high-profile electrode showed superior performance. Overall, an innovative photoelectrode with optimised photon and mass transfer characteristics was designed, manufactured and tested for the treatment of organic contaminants in water. The electrode is considered one of the key components for PEC reactor and can underpin the performance of the system. The proposed high-profile electrode in combination with a flow cell reactor set-up provided baseline for scaling-up of the PEC system, supporting the exploitation of the technology towards real-scale applications.

3D-Printed photoelectrocatalytic system for advanced water treatment

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