PhD Thesis Defence Presentations - Rebecca Dhawle

In the past decade, climate change, evident by the extreme flood and drought-like situations, has impacted the water quality and quantity drastically. This, combined with an increased population, has created a severe scarcity for clean water. The sixth Sustainable Development Goal (SDG) proposed by the United Nations, “Clean Water and Sanitation”, aims to tackle the water crisis by eliminating the use and discharge of the hazardous substances, increasing recycling, and promoting safe reuse of water. Pharmaceutical compounds are one variety of micropollutants (MPs) that have been detected in several water matrices globally. Although they are present at very low concentrations, their tendency to bioaccumulate and remain active poses a serious threat to human health and the environment.

The conventional wastewater treatment plants (WWTPs) usually rely on biological treatment processes and are not sufficiently equipped to remove these micropollutants effectively. Advanced Oxidation Processes (AOPs) therefore have garnered significant attention from the scientific community as an appealing technology to successfully destroy these compounds. AOPs are classified into several different processes based on the production pathway of hydroxyl (^•OH) radicals. Sulphate radical-based AOPs are an alternative to conventional AOPs and use sulphate radicals (SO₄^-•) instead of ^•OH radicals. SO₄^-• are produced from the activation of precursors like persulfate (PS) or peroxymonosulfate (PMS) and benefit from higher oxidation potential, selectivity, and prolonged half-life.

Electrochemical AOPs (eAOPs) have gained popularity since they are chemical free, operate at ambient pressure and temperature, do not produce secondary waste streams, and can be powered from renewable energy sources. The integration of different AOPs increases the amount of reactive oxygen species (ROS) and increases the rate of micropollutant removal. The performance of such an integrated hybrid process is often higher than the sum of performance of the individual processes, leading to a synergistic combination. With this regard, this thesis evaluates the combination of solar photocatalysis with electrochemical oxidation by immobilizing titanium dioxide (TiO₂) on a conductive substrate in a photoelectrochemical (PEC) cell. The objectives of this thesis are (i) successful immobilization of TiO₂ films and their integration into PEC cell, (ii) optimizing the production of hydrogen peroxide (H₂O₂) at the cathode, (iii) evaluating the role of cathodes and other process conditions on eAOPs, (iv) analysing the transformation-products (TPs) and determining the reaction kinetics, and (v) identifying the biological activity and ecotoxicity of the transformation products.

Firstly, TiO₂ was immobilized onto FTO substrates and incorporated into a working two-chamber photo-electrocatalytic (PEC) cell as a working photoanode. This PEC cell was used for the generation of H₂O₂ at the cathode. The performance of the photoanode was further improved by sensitizing the TiO₂ layer with cadmium sulfide. The sensitization allowed for the generation of H₂O₂ entirely with the photo-current generated by the PEC cell without any external bias. The in-situ generated H₂O₂ was collected and used for the discoloration of three dyes in a UV- H₂O₂ system. The PEC cell was then sized up and the cathode chamber was placed under UV irradiation to facilitate the simultaneous generation of H₂O₂ and its activation to ^•OH radicals. Diclofenac (DCF), an anti-inflammatory drug in the concentration 0.5-1.5 mg L^-1, was used as the target pollutant in this system. The results demonstrated that this setup could successfully produce enough H₂O₂ to degrade DCF. The removal of DCF was negatively influenced at higher pH (pH 10) and by the complexity of the water matrix.

Furthermore, the influence of operating parameters such as current density, nature of supporting electrolytes, and pH were investigated on the electrochemical oxidation of 0.5 mg L^-1of losartan (LOS), an anti-hypertensive drug and on 1 mg/L of anastrozole (ANZ), a breast cancer drug using a BDD anode. The degradation of LOS and ANZ followed a pseudo first order kinetics and was favoured in NaCl electrolyte. Although increased electrolyte concentration favoured the removal of LOS, the removal of ANZ was unaffected by increasing the electrolyte concentration. An optimum current density also was determined beyond which the increase in current density had a detrimental effect on the removal of LOS.  The role of cathodes, seldom studied for electrochemical processes, was evaluated for the removal of for LOS. It was found that carbonaceous cathodes showed better removal rates than metal cathodes. PS addition improved the removal of LOS whereas, it had no effect on the removal of ANZ. Abatement of both LOS and ANZ reduced at increased pH, hindered by the presence of humic substances, and in real water matrices. LC/MS analysis was used to identify a total of 9 TPs and subsequently a reaction mechanism for the electrochemical degradation of LOS in NaCl and Na₂SO₄ was proposed. The toxicity of the transformation products was predicted using ECOSAR modelling. 

TiO₂/WO₃ photoanodes with enhanced electrochemical characteristics than pristine TiO₂ and WO₃ were fabricated and used in a single-chamber PEC cell. This setup was used for the removal of 1 mg/L of a synthetic steroid hormone, 17α-ethinyl estradiol (EE2).  The addition of 10 mg/L of PMS to the PEC cell increased the removal of EE2 from 47.9% to 88.8% within 60 min.  The rate constant for the combined PEC PMS process was 0.044 min^-1 which was higher than the rate constant of light-activated PMS process (0.0053 min^-1) and PEC process (0.01 min^-1). The rate of EE2 degradation was increased with an increase in the voltage and PMS concentration. EE2 removal was decreased and reached 62.5 % and 48.5 % when 0.05 M NaCl or 0.05 M NaClO4 was used respectively as the electrolyte. Ecotoxicity using Vibrio fischeri as an indicator decreased, although, not proportionately to the parent compound EE2 in the combined PEC PMS system.

eAOPs were compared with ozonation, an AOP that is currently implemented on large-scale for the tertiary treatment in several countries. The degradation and mineralization of two prevalent pharmaceutical compounds, sulfamethoxazole (SMX) and hydrochlorothiazide (HCTZ) were carried out via anodic oxidation (AO), electro-Fenton (EF), and ozonation. The current density for AO was optimized at 300 mA followed by the optimization of the catalyst dosing of 11.1 mg/L for EF. Both EF and ozonation could degrade both SMX and HCTZ completely while AO achieved a removal of 90% in 60 min. The highest mineralization efficiency of 88% was observed with EF, followed by 74% with AO and 24% by ozonation in 180 min. A detailed investigation into the degradation pathways and transformation products showed identical reaction mechanisms with AO and EF for the removal of SMX and HCTZ. Although ozonation showed the fastest removal of the MPs, it could not sufficiently remove the total organic carbon (TOC). AO and EF emerged as superior technologies in terms of mineralization and were least impacted by the water matrix. 

The biological activity of EE2 before and after electrochemical oxidation was determined using in-vivo bioassays. 5 mg/L of EE2 was degraded using a BDD anode in NaCl and Na₂SO₄ electrolyte. Almost complete removal of EE2 was achieved under 2 min and 120 min, respectively. The TPs formed were identified and quantified using LC-MS. The estrogenic activity of these TPs was assessed using transgenic medaka fish line. The results showed that even after complete degradation of EE2, the estrogenic activity was not completely eradicated. Additionally, degradation of 50 mg/L of DCF was carried out on a BDD anode using Na₂SO₄ as the supporting electrolyte to understand the environmental risks associated with the TPs of DCF. The by-products were identified and quantified using LC-MS. The biological impact of DCF and its TPs was evaluated using the Xenopus Eleutheroembryo Thyroid Assay, employing a transgenic amphibian model to assess thyroid axis activity. Acute toxicity was observed in partially degraded DCF samples indicating that the TPs formed during the electrochemical oxidation exhibited higher toxicity than the parent compound.  However, prolonged degradation time reduced the acute toxicity as well as the enhanced thyroid effects.

The key findings of the experimental work were integrated into a conceptualized reactor design for a novel photoelectrocatalytic reactor for the degradation of micropollutants. The PEC reactor design aimed at facilitating experimental validation and consequently, the real-time application of photoelectrocatalysis as a plausible tertiary treatment technology. The detailed strategy for catalyst regeneration, toxicity assessment, and salt recovery was also discussed. The cost analysis of this novel reactor highlighted the need for further optimization to improve the efficiency of the reactor. The proposed reactor setup can be seen as an intermediate step between bench-scale studies and a full-scale reactor with scope for further improvement and modifications.