Conventional wastewater treatment technologies are ineffective in removing persistent micropollutants, necessitating the development of advanced oxidation processes capable of achieving their efficient degradation. Among emerging approaches, photoelectrocatalysis has attracted increasing attention as a promising technology that combines electrochemical approach with solar energy utilization. In this context, the present thesis investigates the applicability of photoelectrocatalysis for the degradation of pharmaceutical pollutants, with a particular focus on understanding energy consumption, degradation mechanism and safety. The thesis begins with a comprehensive theoretical overview of photoelectrochemical systems, highlighting the fundamental differences between photoelectrocatalytic and conventional electrochemical oxidation. Special emphasis is placed on the energetic advantage of photoelectrocatalysis, which arises from the decoupling of reaction thermodynamics from the applied external potential. Despite this, experimentally measured energy consumption data for photoelectrocatalytic degradation of organic pollutants remain scarce. This work addresses this gap by systematically evaluating the energy efficiency of photoelectrocatalytic pharmaceutical degradation and demonstrating energy consumptions below 3 kWh/m3, with typical values around 1.5 kWh/m3 for unmodified BiVO4 photoanodes. For the first time, the impact of this substitution of oxygen evolution reaction with organic oxidation on solar conversion efficiency of photoelectrochemical system is analyzed in detail, accounting for both thermodynamic and kinetic considerations. The results reveal that replacing water oxidation with a thermodynamically more favorable organic oxidation reaction leads to a decrease in solar conversion efficiency for a given photoanode. However, in this case faster reaction kinetics may allow to achieve higher photocurrents and, thus, higher product fluxes. Moreover, organic oxidation enables the use of photoanodes with narrower band gaps than those required for water splitting, allowing absorption of a larger fraction of the solar spectrum. The formation of key reactive oxygen species is also evaluated, demonstrating that hydroxyl radical generation at high fluxes is unlikely in unbiased photocatalytic systems. Based on the results of literature review, BiVO4 is selected as a model photoanode material for experimental investigations due to its suitable semiconductor properties. Despite these advantages, experimental removal percentages for various pharmaceuticals remain below 60% after two hours of treatment. Preliminary experimental analyses reveal that the performance of BiVO4 is primarily limited by low intrinsic catalytic activity towards oxidation of organic compounds, which manifests itself as low faradaic efficiency even when the process is not controlled by mass-transfer, significant surface recombination resulting in current much lower than the maximum theoretical one, and photocorrosion. To mitigate these limitations, surface modification of BiVO4 with β-FeOOH is investigated as a strategy to improve hole extraction and photoelectrode stability. While this modification leads to a 1.5-2 times photocurrent increase and stability improvement, it unexpectedly results in reduced pharmaceuticals degradation rates (from 50.6% to 34.2% for paracetamol, from 64.0% to 54.6% for sulfamethoxazole, from 53.5% to 55.2% for carbamazepine) and higher energy consumption. Mechanistic analysis demonstrates that β-FeOOH preferentially enhances water oxidation kinetics rather than organic oxidation, thereby increasing competition between the two reactions. These findings underscore that photocurrent density alone is an insufficient metric for predicting photoelectrocatalytic performance. Finally, the role of chloride ions in photoelectrocatalytic degradation is examined on sulfamethoxazole degradation. The presence of chloride significantly accelerates oxidation through the formation of reactive chlorine species, achieving complete degradation in 60 and 20 minutes at chloride concentrations 200 and 600 mg/L Cl-, respectively. However, the formation of highly toxic chlorination products (including halogenated acetic acids) and toxicity increase were detected. A non-linear dependence of degradation pathways on chloride concentration is elucidated for the first time, highlighting the complexity of chlorination-assisted photoelectrocatalytic processes. Overall, this thesis provides a critical and systematic evaluation of photoelectrocatalysis for pharmaceutical pollutant degradation, combining theoretical efficiency analysis with detailed experimental investigations. The results clarify fundamental limitations, challenge common assumptions regarding photocurrent-based performance metrics, and identify key design principles for future development of selective and energy-efficient photoelectrocatalytic systems.
Investigation of Applicability of Photoelectrocatalysis towards Oxidation of Pharmaceutical Pollutants
SKOLOTNEVA, EKATERINA
2026-05-26
Abstract
Conventional wastewater treatment technologies are ineffective in removing persistent micropollutants, necessitating the development of advanced oxidation processes capable of achieving their efficient degradation. Among emerging approaches, photoelectrocatalysis has attracted increasing attention as a promising technology that combines electrochemical approach with solar energy utilization. In this context, the present thesis investigates the applicability of photoelectrocatalysis for the degradation of pharmaceutical pollutants, with a particular focus on understanding energy consumption, degradation mechanism and safety. The thesis begins with a comprehensive theoretical overview of photoelectrochemical systems, highlighting the fundamental differences between photoelectrocatalytic and conventional electrochemical oxidation. Special emphasis is placed on the energetic advantage of photoelectrocatalysis, which arises from the decoupling of reaction thermodynamics from the applied external potential. Despite this, experimentally measured energy consumption data for photoelectrocatalytic degradation of organic pollutants remain scarce. This work addresses this gap by systematically evaluating the energy efficiency of photoelectrocatalytic pharmaceutical degradation and demonstrating energy consumptions below 3 kWh/m3, with typical values around 1.5 kWh/m3 for unmodified BiVO4 photoanodes. For the first time, the impact of this substitution of oxygen evolution reaction with organic oxidation on solar conversion efficiency of photoelectrochemical system is analyzed in detail, accounting for both thermodynamic and kinetic considerations. The results reveal that replacing water oxidation with a thermodynamically more favorable organic oxidation reaction leads to a decrease in solar conversion efficiency for a given photoanode. However, in this case faster reaction kinetics may allow to achieve higher photocurrents and, thus, higher product fluxes. Moreover, organic oxidation enables the use of photoanodes with narrower band gaps than those required for water splitting, allowing absorption of a larger fraction of the solar spectrum. The formation of key reactive oxygen species is also evaluated, demonstrating that hydroxyl radical generation at high fluxes is unlikely in unbiased photocatalytic systems. Based on the results of literature review, BiVO4 is selected as a model photoanode material for experimental investigations due to its suitable semiconductor properties. Despite these advantages, experimental removal percentages for various pharmaceuticals remain below 60% after two hours of treatment. Preliminary experimental analyses reveal that the performance of BiVO4 is primarily limited by low intrinsic catalytic activity towards oxidation of organic compounds, which manifests itself as low faradaic efficiency even when the process is not controlled by mass-transfer, significant surface recombination resulting in current much lower than the maximum theoretical one, and photocorrosion. To mitigate these limitations, surface modification of BiVO4 with β-FeOOH is investigated as a strategy to improve hole extraction and photoelectrode stability. While this modification leads to a 1.5-2 times photocurrent increase and stability improvement, it unexpectedly results in reduced pharmaceuticals degradation rates (from 50.6% to 34.2% for paracetamol, from 64.0% to 54.6% for sulfamethoxazole, from 53.5% to 55.2% for carbamazepine) and higher energy consumption. Mechanistic analysis demonstrates that β-FeOOH preferentially enhances water oxidation kinetics rather than organic oxidation, thereby increasing competition between the two reactions. These findings underscore that photocurrent density alone is an insufficient metric for predicting photoelectrocatalytic performance. Finally, the role of chloride ions in photoelectrocatalytic degradation is examined on sulfamethoxazole degradation. The presence of chloride significantly accelerates oxidation through the formation of reactive chlorine species, achieving complete degradation in 60 and 20 minutes at chloride concentrations 200 and 600 mg/L Cl-, respectively. However, the formation of highly toxic chlorination products (including halogenated acetic acids) and toxicity increase were detected. A non-linear dependence of degradation pathways on chloride concentration is elucidated for the first time, highlighting the complexity of chlorination-assisted photoelectrocatalytic processes. Overall, this thesis provides a critical and systematic evaluation of photoelectrocatalysis for pharmaceutical pollutant degradation, combining theoretical efficiency analysis with detailed experimental investigations. The results clarify fundamental limitations, challenge common assumptions regarding photocurrent-based performance metrics, and identify key design principles for future development of selective and energy-efficient photoelectrocatalytic systems.
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