Degradation of Pharmaceuticals using visible light Photoanode

Abstract

Pharmaceuticals in wastewater are an urgent environmental concern. Many emerging technologies, such as advanced oxidation processes (AOPs), are being explored for their removal. One promising AOP is Photoelectrocatalysis (PEC), which combines photocatalysis with electrochemistry to enhance pollutant degradation by suppressing electron-hole recombination. Bismuth vanadate (BiVO₄) is a promising visible-light-responsive photoanode material due to its narrow bandgap and chemical stability. However, its overall performance is limited by poor charge mobility and rapid recombination of photogenerated electrons and holes. To overcome these limitations, surface modification strategies have been applied, including the use of surfactants to alter and enhance its surface properties. Quaternary ammonium compounds (QACs), widely used for their surfactant properties and structural flexibility, have shown potential in enhancing the photoelectrocatalytic performance of BiVO₄. This study evaluated the effect of six commonly used QACs on BiVO₄ photoanodes. The modified photoanodes were synthesized and comprehensively characterized using several techniques: X-ray Diffraction (XRD) to determine crystal structure, Scanning Electron Microscopy (SEM) to examine surface morphology, Energy Dispersive X-ray Spectroscopy (EDX) for elemental composition, X-ray Photoelectron Spectroscopy (XPS) to analyze surface chemical states, Ultraviolet-Visible (UV-Vis) spectroscopy to assess optical absorption, and Linear Sweep Voltammetry (LSV) to evaluate electrochemical behaviour.
Photoelectrocatalytic degradation experiments were performed using real secondary treated effluent collected from the Horstermeer Wastewater Treatment Plant (WWTP), which was spiked with 20 pharmaceutical compounds. The QAC-modified BiVO₄ photoanodes were tested under simulated solar irradiation. Characterization results confirmed that the monoclinic phase of BiVO₄ was preserved after QAC modification, with minor shifts indicating changes in surface properties. SEM images showed that the structural integrity of BiVO₄ was maintained, while EDX and XPS results revealed an increase in oxygen vacancies, suggesting improved charge transport characteristics. LSV confirmed that photocurrent generation occurred only under illumination, as expected in PEC systems. Among the modified electrodes, the BiVO₄ photoanode modified with DADMAC C18 exhibited the highest pharmaceutical degradation efficiency over a 120-minute PEC run, outperforming even the unmodified BiVO₄ electrode. The ATMAC C18 variant demonstrated rapid initial degradation within the first 15 minutes, while the BAC C18 variant showed relatively poor performance. Kinetic analysis indicated that sulfamethoxazole was the most persistent compound in the pharmaceutical mixture, with the longest half-life. To assess real-world application potential, full-scale PEC reactor designs were reviewed. A circular reactor configuration with annular electrodes was identified as the most suitable due to its balanced light distribution, effective photoelectrocatalytic surface area, and ease of operation. A conceptual full-scale design was proposed to replace the existing UV oxidation system at WWTP Horstermeer, targeting a treatment capacity of 25,000 cubic meters per day. Based on preliminary calculations, the estimated treatment cost was €0.24 per cubic meter, highlighting the potential economic feasibility of implementing PEC technology in municipal wastewater treatment.