Scaling up Photoelectrocatalytic Reactors: A TiO₂ Nanotube-Coated Disc Compound Reactor Effectively Degrades Acetaminophen

Abstract

Multiple discs coated with hierarchically-organized TiO₂ anatase nanotubes served as photoelectrodes in a novel annular photoelectrocatalytic reactor. Electrochemical characterization showed light irradiation enhanced the current response due to photogeneration of charge carriers. The pharmaceutical acetaminophen was used as a representative water micropollutant. The photoelectrocatalysis pseudo-first-order rate constant for acetaminophen was seven orders of magnitude greater than electrocatalytic treatment. Compared against photocatalysis alone, our photoelectrocatalytic reactor at <8 V reduced by two fold, the electric energy per order (E_EO; kWh m⁻³ order⁻¹ for 90% pollutant degradation). Applying a cell potential higher than 8 V detrimentally increased E_EO. Acetaminophen was degraded across a range of initial concentrations, but absorbance at higher concentration diminished photon transport, resulting in higher E_EO. Extended photoelectrocatalytic reactor operation degraded acetaminophen, which was accompanied by 53% mineralization based upon total organic carbon measurements. This proof of concept for our photoelectrocatalytic reactor demonstrated a strategy to increase photo-active surface area in annular reactors.

1. Introduction

The advanced oxidation processes (AOPs) are used in drinking water and both municipal and industrial wastewater purification to transform organic pollutants into less toxic by-products [1]. Among many AOPs, photoelectrocatalysis is an emerging and promising hybrid AOP [2]. This technology provides synergistic benefits from the interaction between photocatalytic and electrocatalytic processes [3,4]. Similar to photocatalysis, photoelectrocatalysis relies on semiconductor materials to photo-generate charge carriers according to Equation (1); applying a constant voltage across semiconductors supported on solid surfaces prevents recombination within the material [5,6]. The photoexcitation of electrons from the filled semiconductor valence band to the empty conduction band generates charge carriers when irradiated with photons of energy superior to the band gap (E_g) [7,8]. Electron (e_cb⁻) photoexcitation also generates a vacancy at the valence band (h_vb⁺). The h_vb⁺ are highly–oxidizing species that can mineralize organic compounds or yield reactive oxygen species (ROS), such as hydroxyl radicals (^•OH), from water oxidation according to Equation (2) [9,10,11].

Semiconductor + hν → h_vb⁺ + e_cb⁻

(1)

h_vb⁺ + H₂O → ^•OH + H⁺

(2)

Charge carriers are unstable due to their excited state and tend to recombine according to Equation (3), which may drastically diminish the availability of oxidants on the semiconductor surface [12,13]. The main contribution of the electrochemically-driven component in the photoelectrocatalytic process is to diminish the extent and even avoid recombination [14,15]. This is achieved by imposing a difference of potential during photoexcitation to induce separation of photogenerated charge carriers [2,16].

h_vb⁺ + e_cb⁻ → heat

(3)

Pollutant degradation in water dramatically increases when semiconductors are employed as photoanodes [2,17,18]. Unlike in slurry reactors, photocatalysts must be mounted on electrically-conducting surfaces. Therefore, a major barrier for photoelectrocatalytic reactors is the surface area of electrodes that can be coated with photocatalysts [19,20]. Previous approaches have considered irradiation outside the cell through transparent conductive glass electrodes of indium tin oxide and fluorine-doped tin oxide (ITO/FTO) coated with semiconductor photocatalysts. However, this approach faces (i) electrode stability problems associated with leaching of the coatings and (ii) photon transport losses due to the absorption by glass [21,22,23]. Herein, a promising alternative reactor design is presented that uses a desirable parallel electrode arrangement in a single annular reactor compartment to promote a homogeneous current distribution [20,24]. Illustrated in Figure 1, multiple disc-shaped photoanodes and cathodes were aligned perpendicular to an annular ultraviolet (UV) lamp placed in the middle of the reactor. This novel reactor design aims to reconcile the needs of electrochemical systems that ensure homogeneous distribution of current density by using parallel electrodes and enabling photoexcitation through efficient light delivery. Removal and mineralization of acetaminophen, a commonly occurring pharmaceutical in treated municipal wastewater [25,26,27], was evaluated in the reactor. This reactor uses electrical energy per order (E_EO, kWh m⁻³ order⁻¹), to assess performance. E_EO is the energy required for 90% pollutant degradation and includes energy for both the photon-driven and electrically-driven processes used in this reactor. Oxidation by-products were identified by high-performance liquid chromatography (HPLC), total organic carbon, and total nitrogen analyses. Comparing photocatalytic to photoelectrocatalytic performance at different applied currents and initial acetaminophen concentrations showed that performance depended on light intensity reaching the electrode surfaces.

2. Materials and Methods

2.1. Electrochemical Cell and Photoelectrode Synthesis

Figure 1 shows the custom-made photoelectrocatalytic reactor configuration used to treat 1 L solutions containing 5 to 50 mg L⁻¹ of acetaminophen. Eight donut-shaped electrode discs were aligned perpendicular to a 14 W UV lamp GPH287T5L/4 (λ = 275 nm; emission spectra shown in Figure 2) encased in a quartz tube (YUP, China). The electrodes were operated potentiostatically using a Tenma 72-8340A potentiostat/galvanostat (Tenma, US). A Masterflex L/S peristaltic pump (Cole-Parmer, US) circulated the solution at 180 mL min⁻¹ to provide mixing and to overcome mass transport limitations of acetaminophen to the reactive surfaces on the donut-shaped disc electrodes.

As depicted in Figure 1b, the donut-shaped discs used as photo-anodes had a total diameter of 71 mm with a defined surface area of 32 cm² per side. The position of the electrodes ensured homogeneous current distribution while ensuring light delivery. Note that the architecture of the system defines hydraulic channels for homogeneous distribution of the solution, whereas the incidence of photons emitted from the immersed light source induces photoexcitation to generate charge carriers. The titanium discs were anodized to produce TiO₂ nanotubes on the surface using previously described methods [28]. Four separate, non-treated, 66 mm in diameter titanium discs were used as cathodes (30 cm² surface area per side). Monopolar connection was attained by using titanium foil (99.99%) as a connector bridge between plates of identical polarity. As shown in the sectional view of the cell (Figure 1d), the electrode configuration allowed a hydraulic pathway during recirculation that maximized solution contact with the electrodes.

Kinetic analysis of acetaminophen degradation followed pseudo-first-order kinetics to determine rate constants (k₁, s⁻¹). Experiments were conducted in triplicate and showed excellent reproducibility. As shown in Equation (4), the rate constant was used to calculate E_EO [29], which is a figure of merit defined by the International Union of Pure and Applied Chemistry (IUPAC) to benchmark AOPs as water treatment technologies in terms of their energy requirements to diminish a pollutant’s content by one order of magnitude (i.e., 90% pollutant removal):

$${\mathrm{E}}_{\mathrm{EO}}\left(\mathrm{kWh}·{\mathrm{m}}^{-3}·{\mathrm{order}}^{-1}\right)=\frac{6.39\times {10}^{-4}\times \left({\mathrm{P}}_{\mathrm{lamp}}+{\mathrm{P}}_{\mathrm{cell}}\right)}{{\mathrm{V}}_{\mathrm{S}}\times k_1}$$

(4)

where the 6.39 × 10⁻⁴ constant accounts for conversion factor (1 h/3600 s/0.4343); Pl_amp and P_cell are the rated power of the lamp and the electrochemical cell (W), respectively; V_S is the solution volume (L); and k₁ is the rate constant (s⁻¹).

Photoanode preparation involved polishing Ti discs with sandpaper of grain sizes P600, P1200, and P2000. Discs were degreased under ultrasonication with three polar solvents for 10 min each: (i) methanol, (ii) isopropyl alcohol, and (iii) acetone. Nanotubes were grown on both sides of the electrode by conducting anodization with two Ti disc cathodes of identical dimensions placed at each side of the anode with 1 cm of interelectrode gap. Anodization was conducted potentiostatically by applying 40 V for 2 h in a solution of ethylene glycol containing 0.3% NH₄F and 2 vol. % of water [28]. The grown nanotubes were thermally annealed at 450 °C for 2 h with a heating ramp of 10 °C min⁻¹ in the muffle furnace Tuve Furnace 21100 (SentroTech, US). Thermal treatment at 450 °C leads to anatase structure formation [30]. Synthesized photoanodes were then implemented in the photoelectrocatalytic cell.

2.2. Chemicals

Acetaminophen (99% pure neat powder) and other carboxylic acid standards were purchased from Sigma-Aldrich. Figure 2 shows the UV–vis spectrum of acetaminophen. Sodium nitrate and ammonium chloride were used as ionic chromatography standards, and sodium sulfate was used as a supporting electrolyte. Analytical grade sulfuric acid and/or sodium hydroxide from Fisher Chemical were employed to adjust the pH. HPLC-grade acetonitrile was purchased from Sigma-Aldrich. Solutions and mobile phases were prepared with nano-pure water obtained from a Millipore Milli-Q system with resistivity >18.2 MΩ cm at 25 °C.

2.3. Analytical Procedures

Scanning electron microscopy (SEM) images of the synthesized TiO₂ nanotubes were recorded using a Jeol JSM-6510LV series (JEOL, US). The Raman spectra was recorded using a SENTERRA II Compact Raman Microscope (Bruker, US) with a laser radiation at 532 nm. Photon irradiance, expressed in μW cm⁻², was measured with a radiometer Avantes AvaSpec 2048 (Avantes, US) spectrometer with a cosine corrector. Linear sweep voltammetry analyses with 100 mV s⁻¹ scan rates were conducted in a three-electrode, one-compartment cell using a PGSTAT302n potentiostat-galvanostat from Metrohm controlled by Autolab Nova 2.1 software (Metrohm, US). The TiO₂ nanotube anode was the working electrode, Pt wire was the counter electrode, and an Ag/AgCl was used as reference electrode.

The pH of the solutions was measured using a Thermo Scientific Orion Star A221 pH meter (Thermo, US). Aliquots were withdrawn during experiments and analyzed. Chromatographic analysis of 25 μL of sample allowed identification and quantification of acetaminophen and by-products (e.g., carboxylic acids, nitrate, and ammonium). Acetaminophen decay was followed by reversed-phase chromatography using a Waters 2695 HPLC coupled to a Waters 2996 Photodiode Array detector, fitted with a Waters LiChrosorb® 10 μm RP18 (100 mm × 4.6 mm) column at 25 °C and a LiChroCART® 4-4 guard column (Waters, US). Chromatograph was operated at isocratic flow using a 35:65 (v/v) acetonitrile/water mixture at 0.6 ml min⁻¹ as mobile phase.

Carboxylic acids yielded during acetaminophen aromatic moiety breakage were identified and quantified by ion-exclusion chromatography using the instrument described above fitted with a Bio-Rad Amnex HPX87H (300 × 7.8 mm) column at 35 °C with a 4.0 mM H₂SO₄ mobile phase at 0.6 mL min⁻¹(Bio-Rad, US). Nitrogenized organic ion formation was quantified by ionic chromatography using a Thermo Dionex ICS-5000DC coupled to a conductivity detector AERS 500 (Thermo, US). Anionic species nitrite and nitrate were quantified using a high capacity hydroxide-selective anion-exchange column Dionex Ionpac AS18 (2 × 250 mm) flowing 30 mM KOH solution as the mobile phase at 0.25 mL min⁻¹ (Thermo, US). Ammonium was quantified using an ammonia TNT plus Vial Test from Hach with 0.015 to 2.00 mg L⁻¹ NH₃-N quantification range.

Mineralization of organics load in treated solutions was evaluated through dissolved organic carbon (DOC) abatement determined with a Shimadzu VCSN total organic carbon (TOC) analyzer. Total nitrogen was measured with a Shimadzu TNM-1 module coupled to the above TOC analyzer.

3. Results and Discussion

3.1. TiO₂ Nanotube Photoelectrode Discs’ Characterization

Figure 3a shows the SEM image of hierarchically organized TiO₂ nanotubes after anodization of pure Ti discs. Homogeneous and well-distributed TiO₂ nanotubes of length 1.8–2.0 μm and diameter ≈ 80 nm were perpendicular to the Ti surface. Raman spectroscopy (Figure 3b) revealed peaks at 146 cm⁻¹, 396 cm⁻¹, 517 cm⁻¹, and 637 cm⁻¹, which is characteristic of anatase nanotubes [31]. Anatase formation by the thermal treatment at 450 °C is consistent with literature [30,32]. The diffractogram of the anodized TiO₂ (see Figure 3c) also depicts the characteristic diffraction peaks associated to anatase crystalline phase, which is in agreement with the conclusion obtained from the Raman spectroscopic analysis. The band gap of 3.2 eV determined is coincident with those previously reported in literature for anatase [33,34].

Voltammetric analysis shows an increased current response of the TiO₂ nanotube discs when the photoanode is irradiated (Figure 3), which is associated with photogeneration of charge carriers [35,36,37]. Photons delivered from the UV light source to the TiO₂ photoanode surface provide the energy required to overcome the band gap (Eg = 3.2 eV) [38,39]. The small overpotential of oxygen evolution with an onset potential of 1.65 V vs Ag/AgCl suggests a low capability for ^•OH electrogeneration from water oxidation (Equation (5)). Therefore, the acetaminophen does not undergo oxidation by direct charge transfer processes within the electrochemical window of TiO₂ anodes. Indeed, the higher TiO₂ oxidation states stabilize ROS as chemisorbed reactive oxides that may induce electrochemical transformation of acetaminophen, but further mineralization is unlikely due to lower oxidation power of chemisorbed ^•OH [40]. The behavior we observed is characteristic of active anode materials according to Comninellis classification [41]. These results prove the photoelectrocatalytic activity of the anodes employed in the multi-electrode multi-disc reactor under a monopolar connection (see Figure 1).

H₂O → ^•OH + H⁺ + e⁻

(5)

3.2. Acetaminophen Degradation by Photocatalysis, Electrocatalysis, and Photoelectrocatalysis

Figure 4 illustrates the acetaminophen loss kinetics under different reactor operational modes. Direct photolysis of acetaminophen (i.e., without disc electrodes) resulted in negligible degradation after 5 h of UV irradiation. Acetaminophen has a very low quantum yield (Φ₂₅₃ = 0.006) and is known to be photostable [42]. Electrocatalytic oxidation of acetaminophen in the dark under constant cell potential of 8.0 V attained only 3% removal by heterogeneous reactions.

Figure 4 shows that photocatalysis with TiO₂ nanotubes abated 72% acetaminophen after 5 h. This proves that perpendicular alignment of the discs to the lamp supports photocatalysis. The acetaminophen photocatalytic abatement was well fit by first-order kinetics, and yielded a rate constant (k₁) of 6.88 × 10⁻⁵ s⁻¹ (R² = 0.998). As well demonstrated in literature, acetaminophen degradation in the TiO₂ photoelectrocatalytic system is mediated by the oxidation capability of photogenerated h_vb⁺ and ^•OH by reactions seen in Equations (1) and (2) [7,8], respectively.

The rate constant for acetaminophen removal in photoelectrocatalytic operational mode (k₁ of 1.42 × 10⁻⁴ s⁻¹) was nearly 3× higher than in photocatalytic mode. Greater than 95% acetaminophen abatement was attained after 5 h under photoelectrocatalytic treatment at E_cell = 8.0 V (Figure 4). The photoelectrocatalysis k₁ was over seven orders of magnitude greater than for electrocatalytic treatment alone. Applying a bias potential enhances the charge carrier separation and diminishes the extent of recombination (Equation (3)) [2,43], which increases the half-life of the oxidants photogenerated on the photoanode surface [13,44]. Moreover, the evolution of O₂ on the anode surface may contribute to selective scavenging e_cb⁻ through Equation (6), which yields superoxide radical (O₂^•−) [16,45]. Yielded O₂^•− is a weaker oxidant with E° = 0.94 V versus a standard hydrogen electrode (SHE) [46], which may contribute to the acetaminophen degradation by (i) selective oxidation of organics and (ii) enhances charge carriers’ stability. Note that the consumption of e_cb⁻ by Equation (7) avoids the risk of recombination Equation (3). The interaction of photocatalytically and electrochemically-driven processes considerably enhances the photoelectrocatalytic treatment performance.

2 H₂O → O₂ + 4 H⁺ + 4 e⁻

(6)

e_cb⁻ + O₂ → O₂^•−

(7)

The higher efficiency of the photoelectrocatalytic system is not only reflected from rate constants but also on energy requirements. Accounting for both lamp and cell power requirements (Equation (4)), E_EO requirement for photocatalysis alone is 130 kWh m⁻³ order⁻¹ compared with 67 kWh m⁻³ order⁻¹ for photoelectrocatalytic treatment, indicating a reduction by two times in energy requirement. Thus, applying a small bias potential is a promising alternative to increase performance of supported photocatalysts.

3.3. Influence of Applied Cell Potential on Reactor Performance.

As supported by above findings, applying a difference of potential synergistically enhanced performance. The h_vb⁺ lifetime on the photoanode surface increases because recombination is inhibited (equation (3)) [19,39]. Applying a constant cell potential (E_cell) promotes extraction of photo-excited e_cb⁻ through an external electrical circuit in an electrochemically-driven process [2]. The E_cell defines the efficient transport and consequent charge carrier separation in the photoelectrocatalytic system [16,47]. Thus, impacts on photoelectrocatalytic k₁ were studied by varying applied E_cell while maintaining a constant photon flux.

Figure 5 shows increasing degradation kinetics with higher E_cell. Increasing photoelectrocatalytic k₁ values of 6.80 × 10⁻⁵ s⁻¹ (R² = 0.998), 1.09 × 10⁻⁴ s⁻¹ (R² = 0.995), 1.42 × 10⁻⁴ s⁻¹ (R² = 0.995), 1.57 × 10⁻⁴ s⁻¹ (R² = 0.998), 1.68 × 10⁻⁴ s⁻¹ (R² = 0.997), and 1.70 × 10⁻⁴ s⁻¹ (R² = 0.995) were determined for E_cell at 0, 4.0, 8.0, 16.0, 32.0, and 62.0 V, respectively. Applying a small E_cell difference of 4.0 V resulted in a 1.6-fold increase in k₁ compared with the pure photocatalytic treatment. Further increases in E_cell provided only minor improvements in acetaminophen abatement. Thus, a maximum separation of charge carriers was attained at a certain applied E_cell, and further increases from this optimum potential did not improve performance.

The above changes in E_cell correspond with E_EO values decreasing from 130 kWh m⁻³ order⁻¹ without a current down to 83 kWh m⁻³ order⁻¹ at 4.0 V and 67 kWh m⁻³ order⁻¹ at 8.0 V. Stabilization of oxidant species on the photoanode surface favors reactions that degrade acetaminophen [2,48]. However, additional increase in E_cell reduces the energy efficiency of the system, skyrocketing the energy requirements by an order of magnitude to 330 kWh m⁻³ order⁻¹ at 62.0 V. This high E_EO can be explained by the concomitant acceleration of parasitic reactions induced by the increase of potential and current circulated during the electrochemically-driven process [49,50]. The excess energy is then consumed on the evolution of oxygen from water oxidation according to Equation (8) and/or dimerization of ^•OH from Equation (9). Effects on the electrode characteristics and its morphology were not observed.

2 H₂O → O₂ + 4 H⁺ + 4 e⁻

(8)

2^•OH → H₂O₂

(9)

3.4. Effect of Acetaminophen Concentration on Photoelectrocatalytic Degradation

Initial pollutant concentrations may affect photoelectrocatalytic treatment performance by competing for surface sites [2,12] or absorbing light in solution, which prevents activation of the semiconductor TiO₂ nanotubes. Figure 6 depicts the degradation kinetics for initial acetaminophen concentrations ranging from 5 mg L⁻¹ to 50 mg L⁻¹ in the photoelectrocatalytic operational mode. Kinetics analyses resulted in photoelectrocatalytic k₁ values of 2.05 × 10⁻⁴ s⁻¹ for 5 mg L⁻¹ (R² = 0.996), 1.42 × 10⁻⁴ s⁻¹ for 10 mg L⁻¹ (R² = 0.995), 6.66 × 10⁻⁵ s⁻¹ for 25 mg L⁻¹ (R² = 0.999), 3.18 × 10⁻⁵ s⁻¹ for 35 mg L⁻¹ (R² = 0.999), and 2.86 × 10⁻⁵ s⁻¹ for 50 mg L⁻¹ (R² = 0.998).

Although acetaminophen has a low quantum yield (Φ₂₅₃ = 0.006), direct absorption of photons by acetaminophen in solution can explain the slower removal observed at higher pollutant concentration (Figure 6) [29,39]. Absorption diminishes photon transport efficiency and limits the amount of light reaching the photoelectrode [23,29]. Photon flux is directly related to the efficient generation of charge carriers by Equation (1); consequently, reducing the photon delivery will also reduce the oxidants photogenerated [6,7]. Furthermore, a driving effect on the deceleration is the higher concentration of organics in solution that compete for h_vb⁺ and ^•OH.

The decrease in photogeneration efficiency and the competition for surface sites increases the energy costs required for operation. As initial concentration increased, so did E_EO for the photoelectrocatalytic treatment: 47 kWh m⁻³ order⁻¹ for 5 mg L⁻¹, 67 kWh m⁻³ order⁻¹ for 10 mg L⁻¹, 144 kWh m⁻³ order⁻¹ for 25 mg L⁻¹, 300 kWh m⁻³ order⁻¹ for 30 mg L⁻¹, and 334 kWh m⁻³ order⁻¹ for 50 mg L⁻¹. It may be inferred that photoelectrocatalytic treatment would be better suited as a polishing step to reduce the concentration of highly-persistent organic pollutants such as pharmaceuticals that may be found in fine chemicals, manufacturing effluents, or hospital effluents. However, photoelectrocatalysis should be conducted in water with high transmittance to allow UV light to reach the electrode surfaces.

3.5. Photoelectrocatalytic Mineralization and Time-Course of Yielded Byproducts.

Acetaminophen was slowly mineralized during photoelectrocatalytic treatment. Figure 7a shows the treated solution attained 53% mineralization at the point when >95% parent compound removal occurred. The remaining TOC suggests that organic by-products are accumulated. Oxidation reactions mediated by h_vb⁺ and ^•OH lead to the aromatic moiety opening and yield short-linear aliphatic carboxylic acids [2]. Ion-exclusion chromatography was used to measure formic, oxamic, and oxalic acids. These highly recalcitrant species are the ultimate products released during AOPs mediated by ^•OH prior to the complete mineralization of organics to CO₂. Figure 8 shows that these carboxylic acids accumulated in solution and reached final concentrations of 33.4 μM formic acid, 2.4 μM oxalic acid, and 0.6 μM oxamic acid. After their formation, the carboxylic acids remained in solution because they are very slowly degraded. This result agrees with previous reports that carboxylic acids are hardly oxidized by h_vb⁺ and ^•OH but are biodegradable [16,40].

As a result of oxidizing the amide functional group, acetaminophen degradation is accompanied by inorganic nitrogen species formation (see Figure 7b inset). Anionic chromatography identified NO₃⁻ release at maximum concentration of 0.005 mM. Nitrite formation was not observed, which is a common trend of AOPs where the high oxidative media of ROS favors complete oxidation towards NO₃⁻ [51,52]. Ammonium was continuously released from the breakage of the amide bond and reached a maximum concentration of 0.021 mM. To better understand the fate of the initial N-organic content, a complete mass balance was conducted (Figure 7b). Compared with the 53% mineralization and volatilization of carbon, only 46% volatilization of nitrogen occurred as the likely result of producing volatile species such as N2 and NOx [53,54,55]. The remaining nitrogen in solution was associated to remaining acetaminophen (0.005 mM), yielded oxamic acid (0.61 μM), and dissolved inorganic nitrogen (0.026 mM); inorganic nitrogen represented 75% of total dissolved nitrogen in solution after photoelectrocatalytic treatment.

4. Conclusions

A monopolar, multiple-disc composite photoelectrocatalytic reactor was constructed to overcome limitations of light transport in photoelectrocatalytic reactors. The parallel arrangement of disc photoanodes and cathodes defined a hydraulic pathway for the recirculated solution while providing the requirements for homogeneous current distribution in electrolytic reactors and light delivery for charge carriers’ photogeneration. This novel reactor system can provide an alternative framework when considering the scaling-up of photoelectrocatalytic water treatment.

Negligible acetaminophen removal was observed by direct photolysis or electrocatalysis. Photoelectrocatalytic treatment was more energy efficient than photocatalysis alone because applying a bias cell potential prevented recombination of ROS on photocatalytic nanotubes’ surfaces. Therefore, E_EO was reduced two fold from 130 kWh m⁻³ order⁻¹ for pure photocatalytic treatment down to 67 kWh m⁻³ order⁻¹ for photoelectrocatalytic treatment.

Applying E_cell improved charge separation and stabilized ROS on the electrode surface. However, when more than 8.0 V was delivered, the electrical current was consumed in parasitic reactions, increasing E_EO. Further increases in E_cell did not improve reactor performance and would have a detrimental impact operational cost.

Pollutant concentration diminished the light transport efficiency, which reduced photon delivery to the photoelectrode surface and reduced charge carrier generation. Even though high concentrations of acetaminophen may be treated in the photoelectrocatalytic reactor, E_EO would increase due to a reduction in treatment performance.

These results demonstrated that scaling-up reactor designs for photoelectrocatalytic treatments must fulfill the needs of light-driven and electrochemically-driven processes to retain performance with respect to efficient energy usage and pollutant removal. The use of multiple, concentric, monopolar discs appears to be a feasible approach. This initial proof of concept provides opportunity for additional photoelectrocatalytic reactor design research. Future work must evaluate ways to improve light transport, which may have been limited by the perpendicular orientation of the electrodes in this design.