Photoelectrocatalytic Oxidation of Sulfamethazine on TiO₂ Electrodes

Abstract

The photoelectrocatalytic degradation and mineralization of sulfamethazine (SMT), a sulfonamide drug, were explored in aqueous solution. Working electrodes with TiO₂ coatings on Ti substrates (TiO₂/Ti) were used, which were produced by the dip coating method. TiO₂ film electrodes were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) following annealing at 500 °C for 1.5 h. To photoelectrochemically characterize them, photocurrents vs. applied potential curves were used. The photoelectrocatalytic efficiency (PEC) of the TiO₂/Ti electrodes regarding the oxidation of SMT has been assessed with reference to degradation and mineralization under different experimental conditions. The selected drug molecule was effectively degraded following the Langmuir–Hinshelwood (L-H) kinetic model. The degradation efficiency was shown to increase with increasing applied potential bias up to +1.5 V vs. Ag/AgCl. It was found to be more favorable in acidic environments compared to alkaline ones. A decrease in the destruction rate constant was recorded when the pH was increased from 3 to 5.6 (natural pH) and 9. The decomposition rate was shown to first increase and subsequently reach a saturation value at high concentrations of SMT, indicating that the degradation also depends on other parameters (e.g., the rate of the charge or the mass transfer on the electrode double layer). The results of the photoelectrocatalytic experiments were compared to those of electrochemical (EC) and photocatalytic (PC) degradation of SMT. A significant enhancement was recorded in the case of the PEC degradation, leading at +1.5 V to an increase of the apparent rate constants of degradation, k, and mineralization, k_TOC, of 153 and 298%, respectively, compared to the simple photocatalytic process.

1. Introduction

Research efforts in the water purification field are continuously growing since water quality control and regulations on different hazardous pollutants have become stricter [1].

Semiconductor photocatalysts offer a simple and cheap process for the abatement of organic compounds under artificial or solar light, and thus an increase in their use has been recorded during the last twenty years [2,3,4]. Because of its interesting characteristics (i.e., chemical stability against both corrosion and photocorrosion and good photocatalytic activity, particularly in its anatase form), titanium dioxide (TiO₂) has been extensively studied and applied, commonly in the form of a slurry of fine particles in a photochemical reactor. The slurry offers a high photoinduced reaction surface and a low recombination rate of the photogenerated e⁻/h⁺ pairs, resulting in high purification efficiency. However, TiO₂ particles cannot be easily separated from the treated wastewater, requiring high operation costs and a complex treatment system. To prevent the need for a separation step, TiO₂ particles may be immobilized onto supporting materials, despite the fact that due to the decrease in catalyst surface area and the diffusion limitations of the organic substance to the catalyst, a decrease in oxidation efficiency is recorded. Different approaches have been investigated to face this problem (e.g., TiO₂ was supported on an electronic conductor and biased positive with the application of an external voltage in a photoelectrochemical cell). In this way, the rate of the photogenerated electron–hole recombination is restricted, and the rate of the surface reactions increases (photoelectrocatalytic oxidation) [5,6,7,8].

SMT is a main sulfonamide antibiotic with a functional group of –SO₂NH₂–. It is broadly used for the control of infectious diseases and for the growth of animals [9,10]. Due to its wide spectrum of antibacterial efficacy and low cost, it is frequently prescribed to humans, aquaculture, and livestock [11,12]. SMT is resistant to biodegradation due to the sulfonamide group, which can inhibit the growth of many Gram-negative and Gram-positive bacteria [13]. Heterocyclic sulfonamides (SAs) are an important group of pharmaceuticals, being the most frequently detected antibiotics in surface waters and wastewaters, followed by fluoroquinolones, tetracyclines, and macrolides [11,14,15,16]. They have been in constant use since the 1930s, posing a significant contamination risk. SMT has been detected in various aquatic matrices, such as groundwater, seawater, and river water, as well as in treated water effluent and reclaimed water [17,18]. It is also persistent and able to accumulate in organisms and soils, and it has been detected in sediments, animal manure, and manure waste lagoons [16,17,19]. SAs have been receiving increasing attention because they can adversely affect both the health of humans and ecosystems [9,17,20,21]. Because SMT interrupts the human endocrine system, may be harmful to marine species, and affects animal reproduction, its presence in aquatic environments is of concern [12]. In addition, its environmental prevalence enhances the spread of resistance in microorganisms, ultimately influencing the efficiency of treatment [22]. Environmental SMT concentrations generally range from nanograms per liter to micrograms per liter [10,12,18,20,21]. SMT concentrations in the range of 15–328 ng/L and 2.05–623.27 ng/L have been detected in tropical waters in Vietnam and river water in China, respectively [10,23]. Other studies reported that SMT was found in water from wells and rivers in concentrations ranging from 0.22 to 2.48 μg/L, respectively [24].

Conventional processes (e.g., coagulation, flocculation, and sedimentation) have been proven inefficient towards the abatement of antibiotics, while physical approaches, such as adsorption, ultrafiltration, and reverse osmosis, are known to be non-destructive, thus transferring pollutants from one phase to another, finally resulting in secondary pollution [10,11,24]. Numerous alternative treatment technologies have been evaluated for the degradation of these emerging organic pollutants [12,23,24,25,26]. Advanced oxidation processes (AOPs) (photo-Fenton, electro-Fenton, sonolysis, etc.) have been proven very effective and are characterized by high rates of degradation in a short time [10,13,26,27]. However, some of them are characterized by high processing costs due to the required energy and/or the production of extra waste (e.g., iron-containing sludge) [9,10]. Thus, the development of a cost-effective, environmentally friendly treatment process for the successful abatement of SMT and other antibiotics and organic pollutants from aquatic bodies is of high importance. Due to its very interesting characteristics (high degradation efficiency, potential for solar energy use, mild conditions of reactions, absence of harmful by-products, and effective catalyst recovery), photoelectrocatalysis (PEC) has been proven to be a promising emerging AOP [17,18,20,28].

However, studies of photoelectrocatalysis (PEC) abatement of SMT are still few. The first report on photo-electrocatalytic degradation of SMT by semiconductor MOFs as catalysts was by Jia et al. (2020). The rate of reaction of ZIF-8/NF-TiO₂ increased by 21.7 times compared with unmodified anatase TiO₂, while the synergistic factor in the photo-electrocatalytic process could reach 3.5 [16]. Recently, three studies on PEC SMT degradation have been reported [18,28,29]. Candia-Onfray et al. (2023) produced various MOF-derived photoanodes (Ru₃(BTC)₂, UiO-66(Zr) NH₂, and Au@ZIF-8, on Ti/TiO₂NT) and applied them for the photo(electro) chemical abatement of CECs, including SMT, in a spiked secondary effluent sample from a municipal wastewater treatment plant [29]. In another study, Wu et al. (2023) produced a series of Bi-Ti/powdered activated carbon (PAC) composites by the sol-hydrothermal method and studied the adsorption and photocatalytic degradation of SMT [28]. An additional recent photoelectrocatalytic study was described by Hu et al. (2023), who produced a self-driven dual-photoelectrode PEC system (TiO₂ NNs-Co₃O₄) made of a TiO₂ nanoneedle array (TiO₂ NNs) photoanode and Co₃O₄ photocathode, which was used for the degradation of SMT [18].

Our present study provides results describing the production of TiO₂ electrodes on Ti substrates, their characterization, and the study of their photoelectrocatalytic activity towards the degradation of SMT, whose structure is shown in Figure 1. Τhe photoelectrocatalytic oxidative degradation of SMT was studied under artificial illumination in a novel photoelectrocatalytic reactor, and different parameters such as pH, concentration of the target drug molecule, and applied potential were investigated. We also aimed to compare the results of the photoelectrocatalytic experiments to those of electrochemical (EC) and photocatalytic (PC) degradation of SMT.

2. Results

2.1. Photoelectrochemical Behavior of the TiO₂/Ti Electrode in Supporting Electrolyte

Figure 2 shows the XRD pattern of the TiO₂/Ti electrode (annealed for 1.5 at 500 °C). A high intensity peak at 2θ = 25.28° and a low intensity peak at 2θ = 28.06°correspond to anatase TiO₂ and the rutile phase, respectively. In accordance with the starting Degussa P-25 material specifications, the anatase form of TiO₂ was predominant in the TiO₂/Ti film. Figure 3 shows the surface morphology of the particulate TiO₂/Ti working electrode. A network of spherical aggregates of a few μm was found. Following annealing, a few tens of nm P-25 particles (ca. 30 nm) have merged into these aggregates.

The current–potential curves of the particulate TiO₂/Ti electrode in 0.1 M Na₂SO₄, under dark conditions, and under UV-A illumination (within the potential range of −0.5 and +2 V vs. Ag/AgCl) are shown in Figure 4.

The TiO₂/Ti electrode exhibits nearly perfect blocking characteristics in the potential range of −0.2 to +1.6 V vs. Ag/AgCl, typically for an n-type semiconductor (Figure 4). Also, due to water decomposition and oxygen evolution, an increase in dark current density has been recorded for potentials higher than +1.6 V.

Contrarily, a remarkable increase in the anodic current density was shown above −0.2 V vs. Ag/AgCl upon illumination. In accordance with the bibliography [30,31,32,33], the illumination of a semiconductor–electrolyte interface with light energy higher than that of its band gap energy results in the production of electron–hole pairs (e⁻/h⁺) in the space charge layer of the semiconductor. The contemporaneous application of a bias potential positive to the flat-band potential generates a bending of the conduction and valence bands, resulting in a more efficacious separation of the photogenerated carriers within the space charge layer. It also augments the photocurrent (I_ph) that begins to flow and likely promotes the oxidative degradation process. The potential gradient effectively forces the electrons to arrive at the counter electrode and leave the photogenerated holes to react with H₂O/OH⁻ to produce OH^• radicals or to directly react with the organics presented in the solution, as shown by the global reactions described by Equations (1)–(5).

Anode (working electrode):

TiO₂ + hν (<400 nm) → TiO₂ − e⁻_cb + TiO₂ − h⁺_vb

(1)

TiO₂ − h⁺_vb + H₂O_s → TiO₂ − OH_s^• + H⁺

(2)

TiO₂ − h⁺_vb + OH⁻_s → TiO₂ − OH_s^•

(3)

TiO₂ − e⁻_cb + TiO₂ − h⁺_vb → recombination

(4)

Cathode (counter electrode):

2H₂O + 2e⁻ → H₂ + 2OH⁻

(5)

cb and vb denote the conduction band and valence band of the photocatalyst, while h⁺ and e⁻ correspond to the photogenerated holes and electrons, respectively.

2.2. Bulk Photoelectrocatalytic Oxidation of Sulfamethazine at the TiO₂/Ti Electrodes

The change in the absorption spectrum of 20 mg L⁻¹ (7.8 × 10⁻⁵ M) SMT during its photoelectrocatalytic abatement at an anodic bias of +1.5 V vs. Ag/AgCl in a 0.1 M Na₂SO₄ solution is shown in Figure 5. The increase in illumination time results in a decrease in the intensity of the absorption maximum at 264 nm, which disappears almost completely within ~3 h, indicating the complete decomposition of the SMT molecule.

Figure 6, on the other hand, shows the performance of the TiO₂/Ti electrode in regard to the reduction of the drug concentration in relation to the illumination time under various experimental conditions. The relative decrease is the ratio of SMT concentration at time t to its starting concentration in the solution at t = 0. Three distinct groups of conditions were investigated: (a) the photocatalytic degradation efficiency without any applied potential; (b) the electrochemical degradation by biasing the electrode with +1.5 V in the dark; and (c) the photoelectrocatalytic degradation, i.e., applying both a +1.5 V potential and UV-A light. As shown in Figure 6 and in

Table 1. Apparent rate constants of degradation, k, and mineralization, k_DOC, of 20 mg L⁻¹ SMT in a 0.1 M Na₂SO₄ solution under different experimental conditions.

Experimental Conditions	Apparent Rate Constant of Degradation k × 103 (min−1)	Apparent Rate Constant of Mineralization kDOC × 103 (min−1)
Photoelectrocatalytic, pH = 5.6, +1.5 V vs. Ag/AgCl	15.65 ± 0.28	4.63 ± 0.06
Photoelectrocatalytic, pH = 3, +1.5 V vs. Ag/AgCl	20.84 ± 0.30	5.78 ± 0.09
Photoelectrocatalytic, pH = 9, +1.5 V vs. Ag/AgCl	11.7 ± 0.34	2.97 ± 0.31
Electrochemical, +1.5 V vs. Ag/AgCl	2.65 ± 0.34	-
Photocatalytic, pH = 5.6	10.2 ± 0.42	1.55 ± 0.22
TiO2/Pt, Photoelectrocatalytic, pH = 5.6, +1.5 V vs. Ag/AgCl	28.32 ± 1.16	8.51 ± 0.74
Thermal-TiO2/Ti, Photoelectrocatalytic, pH = 5.6, +1.5 V vs. Ag/AgCl	16.62 ± 1.83	0.95 ± 0.06
Photoelectrocatalytic, pH = 5.6, +0 V vs. Ag/AgCl	12.74 ± 1.36	1.85 ± 0.12
Photoelectrocatalytic, pH = 5.6, +1.0 V vs. Ag/AgCl	13.49 ± 1.6	2.32 ± 0.19

, where the apparent reaction rate constants of degradation (k) under the different experimental conditions are provided, direct electrochemical oxidation contributed less than 5% to the SMT abatement after 3 h. The pure photocatalytic process, that is, without applying potential, corresponding to the open circuit conditions, resulted in a 72% decrease in SMT concentration after 3 h of illumination. However, when applying an anodic potential of +1.5 V, the photodegradation efficiency was enhanced to 97%, indicating that the applied potential increases the SMT degradation rate remarkably.

Similarly to other organic pollutants, SMT photodecomposition is a complex process with many intermediate products, which can be characterized by higher toxicity compared to the parent compounds, thus being of high importance in water treatment processes. Consequently, it is of crucial importance to achieve complete mineralization of the organic pollutants by applying the selected method. The general equation, which describes the complete SMT oxidation (mineralization), is shown in Equation (6) and is valid after a prolonged irradiation time:

$${\mathrm{C}}_{11}{\mathrm{H}}_{14}{\mathrm{O}}_2{\mathrm{N}}_4\mathrm{S}+20 {\mathrm{O}}_2 \stackrel{}{\to } \mathrm{intermediates} \stackrel{}{\to }11C{\mathrm{O}}_2+4\mathrm{N}{{\mathrm{O}}_3}^{-}+\mathrm{S}{{\mathrm{O}}_4}^{2-}+6{\mathrm{H}}^{+}+4{\mathrm{H}}_2\mathrm{O}$$

(6)

A detailed photoelectrocatalytic SMT oxidation mechanism has been described in previous studies [34,35,36]. A composite of g-C₃N₄ with TNTs (g-C₃N₄/TNTs) has been developed in the study of Ji et al. (2020), showing high photocatalytic efficiency for SMT degradation under simulated solar light, 100% removal within 5 h, decreased toxicity of degradation intermediates/products, a high mineralization rate, and good reusability [26].

Figure 7 shows the level of mineralization as the dissolved organic carbon (DOC) reduction vs. time of illumination for a solution containing 20 mg L⁻¹ SMT, and Table 1 shows the apparent reaction rate constants of mineralization (k_DOC) under the same experimental conditions as Figure 6. The photoelectrocatalytic oxidation at +1.5 V results in a 63% reduction of the initial carbon content of SMT after 4 h of illumination. At the same time, the decomposition was almost complete, indicating the presence of difficult degradable intermediates (Figure 6). For simple photocatalytic oxidation, as in the degradation results, the photomineralization is less efficient in comparison to the one when an anodic potential is applied. Only 26% of the initial organic content contained in the SMT molecule is converted to CO₂ at the same time as the reaction. Under the given experimental conditions, electrochemical oxidation proves to be a very inefficient process of degradation and mineralization of the target molecule.

Experimental data show that photoelectrocatalytic processes are more effective compared to pure electrochemical or photocatalytic ones for drug degradation. An anodic potential higher than that of the flat band potential of TiO₂ results in a significant increase in the efficiency of the photocatalytic activity of the Ti/TiO₂ electrode and consequently increases the reaction rate of SMT degradation. As mentioned before, the application of the positive potential to the TiO₂/Ti electrolyte interface gives a potential gradient within the semiconductor layer, which can drive the photogenerated holes and electrons efficiently apart. Therefore, the charge recombination of the photogenerated carriers decreases, and thus a higher number of positively charged holes is available for the photooxidation of H₂O or the SMT molecule and the intermediate products adsorbed onto the surface of TiO₂.

A few recent studies on PEC SMT degradation have been published. In the study of Candia-Onfray et al. (2023), higher degradation percentages were recorded for Ti/TiO₂NT-Ru₃(BTC)₂ after PEC treatment of 180 min, resulting in the removal of a total concentration of CECs higher than 90%. According to the authors, this finding could be attributed to the higher production of •OH generated by the UV irradiation and the stabilization of the photogenerated charge by the applied current [29]. The Bi-Ti/PAC photocatalyst with a Bi/Ti molar ratio of 10% and calcination temperature of 400 °C was the most efficient in the study of Wu et al. (2023), was stable, and could be recycled for many SMT degradation rounds. SMT degradation rate reached 81.18% (time: 300 min, catalyst dosage: 1.0 g/L, initial SMT concentration: 20 mg/L, pH: 6.0, visible light irradiation at wavelength: 400–780 nm, pseudo first-order rate constant: 0.00531 min⁻¹, regression coefficient of 0.99682) [28]. Finally, Hu and colleagues (2023) showed that under light-emitting diode (LED) illumination, the bias-free TiO₂ NNs-Co₃O₄ PEC system showed exceptional PEC performance, with an internal bias as high as 0.19 V, resulting in almost complete degradation (99.62%) of SMT, with a pseudo-first-order rate constant of 0.042 min⁻¹, showing also excellent reusability performance (99.40% of the fifth cycles) [18].

The rate of the photoelectrocatalytic degradation of SMT at the TiO₂/Ti electrode is affected by different parameters, such as the initial SMT concentration pH, applied potential, light intensity, etc., and some of these factors are discussed below.

Figure 8A shows the effect of the initial SMT concentration on the initial reaction rate (r_o) of photodegradation and mineralization. The r_o values were independently obtained by a linear fit of the C-t data in the range of 10–50 mg L⁻¹ initial SMT concentration. To minimize variations because of the competitive effects of intermediates, pH changes, etc., the calculations of the initial reaction rates were based only on experimental data collected during the first 45 min of illumination. The curve is reminiscent of a Langmuir type isotherm, at which the decomposition rate first increases and subsequently reaches a saturation value at high concentrations of SMT, indicating that the degradation also depends on other parameters, such as, for example, the rate of the charge or the mass transfer on the electrode double layer.

The photocatalytic degradation rate of organic compounds is usually described by a pseudo-first-order kinetic expression, which is rationalized in terms of the Langmuir-Hinshelwood (L-H) model, modified to accommodate reactions occurring at the solid-liquid interface [37,38].

$$r_o=-\frac{dC}{dt}=\frac{k_{r}KC}{1+KC}$$

(7)

where r_o is the initial rate of disappearance of the organic substrate, C is the initial bulk-solute concentration, K corresponds to the equilibrium constant of adsorption of the organic substrate onto TiO₂, and k_r reflects the limiting rate constant of reaction at maximum coverage under the given experimental conditions. This equation can be used when data exhibit sufficient linearity when plotted as follows:

$$\frac{C}{r_o}=\frac{1}{k_{r}K}+\frac{C}{k_r}$$

(8)

Figure 8B shows the dependence of C/r_o values on the respective initial SMT concentrations. The k_r and K values calculated according to Equation (8) from the slope of the straight line (R² = 0.99) and from the intercept with the C/r_o axis were 0.47 mg L⁻¹ min⁻¹ and 0.05 mg L⁻¹, respectively.

Figure 9 presents the effect of the applied potential on the degradation (A), and mineralization (B) of 20 mg L⁻¹ SMT, while in Table 1, the k and k_TOC values under the different potential values are given. As can be seen, an increase in cell voltage leads to an increase in the kinetic constants of degradation and mineralization. By applying +1.5 V, the k and k_TOC values increased by 123% and 250% compared to 0 V cell voltage, respectively, while a 153 % increase in k and a 298% increase in k_TOC were attained in comparison to the simple photocatalytic process. This finding supports the fact that the applied potential is a key factor for photoelectrocatalytic oxidation.

As a result, as the potential increases up to +1.5 V, the rate of SMT oxidation increases, while additionally increasing the potential beyond this value does not enhance the degradation rate. The conditions applied in synthesizing the photoelectrode dictate the potential at which the maximum rate of degradation is achieved, but according to the literature, it seems to be no higher than 2 V. If the potential exceeds this value, this triggers direct electrooxidation of H₂O or SMT.

2.3. The Effect of pH on the Degradation of Sulfamethazine

The solution pH influences the speciation of both the surface functional groups of the TiO₂/Ti electrode and the chemical forms of organic compounds in the solution [39,40]. The solution pH also influences the flat band potential, increasing it by 59 mV per pH unit in the cathodic direction [32], therefore decreasing the oxidative power of the photogenerated holes. All of these pH-dependent parameters may influence the photoelectrocatalytic oxidation of an organic molecule at the TiO₂ electrode.

The effect of the initial solution pH on the degradation (A) and mineralization (B) of 20 mg L⁻¹ SMT is shown in Figure 10, while the k and k_DOC values under the different pH values are provided in Table 1. An applied potential across the photoanode/electrolyte interface of +1.5 V and a supporting electrolyte concentration of 0.1 M were used for all experiments. As can be seen in both Figure 10 and Table 1, as it concerns SMT degradation, a decrease in the destruction rate constant was recorded when the pH was increased from 3 to 5.6 (natural pH) and 9. This finding is in line with Yang’s results on pentachlorophenol degradation [41], as well as with Kim and Anderson’s findings [42], where the maximum photoelectrocatalytic degradation of HCOOH was recorded at pH = 3.4.

A novel clay (bentonite)-supported Ag⁰ nanoparticles (NPs)-doped TiO₂ nanocomposite (Clay/TiO₂/Ag⁰ (NPs) thin film) was applied for the photocatalytic breakdown of SMT from aqueous solution under visible light and UV-A radiation. It has been shown that pH 6 was the maximum pH value for an efficient SMT breakdown, while the reduction in antibiotic concentration (0.5–20.0 mg/L) highly supported the degradation efficiency, with the process following pseudo-first-order kinetics [12]. Graphene modified anatase/titanate nanosheets (G/A/TNS) were applied for solar-light-driven photocatalytic breakdown of SMT, resulting in a degradation of 96.1% in 4 h at pH 6, with a pseudo-first-order model describing the reaction kinetics and a rate constant (k1) for SMT degradation by G/A/TNS-0.5 of 0.493 h⁻¹ [43]. The kinetics of photo-decomposition of SMT in the absence and presence of TiO₂ at pH 3, 5.5, and 10 were also assessed by Tzeng et al. (2016). Apparent rate constants were well described by the pseudo-first-order kinetic model and ranged from 0.24 to 1.61 h⁻¹. It has been shown that photolytic reactions dominate SMT decomposition at pH 10. However, the overall photo-decomposition was increased at pH 5.5 because the SMT adsorption on TiO₂ surfaces was significantly enhanced, supporting the photo-catalytic degradation of SMT by OH radicals [9]. Ayala-Durán et al. (2020) evaluated iron mining residue as a potential catalyst for heterogeneous Fenton/photo-Fenton degradation of sulfonamide antibiotics. Authors showed that degradation was significantly dependent on the initial pH value, with the highest efficiency recorded at pH 2.5, resulting in a concentration below the detection limit of sulfathiazole within 30 min (under black light irradiation, using 0.3 g L⁻¹ residue, and H₂O₂ consumption of 0.2 mmol L⁻¹). Catalytic activity was maintained high during up to five cycles [44].

2.4. The Effect of the Type of the Electrodes on the Degradation of Sulfamethazine

Additionally, experiments with P-25 TiO₂/Ti photoelectrodes, Pt-modified P-25 TiO₂/Ti, and thermally grown anatase photoanodes were also studied. The former were produced by a photocatalytic method that involves the reduction of Pt ions onto a TiO₂ surface upon UV-A illumination [45]. The latter were thermally grown by sintering TiO₂ P-25 at 500 °C. As can be seen in Figure 11 and Table 1, the presence of Pt islands on the TiO₂ particles causes an increase in the photoelectrocatalytic efficiency due to the better charge transfer of the photogenerated carriers through the electrode layer. This leads to a complete reduction of the SMT molecules in the solution after just 90 min of illumination, while the associated reaction rate constant is significantly increased (see Table 1). On the contrary, the thermal electrode shows a weaker photoelectrocatalytic behavior, leading after 3 h only to a ~50% reduction of the SMT concentration and a 21% reduction of the dissolved organic carbon.

SMT photodegradation in the presence of TiO₂, P25, and ZnO under UV irradiation was examined by Aissani et al. (2018). Adsorption experiments in the dark showed no adsorption of SMT on the catalysts, while direct photolysis did not significantly degrade SMT. Moreover, the pH effect was negligible in the range 4–9, and the removal efficiency was found to be influenced by the amount of catalyst (optimal values were 0.5 and 0.25 g/L for TiO₂ P25 and ZnO, respectively). A negative effect on its degradation was caused by an increase in the initial SMT concentration, while a second-order kinetic model described the experimental data in the presence of TiO₂ [10]. The photocatalytic degradation of SMT in TiO₂ suspension was studied both experimentally and theoretically under UV irradiation by Zhang (2014). SMT degradation was found to increase with prolonged reaction times [17]. Photolytic and photocatalytic degradation of SMT dissolved in Milli-Q water and in synthetic wastewater were described by Babić et al. (2015). Different processes were investigated, including direct photolysis, UV/H₂O₂, UV/TiO₂, and UV/TiO₂/H₂O₂ using UV-A and UV-C radiation. Pseudo-first-order kinetics were found to describe SMT degradation in all studied processes. UV-C/TiO₂/H₂O₂ was the most efficient process, resulting in a complete SMT abatement in only 10 min [15]. Large-area N-TiO₂/GR layer materials were found to exhibit enhanced photocatalytic activity for the degradation of different antibiotics, including SMT, with graphene being able to function as an effective “electron pump” for the photodegradation, which can enhance the separation of carriers [46]. A reactor composed of four consecutive stainless-steel plates immobilized by tungsten-doped TiO₂ (WeTiO₂) using polysiloxane has been constructed by Fouad et al. (2020) for the degradation of SMT. The residual SMT concentration was below the detection limit after 30 min of photocatalysis (initial concentration: 17.42 mg/L, flow rate: 300 mL/min, pH: 4.0) [23].

3. Materials and Methods

3.1. Chemicals

Sulfamethazine (C₁₁H₁₄O₂N₄S, 4-Amino-N-(4,6-dimethyl-2-pyrimidinyl) benzene-sulfonamide) was a product of Sigma-Aldrich Chemie GmbH (Taufkirchen, Germany) and was used as received.

TiO₂ was from Degussa-Hüls AG (Frankfurt, Germany) (TiO₂ P-25 Degussa, anatase/rutile: 3.6/1, BET surface area 56 m² g⁻¹, nonporous). H₂SO₄ and NaOH used for pH adjustment, as well as anhydrous Na₂SO₄ (p. a. > 99%), were from Merck (Darmstadt, Germany). Ti plates (0.5 mm thick) were from Alfa Aesar (Tewksbury, MA, USA) (99.5% metal basis). Doubly distilled water was used for this study. Aqueous stock solutions of sulfamethazine (200 mg L⁻¹) were prepared weekly, protected from light, and stored at 25 °C.

3.2. TiO₂/Ti Electrode Preparation

The procedure for the preparation of TiO₂/Ti electrodes has been previously described in detail [34,47]. Various parameters, e.g., the preparation method, heat treatment conditions, etc., affect the photoelectrochemical efficiency of a TiO₂ electrode. In our study, by using the dip coating method, the best results regarding the photoelectrocatalytic efficiency were recorded for specimens treated between 400 and 700 °C and for time intervals of 1.5 to 10 h with electrodes annealed at 500 °C for 1.5 h. Thus, all bulk photoelectrocatalytic experiments on SMT degradation were performed using these electrodes.

3.3. Experimental Setup and Procedures

For the electrochemical/photoelectrochemical characterization of the produced TiO₂ films, a classical three-compartment photoelectrochemical cell was used.

The detailed experimental setup and the electrochemical reactor for the bulk photoelectrolysis experiments have been described previously [48]. It consisted of a 500 cm³ cylindrical cell (i.d. = 7.8 cm; height = 14.8 cm) with a removable cap. The same Osram Dulux 9 W/78 UVA lamp was placed in a cylindrical borosilicate glass sleeve and introduced in the middle of the reactor. The light intensity on the TiO₂ electrode surface position was measured as 3.9 mW cm⁻² with a photometer (Solar Light Co., Glenside, PA, USA, PMA 2100), while the flux of the UV-A photons emitted into the 340 cm³ reaction solution was found to be 1.25 × 10⁻⁴ Einstein L⁻¹ min⁻¹ by ferrioxalate actinometry [30]. The TiO₂/Ti electrode with a 220 cm² surface area was fitted between the cylindrical sleeve and the inner wall of the cell. A stainless steel wire was coiled around the sleeve and used as a counter electrode, while an Ag/AgCl electrode equipped with a salt bridge made of a thin thermoplastic tube ending in a Vycor^® tip was used as a reference electrode. The potentiostat Wenking POS 73 (Bank Elektronik, Pohlheim, Germany) was used to obtain the applied potential. The photoelectrochemical reactor was placed in a dark chamber in order to avoid interference from the daylight. All experiments were carried out at a temperature of 25 °C.

To check the reproducibility of the experimental results, some photocatalytic experiments were repeated three times. The reproducibility of the optical density values was within ±5%, while that of the dissolved organic carbon (DOC) ± 10%.

3.4. Analytical Methods

Scanning Electron Microscopy (SEM) was carried out using a JSM 733 microscope (Tokyo, Japan). A SHIMADZU X-ray diffractometer (Lab X, XRD-6000, Kyoto, Japan) was used for X-ray diffraction (XRD) coating characterization.

Absorption spectra in the ultraviolet and visible ranges were recorded with a Shimadzu PharmaSpec UV-100 spectrophotometer (Kyoto, Japan). A total organic carbon analyzer (Shimadzu Instruments, model V_CSH TOC Analyzer, Kyoto, Japan) was used to monitor the dissolved organic carbon (DOC) reduction. During the degradation experiments, samples of 5 mL for the absorption spectra and 6 mL for the DOC analysis were collected from the reactor at the determined time intervals.

4. Conclusions

In this work, the photoelectrocatalytic oxidative degradation of sulfamethazine, a sulfonamide drug, has been studied under artificial illumination in a novel photoelectrocatalytic reactor. It has been proven that the particulate TiO₂/Ti electrode could be efficient for degradation and mineralization, despite the fact that more active electrodes are required for practical achievement. The degradation efficiency is faster by photoelectrocatalysis compared to photocatalysis or electrochemical oxidation alone. This finding supports the fact that there is a synergetic effect on SMT degradation when an appropriate irradiation and a potential difference are applied simultaneously on the particulate TiO₂/Ti electrode, leading to a 153% increase in the rate constant of degradation and a 298% increase in the rate constant of mineralization in comparison to the simple photocatalytic process. The SMT photooxidation followed first-order kinetics according to the Langmuir–Hinshelwood model, while parameters such as pH, concentration of the target drug molecule, and applied potential play a key role in influencing the reaction rate constant.