Unveiling Synergistic Effects for the Optimizing Photoelectro-Fenton Process for Enhanced Terbutryn Herbicide Degradation

Abstract

Escalating herbicide pollution in natural water bodies necessitates further exploration of effective remediation strategies. This study investigated the electro-degradation of Terbutryn (TBT) at concentrations comparable to those encountered in agricultural practices. Anodic oxidation (AO), electro-Fenton (EF), and photoelectron-Fenton (PEF) were employed for TBT abatement. AO achieved moderate removal (68%), EF significantly improved efficiency (99%), and PEF surpassed both, reaching near complete removal (99.4%) by combining EF with UV light-induced •OH generation. Statistical analysis confirmed that optimizing treatment conditions was crucial. All three factors (current density, Fe²⁺ concentration, and initial TBT concentration) independently affected the PEF process ability to remove TBT pollutants. However, the interplay between these factors was even more important. Sufficient Fe²⁺ was critical for high TBT concentrations, and a balance between current density, Fe²⁺, and initial TBT concentration was necessary. Excessive levels of any could hinder COD removal. High-performance liquid chromatography (HPLC) was employed to monitor the degradation profile of by-products, including desthiomethyl Terbutryn, 2-hydroxy Terbutryn, and cyanuric acid. The analysis of these degradation products facilitated the proposal of a degradation pathway for Terbutryn. PEF stands out as a viable approach for TBT removal, especially in high-TBT wastewater.

1. Introduction

Terbutryn (TBT), a triazine herbicide, has been extensively employed for broadleaf weed control in various crops [1]. While effective in agriculture, concerns regarding its environmental impact and potential health risks have led to its prohibition in the European Union since 2008 (European directive 2008/105/EC) [2,3]. Despite this ban, TBT remains used in several countries, including the United States, Mexico, Argentina [4], and Ecuador [5].

The low biodegradability and persistence of Terbutryn (Table 1) [6] in water pose significant environmental challenges. Its mobility and runoff characteristics result in its detection of soil, plants, and surface water, raising concerns about potential contamination and human health impacts [7,8]. Studies have linked TBT exposure to carcinogenicity, endocrine disruption, and immune suppression in males [9,10]. Therefore, developing effective methods for degrading TBT is crucial for environmental remediation efforts [11].

Advanced oxidation processes (AOPs) offer a promising alternative for removing persistent organic pollutants (POPs) like TBT [12]. These processes leverage the in situ generation of highly reactive hydroxyl radicals (•OH) to degrade pollutants into CO₂, H₂O, and short-chain carboxylates (Equation (1)). Anodic oxidation (AO), a well-established AOP, utilizes an electrolytic cell to directly or indirectly oxidize organic pollutants, promoting the reaction of pollutants (R) with electrogenerated species like hydroxyl radicals (•OH) (Equation (1)) [13].

a(•OH) + R → mCO₂ + nH₂ + xH⁺ + ye⁻(1)

The electro-Fenton (EF) process, another AOP, generates •OH radicals in a solution through the Fenton reaction between ferrous ion and hydrogen peroxide (Equation (2)). This catalytic reaction is sustained by the continuous regeneration of Fe²⁺ ions (Equation (3)). A key advantage of EF is the in situ and constant supply of H₂O₂ within the contaminated solution. This occurs through the two-electron reduction of injected oxygen (O₂) at the cathode (Equation (4)) [14,15].

Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻ (2)

Fe³⁺ + e⁻ → Fe²⁺(3)

O₂ (g) + 2H⁺ + 2e⁻ → H₂O₂ (4)

Photoelectro-Fenton (PEF) further enhances the degradation process by exposing the solution to UVA light. This irradiation promotes the generation of •OH radicals through the photolysis of the Fe(OH)²⁺, the dominant Fe³⁺ species at pH 3.0 (Equation (5)). Additionally, UVA photons facilitate the decomposition of Fe(III)–carboxylate complexes, accelerating the overall mineralization process (Equation (6)) [16]. This study evaluates TBT’s degradation rates and COD removal at concentrations relevant to agricultural applications, comparing the performance of anodic oxidation, electro-Fenton, and photoelectro-Fenton. AO, EF, and PEF have shown promise in degrading various organic pollutants. Still, investigations focused on TBT degradation at high concentrations, similar to those used in agricultural applications for weed management, using these techniques remain limited (Table 2). This scarcity of research underscores the need for further studies to comprehensively understand the efficacy and mechanisms of EAOPs in addressing TBT contamination under doses used in agriculture. For example, one study used a dose of TBT at 3000–6000 mg of TBT applied per hectare, prepared in 60 L of water [17].

[Fe (OH)]²⁺ + hv → Fe²⁺ + •OH(5)

[Fe(OOCR)]²⁺ + hv → Fe²⁺ + CO₂ + R•(6)

Furthermore, a statistical analysis is employed to delve deeper into the interplay between critical factors influencing degradation efficiency in the PEF process at a high TBT concentration, which include current density, Fe²⁺ catalyst concentration, and initial TBT concentration. Understanding these interactions is crucial for optimizing the PEF treatment for remediation applications targeting residual TBT contamination. The study demonstrated the limitations of AO in TBT removal and explored the potential of EF and PEF for efficient degradation. The statistical analysis provides valuable insights for optimizing PEF treatment conditions, leading to cost-effective remediations of residual TBT in water sources.

This research contributes to several Sustainable Development Goals (SDGs) by promoting environmental sustainability and human well-being. Specifically, it addresses SDG 6: Clean Water and Sanitation by developing efficient methods to degrade harmful pesticides like Terbutryn, thereby reducing water pollution. Additionally, it contributes to SDG 12: Responsible Consumption and Production by promoting sustainable agricultural practices and minimizing the environmental impact of pesticide use. By advancing in ecological science and technology, this research contributes to a more sustainable future for future generations.

2. Experimental Procedures

2.1. Reagents

Syngenta supplied a commercial TBT product (250 g L⁻¹). Desthiomethyl Terbutryn, 2-hydroxy Terbutryn, and cyanuric acid were purchased in analytical grade from Aldrich, Panreac, and Merck. Na₂SO₄ from Karal was a supporting electrolyte, and FeSO₄·7H₂O from JT Baker acted as the catalyst. HPLC-grade methanol, acetonitrile, and H₂O₂ were procured from JT Baker and Sigma-Aldrich (St. Louis, MI, USA) for the HPLC method. Drinking water was used in all experiments with a pH of 7.4 and electrical conductivity of 487 μS cm⁻¹ at 25 °C to prepare the solutions (more parameters in Table 3). The pH of these solutions was adjusted to 3.0 using H₂SO₄ from Merck.

2.2. Electrolytic Systems

The electrochemical degradation tests were conducted in a 4 L electrochemical reactor using a flow rate (Q) of 7 L min⁻¹ (Reynolds number, Re = 8317). The detailed characteristics of this cell are provided in Table 4. The complete electrochemical flow cell setup is shown in Figure 1. All components, including the electric circuit, valves, and connectors, were constructed from PVC (0.5-inch diameter). A peristaltic pump (Shurflo, model 2088-592-054) with a capacity of 45 PSI and a flow rate of 12 L min⁻¹ was used for solution recirculation. Flow rates were monitored using a Kobold rotameter (model KSM 4001).

The experiments employed galvanostatic conditions with a constant current density (j) applied using a BK Precision 1627A power supply. Solutions containing 25 mg L⁻¹, 50 mg L⁻¹, and 100 mg L⁻¹ of TBT in 0.050 mM Na₂SO₄ were degraded at pH 3.0 by AO, EF, and PEF at (j) values of 7.0, 15.0, and 31.0 mA cm⁻² for 180 min. A BDD anode and a flat plate sheet of stainless steel serving a cathode were used in anodic oxidation. For EF and PEF experiments, BDD electrodes were used for both the anode and cathode. Oxygen was supplied to the reactor by an air pump (Elite 799) at a flow rate of 1 Lh⁻¹.

In EF and PEF experiments, specific conditions were implemented to optimize degradation as follows: 0.5 mM, 0.7 mM, and 0.9 mM of Fe²⁺ catalysts were added to facilitate the Fenton reaction, and the solution pH was adjusted to 3.0 using 1 M of H₂SO₄. For PEF only, the solution was additionally exposed to UV radiation (λ = 250–400 nm) emitted by a Xenon Lamp (5 Watts).

2.3. Analytical Procedures

The degradation of TBT was monitored using an Agilent 1260 Infinity HPLC system (Santa Clara, CA, USA) set to a wavelength of 223 nm. An analytical column, Agilent Eclipse C18 PAH (250 mm × 4.6 mm, 3 μm particle size), was employed for separation. The mobile phase consisted of a binary mixture of H₂O and CH₃CN in a volume ratio of 55:45. The flow rate was set to 1 mL min⁻¹. The separation was achieved under isocratic elution conditions. The resulting chromatograms displayed a peak for TBT eluting at a retention time of 8.94 min. The degradation rate was calculated using Equation (7) [18].

(7)$\% Degradation={\displaystyle \frac{\left(TB{T}_0\right)-\left(TB{T}_t\right)}{\left(TB{T}_0\right)}}\times 100$

where TBT₀ and TBT_t are the herbicide concentrations at an initial time and time t.

HPLC equipped with a Hypersil ODS column (125 mm length, 5 μm internal diameter) was used to detect aromatic by-products formed during degradation. The separation was achieved using an isocratic mobile phase composed of a 2:98 (V/V) mixture of methanol and phosphate-buffered solution (pH 6.9). The flow rate of the mobile phase was maintained at a constant 1.0 mM min⁻¹. Detection of the separated by-products was performed at a wavelength of 223 mm. Cyanuric acid was eluted first with a retention time of 1.77 min, followed by desthiomethyl Terbutryn at 3.64 min and 2-hydroxy Terbutryn at 4.46 min.

Ion exclusion high-performance liquid chromatography (HPLC) was employed to identify the carboxylic acids resulting from the degradation of Terbutryn. An Agilent 1260 Infinity System equipped with a Bio-Rad Aminex HPX 87H column (300 mm × 7.8 mm) and a diode array detector set at λ = 210 nm was used for the analysis. Aliquots of 20 μL were injected under isocratic conditions using a mobile phase of 5 mM H₂SO₄ at a flow rate of 0.8 mL min⁻¹.

The concentration of H₂O₂ during the trials was determined by UV-Vis titration with titanium (IV) oxysulfate [Ti(SO₄)₂] at λ = 407 nm, following the DIN 38409 H15 method. The current efficiency (CE) of the H₂O₂ generation can be described by Equation (8) [19,20].

(8)$\%C{E}_{H_2{O}_2}={\displaystyle \frac{2F{ C}_{H_2{O}_2}V}{I t}}\times 100$

where 2 is the number of electrons required for O₂ reduction to H₂O₂ production. F is the Faraday constant (96,487 C mol⁻¹), ${ C}_{H_2{O}_2}$ is the H₂O₂ concentration (mol L⁻¹), V is the volume of the solution (L), I is the current intensity (A), and t is the reaction time (s). Before H₂O₂ concentration determination, the BDD cathode was electrochemically cleaned by applying a current density of 25 mA cm⁻² for 10 minutes in a 0.1 M HClO₄ solution [21].

For evaluating the efficiency of the electrochemical process, Equation (9) serves as a tool, and it provides a quantitative measure of the process’s utilization of the electrical charge, a key parameter for assessing energy consumption (E_C) [22].

(9)$E_C\left({\displaystyle \frac{kWh}{m^3}}\right)={\displaystyle \frac{E_{cell} \ast I \ast t}{V_s}}$

where E_cell is the average potential difference in the cell (V), I is the average applied current (A), t is the electrolysis time (h), and Vs is the solution volume (m³).

2.4. Experimental Design

To statistically analyze the experimental results, a 3³ factorial design was implemented to maximize the removal of the chemical organic demand (COD) and degradation rate constant (DRC). This design evaluated three independent factors, which were the applied current density, Fe²⁺ catalyst, and TBT initial concentration, each at three different levels (Table 5). The accuracy and quality of the statistical analysis were ensured by evaluating the relationship between R² and adjusted R² coefficients.

A three-level Box–Behnken design comprised 27 experimental runs with duplicate trials and five central points for enhanced accuracy. The impact of these parameters on the degradation rate constant (DRC) and COD reduction percentage (% COD) was statistically analyzed using analysis of variance (ANOVA) with a 95% confidence level, Pareto charts, and response surface modeling (RSM).

RSM was used to investigate potential interactions and individual effects of the three factors (Fe²⁺ concentration, current density, and TBT concentration) on response variables, the DRC (Equation (7)) and COD (Equation (10)) [23]. This approach allowed us to identify optimal operating conditions for efficient PEF treatment. Further details on RSM methodology can be found in Villaseñor-Basulto et al., 2022 [24].

(10)$\%COD={\displaystyle \frac{{COD}_0-{COD}_t}{{COD}_0}} \ast 100$

RSM utilized a second-order polynomial model to analyze the relationship between the experimental factors and response variables (Equation (11)) [24].

(11)$Y_i={\beta }_0+{\sum }_{i=1}^n{\beta }_i{x}_i+{\sum }_{i=1}^n{\beta }_{ii}{x}_i^2+{\sum }_{j=i+1}^n{\beta }_{ij}{x}_i{x}_j$

where Yi is the response variable, xi and xj are independent variables, β0 is the regression coefficient for the intercept term, βi is the regression coefficient for the linear terms of each variable, βii is the regression coefficient for the squared terms of each variable, βij are regression coefficients for the interaction terms between variables (xi*xj), and εi is the error term.

3. Results and Discussion

3.1. Anodic Oxidation Process

Terbutryn degradation was initially evaluated using anodic oxidation in a pre-pilot flow reactor (see Figure 1). Experiments were conducted with initial TBT concentrations ranging from 25 to 100 mg L⁻¹ over 180 min. As shown in Figure 2a, the degradation rate increased with increasing current density, with the highest degradation (68%) achieved at 31.0 mA cm⁻² (▲). This trend is further supported by the pseudo-first-order kinetic analysis in Figure 2b, where the highest rate constant (k_deg) (see Table 6) was observed at the highest current density. The enhanced degradation of higher current densities can be attributed to the increased generation of •OH, a highly reactive oxidizing species. The excellent correlation between the experimental data and the pseudo-first-order kinetic model confirms the applicability of this approach to describe the degradation process, which is consistent with previous findings [25,26,27]. The decrease in TBT concentration for these experiments was analyzed using Equation (12) [28].

Ln (C₀/C_t) = k_deg t(12)

where C₀ is the initial concentration, C_t is the concentration at time t, and k_deg is the apparent rate constant.

To further understand the impact of initial TBT concentration on degradation efficiency, experiments were conducted at an initial concentration of 100 mg L⁻¹ (Figure 3a). While the overall degradation rates were lower than the 25 mg L⁻¹ (Figure 2a) experiments, a similar trend was observed, with higher current densities leading to increased TBT removal. At a current density of 31 mA cm⁻² (▲), a maximum degradation of 58% was achieved. The corresponding pseudo-first-order rate constants (k_deg), summarized in Table 6, further confirm the positive correlation between current density and degradation efficiency.

Figure 2 and Figure 3 demonstrate the relatively slow degradation of the substrate and the positive impact of increasing current density on its efficiency. This approach allowed the system’s capacity and limitations to be probed under various conditions. As j increases from 7.0 to 31.0 mA cm⁻², the degradation rate accelerates, likely due to the enhanced reaction of TBT with the higher concentration of •OH generated on the anode, as described in Equation (13) [29]. A complete degradation of TBT was not achieved under any of the tested conditions. These findings suggest that the available •OH may not be sufficient to completely oxidize all TBT molecules, indicating potential limitations in the AO process for complete TBT removal. These results corroborate the findings of other studies investigating advanced oxidation processes [30].

BDD + H₂O → BDD (•OH) + H⁺ + e⁻(13)

3.2. Electrochemical H₂O₂ Generation

Hydrogen peroxide (H₂O₂) plays a critical role as a precursor for •OH generation, essential for degrading the herbicide. Therefore, understanding its initial and ongoing production is vital for ensuring sufficient •OH generation to effectively degrade the contaminant. Figure 4a depicts the electrogeneration of H₂O₂ at pH 3.0 under various current densities (7.0 (■), 15.0 (•), and 31.0 mA cm⁻² (▲)) and a constant flow rate (7 L min⁻¹). The maximum accumulated H₂O₂ concentration of 1.767 mM was achieved at a current density of 31.0 mA cm⁻², highlighting the direct correlation between the applied current density and H₂O₂ production. This finding aligns with previous studies, such as those by Cruz-Gonzalez et al. (1.94 mM) [31], García et al. (1.79 mM) [32], and Isarain-Chavez (1.88 mM) [33], which reported comparable H₂O₂ production under similar conditions. As observed, the concentration of H₂O₂ progressively increased due to oxygen reduction (Equation (4)) until reaching a steady state after 180 min. The observed decrease in the H₂O₂ accumulation rate can be attributed to the following two competing parasitic reactions: (i) H₂O₂ degradation at the anode (Equation (14)) and (ii) its reduction to water at the cathode (Equation (15)) [34,35].

(14)$BDD+H_2{O}_2\to BDD\left(H{O}_2\right)+H^{+}+e^{-}$

(15)$H_2{O}_2+2H^{+}+2e^{-}\to 2H_{2}O$

Figure 4b illustrates the current efficiency (CE) for H₂O₂ electro-generation. The current values obtained were 37.62% at j = 7.0 mA cm⁻² (■), 32.61% for j = 15.0 mA cm⁻² (•), and 24.94% for 31.0 mA cm⁻² (▲). The decrease in CE at higher current densities suggests an increase in competing reduction reactions at the BDD cathode, such as a four-electron reduction of O₂ (Equation (16)) and a direct proton reduction to H₂ (Equation (17)) [36].

(16)$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-}$

(17)$2H^{+}+2e^{-}\to H_2$

3.3. Electro-Fenton Process

Given the incomplete degradation (less than 100%) achieved by the AO of TBT at high concentrations, alternative electrochemical oxidation processes, EF and PEF, were investigated for their potential to degrade TBT at high contaminant levels efficiently.

Experiments were conducted in drinking water using solutions containing 25, 50, and 100 mg L⁻¹ of TBT in 0.05 M Na₂SO₄ at pH 3.0, adding 0.5, 0.7, and 0.9 mM Fe²⁺. Figure 5a depicts the normalized degradation of 100 mg L⁻¹ TBT at different current densities in the presence of 0.5 mM Fe²⁺. As observed, the degradation rate increased with increasing current density. The highest current density (j = 31.0 mA cm⁻²) (▲) achieved a significantly faster degradation rate (95.8%) compared to lower current densities. This can be attributed to the enhanced generation of hydroxyl radicals, according to Equation (2) [37]. The observed exponential decrease in TBT concentration follows pseudo-first-order kinetics, as shown in Figure 5b.

The effectiveness of EF was compared to AO for TBT degradation. Using both methods, solutions containing 100 mg L⁻¹ TBT were treated at j = 7.0, 15.0, and 31.0 mA cm⁻² (Figure 6). At the highest current density (▲), AO achieved only 58% degradation. In contrast, EF under the same conditions (►) achieved 95.8% degradation, with k_deg increasing from 0.0046 to 0.0179 min⁻¹ (see Figure 6b). This trend is consistent across all studied current densities. These results support the generation of a greater quantity of •OH during the EF process. In EF, •OH is produced through two pathways (Equations (2) and (13)); consequently, a greater quantity of •OH can lead to a higher degree of TBT degradation. However, complete TBT degradation (100%) was achieved only at the lowest initial concentration of 25 mg L⁻¹ (see Table 6). At higher concentrations (50 and 100 mg L⁻¹), degradation was incomplete due to limitations in hydroxyl radical production and competing parasitic reactions, for example, the oxidation of •OH to O₂, the dimerization of O₂ to H₂O₂, and the transformation of H₂O₂ into HO₂•, as illustrated by Equations (18)–(20) [35,37,38].

(18)$2BDD(•OH)\to 2BDD+O_{2\left(g\right)}+2H^{+}+{2e}^{-}$

(19)$2BDD(•OH)\to 2BDD+H_2{O}_2$

(20)$BDD(•OH)+H_2{O}_2\to BDD\left(H{O}_2\bullet \right)+H_{2}O$

3.4. Effect of Fe²⁺ Concentration on EF Process

The concentration of Fe²⁺ plays a critical role in the EF process. A small amount of Fe²⁺ ions are strategically added to the solution and these Fe²⁺ ions react with H₂O_2, generating •OH and Fe³⁺ (Equation (2)) [32]. This reaction is catalytic due to the regeneration of Fe²⁺. However, the concentration of Fe²⁺ significantly impacts the process efficiency. An inadequate amount of Fe²⁺ limits the initial generation of hydroxyl radicals. Conversely, excessive Fe²⁺ can lead to undesirable side reactions that consume •OH and reduce effectiveness [39]. To explore the effect of Fe²⁺ concentration on the degradation process, experiments were conducted using solutions containing varying TBT (25, 50, 100 mg L⁻¹). The influence of Fe²⁺ was assessed by employing three different Fe²⁺ concentrations (0.5 mM, 0.7 mM, and 0.9 mM) in each solution.

Figure 7a illustrates the influence of Fe²⁺ concentration on 100 mg L⁻¹ TBT degradation. All experiments were conducted at a constant current density of 31 mA cm⁻². With an initial Fe²⁺ concentration of 0.5 mM (■), 95.84% degradation was achieved. Increasing the Fe²⁺ concentration to 0.7 mM (▲) and 0.9 mM (•) resulted in a slight improvement in abatement, reaching 98.52% and 99.05%, respectively. Figure 7b presents the corresponding degradation rate constants (k_deg) (see Table 6). The most outstanding k_deg value was 0.026 min⁻¹ for 0.9 mM Fe²⁺. These values indicate a positive correlation between Fe²⁺ concentration and the degradation rate.

The EF process exhibited similar degradation patterns of TBT with 0.5 mM, 0.7 mM, and 0.9 mM concentrations because the progressive addition of Fe²⁺ accelerated the Fenton reaction (Equation (2)). This boost is due to the production of more significant amounts of •OH, which enhance the degradation rate. However, further addiction of Fe²⁺ from 0.7 mM to 0.9 mM had the opposite effect, gradually inhibiting the EF process. This inhibition could be attributed to the excess Fe²⁺ consuming •OH (Equation (21)).

Fe²⁺ + •OH → Fe³⁺ + OH⁻(21)

3.5. Photoelectro-Fenton Process

Photoelectron-Fenton (PEF) has demonstrated the potential to outperform EF due to its ability to regenerate Fe²⁺ [40,41], a critical factor in the Fenton reaction [42]. To analyze this advantage, experiments were conducted using PEF to investigate the influence of various parameters on the degradation of TBT. The parameters included current density (7.0, 15.0, and 31.0 mA cm⁻²), initial TBT concentration (25, 50, and 100 mg L⁻¹), and Fe²⁺ catalyst concentration (0.5, 0.7, and 0.9 mM). The results of these experiments were compared to those obtained in AO and EF to evaluate the degradation efficiency of high TBT concentrations in the PEF process.

Figure 8a compares the degradation efficiency of 100 mg L⁻¹ TBT using AO, EF, and PEF. All experiments were conducted at a constant current density of 31 mA cm⁻². AO (■) achieved a moderate degradation of 56.31%. EF (▲) demonstrated a significantly higher degradation rate of 95.85% with 0.5 mM of catalyst. On the other hand, PEF (•) presents a slight improvement over EF, reaching 96.30% degradation. Figure 8b and Table 6 show the corresponding degradation rate constants. The significantly higher k_deg values of EF and PEF compared to AO highlight the potential of these electrochemical processes for efficiently removing high TBT concentrations. This finding aligns with previous studies [43,44,45]. While anodic oxidation relies solely on hydroxyl radicals (•OH) generated directly at the anode, EF utilizes Fenton’s reaction, where iron (Fe²⁺) reacts with H₂O₂ to produce more hydroxyl radicals; this significantly increases the concentration of these oxidants in the solution. PEF combines the Fenton reaction with the benefits of UV radiation. UV radiation activates the Fe²⁺ catalyst (Equations (5) and (6)), potentially leading to an even faster generation of •OH than EF alone [45,46].

3.6. Effect of Fe²⁺ Concentration on PEF Process

Figure 9a illustrates the influence of Fe²⁺ concentration on the degradation of Terbutryn. The experiments were conducted with a constant TBT concentration of 100 mg L⁻¹ and varied the Fe²⁺ concentrations as follows: 0.5 mM (■), 0.7 mM (•), and 0.9 mM (▲). As the Fe²⁺ concentration increased, the abatement rate of TBT also increased. At the lowest concentration (0.5 mM), 96.28% of TBT was degraded. Increasing the Fe²⁺ to 0.7 mM and 0.9 mM resulted in a slight improvement in degradation, achieving 98.43% and 99.39%, respectively. Figure 9a reveals a clear trend. As the Fe²⁺ concentration increases, the degradation efficiency rises. Figure 9b complements this finding by presenting the corresponding degradation rate constants (k_deg). EF and PEF exhibit a positive correlation between Fe²⁺ concentration and k_deg. The values for EF range from 0.0179 min⁻¹ (0.5 mM Fe²⁺) to 0.0260 min⁻¹ (0.9 mM Fe²⁺), while PEF shows slightly higher values, ranging from 0.0183 min⁻¹ (0.5 mM Fe²⁺) to 0.0288 min⁻¹ (0.9 mM Fe²⁺). The consistently high R² values (0.999) across all data points further solidify the correlation between Fe²⁺ and the degradation rate.

While Fe²⁺ provides more catalysts for the Fenton reaction (Equation (2)), it can also become a competitor for •OH. Fe²⁺ reacts with •OH and produces (Fe(OH)²⁺) (Equation (5)), reducing the number of radicals available for TBT degradation. This competition becomes more significant at higher Fe²⁺ concentrations, limiting the proportional increase in the degradation rate. In PEF, UV light accelerates the regeneration of Fe²⁺ from Fe(OH)²⁺ into an active form through photoreduction (Equation (5)), allowing for more •OH production. Additionally, UV light directly breaks down the Fe(III) complex formed with the generated carboxylic acids (Equation (6)). Nevertheless, high TBT concentrations create a denser solution, potentially reducing light penetration. If light cannot react with Fe(OH)²⁺ and Fe(OOCR)²⁺ (Equations (5) and (6)) within the solution, the •OH regeneration might be hindered, and this limits the overall benefit of adding more Fe²⁺ [47,48].

3.7. COD Decay and Energetic Determination

This study evaluated the effectiveness of EF and PEF in reducing the chemical oxygen demand (COD) of a solution containing 100 mg L⁻¹ of Terbutryn. As shown in Figure 10a, PEF (▲) achieved the most significant reduction (90%) in COD within 300 min, surpassing the performance of EF (•) and AO (■). This superior performance can be attributed to the potent oxidizing capacity of hydroxyl radicals generated in the PEF process. These •OH radicals are formed through a photolytic process of UV radiation (Equation (5)).

However, there is minimal difference between the COD removal efficiencies of the EF and PEF processes. This can be attributed to the limited penetration of UV light. However, the solution, particularly at high TBT concentrations, reduces the generation of additional •OH through the photolysis of Fe (III) complexes (Equation (5)). As a result, the Fenton reaction remains the dominant mechanism for both processes, leading to similar degradation rates [38,39].

COD removal efficiency exhibited a decrease over time. This slowdown is likely due to two factors, namely (i) the diminishing amount of readily degraded organic matter and (ii) the formation of more complex by-products resistant to oxidation. The presence of these by-products explains the observed difficulty in achieving complete COD reduction in the analyzed samples. To evaluate the energy consumption of the process, the energy consumption per unit mass of COD removed (EC_COD, in kWh (g(COD)⁻¹) was calculated based on Equation (22) [49].

(22)$E{C}_{COD}(kWh(gCOD{)}^{-1})={\displaystyle \frac{2.7\times 10^{-7}{E}_{cell }I t}{V_s ∆COD}}$

where 2.7 × 10⁻⁷ is a factor of conversion of W s to kWh, E_cell is the potential difference between the anode and cathode (V), I is the applied current (A), t is the electrolysis time (s), Vs is the solution volume (L), and ∆COD is the experimental COD decay (g L⁻¹).

Figure 10b explores the energy consumption associated with the treatments. It illustrates the correlation between the energy consumption per unit mass of COD removed (EC_COD) and the electrolysis time. The EF process exhibits a higher energy consumption (0.43 kWh (g COD⁻¹)) compared to photoelectro-Fenton (0.37 kWh (g COD⁻¹)). This finding underscores the advantage of PEF, as it achieves a remarkable COD removal rate of approximately 90% with a lower energy consumption than EF, potentially optimizing the overall process for removing high TBT concentrations. These findings corroborate previous research [19,50], demonstrating that PEF effectively reduces COD while consuming less energy.

The lower energy consumption of PEF can be attributed to its ability to leverage UV radiation for the continuous regeneration of the iron catalyst (Fe²⁺). These Fe²⁺ are the driving force behind the Fenton reaction, the primary mechanism responsible for TBT degradation [51]. In contrast, EF lacks this continuous regeneration cycle, necessitating a higher initial concentration of Fe²⁺. However, excessively high Fe²⁺ concentrations can hinder the degradation process. Additionally, PEF’s dependence on UV light translates to a lower specific energy consumption per unit mass of COD removed than EF, which relies solely on electrical input. This characteristic suggests that PEF may be a more sustainable and potentially more cost-effective approach for TBT removal.

3.8. Synergistic Effect

The integration of EF with UV light (PEF) appears to induce a synergistic effect. This synergy manifests as an enhanced degradation of TBT when both processes are employed concurrently compared to their applications. To quantify the efficiency improvement gained by combining EF and UV radiation, a critical parameter termed the synergistic index (S_index) was evaluated [49].

The S_index reflects the collaborative effect between EF and UV light in PEF. Its calculation is based on the observed reduction in TBT concentration data. This process involves determining all relevant kinetic constants associated with the individual and combined reactions. These constants are incorporated into a mathematical model described by Equation (23) [52].

(23)$S_{index}={\displaystyle \frac{k_{PEF}}{k_{UVA}+k_{EF}}}$

Figure 11a depicts the degradation rates of TBT for AO (■), EF (•), and PEF (▲) processes at different current densities. A modest increase in the degradation rate is observed for all three processes as the current density increases. The degradation rate constants for EF were 0.0114, 0.0149, and 0.0179 min⁻¹, respectively. Similarly, PEF exhibits rate constants of 0.0123, 0.0160, and 0.0190 min⁻¹, respectively. These findings suggest that increased current density positively influenced the degradation rate in all three processes.

Figure 11b illustrates the relationship between current density and the S_index of PEF processes. When the initial TBT concentration is 25 mg L⁻¹, the S_index for PEF ranges from 1.10 to 1.28 as the current density increases. This observation suggests that the synergistic effect arising from the combination of EF and UV irradiation in PEF leads to a greater abundance of •OH. These highly reactive radicals play a significant role in organic matter degradation. Additionally, the UV light in PEF likely activates the •OH, further enhancing the process elimination efficiency [13,53].

3.9. Evolution of Primary Aromatic Products

Figure 12 depicts the concentration profiles of the primary aromatic products identified by HPLC analysis during the electro-Fenton treatment of 100 mg L⁻¹ TBT solution at various current densities. These products were desthiomethyl Terbutryn (DTT), 2-hydroxy Terbutryn (HTT), and cyanuric acid (CNA). Their formation is attributed to eliminating the methylthio group (-SCH₃) by •OH generated in the PEF, as reported in previous studies [54,55,56]. A significant increase in the concentration of primary products is observed between 110 and 130 min of electrolysis, regardless of the applied current density. This suggests the intensification of •OH attack on the primary products formed during this timeframe.

Desthiomethyl Terbutryn (DTT) is the most readily formed product, reaching a maximum concentration of approximately 75 μ L⁻¹ at the highest current density (31.0 mA cm⁻² ▲). Lower concentrations of 40 μL⁻¹ (•) and 20 μ L⁻¹ (■) were observed at j = 15.0 mA cm⁻² and 7.0 mA cm⁻², respectively (Figure 12a). The formation of DTT can be attributed to the selective removal of the methylthio group by •OH radicals.

2-hydroxy Terbutryn (HTT) originated from substituting the methylthio group with a hydroxy group by •OH radicals [57]. As shown in Figure 12b, HTT reached a maximum concentration of around 60 μ L⁻¹ at j= 31.0 mA cm⁻² (▲). Figure 12c demonstrates a higher cyanuric acid (CNA) concentration produced at the highest current density (31 mA cm⁻² ▲). This trend aligns with the expected increase in •OH generation at higher current densities.

Identifying desthiomethyl Terbutryn, 2-hydroxy Terbutryn, and cyanuric acid as degradation products during the electrooxidation of TBT is consistent with the findings of previous studies [54,55]. Based on these observations, a proposed degradation pathway for TBT is presented in Figure 13.

Terbutryn exhibits a sequential degradation pathway, as shown in Figure 13. The initial degradation process proceeds relatively constantly, maintaining a near-linear profile until the later stages. However, a significant slowdown and deviation from linearity are observed once approximately 90% of the initial Terbutryn has degraded.

The initial degradation step involves the selective removal of the methylthio group, leading to the formation of desthiomethyl Terbutryn. This preferential cleavage aligns with the degradation pattern observed for other s-triazine herbicides, as reported in previous studies [54,55]. Notably, desthiomethyl Terbutryn emerges as the primary degradation product of Terbutryn.

Subsequently, 2-hydroxy Terbutryn is formed by substituting the remaining methylthio group with a hydroxyl group. Although intermediates like dezisobuthyl-hydroxy Terbutryn and dezethyl-hydroxy Terbutryn were not definitively identified due to limitations in available analytic standards, the complete oxidation of the two alkyl chains ultimately leads to the formation of cyanuric acid, the final degradation product. This degradation pathway aligns with the findings of previous research on Terbutryn transformation products [56,57].

3.10. Response Surface Analysis

The application of an ANOVA to the PEF process data revealed a statistically significant model (p-value < 0.0001). This indicates that the model effectively describes the relationship between the independent variables (current density, Fe²⁺ catalyst, TBT initial concentration) and the dependent variables (COD removal and degradation constant removal) in the PEF process.

Within this model, all three independent factors significantly influenced COD removal (p-value = 0.0001), with a high coefficient of determination (R² = 99.41%), signifying a strong correlation between the model predictions and experimental observations.

Two dependent factors, the COD removal and degradation constant removal, were evaluated in the PEF process. As shown in Table 7, both factors were statistically significant (p-value = 0.0001), indicating that the model effectively captured their relationship with the chosen independent variables.

The optimal PEF operating conditions for maximizing COD removal and the degradation constant were identified as a current density of 31.0 mA cm⁻², Fe²⁺ concentration of 0.9 mM, and 25 mg L⁻¹ of TBT (Figure 14). This configuration resulted in a final energy consumption of 0.38 kWh (COD⁻¹). While an initial exploration identified 25 mg L⁻¹ as a potentially effective TBT initial concentration for the PEF process (Table 6), the statistical analysis using ANOVA revealed a more nuanced picture. The developed model demonstrated that the PEF process can achieve significant TBT degradation even at higher initial TBT concentrations (doses used in agriculture). As shown in Figure 14, the model predicts the possibility of reaching up to 90% abatement, highlighting the process’s potential for treating wastewater containing high TBT levels.

Table 8 presents the representative model equations of both dependent factors, along with comparisons between the observed and predicted values for each response variable in the PEF process (Figure 14). The high R² values (99.41% for COD removal and 98.54% for DRC removal) depicted in Figure 15 suggest a strong agreement between the model’s predictions and the experimentally obtained data. This reinforces the validity of the chosen model for describing and optimizing the PEF process for TBT degradation. These findings underscore the importance of employing a statistically robust model for optimizing treatment conditions and maximizing degradation efficiency [58].

3.11. Effect of Process Variables

PEF treatment demonstrated a statistically significant influence of current density, Fe²⁺ catalyst concentration, and initial TBT concentration on COD removal (p-value 0.0001, Table 9). This finding indicates that all three factors independently affect the process’s ability to remove COD and degrade TBT. As shown in Table 9, the interactions between these parameters also exhibited varying degrees of impact on the treatment’s effectiveness, as evidenced by the different p-values associated with each interaction term.

These results suggest the interaction between the Fe²⁺ catalyst and TBT initial concentrations had a more significant effect on COD removal (p-value 0.0306). This result is rationalized by Equation (2), in which Fe²⁺ is related to hydroxyl radical production by the Fenton reaction. This statistical result indicates that more Fe²⁺ is needed for COD removal in agreement with the kinetic results. The interplay between Fe²⁺ concentration and current density likely plays a crucial role in influencing COD removal during the PEF process. Higher current densities can generate a greater abundance of hydroxyl radicals (•OH) through the electro-Fenton reaction [37]. These highly reactive radicals are responsible for degrading organic pollutants like COD. However, Fe²⁺ acts as a catalyst in this reaction cycle, facilitating the regeneration of •OH radicals. Therefore, an optimal balance between current density and Fe²⁺ concentration is essential. Insufficient Fe²⁺ might limit the regeneration of •OH radicals, hindering COD removal.

Conversely, excessively high Fe²⁺ concentrations could lead to the scavenging of •OH radicals by parasitic reactions (Equations (18)–(20)), thereby reducing their availability for COD degradation [51]. The interaction between initial TBT concentration and current density in the PEF process significantly impacts COD removal. Higher current densities can generate more hydroxyl radicals (•OH), the primary oxidants responsible for organic pollutant degradation. However, as the initial TBT concentration increases, a higher concentration of •OH radicals may be necessary to degrade the TBT molecules and achieve sufficient COD reduction effectively. Conversely, a lower generation of •OH radicals from a lower current density might be sufficient for efficient COD removal at lower initial TBT concentrations. This interplay highlights the importance of optimizing current density and initial TBT concentration to produce enough •OH radicals for effective COD reduction while avoiding unnecessary energy consumption associated with excessively high current densities [59].

4. Conclusions

This study demonstrated the effectiveness of electrochemical advanced oxidation processes for the degradation of Terbutryn, a persistent herbicide. While anodic oxidation showed limited degradation capabilities, electro-Fenton and photoelectro-Fenton proved significantly more efficient. The statistical analysis revealed the complex interplay between current density, Fe²⁺ concentration, and initial TBT concentration in optimizing the PEF process. By carefully balancing these factors, Terbutryn degradation and COD removal levels can be achieved.

These findings have important implications for treating wastewater contaminated with Terbutryn and other persistent organic pollutants. The PEF process offers a promising solution for the remediation of contaminated water bodies, especially considering the limited research on Terbutryn degradation using advanced oxidation processes. Further research is needed to scale up this technology and to investigate its applicability to real wastewater treatment scenarios.

Furthermore, the research can contribute to developing sustainable and environmentally friendly water treatment technologies, helping protect water resources and human health, which is aligned with the Sustainable Development Goals of Clean Water and Sanitation and Responsible Consumption and Production.

Author Contributions

Conceptualization, N.B.-Y., M.O.A.P.-A. and J.M.P.-H.; formal analysis, N.B.-Y. and M.O.A.P.-A.; investigation, N.B.-Y. and M.O.A.P.-A.; writing—original draft preparation, N.B.-Y., M.O.A.P.-A., J.M.P.-H. and C.E.B.-D.; writing—review and editing, C.E.B.-D. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors appreciate the financial support for this research through the CIIC-UG grant (026/2024), Laboratorio Nacional UG-UAA-CONHACyT, LABORATORIO NACIONAL CONAHCYT DE CIENCIA Y TECNOLOGÍA DEL AGUA (LNCyTA), number 101, as Laboratorio Nacional Conahcyt, and the specific agreement for interinstitutional cooperation between the National University of Chimborazo-Ecuador and the University of Guanajuato-Mexico.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. (a) Experimental setup of pre-pilot flow plant to Terbutryn degradation by AO, EF, and PEF. (A) Flow cell FM01-LC, (B) reservoir, (C) peristaltic pump, and (D) power supply. (b) Scheme of flow cell FM01-LC: (A) support plate, (B) gasket, (C) anode, (D) channel distributor, (E) turbulence promotor, and (F) cathode.

Figure 2. (a) Normalized degradation of 25 mg L−1 Terbutryn degradation by AO after 180 min electrolysis time at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 3. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by AO after 180 min electrolysis time at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 4. (a) H2O2 concentration after 300 min of electrolysis at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲) and a liquid flow rate of 7 L min−1; (b) current efficiency over time.

Figure 5. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by EF after 180 min electrolysis time at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 6. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by AO at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲); and EF at j = 7.0 mA cm−2 (▼), 15.0 mA cm−2 (■), and 31.0 mA cm−2 (►) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 7. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by EF at j = 31.0 mA cm−2 with Fe2+ catalyst concentrations of 0.5 mM (■), 0.7 mM (•), and 0.9 mM (▲) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 8. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by AO (■), EF (•), and PEF (▲) at j = 31.0 mA cm−2 and 0.5 mM Fe2+ for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 9. (a) Normalized degradation of 100 mgL−1 Terbutryn degradation by AO at j = 31.0 mA cm−2 with Fe2+ catalyst concentrations of 0.5 mM (■), 0.7 mM (•), and 0.9 mM (▲) for solution volume of 4 L in 0.05 M Na2SO4 at pH 3 treated in flow cell with liquid flow rate of 7 L min−1; (b) corresponds to kinetics behaviors.

Figure 10. Time course of (a) normalized COD and (b) energy consumption per unit of COD mass calculated from Equation (22) for AO (■), EF (•), and PEF (▲).

Figure 11. (a) An apparent kinetic constant (kdeg) in the treatment of Terbutryn by AO (■), EF (•), and PEF (▲) processes. (b) A synergistic effect was estimated at different current densities during PEF in 25 mg L−1 (■), 50 mg L−1 (•), and 100 mg L−1 (▲).

Figure 12. Concentration of desthiomethyl Terbutryn (a), 2-hydroxyl Terbutryn (b), and cyanuric acid (c) during EF-BDD electrolysis of 100 mg L−1 TBT at j = 7.0 mA cm−2 (■), 15.0 mA cm−2 (•), and 31.0 mA cm−2 (▲).

Figure 13. Possible degradation pathway.

Figure 14. Photoelectro-Fenton optimization for achieving desired COD removal values. (a) j (mA cm−2) vs. Fe2+ (mM), (b) TBT (mg L−1) vs. Fe2+ (mM) and (c) TBT (mg L−1) vs. j (mA cm−2).

Figure 15. Observed versus predicted correlation of COD removal.

Table 1

Physicochemical properties of Terbutryn.

Parameter	Feature	Reference
Name	2-N-tert-butyl-4-Nethyl-6-methylsulfanyl-1,3,5-triazine-2,4-diamine	[6]
Molecular formula	C10H19N5S	[6]
Chemical structure	[Image omitted. Please see PDF.]	[7]
Molecular weight (fw)	241.36 g mol−1	[9]
Melting point (mp)	104 °C	[9]
Solubility in water at 25 C (sw)	51 mg L−1	[9]
Half-life (by biodegradation)	5824 days	[9]
Henry’s law constant (KH)	1.15 × 10−8Atm m3 mol−1	[9]
Negative logarithm of dissociation constant (pKa)	4.3	[9]
Octanol/water partion coefficient logarithm (log Kow)	3.74	[9]
Animal testing, acute toxicity, rabbit dermal LD 50	>10,200 mg Kg−1	[9]
Animal testing, acute toxicity, mouse oral LD50	3884 mg Kg−1	[9]
Carcinogenicity IARC	Negative	[9]

Table 2

Electrochemical advanced oxidation process performance in Terbutryn degradation.

EAOP	Electrochemical System	Electrodes(Anode–Cathode)	%Degradation	ElectrolysisTime(min)	Current Density	C0of TBT	Reference
Anodic Oxidation	Pilot plant (30 L)	BDD–gas diffusion	84.1	75	73.6 mA cm−2	200 μg L−1	[3]
Electro-Fenton	Pilot plant (30 L)	BDD–gas diffusion	84.2	45	73.6 mA cm−2	200 μg L−1	[3]
Electrochemically Assisted Photocatalysis	Electrochemical cell (0.19 L)	Ti/TiO2–carbon felt	63	60	12.5 μA cm−2	500 μg L−1	[8]
Anodic Oxidation	Pilot plant (30 L)	BDD–carbon PTEF	70	30	74 mA cm−2	500 μg L−1	[11]
Electro-Fenton	Pilot plant (30 L)	BDD–carbon PTEF	88	30	74 mA cm−2	200 μg L−1	[11]
Solar Photoelectro-Fenton	Pilot plant (75 L)	BDD–carbon PTEF	85	180	74 mA cm−2	500 μg L−1	[11]

Table 3

Characterization of drinking water for experimental use.

Parameter	Value
Total dissolved solids	233 mg L−1
Temperature	23–26 °C
Salinity	0.042%
Specific density	1.000
pH	7.4
Oxidation–reduction potential	144 mV–152 mV
Electrical conductivity	487 μS cm−1
Total hardness	78.046 mg L−1
Harness to Mg	19.451 mg L−1
Hardness to Ca	58.046 mg L−1

Table 4

Experimental details of FM01-LC cell flow.

Electrode Length, L	16 cm
Electrode height, B	4 cm
Electrode spacing, S	0.55 cm
Anode area, BDD	64 cm2
Cathode area, BDD and stainless steel	64 cm2

Table 5

Levels of independent factors from the photoelectro-Fenton treatment.

Independent Factor	Unit	Level of the Factor
		−1	0	1
Current density	mA cm−2	7	15	31
Fe2+ catalyst	mM	0.5	0.7	0.9
TBT initial concentration	mg L−1	25	50	100

Table 6

Selected results of different EAOPs in TBT degradation.

EAOP	Experimental Conditions	K (min−1) (R2 = 0.999)	Energy Consumption	Reference
AO	E time 75 min. j = 73.6 mA cm−2. Removal = 84.1%.			[3]
EF	E time 45 min. j = 73.6 mA cm−2. [Fe2+] = 0.5 mM. Removal = 84.2%.		3.45kWh m−3	[3]
PEC	E time 60 min. j = 12.5 uA cm−2. Removal = 63%.	0.0115		[8]
AO	E time 30 min. j = 74 mA cm−2. Removal = 70%.		13.9kWh m−3	[11]
EF	E time 30 min. j = 74 mA cm−2. [Fe2+] = 0.1 mM. Removal = 88%.		15.6kWh m−3	[11]
SPEF	E time 180 min. j = 74 mA cm−2. [Fe2+] = 0.1 mM.Removal = 85%.		5.9kWh m−3	[11]
AO	j = 7 mA cm−2. C0 = 25 mg L−1. Removal = 50.89%.	0.004		This work
	j = 15 mA cm−2. C0 = 25 mg L−1. Removal = 57.86%.	0.0048
	j = 31 mA cm−2. C0 = 25 mg L−1. Removal = 67.36%.	0.0062
	j = 7 mA cm−2. C0 = 50 mg L−1. Removal = 44.90%.	0.0032
	j = 15 mA cm−2. C0 = 50 mg L−1. Removal = 56.80%.	0.0046
	j = 31 mA cm−2. C0 = 50 mg L−1. Removal = 66.11%.	0.0058
	j = 7 mA cm−2. C0 = 100 mg L−1. Removal = 31.48%.	0.0021
	j = 15 mA cm−2. C0 = 100 mg L−1. Removal = 52.20%.	0.0041
	j = 31 mA cm−2. C0 = 100 mg L−1. Removal = 56.31%.	0.0046	0.73kWh (g COD−1)
EF	j = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 99.82%.	0.035		This work
	j = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 99.85%.	0.038
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 99.98%.	0.048
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 93.28%.	0.015
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 95.31%.	0.017
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 97.71%.	0.021
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 87.35%.	0.0114
	j = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 93.07%.	0.0149
	J = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 98.54%.	0.0179
	j = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 99.98%.	0.049
	J = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 100%.	0.052
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 100%.	0.0548
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 96.42%.	0.0184
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 97.94%.	0.0214
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 99.02%.	0.0258
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 90.66%.	0.013
	J = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 96.06%.	0.018
	j = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 98.52%.	0.0235
	j = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.0558
	j = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.0586
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.0628
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 98.84%.	0.0249
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 99.52%.	0.0299
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 99.78%.	0.0345
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 92.62%.	0.0142
	j = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 97.06%.	0.198
	j = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 99.05%.	0.026	0.439kWh (g COD−1)
PEF	j = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 99.64%.	0.0311		This work
	j = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 98.81%.	0.0342
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.5 mM. Removal = 99.94%.	0.0371
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 93.94%.	0.0155
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 96.72%.	0.019
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.5 mM. Removal = 97.31%.	0.0197
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 90.36%.	0.013
	j = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 92.91%.	0.0147
	j = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.5 mM. Removal = 96.28%.	0.0183
	j = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 99.97%.	0.0466
	j = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 100%.	0.0532
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.7 mM. Removal = 100%.	0.0556
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 97.54%.	0.0207
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 98.81%.	0.0248
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.7 mM. Removal = 99.55%.	0.0305
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 93.37%.	0.0152
	j = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 96.21%.	0.0181
	j = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.7 mM. Removal = 98.43%.	0.0232
	J = 7 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.0526
	j = 15 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.05840
	j = 31 mA cm−2. C0 = 25 mg L−1. [Fe2+] = 0.9 mM. Removal = 100%.	0.0619
	j = 7 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 99.05%.	0.0265
	j = 15 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 95.58%.	0.0308
	j = 31 mA cm−2. C0 = 50 mg L−1. [Fe2+] = 0.9 mM. Removal = 97.80%.	0.0345
	j = 7 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 96.26%.	0.0182
	j = 15 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 98.37%.	0.023
	j = 31 mA cm−2. C0 = 100 mg L−1. [Fe2+] = 0.9 mM. Removal = 99.40%.	0.0288	0.372kWh (g COD−1)

Notes: E time: electrolysis time. Each experiment in this study was conducted for 180 min of electrolysis. C₀: initial concentration.

Table 7

Analysis of variance of photoelectro-Fenton process.

Model	Photoelectro-Fenton
	COD removal	Degradation constant removal
p-value	0.000	0.0001
Stnd. error	1.4163	0.0021
R2	99.41	98.54
Adjusted R2	99.11	97.76

Table 8

Modified equation with significant terms for dependent variables.

Dependent Variable	BDD
Degradation rate constant	DCR (min−1) = 0.0165672 + 0.114684*(Fe2+) + 0.000455456*(j) − 0.00140564*(TBT) − 0.0419444*(Fe2+)2 +0.00043378*(Fe2+)*(j) − 0.00048119*(Fe2+)*(TBT) − 0.0000108507*(j)2 − 3.2568E-7*(j)*(TBT) + 0.0000109659*(TBT)2
Chemical oxygen demand	%COD = 76.7696 + 44.2188*(Fe2+) + 0.699239*(j) − 1.84927*(TBT) − 6.94444*(Fe2+)2 + 0.0520833*(Fe2+)*(j) −0.12619*(Fe2+)*(TBT) − 0.0101273*(j2) + 0.000493197*(j)*(TBT) + 0.0121481*(TBT2)

Table 9

Analysis of variance for %COD.

Source	p-Value
A: Fe2+	0.0001
B: Current density	0.0001
C: TBT	0.0001
AB	0.7593
AC	0.0306
BC	0.5808

References

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