Electrochemical degradation of methylene blue dye using a graphite doped PbO₂ anode: Optimization of operational parameters, degradation pathway and improving the biodegradability of textile wastewater

2.1 Chemicals

Methylene blue (≥97%), lead nitrate (≥99%), nitric acid (≥90%), sodium sulfate (≥99%), sodium hydroxide pellets (≥98%) and sulfuric acid (≥98%) were purchased from Merck company. The graphite substrates were purchased from Atlas Equipment Company (Iran). All chemical reagents used in this study were of analytical grade and used without further purification.

2.2 Preparation and characterization of G/β-PbO₂ anode

The electrochemical precipitation method was used to prepare G/β-PbO₂ anode. Firstly, the graphite plates (dimensions of 10 cm × 3 cm × 0.2 cm) were polished by fine grade sandpaper (400 grid) and then rinsed with deionized water. In the following, the polished graphite plates were sonicated in the 40% NaOH solution and 1:1 (V/V) HNO₃/H₂SO₄ for 60 min. This can be very useful for the stability of the total β-PbO₂ electrodeposited. Finally, the prepared graphite plates were washed several times with deionized water and then transferred to an electrodeposition constant-current cell. The electro-deposition and constant-current electrolysis were powered by a DC power source (MEGATEK, MP-3005, China). Electrolyte solution containing 500 mL of 0.5 M Pb(NO₃)₂ and 0.1 M HNO₃ (Dargahi et al., 2018). The pre-treated graphite plate was used as the anode and the stainless-steel plate of the same area was used as the cathode. The electrochemical deposition of β-PbO₂ on graphite was carried out under an applied current of 7.5 mA/cm² for 180 min (Ansari and Nematollahi, 2018). After the electrodeposition process, the prepared G/β-PbO₂ was rinsed several times with deionized water (Chen et al., 2016). PbO₂ electrodeposition reactions are as follows (Li et al., 2011):

The average amount of electrodeposited β-PbO₂ on the electrodes surface calculated by Faraday law (Eq. (8)) (Ansari and Nematollahi, 2018; Nasirpouri, 2017).

Scanning electron microscopy (SEM) analysis along with energy-dispersive X-ray spectroscopy (EDX) (model HITACHI S-4160, Japan) was used to investigate the surface morphology and chemical composition of the prepared G/β-PbO₂ anode. Phase type and crystallite structure of lead dioxide film precipitated on the graphite substrate surface were investigated by X-ray diffraction analysis (Rigaku RINT2200, Japan).

2.3 Electrochemical degradation experiments and analytical methods

All electrochemical degradation experiments were performed in a 250 mL-batch reactor. A stainless steel 316 (SS316) plate with dimensions equal to the G/β-PbO₂ anode was used as the cathode. Two G/β-PbO₂ anodes and a SS316 cathode were placed parallel to the sample solution at a distance of 2 cm from each other. The reactor contents were mixed by a magnetic stirrer at a constant speed of 250 rpm. The schematic of the electrochemical degradation setup is illustrated in Fig. 1. H₂SO₄ and NaOH were used to adjust the pH of the sample solution (pH meter; SensION, HACH). Na₂SO₄ was used as the supporting electrolyte at different concentrations. After adjusting the pH of the solutions containing MB and Na₂SO₄ electrolyte, experiments were conducted galvanostatically with a DC power supply (DAZHENG PS-305D, China). Finally, after the end of the electrochemical degradation reaction, an appropriate aliquot of the reaction solution was taken for analysis. The real wastewater samples were obtained from a local textile industry (Nakh Rang company). Samples were collected before entering the effluent to the wastewater treatment plant. The provided samples were kept in a dark container at 4 °C for a week. The characteristics of raw and treated wastewater are presented in Table 5.

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Fig. 1

Schematic of the experimental setup (1. SS316 cathode, 2. G/β-PbO₂ anodes, 3. Magnetic stirrer, 4. Magnet, 5. DC power supply).

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Dye removal was measured by a UV–vis spectrophotometer (DR6000, HACH) at a maximum wavelength of 660 nm (Wu et al., 2017). Chemical oxygen demand (COD) was measured using digestion vials (Ultra-Range COD reagent, Range: 20–1500 mg/L COD, Hack, CO, USA). In this method, 2 mL of the reaction sample was added to the vial and thoroughly mixed with COD reagent. It was then heated in the digestion reactor at 150 °C for 2 h. Finally, the COD of the sample was measured by a spectrophotometer (DR6000, HACH). The biological oxygen demand (BOD₅) of the samples was also measured manometrically with a respirometer (BSB-Controller model 620 T, WTW). An accurate index for defining the biodegradability of effluent is the ${\mathrm{B}\mathrm{O}\mathrm{D}}_5/\mathrm{C}\mathrm{O}\mathrm{D}$ ratio. Accordingly, wastewaters with ${\mathrm{B}\mathrm{O}\mathrm{D}}_5/\mathrm{C}\mathrm{O}\mathrm{D}\ge 0.4$ is considered to be biodegradable (Garcia-Segura et al., 2017). MB mineralization was measured by a TOC (Total Organic Carbon) analyzer (Elementar Analysensysteme GmbH, Germany). Intermediate compounds produced during electrochemical oxidation of MB dye were identified by LC-MS analysis (Shimadzu LCMS 2010 A). The cyclic voltammetry experiments were performed at room temperate in a conventional three-electrode one compartment glass cell (Autolab PGSTAT-20 instrument monitored with the Electrochemical System Software (Nova)). The working electrode used in the voltammetry experiments was a glassy carbon disc (1.8 mm² area). A platinum wire was used as the counter electrode and an Ag/AgCl (3 M) was used as the reference electrode. All of the electrodes were manufactured by AZAR Electrode Company (Urmia, Iran). The concentration of Pb²⁺ in the treated sample was measured by cyclic voltammetry and inductively coupled plasma optical emission spectrometer (ICP-OES, 730 ES, Varian Scientific Instruments). The kinetic of dye removal in the oxidation system was calculated using Eq. (9).

2.4 Experimental design and process optimization

Response Surface Methodology (RSM) is one of the most widely used methods in the design of experiments for water and wastewater treatment processes (Rahmani et al., 2020). CCD is one of the most widely used RSM technique for modeling the effect of independent variables on system response (Shokoohi et al., 2017). In this study, the effects of four independent variables, i.e., initial solution pH, reaction time, current density, and Na₂SO₄ concentration on system response (MB removal) were modeled by CCD. For this purpose, the effect of four independent variables was investigated at five levels (−α, −1, 0, +1, +α). Table 1 shows the independent variables and their operating ranges. According to Eq. (10), a 5-level-4-factor full CCD contains 30 experiments, of which 16 tests are related to factorial points, 10 tests are indicative of axial points, and 4 tests indicate the repetition in central points. The full CCD scheme with the actual values of the independent variables and the predicted dye removal efficiency can be seen in Table 2. Experimental design and data analysis were performed by Design-Expert 12.0 software (Shokoohi et al., 2019).

In this equation, N is the number of experiments designed; K represents the number of independent variables, which is equal to 4 in this study, and C is the number of the experiment repetitions at the central points. 2^K and 2 K are the numbers of experiments in the axial and factorial points, respectively. The adequacy of the proposed model for predicting system response and the significance level of linear, interactive, and quadratic effects were determined by the F-value and p-value obtained by ANOVA analysis of variance. Model quality and accuracy were determined by R² and adj-R² coefficients. Interactive effects of parameters on system response were investigated by 3D and contour response surface plots.

GA and CCD techniques were used to obtain the optimal points of the independent variables and to predict the maximum removal efficiency of MB in the electrochemical degradation system with G/β-PbO₂ anode. In both methods, the basis of the optimization is the final proposed equation of the quadratic model to investigate the effect of input variables on the system response. The GA method is based on Charles Darwin’s survival of the fittest theory. The technique is started by generating a random population that has suitable solutions to solve a problem. The best solutions created by a population are used to generate a new population, with the probability that the new population is better than the previous population. To obtain optimal conditions, the above steps continue to improve the population as much as possible (Salari et al., 2019). All optimization steps were performed by the GA technique using MATLAB R2013a software. Finally, the predictions of both GA and CCD methods were compared.

3.1 Characterization of G/β-PbO₂ anode

SEM images of the graphite plate coated with PbO₂ can be observed at three different magnifications in Fig. 2(a)–(c). As can be seen, PbO₂ is coated on the graphite substrate uniformly and without cracking, mainly in the form of pyramidal clusters and highly compressed structures (Li et al., 2016). The XRD diffractogram of the G/β-PbO₂ anode has shown in Fig. 2(d). The diffraction peaks at 2θ = 25.4°, 32°, 36.2°, 40.4°, 45°, 49.2°, 52.2°, 58.9°, 62.7°, and 67° belong to tetragonal β-PbO₂ which can be indexed with standard JCPDS data card no. 89–2805 (Li et al., 2014). Also, the diffraction peaks observed at 23.6°, 28.5°, 33.1°, 50.7°, 52.2°, and 56.5° belong to the α-PbO₂ phase (JCPDS 72-2440). XRD diffractogram of the G/β-PbO₂ was also analyzed with X'Pert HighScore software. The relevant results are shown in Fig. 2(e). These results show that the Wt.% of α-PbO₂ phase (Scrutinyite) is 11.7% and the Wt.% of β-PbO₂ phase (Plattnerite) is 88.3%, so the dominant form of PbO₂ on the graphite surface is the β-PbO₂ with the tetragonal structure (Hao et al., 2014). The average crystallite size (D) of β-PbO₂ was calculated by the Debye-Scherrer formula (Eq. (11)) and X'Pert HighScore software (Mandal et al., 2020).

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Fig. 2

(a), (b) and (c) SEM images of the G/β-PbO₂ anode at three different magnifications; (d) XRD diffractogram of the G/β-PbO₂ anode; (e) The result of X'Pert HighScore software.

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Fig. 3

EDX spectra of the G/β-PbO₂ anode.

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3.2 Modeling of MB degradation and process optimization

To investigate the MB removal in the electrochemical oxidation system using G/β-PbO₂ anode, 30 experiments were presented by CCD experimental design. Table 2 shows the full CCD matrix with the experimental and predicted values of MB removal. Changes in system response to independent parameter changes were first examined by the software. Accordingly, in this study, the MB removal follows a quadratic model. The significance level (p-value) was considered ≤0.05 for the model and all related statistics. Therefore, to achieve a compact model, parameters with p-value ˃0.05 were excluded from the proposed model. Accordingly, four linear effects (A, B, C, and D), three interactive effects (AC, BC, and CD) and four quadratic effects (A², B², C², and D²) were significant in this study. The results of the data analysis (ANOVA) and Fit Statistics for the reduced quadratic model are shown in Table 3.

The F-value and p-value values of the model are 1674.82 and <0.0001, respectively, which indicates that the reduced model is significant and adequate to predict the system response (Salari et al., 2019). The Lack of Fit F-value of 0.6541 implies that the Lack of Fit is not significant compared to the pure error. There is a 75.21% chance that a Lack of Fit F-value this large could occur due to noise. Non-significant lack of fit is good. In other words, the model has well fitted with the available data (Rahmani et al., 2019). The validity of the proposed model was confirmed by high (close to 1) values of the R² and adj-R² (Ravikumar et al., 2005). In this study, the values of R² and adj-R² are 0.999 and 0.9984, respectively; on the other hand, if the values of R² and adj-R² are close together, it confirms that there is no unnecessary parameter in the proposed model. The Adeq precision parameter statistically represents the signal-to-noise ratio, and values ˃4 are favorable for the model. In this study, this parameter is 168.96. A model whose variance coefficient (C.V.%) is less than 10% is considered as a reproducible model. In this study, the value of 0.37% indicates a high reproducibility of the modified model. The final equations of the reduced model in terms of actual and coded factors can be observed in Eqs. (12) and (13).

To further evaluate the adequacy of the proposed model, diagnostic plots were used. The difference between the values obtained from the experimental and predicted values by the model is called residual. The normality of the data can be detected by the normal probability plot of the residuals. Fig. 4(a) shows that all data points are approximately on a central line, that is, the data are normally distributed in the model prediction. Based on Fig. 4(b) and (c), all residuals have randomly scattered within the standard deviation range (±3.72) and none of the points are out of range. This indicates that there is no need to repeat any of the runs. As shown in Fig. 4(d), the lowest point in the Box-Cox plot (Best lambda = 1) confirms that there is no need to transfer system response (Antonopoulou et al., 2017; Cho and Zoh, 2007).

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Fig. 4

The diagnostic plots for validation of obtained model: (a) normal plot of residuals, (b) residuals versus predicted, (c) residuals versus run order and (d) Box-Cox Plot.

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To predict the maximum removal efficiency of MB in the electrochemical oxidation system with G/β-PbO₂ anode, process optimization was performed by both CCD and GA methods. In the CCD method, four independent variables were set as input parameters to the model in “in range” mode, and MB removal efficiency was set in “maximize” mode (MB removal = 100%) (Dargahi et al., 2018; Shokoohi et al., 2019). Among the solutions presented, the solution with the highest desirability was selected as the optimal conditions of the oxidation system. The optimal conditions obtained by the CCD method are presented in Fig. 5(a). According to the prediction of the model, the maximum MB removal efficiency, in this case, is 97.68%. An experiment with three repetitions was performed under optimum conditions, in which the MB removal efficiency was 96.2 ± 0.14. In the GA method, the final equation based on the actual factors (Eq. (12)) was used as the input fitness function, and the input variables were defined in their high and low values (−α and +α) (Eqs. (14)–(17)). Population size and type were 250 and double vectors, respectively, and the number of generations was set to 250.

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Fig. 5

(a) The optimal points obtained by CCD method; (b), (c), (d) and (e) The optimal points obtained by GA method; (f) The Pareto effect of each term on MB removal.

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Fig. 5(b) shows the fitness values versus the number of generations. As can be seen, as the number of generations increases, the average fitness decreases gradually. Finally, after 80 repetitions, the optimum conditions for the removal of MB in the electrochemical oxidation system are presented in Fig. 5(c). Under these conditions, the maximum predicted and experimental removal efficiencies were 98.58 and 97.4 ± 0.17%, respectively. Fig. 5(d) and (e) display the average distance between individuals and plots of best, worst and mean scores, respectively. By comparing the predicted and experimental removal efficiency values, both optimization methods were found to be able to the prediction of the system response with high accuracy.

To determine the effect of each parameter on the system response (MB dye removal efficiency), the Pareto effect for each parameter (P_i) was calculated by Eq. (18).

In this equation, β_i is the regression coefficient of each parameter in the final model equation in terms of coded factors, which were represented in Eq. (13). As shown in Fig. 5(f), the highest effect, among the four independent variables, were current density (+59.88%), reaction time (+11.22%), Na₂SO₄ concentration (+8.28%) and solution pH (−2.30%), respectively. Among the interactive effects, the highest percentage of the effect belongs to the interaction between reaction time and current density (BC). Among the quadratic effects, the most significant was the solution pH (A²).

3.3 Effect of independent variables on MB removal efficiency

3.3.1

3.3.1 The effect of solution pH

Interactive effect between initial solution pH and reaction time has shown in Fig. 6(a) and (b). At an initial MB concentration of 60 mg/L, a current density of 10 mA/cm² and Na₂SO₄ concentration of 78.8 mg/L, the MB removal efficiency increases with decreasing pH and increasing reaction time. As shown in Fig. 6(b), the highest MB removal efficiency (97.67%) was predicted at the pH of 5.75 and the reaction time of 50 min. In contrast, the lowest removal efficiency of 79.67% was observed at the pH of 11 and the reaction time of 10 min. One of the most important parameters influencing the development of electrochemical processes is the initial solution pH; as mentioned earlier, PbO₂ anode is classified in the group of non-active anodes with high oxygen evolution potential. The main mechanism of degradation of organic pollutants in this type of the anodes is the formation of ${\mathrm{H}\mathrm{O}}^{·}$ , the ${\mathrm{H}\mathrm{O}}^{·}$ produced on the surface of PbO₂ anodes have a short life time, and then, it is not participating for oxidizing in the solution. Actually, it acts in the Nernst layer, close to the anode (reaction cage) (Abu Ghalwa et al., 2016). On the other hand, the ${\mathrm{H}\mathrm{O}}^{·}$ oxidation potential is strongly dependent on the pH of the aqueous solution, and with increasing reaction solution pH, its oxidation potential is dramatically reduced from 2.4 to 2.7 V under acidic conditions to approximately 1–1.29 V under alkaline conditions (Neta et al., 1988).

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Fig. 6

(a) 3D and (b) contour response surface plots showing the interaction of pH and reaction time (MB = 60 mg/L, Current density = 10 mA/cm², Na₂SO₄ = 78.8 mg/L); (c) 3D and (d) contour response surface plots showing the interaction of pH and current density (MB = 60 mg/L, Reaction time = 50 min, Na₂SO₄ = 78.8 mg/L).

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In the anodic oxidation system, there is always a competition for the reaction of oxygen evolution and oxidative degradation of organic matter on the anode. Increasing the pH of the solution reduces the oxygen evolution potential and, consequently, elevates the rate of oxygen evolution at the anode surface; this subdues the rate of diffusion of organic pollutants to the anode, which ultimately reduces the oxidation rate of the organic matter at the anode surface (Zhao et al., 2019). Conversely, under acidic conditions, as the oxygen evolution reaction subsided, the electrochemical conditions for the degradation of organic pollutants at the anode surface are strengthen (Neto and De Andrade, 2009). In addition, under alkaline conditions, due to the high consumption rate of supporting electrolyte, the conductivity of the solution falls due to the lack of sufficient electrolyte, and thus, the degradation efficiency of organic pollutants further declines (Dai et al., 2013). In a study performed by Fernanda et al., for degradation of dimethyl phthalate ester using fluoride-doped Ti/β-PbO₂ anode, the kinetics of the contaminant removal at pH = 3 was higher than pH values of 7 and 10 (Souza et al., 2014). In a study conducted by Samet for the electrochemical degradation of 4-chloroguaiacol with Nb/PbO₂ anodes, an increase in pH in the range of 2–6 reduced the kinetics of contaminant removal (Samet et al., 2006). In addition, the highest electrochemical degradation efficiency of Methyl Orange using Nb/PbO₂ anode was achieved at an initial pH of 6 (Yang et al., 2016).

3.3.3

3.3.3 The effect of Na₂SO₄ concentration

The use of a supporting electrolyte to improve the electrical current transfer in the solution mass is an indispensable factor in the electrochemical oxidation processes of organic compounds; in various studies, NaCl (Aquino et al., 2014) and Na₂SO₄ (Yang et al., 2016) have been employed for this purpose. In the present study, the effect of different concentrations of Na₂SO₄ electrolyte on MB removal efficiency was investigated in the range of 20–100 mg/L. As shown in Fig. 7(a) and (b), it is observed that, at a constant current density of 10 mA/cm², the dye removal efficiency was improved from 84% to about 97.7% by increasing Na₂SO₄ concentration from 20 mg/L to 78 mg/L. However, a further increase in Na₂SO₄ electrolyte concentration from 80 to 100 mg/L slightly diminished the removal efficiency of MB. Therefore, the optimum Na₂SO₄ electrolyte concentration was 78.8 mg/L in the present study. Increasing the electrolyte concentration of Na₂SO₄ improves the conductivity of the solution by facilitating electron transfer and increasing the rate of production of ${\mathrm{H}\mathrm{O}}^{·}$ at the anode surface. On the other hand, based on the reports, BDD and PbO₂ anodes are capable of producing radicals from anions in solution. Eqs. (19) and (20) show the formation of sulfate and chlorine radicals (Sirés et al., 2014):

(19)

$${\mathrm{S}\mathrm{O}}_4^{2-}\to {\mathrm{S}\mathrm{O}}_4^{·-}+{\mathrm{e}}^{-}$$

(20)

$${\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}^{·}+{\mathrm{e}}^{-}$$

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Fig. 7

(a) 3D and (b) contour response surface plot showing the interaction effect of Na₂SO₄ concentration and current density (MB = 60 mg/L, Reaction time = 50 min, pH = 5.75).

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However, at high concentrations of Na₂SO₄, excess sulfate ions ( ${\mathrm{S}\mathrm{O}}_4^{2-}$ ) migrate to the anode and accumulate at its surface. Occupying the active sites on the surface of the anode with ${\mathrm{S}\mathrm{O}}_4^{2-}$ ions results in inactivation of the electrocatalytic property of the anode, and as a result, it is led to reducing the organic pollutant removal efficiency (Dai et al., 2016). In a study conducted by Yao et al., for electrochemical removal of thiamethoxam using PbO₂-CeO₂ anode, an increase in Na₂SO₄ electrolyte concentration decreased the removal efficiency of the contaminant (Yao et al., 2018). In addition, Yang studied the removal of Methyl Orange using Nb/PbO₂ anode and determined the optimum Na₂SO₄ electrolyte concentration as 0.08 mol/L (Yang et al., 2016).

3.4 Comparison performance of G/β-PbO₂ and graphite anode and degradation kinetic

To determine the effect of β-PbO₂ electrocatalyst on MB dye removal, the performance of graphite and G/β-PbO₂ anodes were compared for electrochemical MB removal at three concentrations of 60, 120 and 180 mg/L under optimum conditions (pH = 5.75, Current density = 10 mA/cm², Na₂SO₄ = 78.8 mg/L). The results of this comparison are shown in Fig. 8. The comparison of Fig. 8(a) and (b) indicates that the MB removal efficiency by the G/β-PbO₂ anode is significantly higher than that of graphite anode. So that at MB concentration of 60 mg/L, the removal efficiencies for graphite and G/β-PbO₂ anodes at the reaction time of 10 min were 25% and 87%, respectively. At the same conditions, the removal efficiencies for graphite and G/β-PbO₂ anodes at the end of 60 min were 68.3% and 96.4%, respectively. Graphite anode is classified in the group of active anodes with low oxygen evolution potential based on nature, hence the dominant mechanism for oxidation and degradation of the contaminant by this anode is direct electron transfer from the organic contaminant to the anode surface (Abdalrhman et al., 2019). The graphs related to the kinetics of MB degradation in the electrochemical system with G/β-PbO₂ and graphite anodes are shown in Fig. 8(c) and (d), respectively. Fig. 8(e) shows the kinetic coefficients of electrochemical removal of MB dye under optimum operational conditions. As can be seen, the electrochemical removal of MB dye by graphite and G/β-PbO₂ anode well corresponds to the pseudo-first-order kinetic model (R² > 0.9). Additionally, the kinetic coefficients (k_obs) of electrochemical removal of MB dye by G/β-PbO₂ anode was decreased by about 1.42 times with increasing initial dye concentration from 60 to 180 mg/L. In addition, at a constant MB dye concentration of 60 mg/L, the kinetic coefficient of electrochemical dye removal by the G/β-PbO₂ anode is about 2.62 times higher than that of graphite anode. Samarghandi et al., have used graphite anode to remove 2,4-D herbicides; based on their reports, increasing the current density in the range of 1 to 5 mA/cm² has resulted in an increase in the oxidation system efficiency, so that 2,4-D removal efficiency at initial concentrations of 50 and 100 mg/L under the conditions including the current density of 5 mA/cm², pH of 3 and 80 min were 98% and 69%, respectively (Samarghandi et al., 2019). Also, in Fig. 8(a) and (b), it is reasonably clear that for both anodes, an increase in the initial concentration of MB dye from 60 to 180 mg/L reduces the removal efficiency. For the G/β-PbO₂ anode, at the reaction time of 60 min, MB dye removal efficiency at initial dye concentrations of 60 and 180 mg/L was 96.4 and 85%, respectively. Under the same conditions, removal efficiencies using graphite anode was 68.3% and 46%, respectively. At low concentrations of organic pollutant, the kinetics of electrochemical degradation reactions are faster than the rate of contaminant mass transfer to the anode surface; thus, the efficiency of removal of organic pollutants is increased and in other words, the performance of the electrochemical removal reaction is improved. However, under constant operational conditions, by increasing the initial MB concentration, more contaminants get closer to the anode surface, while the ${\mathrm{H}\mathrm{O}}^{·}$ is not sufficient to degrade them, and thus, removal efficiency is reduced. On the other hand, by increasing the initial concentration of the pollutant, more intermediates are produced which compete with the parent molecules of the pollutant through reaction with the ${\mathrm{H}\mathrm{O}}^{·}$ . Hence, the ratio of radical to pollutant will further decrease (Song et al., 2010). On the other hand, some intermediates produced during oxidation may be adsorbed onto the anode surface, which may lead to inactivate the electrocatalyst sites (Lin et al., 2013; Wang et al., 2010).

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Fig. 8

Effect of different concentrations of MB on removal efficiency and reaction kinetics in the electrochemical oxidation system using (a) and (c) G/β-PbO₂ anode and (b) and (d) graphite anode. (e) Kinetic coefficients of electrochemical removal of MB dye in electrochemical oxidation system using G/β-PbO₂ and graphite anodes.

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3.5 Mineralization, and degradation mechanism of MB

TOC analysis was used to confirm MB mineralization in the electrochemical oxidation system with G/β-PbO₂ anode. Fig. 9(a) shows the TOC reduction rate in optimal conditions as a function of time in the range of 0 to 60 min. According to the results, 60 min is required for the complete mineralization of MB. The cyclic voltammograms of MB during the anodic oxidation with G/β-PbO₂ anode are shown in Fig. 9(b). As can be seen, within 0 to 60 min, the oxidation and reduction peaks of MB have been steadily decreased. These results confirm the MB degradation during the process.

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Fig. 9

(a) TOC reduction rate in electrochemical oxidation system with G/β-PbO₂ anode; (b) cyclic voltammograms of MB during electrochemical oxidation process with the G/β-PbO₂ anode.

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Intermediates produced by electrochemical degradation of MB were identified by LC-MC analysis. Table 4 shows the chemical structures and names of the identified intermediate compounds. According to the results, the most abundant intermediates produced were compounds with m/z < 150, which confirms the degradation of MB (m/z = 319). Meanwhile, cyclohexane, cyclohexa-2,5-dien-1-ylium, and N-(sec-butyl) aniline was identified as the most abundant compounds. Finally, a degradation pathway was proposed for the degradation of MB in the electrochemical oxidation system using G/β-PbO₂ anode. Fig. 10 demonstrates that ROS ( ${\mathrm{H}\mathrm{O}}^{·}$ ) can cause the hydroxylation process by adding OH to the MB compound. This effect can be observed in the formation of 7-(methyl-l4-azaneyl)-5l4-phenothiazine-3,5,5 (10H)-triol (m/z = 279), 7-(methyl-l4-azaneyl)-5,10-dihydro-5l4-phenothiazine-3,5-diol (m/z = 263) and 4-((4-(methyl-l4-azaneyl)phenyl)amino) phenol (m/z = 215). The other role of active oxygen groups is to oxidize and hydrolyze the compounds in the presence of water. This effect is proved by the formation of 4-(phenyl-l4-azaneylidene) cyclohexa-2,5-dien-1-imine (m/z = 181) and cyclohexa-2,5-dien-1-ylidene(phenyl)-l4-azane (m/z = 170). The ${\mathrm{H}\mathrm{O}}^{·}$ radical is able to cleavage the rings in the structure of the intermediates through ring-opening reactions and eventually produce the pent-1-ene compound. Finally, CO₂ and H₂O can be considered as final products of MB degradation.

Table 4 Intermediate compounds identified during the MB electro degradation.

Structure and chemical name	m/z	Structure and chemical name	m/z
	319		149
Methylene Blue		N-(sec-butyl) aniline
	279		119
7-(Methyl-l4-azaneyl)-5l4-phenothiazine-3,5,5(10H)-triol		N-vinyl aniline
	263		101
7-(Methyl-l4-azaneyl)-5,10-dihydro-5 l4-phenothiazine-3,5-diol		Cyclohexanol
	215		85
4-((4-(Methyl-l4-azaneyl) phenyl) amino) phenol		Cyclohexane
	181		79
4-(Phenyl-l4-azaneylidene) cyclohexa-2,5-dien-1-imine		Cyclohexa-2,5-dien-1-ylium
	170		71
Cyclohexa-2,5-dien-1-ylidene(phenyl)-l4-azane		Pent-1-ene

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Fig. 10

Proposed pathway for degradation of MB in electrochemical oxidation process with the G/β-PbO₂ anode.

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3.6 Reusability of G/β-PbO₂ anode

High electrocatalytic activity along with high reusability are the main characteristics of an ideal anode for use in electrochemical oxidation processes (Li et al., 2017). In this study, the reusability of the prepared G/β-PbO₂ anode was investigated for fifteen consecutive electrochemical oxidation experiments under optimal conditions. As shown in Fig. 11(a), after fifteen reuses of G/β-PbO₂ anode, the removal efficiency of MB was reduced by only about 2%. These findings exhibit the high reusability of G/β-PbO₂ anode (Xu et al., 2019). The concentration of lead released from the G/β-PbO₂ anode at the end of the 15th oxidation cycle was probed by cyclic voltammetry analysis. The relevant results are presented in Fig. 11(b). If lead enters the treated sample, its peak should be seen in the cyclic voltammetry. As can be seen, no peak is observed on the voltammogram “a” which is related to the treated solution. However, after adding 0.5 mg/L of Pb (NO₃)₂ to the treated solution, the oxidation peak of lead was observed at −0.5 V vs Ag/AgCl (voltammogram “b”). By comparing the two voltammograms, it is concluded that no lead ions leak from the anode surface to the sample solution. These results confirm the stability of the prepared anode. However, ICP analysis showed that the concentration of lead leaked into the solution is 0.0015 mg/L.

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Fig. 11

(a) Reusability of G/β-PbO₂ anode for electrochemical degradation of MB; (b) Cyclic voltammetry of treated sample and treated sample containing 0.5 mg/L of Pb (NO₃)₂ at glassy carbon electrode (Scan rate = 100 mV/s, Temperature: 25 ± 1 °C).

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3.7 Textile wastewater treatment

After investigating and optimizing the influence of important operational parameters on the efficiency of the MB electrochemical oxidation process using G/β-PbO₂ anode, the performance of the optimized process was evaluated for the treatment of real wastewater of local textile industry (Nakh rang Co.). Required samples were taken from the wastewater stream at the outlet of the industrial plant and before entering the treatment plant, and the pH was immediately measured. The samples were treated by two different methods.

In method I, the pH of the samples was adjusted to the optimum value prior to the treatment process (pH = 5.7), however, in method II, the pH of the samples was the same value recorded in the treatment plant (pH = 8.2). As shown in Table 5, the final pH of the samples was reduced after treatment by both methods. In electrochemical oxidation systems, a decrease in pH during the electrolysis process has been reported. In a study, Xu evaluated the degradation of perfluorooctanoic acid by Zr-PbO₂ anode and reported that the final pH of the studied samples was decreased from about 4.8 to about 3 (Xu et al., 2017). Also, according to Wang et al., the final pH of the samples in the electrochemical degradation system of ciprofloxacin by the SnO₂-Sb/Ti anode was always associated with a decrease; in addition, the decrease in pH values was higher for the samples with higher initial pH. An appropriate indicator for determining the biodegradability of wastewater is its $\mathrm{B}\mathrm{O}\mathrm{D}/\mathrm{C}\mathrm{O}\mathrm{D}$ ratio. Accordingly, wastewater with $\mathrm{B}\mathrm{O}\mathrm{D}/\mathrm{C}\mathrm{O}\mathrm{D}\ge 0.4$ is considered to be biodegradable (Shabanloo et al., 2020). As can be seen, the biodegradability of the textile wastewater sample after treatment by two methods I and II reached about 0.48 and 0.37, respectively, which symbolizes an increase in its biodegradability. In addition, as can be seen, the removal efficiencies of BOD and COD by treatment method I were about 33.4% and 62.3% and were about 27% and 46% by treatment method II, respectively. The higher COD removal efficiency, compared to BOD removal efficiency, specifies the presence of non-biodegradable organic compounds in the samples studied.