The treatment of textile wastewater has attracted much attention due to its toxicity and threat to the environment and human life [1]. Electrochemical oxidation has been regarded as an attractive approach because of its versatility, environmental compatibility, safety, automation, cost effectiveness, and energy efficiency [2–6]. Therefore, it has been widely used in the degradation of organic pollutants [7–9]. It is well known that the electrode materials play an important part in electrochemical oxidation processes [10]. Numerous electrode materials including PbO₂ [11–13], SnO₂ [14, 15], RuO₂ [16, 17], IrO₂ [18, 19], and boron-doped diamond (BDD) [20, 21] have been extensively studied. Among these anode materials, PbO₂ has been regarded as a promising metal oxide electrode material due to its low cost, excellent conductivity, chemical stability, and high over-potential for the oxygen evolution reaction [22, 23]. Many efforts have been devoted to improving the electrocatalytic activity and stability of PbO₂. Doping with metal or non-metal elements (Cu [24], Bi [25], Ce [26], Fe [27], F [28], etc.) or other compounds (TiO₂ [29], SnO₂ [30], ZrO₂ [31], Co₃O₄ [32], TiN [33], etc.) has been widely discussed in the last several years.

However, the application of PbO₂ electrodes faces the problem of electrochemical corrosion. Previous research showed that the introduction of an Sb-SnO₂ interlayer could increase the chemical stability and electrocatalytic activity of the anode [34]. Moreover, electrode substrate microstructure modification has also been demonstrated to be an efficient approach to enhance the stability and electrocatalytic properties [35]. Vertically oriented TiO₂ nanotube arrays (TiO₂ NTAs) are regarded as an effective microstructure modification due to their big specific surface area, high uniformity, and good hydrophilicity. It was found that the special TiO₂ NTA architecture could improve the loading of Sb-SnO₂ and thus enhance the electrocatalytic activity of a PbO₂ electrode [36]. Fabrication of an Sb-SnO₂ coating by electrodeposition can produce a more uniform and dense surface. However, the surface tension on TiO₂ NTAs should be suppressed during the electrodeposition process. In this study, an Sb-SnO₂ coating was prepared by a microwave method to decrease the surface tension.

Rhodamine B (RhB) has been widely used in many biotechnology applications and industrial processes, such as dye laser production, paper dyeing, flow cytometry, ELISA, and fluorescence microscopy. The treatment of RhB is difficult due to its high discharge quantity and because it is hard to biodegrade. To address the environmental influence of this pollutant, an electrochemical oxidation technique has been developed. However, the research towards the specific degradation process of RhB during electrocatalytic oxidation is still imperfect. Therefore, it is necessary to study the influencing factors and intermediate products of RhB in the electrochemical process.

Figure 1 shows the structure of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode. It was found that the TiO₂ NTAs served as a tubal template; the Sb-SnO₂ coating embedded in the TiO₂ NTAs acted as the interlayer and PbO₂ containing an α-PbO₂ inner layer and a β-PbO₂ outer layer was employed as the catalytic surface layer. It was expected that this multilayer microstructure would significantly enhance the electrocatalytic activity. This paper is focused on the electrocatalytic properties and stability of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode for the treatment of textile wastewater. RhB was chosen as the model organic pollutant to investigate the electrocatalytic performance of the electrode. The removal of chemical oxygen demand (COD) and color was analyzed to study the decomposition of RhB. The influences of key operating variables such as initial concentration, current density, initial pH, temperature, and chloride ion concentration on the degradation efficiency were investigated in detail. The degradation kinetics and the energy efficiency of the electrode were evaluated during the electrocatalytic oxidation process. Moreover, the safety and stability of the electrode are also discussed. Finally, the degradation mechanism of RhB during electrochemical oxidation was proposed.

The PbO₂/Sb-SnO₂/TiO₂ NTAs electrode was fabricated on the basis of our previous research [37]. Anodization was performed at 50 V for 1 h in the electrolyte (0.3 wt% NH₄F + 3 vol% H₂O + ethylene glycol). The as-prepared TiO₂ NTAs electrode was annealed in atmosphere at 500 ℃ for 2 h. Then, Sb and Sn were deposited onto the TiO₂ NTAs by a microwave method, and the Sb-SnO₂ layer was formed by annealing at 500 ℃ for 1 h. Finally, the PbO₂ coating was fabricated by electrodeposition with an inner layer (α-PbO₂) and an outer layer (β-PbO₂). The electrodeposition process was conducted in a two-electrode system under a current of 10 mA∙cm⁻². The α-PbO₂ and β-PbO₂ layers were prepared by electrodeposition at 40 ℃ for 30 min and 65 ℃ for 120 min, respectively.

Field-emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM-6700F instrument equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. X-ray diffraction (XRD) patterns were obtained from an X'pert PRO MRD diffractometer (Cu K_α source; λ = 0.15416 nm) with a scanning angle (2θ) range of 10°–80°.

The electrochemical properties of the as-prepared PbO₂/Sb-SnO₂/TiO₂ NTAs electrode were investigated on an electrochemical workstation (CHI 660D, Chenhua). The traditional three-electrode configuration, in which a saturated Ag/AgCl electrode was the reference electrode, a Pt sheet was the counter electrode, and the as-synthesized PbO₂/Sb-SnO₂/TiO₂ NTAs electrode was the working electrode, was used. Linear sweep voltammetry (LSV) measurement was conducted in 0.5 mol·L⁻¹ H₂SO₄ aqueous solutions at a scan rate of 50 mV·s⁻¹. Accelerated life measurement was carried out to investigate the lifetime of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode in 0.5 mol·L⁻¹ H₂SO₄ aqueous solutions with a current density of 0.5 A·cm⁻². In this process, the electrode was supposed to be deactivated when the cell potential was higher than 10 V. Cyclic voltammetry (CV) experiments were conducted in RhB solutions at a scan rate of 20 mV·s⁻¹ (reference electrode: Ag/AgCl; counter electrode: Cu sheet; working electrode: PbO₂/Sb-SnO₂/TiO₂ NTAs electrode).

Electrochemical oxidation was conducted using the two-electrode configuration, as shown in Fig. 2. The PbO₂/Sb-SnO₂/TiO₂ NTAs electrode was used as the anode and a copper sheet was employed as the cathode. The volume of the electrolyte solutions was 200 mL. A UV-visible spectrophotometer (UV2600A, Unico, Shanghai) was employed to measure the absorbance spectra of RhB and the maximum absorption peak of RhB was at 554 nm. COD was measured using a water quality tester (ET99722 Lovibond). The color removal efficiency (η) and COD removal of RhB were determined according to equations (1) and (2):

(1)

(2)

where A₀ and A_t were the absorbance values of RhB at 554 nm before electrolysis and after an electrolysis time t, respectively; COD₀ and COD_t were the chemical oxygen demands at time 0 and t, respectively.

The average energy demand per gram of RhB (EC_p, kWh·gRhB⁻¹) and the energy consumption per COD (EC_COD, kWh·gCOD⁻¹) were calculated using equations (3) and (4):

(3)

(4)

where U is the cell voltage (V), I is the current (A), t is the electrolysis time (s), V is the volume of the electrolyte, and 5/18 is the conversion coefficient of J to kWh.

A solid-phase micro-extraction-gas chromatography-mass spectrometry (SPME-GC-MS) technique was used to determine the intermediates in the electrocatalytic degradation process. The samples that were adsorbed on the SPME syringe were automatically injected into a TRACE GC Ultra (Thermo/Finnigan, Milan, Italy) gas chromatograph and a TraceISQ (Thermo/Finnigan, Milan, Italy) mass spectrometric detector. The injection port was set to splitless mode and kept at 260 ℃. The oven temperature was maintained at 60 ℃ for 3 min and then ramped to 280 ℃ at 10 ℃·min⁻¹. The flow was set to continuous flow mode with a carrier gas (He, 99.999%) rate of 1 mL·min⁻¹. The ion source temperature and interface temperature were maintained at 230 ℃ and 280 ℃, respectively.

Electrode characterization through SEM, XRD, LSV, and accelerated life was conducted to investigate the surface morphology, crystalline structure, oxygen evolution potential, and stability of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode. It was found that highly ordered TiO₂ NTAs were formed on the Ti substrate. Moreover, the TiO₂ NTA surface was completely covered by the Sb-SnO₂ coating after Sb and Sn were deposited using microwaves, as shown in the right corner of Fig. 3a. The XRD pattern reveals that the TiO₂ NTAs electrode exhibits the diffraction peaks of anatase TiO₂. It is clear that no peaks corresponding to TiO₂ were detected after deposition of Sb and Sn, suggesting better coverage of the Sb-SnO₂ coating. This is in accord with the SEM results. Fig. 3b shows the surface morphology of the PbO₂/Sb-SnO₂/TiO₂ NTAs. The PbO₂ coatings contain an inner layer (α-PbO₂) and an outer layer (β-PbO₂). It was observed that α-PbO₂ and β-PbO₂ exhibit spicules and a hill-like surface, respectively. In this composite electrode, the TiO₂ NTAs served as a tubal template, the Sb-SnO₂ coating embedded in the TiO₂ NTAs acted as an interlayer to enhance the electrocatalytic activity and electrochemical stability of the PbO₂ coating, the α-PbO₂ inner layer was applied to provide better contact between the particles for its compact structure, and the β-PbO₂ outer layer was employed as the catalytic surface layer to enhance the electrocatalytic activity and service lifetime. This well-aligned multilayer microstructure could enhance the effective area and loading amount, which was expected to result in better electrocatalytic activity during the oxidation reactions of organic dye molecules.

The characteristic reflections of β-PbO₂ in the PbO₂/Sb-SnO₂/TiO₂ NTAs were detected in the XRD results and there were no other diffraction peaks. The corresponding plane of β-PbO₂ is clearly shown in Fig. 3c. This phenomenon indicates that the outer layer is thoroughly covered by a thick β-PbO₂ layer. According to the Debye-Scherrer equation, the average grain size of β-PbO₂ is 14.7 nm, which is smaller than the reported size (29.1 nm) resulting from the one-step electrodeposited method [38]. This may be attributed to the introduction of the inner α-PbO₂ layer. The compact structure of the α-PbO₂ layer may affect the growth of β-PbO₂ and thus decrease the grain size.

The oxygen evolution potential of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode is 1.7 V (vs. Ag/AgCl), as shown in the inset of Fig. 3d. Moreover, it was also observed that the electrode exhibits a lifetime of 815 h under a current density of 0.5 A·cm⁻² in 0.5 mol·L⁻¹ H₂SO₄ aqueous solutions. SEM of the deactivated PbO₂/Sb-SnO₂/TiO₂ NTAs electrode is also shown in Fig. S1 (see the Supporting Information). It was observed that the coating fell off from the substrate, resulting in the deactivation of the electrode. Moreover, we found two plateaus in the accelerated life curve. The first plateau and the second plateau were attributed to the anti-corrosion of the PbO₂ coating and the generation of a solid solution by PbO₂ and Sb-SnO₂, respectively. The EDS result in Fig. S2 further demonstrates the formation of a solid solution by PbO₂ and Sb-SnO₂. Fig. 3e shows the CV curve of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode in a RhB solution. The CV curve shows that one oxidation peak corresponding to RhB oxidation appears at 1.26 V (vs. Ag/AgCl). There is no reduction peak in the CV curve, which suggests that the RhB oxidation is an irreversible reaction. It should be noted that water is not electrolyzed during this process because the voltage is lower than the water decomposition voltage.

The current density plays a significant role in the electrocatalytic oxidation process. Current density values ranging from 20 to 60 mA·cm⁻² were applied to evaluate its effect on the electrochemical degradation of RhB. Fig. 4 shows the relationships between color and COD removal efficiency and electrolysis time for different current densities. It was found that the color removal of RhB increased from 67.7% to 99.8% when the current density increased from 20 to 60 mA·cm⁻² after 120 min. COD removal efficiency also increased from 42.5% to 77.2%. This can be attributed to the higher yields of strong oxidation free radicals. The active radicals will attack the chromophoric structure of RhB, which is then mineralized to small molecules. The experimental results show that 99.7% of color is removed after 180 min of electrolysis at 40 mA·cm⁻², and only 80.2% of COD is removed under the same conditions. The probable reason is that the chromophore of RhB is destroyed to form the soluble organic intermediates that made a contribution to the COD. Fig. S3a shows the kinetics of RhB electrochemical degradation at different current densities. It should be noted that the reaction can be divided into two stages and both of them obey pseudo-first-order kinetics. This can be attributed to the competition between the intermediates and RhB molecules during the electrolysis process. The amount of RhB molecules decreases with the degradation time. However, the species of the intermediates increase with increases in electrolysis time. Different intermediates had different degradation rates and the overall degradation rate was determined by all of the kinds of molecules in the solutions. Therefore, two different stages occur during the electrochemical degradation process. Table 1 displays the pseudo-first-order rate constants (k₁, k₂) in different stages. It was observed that k₁ increased with increasing current density in the range of 20–60 mA·cm⁻². However, the value of k₂ decreased when the current density was 60 mA·cm⁻², which can be ascribed to the low substrate concentration. From Fig. 4a, we found that the removal efficiency reached 94.1% at 60 min. This implies that the number of degradable substances is very low in the solutions. The low substrate concentration leads to a decrease in the degradation rate. Moreover, COD removal also follows pseudo-first-order kinetics and the rate constant (k_COD) increases with the current density. The k_COD value increased from 5.36 × 10⁻³ to 12.2 × 10⁻³ min⁻¹, which implies that RhB degradation efficiency increases with the current density. The EC_p and EC_COD at 120 min are listed in Table 1 towards investigation of the energy consumption. It was found that EC_p and EC_COD increase with the current density. These results show that higher current densities are beneficial for RhB removal. However, high current densities result in high energy consumption. Therefore, it is not necessary to employ an excessively high current density.

It is necessary to investigate the influence of initial dye concentration on the electrochemical oxidation process because practical wastewater contains various concentrations of RhB. As shown in Fig. 5, it can be found that the color removal efficiency and COD removal both decrease with increasing initial concentration. According to the literature [39], the electrocatalytic oxidation process is controlled by mass transfer. At low concentrations, the electrochemical process is faster than diffusion. RhB molecules can be totally degraded on the electrode surface. Nevertheless, active free radicals generated on the electrode surface are insufficient for pollutant degradation with increasing concentration. With an increase in concentration, more organic substances are transferred to the electrode surface. However, the amount of hydroxyl radicals produced on the electrode was not enough for readily degrading pollutants. Therefore, color removal efficiency decreases with increases in initial concentration. It should be noted that COD removal efficiency is very close when the concentration is 80 and 100 mg·L⁻¹. This phenomenon may be caused by the deficiency of active free radicals. It is assumed that too many intermediates are accumulated on the electrode surface during the electrochemical oxidation. COD removal is limited by the amount of active free radicals. Table S1 shows the apparent kinetic coefficients of initial concentration, which were obtained from Fig. S4a. The value of k₂ significantly decreases with increasing RhB concentration, which further confirms that the electrocatalytic process is determined by mass transfer. Moreover, Fig. S4a shows a negative relationship between EC_p and electrolysis time, which indicates that electrochemical oxidation consumes less energy for high-concentration wastewater than for low-concentration wastewater. This result reveals that a high concentration of RhB is not favorable for its degradation. However, it is worthy to note that the electrocatalytic oxidation technique exhibits promise for application in the treatment of high-concentration wastewater due to its low energy consumption.

Initial pH can affect coexisting ions or the morphology of organic matter; thus, it is an important factor when evaluating the electrocatalytic oxidation. The effect of initial pH on the electrochemical oxidation is shown in Fig. 6. It can be observed that the maximum color removal of RhB was obtained under neutral conditions (pH = 7, η = 99.7%, COD removal efficiency = 80.2%). When the pH was 2, 7, and 11, the corresponding color removal efficiencies were 75.2%, 92.0%, and 66.8%, respectively (t = 120 min). The different forms of RhB at different pH levels may affect the generation of active radicals. It can be observed that acidic conditions (pH = 2) and alkaline conditions (pH 11) are not favorable for the electrochemical degradation. Fig. S5a shows that the degradation at different pH levels follows pseudo-first-order kinetics. The values of k₁, k₂, and k_COD in Table 2 further prove that degradation in neutral conditions is better than in acidic or alkaline conditions. As shown in Fig. S5b, when the pH was 2, 7, and 11, the corresponding EC_p was 0.564, 0.480, and 0.632 kWh·gRhB⁻¹, respectively. From Fig. S5b, we learned that the lowest EC_p was acquired in neutral conditions. Both acidic and alkaline conditions could enhance the energy consumption. These results indicate that the electrocatalytic degradation can work well in a wide pH range and the best degradation efficiency can be obtained in neutral conditions. Therefore, pH adjustment is not necessary during the RhB degradation process.

The effect of temperature on electrochemical degradation was investigated because the temperature is of importance for oxidation reactions. As shown in Fig. 7, it was observed that a low temperature is beneficial for electrolysis. The color removal efficiency of RhB decreased from 99.8% to 72.2% and COD removal efficiency decreased from 65.7% to 52.1% when the temperature was increased from 0 ℃ to 30 ℃ after 100 min. We employed pseudo-first-order kinetics to simulate the kinetic reaction (Fig. S6a). The data listed in Table S2 show that the value of k₁ slightly changes with the temperature. However, the value of k₂ dramatically decreases with increases in temperature, which reveals that a low temperature can improve RhB degradation. When the temperature was 0, 10, 20, and 30 ℃, the corresponding k_COD was 11.2, 9.19, 8.54, and 6.39 min⁻¹. The change of the k_COD implies that a low temperature can enhance the performance of the electrochemical oxidation. The probable reason is that the rates of side reactions such as the hydrogen evolution reaction and the oxygen evolution reaction may increase with the temperature, thus decreasing the efficiency of the electrocatalytic oxidation process. The relationship between EC_p and time is also shown in Fig. S6b. It can be observed that EC_p first decreased and then increased with time when the temperature was 0 or 10 ℃. However, EC_p increased with time when the temperature was 20 or 30 ℃. This phenomenon can be explained by the relationship between U × t and η according to equation (3). From the above results, we speculate that a low temperature can accelerate the electrocatalytic oxidation because the rates of side reactions (hydrogen evolution reaction and oxygen evolution reaction) increase with increasing temperature. The enhanced side reactions result in decreasing degradation efficiency. However, we need to consume extra energy to change the temperature, which increases the cost of degradation. Therefore, room temperature is the best choice for us for the electrocatalytic oxidation process.

It is necessary to investigate the influence of chloride ions on electrochemical oxidation because wastewater usually contains chloride ions. The degradation efficiency (η and COD removal efficiency) significantly increases with the addition of chloride ions, as shown in Fig. 8. A higher concentration of chloride ions leads to a faster electrocatalytic oxidation process. It is well known that chloride ions can be oxidized to active chlorine on an electrode surface during electrolysis. The enhanced degradation efficiency can be attributed to the strong oxidizability of active chlorine. The molecular structure of RhB can be destroyed owing to the strong oxidizability, thus improving the RhB removal efficiency. The dynamics simulation is displayed in Fig. S7, from which we found that k₁ and k₂ increase with the chloride ion concentration. The kinetic constants of RhB degradation listed in Table S3 further confirm that the addition of chloride ions is helpful to the electrocatalytic degradation process. When the concentration of chloride ions was 1, 5, and 10 mmol·L⁻¹, the corresponding k_COD was 13.6, 78.1, and 99.9 min⁻¹, respectively. From these results, we speculate that the addition of chloride ions can improve the RhB removal efficiency due to the generation of active chlorine. It should be noted that the color and COD removal efficiencies have positive relationships with the concentration of chloride ions. Moreover, it was observed that EC_p decreases with the addition of chloride ions, which can be attributed to the enhanced removal efficiency. Therefore, we conclude that the removal efficiency increases with an increase in chloride ion concentration. However, halogenated by-products may be generated due to the addition of chloride ions; thus, it is worth weighing whether to add the chloride ions.

SEM and EDS of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode after the degradation experiment were performed to evaluate its stability, as shown in Fig. 9. It was found that the morphology of the electrode slightly changes and some particles are adsorbed on the surface. EDS measurement indicated that the adsorbed particles may come from the sodium sulfate or organic matter in the electrolyte. This result shows that the electrode has strong stability and is promising for application in wastewater treatment.

Safety of the electrode is very important [40]. Therefore, the risk of secondary water pollution should be carefully considered due to Pb²⁺ leaching from the anode. An ICPE technique was employed to investigate the concentration of leached Pb²⁺ at different current densities after electrolysis, as listed in Table 3. It was observed that the concentration of leached Pb²⁺ cannot be detected because of its detectable limit when the current density is below 40 mA·cm⁻². Little Pb²⁺ was detected when the current density was increased to 50 or 60 mA·cm⁻². The concentrations of leached Pb²⁺ listed in Table 3 meet the national standard for the wastewater discharge guideline (GB8978-2015). This result reveals that the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode is safe for electrochemical degradation and the risk of secondary water pollution caused by Pb²⁺ can be ignored. Moreover, our previous research also showed that a modified PbO₂ electrode exhibits strong stability [33, 41, 42]. However, it should be noted that the optimal current density should be controlled at below 40 mA·cm⁻² in practical wastewater treatment due to the high energy consumption.

Fig. S8 shows the GC-MS spectrogram and Table S4 shows the identified intermediates for the electrocatalytic oxidation at different degradation times. Based on the identified intermediates obtained by the SPME-GC-MS technique, a possible pathway for the electrochemical degradation of RhB was proposed, as shown in Fig. 10. As the electrocatalytic degradation proceeds, various new intermediate products can be detected at different times. According to Table S4, it was found that benzene rings and poly-heterocyclic compounds are detected during the first 20 min, which can be attributed to the destruction of the conjugated chromophoric structure. Then, some long chain esters or ketone compounds are generated with increasing degradation time due to the oxidation of active free radicals. Moreover, smaller carboxylic acids or esters further form as the electrochemical oxidation proceeds and the contents of these intermediates decrease with the degradation time. Finally, most of them eventually degrade to CO₂ and H₂O at the end of the process due to the destruction of the reactive species. In summary, the degradation process undergoes the following stages: destruction of the conjugated chromophoric structure to form benzene rings or poly-heterocyclic compounds, opening of rings to generate long chain esters or ketones, destruction of longer chains by reactive species to produce smaller molecules (carboxylic acids or esters), and finally mineralization to CO₂ and H₂O.

The current work revealed that a PbO₂/Sb-SnO₂/TiO₂ NTAs electrode with a multilayer microstructure can be successfully employed for the electrochemical degradation of RhB. SEM, XRD, and electrochemical measurements were carried out to understand the morphology, structure, and electrochemical performance of the electrode. Moreover, the influences of key operating parameters (initial RhB concentration, current density, initial pH, temperature, and chloride ion concentration) on RhB removal were systematically investigated to evaluate the electrocatalytic activity of the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode. It was found that RhB removal (η and COD removal efficiency) increases with increasing current density and chloride ion concentration. In contrast, RhB removal increases with decreasing initial RhB concentration and temperature. It should be noted that neutral conditions are helpful to electrochemical oxidation. Moreover, we also observed that the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode exhibits high safety and stability. The risk of secondary water pollution caused by Pb²⁺ can be neglected. SPME-GC-MS results suggested that the conjugated chromophoric structure of the RhB molecule is destroyed by active free radicals during the initial stage. Then, the intermediate products are gradually oxidized to form small esters, ketones, and other small organic compounds and finally mineralized. In summary, the PbO₂/Sb-SnO₂/TiO₂ NTAs electrode exhibits enhanced stability and electrocatalytic activity in the electrochemical degradation process and has a potential application in wastewater treatment.

Acknowledgments

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 21507104) and Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017JM2015).