Dimensional Stable Lead Electrode Modified by SDS for Efficient Degradation of Bisphenol A.

The dimensional stable lead electrodes modified by sodium dodecyl sulfate (SDS) were prepared with electrochemical deposition and it shows that a more compact, uniform, and smooth film of Ti/PbO2(F+SDS) electrode reduces the corroded crystal faces and surface defects. Characterization results demonstrate that the oxygen evolution potential (OEP) of the Ti/PbO2(F+SDS) electrode was higher, and its service life is almost 1.6 times longer than the Ti/PbO2(F) electrode. Compared with Ti/PbO2(F) electrode, the soluble Pb concentration of the Ti/PbO2(F+SDS) electrode decreased 15.1%, which indicates that the decrease of crystal surface defects leads to the reduction of Pb ions so that the excellent corrosion resistance electrodes cause low environmental risks. The modified electrodes were used as anodes to degrading bisphenol A (BPA) when an electrolyte is 0.2 M Na2SO4, the applied voltage is 5 V and electrodes distance is 3 cm, 20 mg/L BPA for electrolysis time of 180 min can reach up to 97.6%.

Introduction

Advanced oxidation process (AOP) is a kind of technology that converts organic molecules into small molecules, CO2, H2O, etc. by reactive hydroxyl radicals reacting with organic matter in wastewater so that complex organic pollutants can be oxidized and removed effectively.[1,2] Electrocatalytic oxidation technology is one of the AOPs technologies and has many outstanding advantages: high activity, strong oxidizing ability, high degradation efficiency, simple operation, low cost, and environmentally friendliness.[3−6] Anodic material exists as a core element in the process of electrochemical oxidation and has attracted the attention of more and more researchers. A kind of popular anodic material is titanium-based lead dioxide (Ti/PbO2) electrode, which is widely used for its good conductivity, high oxygen evolution potential, low cost, and high electrocatalytic efficiency.[6−8] Currently, preparing Ti/PbO2 electrode is mainly electrochemical deposition.[9−13] Although it has been widely used in industrial production, there are still some defects that need to be overcome, such as low adhesion of the plating layer, peeling easily, instability in the electrolysis process, and weak corrosion resistance.[9] Numerous studies[6,8,10−19] have shown that the catalytic activity and stability of the electrodes can be improved by doping specific metal elements (Bi, Co, Fe, Ni, Ce), manufacturing nano and microstructured deposits and forming oxide composites.

Further research on the modification of the Ti/PbO2 electrode is a focus. To prevent oxidation of the substrate, increase the bonding force between the lead dioxide coating and the substrate, and reduce interfacial resistance and internal stress, an electrode having an intermediate layer (Sb–SnO2) may be prepared on the substrate.[20] It is mentioned in the literature[10,11,15,21,22] that F can effectively improve the bonding strength and stability of the Ti/PbO2 film and increase the oxygen evolution potential, while the addition of sodium dodecylbenzene sulfonate (a common anion surfactant, SDBS)[10,23] can change the surface of the electrode in addition to improving the bonding force. In the present review, we report that a dimensional stable lead electrode having the Sb–SnO2 film is modified by Nafion (anionic polyelectrolyte)[10] and sodium dodecyl sulfate (SDS) prepared by the electrodeposition method. The characterization of Ti/PbO2 electrodes was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), steady-state polarization curves, and we investigate the electrode stability by electrode accelerated life test.

In recent years, endocrine-disrupting compounds (EDCs) have attracted widespread attention for interfering with the normal function of the endocrine system of humans and wild animals. It is observed from different studies that we are surrounded by a wide range of EDCs in our daily life.[24,25] As a known EDC, bisphenol A (2,2-bis(4-hydroxyphenyl)propane, BPA) is widely used in the manufacture of epoxy, polycarbonate, and many plastic products (food and drink packaging, food, and beverage cans, water bottles, PVC pipes, etc.).[12,25−28] The residual BPA was possibly released into the water due to incomplete reaction resulting in the widespread existence of BPA. Hence, the purification of residual BPA in wastewater with low degradation rate needs a considerable amount of attention and it is urgent to minimize its contamination and remove it effectively.[13,25,29]

The dimensional stable lead electrode modified by SDS was used as anodes to degrading BPA, the effect of electrolyte, electrolyte concentration, BPA initial concentration, electrode distance, applied voltage, and current density of degradation reaction were systematically studied. When electrolyte was 0.2 M Na2SO4 solution, the applied voltage was 5 V, initial concentration of BPA was 20 mg/L, electrodes distance was 3 cm, after 180 min, BPA removal efficiency reached up to 97.6%.

Results and Discussion

Characterization of the Electrodes

SEM Characterization

The SEM images of electrodes with 500 and 3000 times magnification were shown in Figure a–f, which could be helpful to observe the morphology of the electrodes. Figure a,b shows that the PbO2 film presents pyramid structure and crystals are closely arranged, and as shown in Figure b, an obvious change in the PbO2 film occurs by adding SDS. Evidently, preparing Ti/PbO2(F+SDS) in an electroplating bath of an extra 0.05 g of SDS electrode forms a more compact, uniform, and smooth film, when the magnification was 3000 times in Figure d, crystals can still be observed in the pyramid structure. As we all known, SDS is typical anionic surfactant that can make the Pb ions disperse evenly in the solution to obtain the coating with the symmetrical distribution of PbO2 particles, forming a uniform, compact, and smooth film. As we all know, SDS is a typical anionic surfactant that can make the Pb ions disperse evenly in the solution to obtain the coating with the symmetrical distribution of PbO2 particles, forming a uniform, compact, and smooth film. Due to the effect of SDS, the PbO2 particles were separated from each other, which could not agglomerate in large size. Thus, the size of PbO2 particles became smaller with additional SDS.[30] As shown in Figure a–e, the distribution of PbO2 particles became denser with an increase of Nafion, while excessive Nafion will cause damage to the pyramid structure (as shown in Figure f).

Figure 1

(a–f) SEM images of Ti/PbO2(F) and Ti/PbO2(F+SDS) electrode; (g) XRD images of Ti/PbO2(F) and Ti/PbO2(F+SDS) electrodes.

XRD Characterization

The crystal structure of the electrodes were examined by XRD, and a comparison of XRD patterns of electrodes is shown in Figure g. All the diffraction peaks were corresponding to the standard card and well-indexed to β-PbO2, which were assigned to the (110), (101), (200), (211), (220), (301), (320), and (400) planes of β-PbO2 at 25.28, 31.92, 36.10, 49.04, 52.08, 58.86, 62.48, 66.96, and 74.42°, respectively.[12,31] The diffraction peaks intensities of (101) and (301) planes of Ti/PbO2(F+SDS) electrode were larger than that of Ti/PbO2(F) electrode, which means the increased degree of order of β-PbO2 with adding SDS.[30] However, the diffraction peak intensities of (110), (211), (220), (301), and (400) planes of Ti/PbO2(F+SDS) electrode decreased with the increase of Nafion,[10] which reduces the corroded crystal faces and surface defects. Generally, the increase of surface defects leads to the increase of hydroxyl oxygen, resulting in more free Pb ions existing in solution, which could cause the dissolution of Pb. While Nafion and SDS make the electrode surface smoother reducing the surface defects, in this condition, Ti/PbO2(F+SDS) electrode could be more stable for less dissolved Pb ions.

Electrochemical Performance of the Electrodes

Steady-State Polarization Curves

The steady-state polarization curves of the electrode tests were measured in 0.1 mol/L of HClO4 at a scan rate of 1 mV/s. As shown in Figure a, Ti/PbO2(F, 30 μL) electrode has the highest oxygen evolution overpotential (OEP). The oxygen evolution overpotential variation with SDS content is shown in Figure b when the F content was 10 μL, the Ti/PbO2(SDS, 0.05 g) OEP was slightly higher than that of Ti/PbO2(SDS, 0.10 g). The modified electrode OEP by Nafion and SDS measured by mean of steady-state polarization curves and the OEP of Ti/PbO2(F+SDS) was as shown in Figure c. The results demonstrated that the existence of F could somewhat increase the OEP of Ti/PbO2 inhibiting the oxygen evolution reaction to improve the stability of electrodes effectively, and the OEP of electrodes could be increased due to existing SDS, also.[19,23,30,31]

Figure 2

(a–c) Steady-state polarization curves in 0.1 M HClO4 at a scan rate of 1 mV/s for Ti/PbO2(F) and Ti/PbO2(F+SDS) electrodes; (d) slopes of the current to different scan rates.

Electrochemical Surface Area

The difference in the oxidation activity of PbO2 electrodes made from anode area was researched by the electrochemical surface area (ECSA). ECSA is generally considered to be a significant parameter for OER performance, which is proportional to the active sites involved in electrochemical reactions, the specific calculation method references the literature.[32] The experiments were carried out in 0.1 mol/L HClO4 between 1.4 and 1.6 V (vs SCE) with scan rates of 20, 40, 60, 80, and 100 mV/s. As shown in Figure d, we can estimate that the ECSA of Ti/PbO2(F) is 0.04 cm2 and that of Ti/PbO2(F+SDS) is 0.15 cm2. Thus, the introduction of Nafion and SDS not only reduces the corroded crystal faces and surface defects but also makes the nature in the oxidation activity of Ti/PbO2 anode have a change.

Electrode Accelerated Lifetime Test

As known, the electrode stability is an important factor for practical application in an industrial scale, while testing electrode stability is usually time-consuming. Therefore, the accelerated life test for electrodes (as Figure shown) was carried out in 2.0 M H2SO4 solution at a current density of 1 A/cm2 at 60 °C for reducing test time at the electrolyte volume of 100 mL, which both repeated for three times. At the beginning of the test, its potential was about 4 V and a sharp increase during the last hours was observed. For Ti/PbO2(F) electrode, as line 1, 2, 3 shows, the service life was approximately 119, 128, and 143 h, respectively, and for line 4, 5, 6, for Ti/PbO2(F+SDS) electrode, it can be estimated that the service life was 188, 223, and 228 h, respectively. It was obvious that the accelerated lifetime of the Ti/PbO2(F+SDS) electrode was 1.6–1.7 times than that of the Ti/PbO2(F) electrode.

Figure 3

(a) Potential variation with electrolysis time in accelerated life test for Ti/PbO2(F) (1, 2, 3) and Ti/PbO2(F+SDS) (4, 5, 6); (b) XRD patterns for Ti/PbO2 of accelerated life test for 60, 120 h.

Actually, Ti/PbO2(F+SDS) electrode has the denser and smoother surface than Ti/PbO2(F) electrode, which can inhibit the penetration of electrolytes into the interior of the electrode, therefore, less corrosion would occur and the effect on the conductivity is slight. Ti/PbO2(F+SDS) electrode has a higher oxygen evolution potential than Ti/PbO2(F) electrode and oxygen evolution reaction on Ti/PbO2(F+SDS) electrode is reduced, which explains the longer service life.[12,19]

Generally, the service life of electrodes should be closely related to using conditions (current density, temperature, pH, etc.), and accessing the actual service life in industrial applications is certainly difficult. At present, there is an empirical judgment[33] in the academic community about the relationship between actual service life and its using conditions, which was described as followswhere τ2 is the calculated life (h), τ1 is the accelerated life (h), i1 is the current density of accelerated test (A/m2), i2 is the current density of using under actual conditions (A/m2), and i2 is generally 1000 A/m2.

Thus, take the maximum value of τ1 for a example, τ2 of Ti/PbO2(F) was obtained to be 14 300 h (1.63a) and τ2 of Ti/PbO2(F+SDS) was 22 800 h (2.60a), the service life of Ti/PbO2(F+SDS) electrode is about 1.6 times longer than Ti/PbO2(F) electrode. Soluble lead concentration in electrolytic solution measured by ICP was reduced by 15.1% at Ti/PbO2(F+SDS) electrode.

The crystal structure of Ti/PbO2 during the accelerated life test was examined by XRD, and a comparison of XRD patterns of 60, 120 h was shown in Figure b. The diffraction peak intensities of (110), (101), (211), (220), and (400) planes of Ti/PbO2 electrode decreased with time going by, which means the decreased degree of order of β-PbO2.

Effect of Experimental Conditions of BPA Degradation

Effect of Electrolyte

The effect of electrolyte for BPA degradation at BPA initial concentration of 20 mg/L is shown in Figure , electrode distance of 3 cm, applied voltage of 5 V, and electrolysis time of 180 min. The electrolytes were NaCl, Na2SO4, NaH2PO4, and Na2HPO4, respectively, and the concentration of electrolyte was 0.02 mol/L.

Figure 4

(a) Degradation of BPA in different electrolyte types by Ti/PbO2(F); (b) the degradation of BPA in different electrolyte types by Ti/PbO2(F+SDS); (c) the BPA removal efficiency in different electrolyte types by two electrodes for 60 min; (d) the BPA removal efficiency in different electrolyte types by two electrodes for 180 min.

The electrolyte types made a great influence on BPA degradation. As shown in Figure a, the removal efficiency of BPA was the best in NaCl solution and it reached up to 93.54% by Ti/PbO2(F) for 180 min, which was much higher than others (44.8% in Na2SO4, 22.4% in NaH2PO4, 29.5% in Na2HPO4). At the same time, as shown in Figure b, the trend of BPA removal efficiency by Ti/PbO2(F+SDS) was same as Ti/PbO2(F), it performed as that BPA removal efficiency was 92.0% in NaCl, 43.9% in Na2SO4, 25.3% in NaH2PO4, and 21.8% in Na2HPO4. The electrocatalytic oxidation performance of two electrodes has only slight differences as shown in Figure c,d. The relationship of BPA removal efficiency by Ti/PbO2(F) for 60 min in different electrolyte types was NaCl (60.2%) > Na2HPO4 (16.9%) > Na2SO4 (15.7%) > NaH2PO4 (13.6%), which was different from the degradation efficiency by Ti/PbO2(F+SDS) [NaCl (53.0%) > Na2SO4 (19.7%) > NaH2PO4 (13.2%) > Na2HPO4 (12.6%)]. The Ti/PbO2(F+SDS) showed better activity in Na2SO4 solution than Ti/PbO2(F) while it was opposite in NaCl solution for 60 min. NaH2PO4 and Na2HPO4 were not considered as an electrolyte for the next research for low BPA removal efficiency with them. And in the remaining two electrolyte types, the electrocatalytic oxidation performance of two electrodes performed consistently, which was that BPA removal efficiency by Ti/PbO2(F) was slightly higher than Ti/PbO2(F+SDS).

It is clear that BPA removal efficiency is higher in NaCl solution than that in Na2SO4 solution. Whereas, it is because that chlorite ions, hypochlorite ions and other strong oxidation species that produced by Cl– reacting with OH radical make the great effect on degradation of BPA.[34] Thus, to eliminate the influence of Cl–, we choose Na2SO4 solution as electrolyte for reflecting the oxygen evolution activity of Ti/PbO2 electrode more accurately.

Effect of Electrolyte Concentration

In this paper, Na2SO4 was chosen to be the electrolyte of BPA degradation and the degradation curves in different Na2SO4 concentrations by the electrodes were as shown in Figure . Figure a presents the BPA removal efficiency variation with electrolysis time by Ti/PbO2(F) and it is obvious that the higher Na2SO4 concentration has a better degradation effect. Even, when the concentrations of Na2SO4 were 0.15 M and 0.2 M, BPA removal efficiency reached up to 100% after 120 min. For Ti/PbO2(F+SDS), as shown in Figure b, the change in law of BPA removal efficiency with increasing Na2SO4 concentration was the same as Ti/PbO2(F), however, when Na2SO4 concentration was 0.2 M, BPA removal efficiency was 97.6%, which was lower. A comparison of the degradation effects of the electrodes could be observed intuitively in Figure c, as Na2SO4 concentration increased, the BPA removal effect of the Ti/PbO2(F) electrode was better than Ti/PbO2(F+SDS) electrode more distinctly. The result could be explained as improving the stability of electrodes by inhibiting the oxygen evolution reaction and further reducing the generation of hydroxyl radicals, which made the Ti/PbO2(F+SDS) electrode to have lower electrochemical activity.

Figure 5

(a, b) Degradation of BPA in different Na2SO4 concentrations by Ti/PbO2(F) and Ti/PbO2(F+SDS); (c) the BPA removal efficiency in different Na2SO4 concentrations by two electrodes for 180 min.

Effect of Initial Concentration of BPA

The effect of initial BPA concentration on the electrodes is shown in Figure a,b. The initial BPA concentrations for study were 10, 15, 20, 25, and 30 mg/L in 0.2 M Na2SO4 solution, respectively, the electrode distance was 3.0 cm and the applied voltage was 5 V. It can be observed that the BPA removal efficiency can reach up to 100% after 120 min, while the BPA degradation rate became more and more slow with initial increase in BPA concentration. It is a fact that the unusual dependence of the reaction rate from BPA initial concentration exists in the degradation process. As well known, the degradation reaction rate was mainly depended on the speed of organic matter migration to the surface of the electrode. It presents that the reaction rate decreases with the increase in initial BPA concentration. As the initial BPA concentration increases, it transfers more BPA to the surface of the electrode so that it could prevent contact between the BPA and the active species, which result in a decrease in reaction rate.[12,19,23]

Figure 6

Effect of initial BPA concentrations (a, b), electrode distance (c, d), applied voltage (e, f) and current density (g, h) for degradation of BPA by Ti/PbO2(F) and Ti/PbO2(F+SDS).

Effect of Electrode Distance

As shown in Figure c,d, the effect of electrode distance on BPA degradation was investigated in 0.2 M Na2SO4 solution at electrode distance of 2.0, 2.5, and 3.0 cm, respectively, the initial BPA concentration was 20 mg/L and the applied voltage was 5 V. The BPA removal efficiency was observed to reach up to 100% after 180 min and the difference of different electrode distance was not very obvious, the Ti/PbO2(F) electrode showed faster degradation rate than the Ti/PbO2(F+SDS) electrode. When the electrode distance is 2 cm, the Ti/PbO2(F) electrode has the fastest degradation rate of BPA degradation, for the Ti/PbO2(F+SDS) electrode, the electrode distance of 2.5 cm displayed the best result. The effect of electrode distance on BPA degradation process could be related to the resistance of the system, and the electrode distance chosen to be researched is 3.0 cm whose BPA removal efficiency was ideal within the determined electrolysis time.[19]

Effect of Applied Voltage

The effect of applied voltage on BPA degradation was shown in Figure e,f in 0.2 M Na2SO4 solution at electrode distance of 3.0 cm, the initial BPA concentration was 20 mg/L and the applied voltages were 3, 4, 5, and 6 V, respectively. Clearly, when applied voltage increases, BPA removal efficiency increased for Ti/PbO2(F) electrode and Ti/PbO2(F+SDS) electrode. For Ti/PbO2(F) electrode, BPA removal efficiencies were both 100% at the applied voltages of 5 and 6 V after 180 min, simultaneously, 66.7 and 89.3% of BPA was removed at the applied voltages of 3 and 4 V, respectively. For Ti/PbO2(F+SDS) electrode, when the electrolysis time was 180 min, 69.4, 85.2, 97.6, and 100% of BPA could be removed at the applied voltages of 3, 4, 5, and 6 V, respectively. Whereas, the increase of the applied voltage brings out more energy consumption and the electrode service life is also affected by an increased applied voltage.[12,35] Hence, to save energy, the applied voltage of 5 V was chosen in the experiment ensuring complete degradation of BPA within 180 min of electrolysis time.

Effect of Current Density

The effect of current density was as shown in Figure g,h in 0.2 M Na2SO4 solution at an electrode distance of 3.0 cm at the applied voltage of 5 V, the initial BPA concentration was 20 mg/L and the current densities were 20, 40, 60, and 80 mA/cm2, respectively. Similarly, BPA removal efficiency by Ti/PbO2(F) electrode was a little higher than that by the Ti/PbO2(F+SDS) electrode. When the current density was 20 mA/cm2, 77.2% of BPA was removed in the degradation system by Ti/PbO2(F) electrode and BPA removal efficiency was 69.4% by Ti/PbO2(F+SDS) electrode after 180 min, and BPA removal efficiency was 97.9% by Ti/PbO2(F) electrode and 97.8% of BPA was removed by Ti/PbO2(F+SDS) electrode at the current density of 40 mA/cm2. When the current densities were 60 and 80 mA/cm2, BPA could be removed completely within 180 min, and BPA removal efficiency has reached up to 100% with electrolysis time of 120 min. The reason why the current density was studied is that it is related to the generation of hydroxyl radicals.[36] As Figure g,h shows, the BPA removal efficiency increased with a current density, which could be explained by the fact that higher current density promoted the reaction of oxygen evolution.[37] Like the applied voltage, too high current density is not advisable for BPA degradation considering energy consumption.

BPA Degradation Analysis

As for the oxidation mechanism of organic matter, electrocatalytic oxidation can be classified into direct oxidation and indirect oxidation. In the direct electrochemical oxidation process, contaminants are adsorbed to the surface of the anode and then destroyed and removed by the anodic electron transfer reaction. The oxidation reaction process of organic matter on the anode is as follows

Obviously, there are two kinds of active oxygen in the state of adsorbed ·OH and high-valence oxide in the crystal lattice. To increase the electrochemical incineration activity, a high concentration of active adsorption sites is required on the surface of the electrode, and the concentration of adsorbed hydroxyl radicals is sufficiently high.

Indirect electrochemical oxidation is a method that utilizes strong oxidants to degrade pollutants during electrochemical reactions, which produces superoxide-free radicals such as O2·, H2O2, HO·, and other active groups to oxidize organic matter in water through the action of electrodes and catalytic materials. The surface of electrode adsorbs a large amount of hydroxyl radicals, and chemical reaction can be performed as follows:

According to the relevant literature,[38] BPA is removed via the synergistic action of direct and indirect electrocatalytic oxidation pathways. In the direct oxidation process, BPA is adsorbed on the surface of the electrode, and the electrons are directly oxidatively degraded by the electrode catalysis, and the indirect oxidative degradation of BPA was carried out by the formation of HO· and other oxidation intermediates during the electrolysis process.

When the electrolyte was 0.2 M Na2SO4 solution, applied voltage was 5 V, the electrode distance was 3 cm, BPA initial concentration was 20 mg/L, and BPA concentration was analyzed by HPLC. After 180 min, the presence of BPA was not detected in the solution, indicating that BPA had been degraded and converted to other substances. The product from degradation of BPA was studied by LC/MS, the results show the presence of benzoquinone and some small linear organic compounds. Therefore, it was confirmed that the Ti/PbO2 electrode was effective for BPA degradation and the general pathway[11,38] of BPA degradation can be explained as shown below (Figure ):

Figure 7

Reaction pathway for electrochemical degradation of BPA.

Conclusions

The Ti/PbO2(F) and Ti/PbO2(F+SDS) electrodes were prepared successfully with electrochemical deposition. It shows that adding SDS in electroplating bath can form a more compact, uniform, and smooth film of the Ti/PbO2 electrode whose particle size becomes smaller. Hence, it can be obtained that the morphology and crystallization could be changed by adding SDS and different contents of Nafion, and it could reduce the corroded crystal faces and surface defects.

The OEP of the dimensional stable electrode modified by SDS is higher than that of the Ti/PbO2(F) electrode. The existence of F and SDS could increase the OEP of Ti/PbO2 inhibiting the oxygen evolution reaction to improve the stability of electrodes effectively, which reduces the process of oxygen evolution resulting in prolonging the service life according to electrode accelerated lifetime test, and the service life of Ti/PbO2(F+SDS) electrode is about 1.6 times longer than Ti/PbO2(F) electrode. Pb concentrations in electrolytic solution were measured by ICP, which decreased 15.1% for Ti/PbO2(F+SDS) electrode, and it indicates that the reducing of Pb ions shows the excellent corrosion resistance electrodes causing low environmental risks.

When the electrolyte was 0.2 M Na2SO4 solution, applied voltage was 5 V, the electrode distance was 3 cm, BPA initial concentration was 20 mg/L, BPA removal efficiency for 180 min can reach up to 100 and 97.6% for Ti/PbO2(F) and Ti/PbO2(F+SDS) electrodes, respectively. Consequently, the dimensional stable electrode modified by SDS has a longer service life and lower environmental risks, simultaneously, which also guarantees efficient removal of bisphenol A.

Experimental Section

Material and Methods

Chemicals

Lead nitrate, nitric acid, cupric nitrate, 5% Nafion, sodium dodecyl sulfate, sulfuric acid, perchloric acid, sodium sulphate, sodium chloride, sodium phosphate dibasic, sodium dihydrogen phosphate, bisphenol A. All the chemicals were analytical grade regent.

Analytical Instruments

Platinum electrode holder (JJ110, Shanghai Yueci Electronic Technology Co., Ltd.), DC Power Supply (LW3J5D2, Shanghai Liyou Electric Co., Ltd.), pH meter (PHSJ-3F, Instrument Electric Scientific Instrument Co., Ltd.), thermostatic magnetic stirrer (MYPH-2, Shanghai Meiyingpu Instrument & Meter Manufacturing Co., Ltd.), electric blast drying oven (DGH-7075A, Shanghai Yiheng Instrument Co., Ltd.), box-type resistance furnace (SX2-4-10, Shanghai Yiheng Instrument Co., Ltd.).

Electrochemical workstation (CHI660, Shanghai Chenhua Instrument Co., Ltd.) was used for electrochemical performance test of the electrodes with the three-electrode system at room temperature. All potentials were referred to the SCE.

X-ray diffraction (XRD) on a D/max 2550 V X-ray diffractometer (Japan) determined the phase structure of electrodes; scanning electron microscope (SEM, Nova NanoS) was used to observe the surface morphology and size of the electrodes; inductively coupled plasma emission spectrometer (ICP, Varian 710 ES) detected the ions concentrations in the solution.

BPA concentration was analyzed by high-performance liquid chromatography (Shimadzu Prominence LC-20A HPLC, Japan). The mobile phase was a mixed solution, which contained methanol and ultrapure water with the ratio of 75:25 (V/V), whose flow rate was 1.0 mL/min. Analysis conditions were: the absorption wavelength was 230 nm, the injection volume of BPA solution was 20 μL, and the column temperature was 35 °C.

Electrode Preparation

Pretreatment of Titanium Plate

The titanium plate was pretreated following these steps. First, the titanium substrate was cut into such a size that was 3 cm × 1 cm × 0.1 cm and cleaned thoroughly with deionized water. Then, it was heated with 1.0 mol/L of NaOH at 90 °C for 30 min to remove surface oil and boiled with 10% oxalic acid for 2 h. Afterward, the titanium plate was rinsed with deionized water and ethanol for 2–3 times. Finally, the cleaned Ti plate was stored in ethanol solution.

Preparation of Sb–SnO2 Film

The Sb–SnO2 film was prepared by thermal decomposition.[20] The polymer precursor solution was uniformly coated on the two sides of the titanium plate with a brush. The titanium plate was dried in an electric blast drying oven at 130 °C for 10 min, then it was placed in a box-type resistance furnace at 500 °C for 15 min, and after little cooling, the surface ash was blown off and the next coating was carried out. The modification procedure was repeated 10 times, the last firing time was extended to 1 h, and the sample was cooled with the furnace thereby producing a Sb–SnO2 film layer.

Preparation of Ti/PbO2 Electrode

In this study, the Ti/PbO2 electrodes were prepared by electrochemical deposition in two kinds of electrodeposition solutions. The Ti plate covered with Sb–SnO2 film was inserted into the electrodeposition solution containing 0.5 mol/L of Pb(NO3)2, 0.1 mol/L of Cu(NO3)2, 0.1 mol/L of HNO3, and 10 μL of 5% Nafion. The electrochemical deposition of PbO2 was accomplished under current density 80 mA/cm2 for 30 min and then 60 mA/cm2 for 30 min at 60 °C. In this way, we can get the Ti/PbO2(F) electrode. And by adding 30 μL of 5% Nafion and 0.05 g of SDS to the deposition solution, the Ti/PbO2(F+SDS) electrode was prepared.

More details for a selection of the applied solution compositions and electrolysis conditions for preparation of the PbO2 anode are presented in the Supporting Information (Scheme S1). And the current efficiency of PbO2 during the deposition process are shown in the Supporting Information (Table S2).

Electrode Accelerated Life Test

The accelerated life test was a fast method to investigate electrode stability. The tested electrode was anode and titanium plate was used as cathode, which was performed at 1 A cm2 in 2 mol/L of H2SO4 solution at 60 °C. The anode potential was monitored and recorded periodically, and it is generally considered that the electrode was deactivated until the electrode potential increased to 5 V from its initial voltage.[39]

Degradation of BPA

The degradation of BPA was carried out in an electrolytic cell with electrolyte solution volume of 100 mL. The prepared Ti/PbO2(F) and Ti/PbO2(F+SDS) electrodes were anodes and the cathode was stainless steel mesh. The degrading solution consists of certain concentrations of BPA solution and electrolyte solution. Under the condition of stirring rate of 250 rpm, various factors including electrolyte, electrolyte concentration, initial concentration of BPA, electrode distance, applied voltage, and current density were studied. The experiments lasted for 3 h by the state of direct current (DC) and the samples were taken at 0, 10, 20, 30, 60, 90, 120, 150, and 180 min. BPA concentration was analyzed by HPLC and can be calculated from the peak area that is proportional to the BPA concentration.