1 Introduction

Worldwide, approximately 70% of the world's dye production corresponds to azo compounds, coming mainly from industries such as textiles or tanning, which are released every day [1, 2]. This kind of dyes has a complex chemical structure containing one or more azo groups (−N=N−) as a chromophore, connected to aromatic systems with lateral groups, including −OH, −SO3−, and −CH3, among others [3]. In many cases, the wastewater generated by this industry is discharged into municipal wastewater treatment plants without any pretreatment [4]. The negative effects of azo dyes on both humans and aquatic life have led to urgent calls for the treatment of effluents containing azo dyes in order to remove them or to turn them into safe and useful products [5, 6]. To treat these wastewaters effectively, the so-called advanced oxidation processes (AOPs) are widely applied. The AOP techniques include photocatalysis, Fenton oxidation, electrocatalytic oxidation, and a combination of these processes [7, 8].

Electrocatalytic oxidation technology represents an interesting approach for the treatment of hazardous, toxic, and highly concentrated organic wastewater due to its high efficiency, ease of control, versatility, low time consumption, and environmental sustainability [9,10,11]. During the electrocatalytic process, the anode material is a key factor in the oxidation efficiency of organic pollutants [12, 13]. Several types of electrodes have been studied, including platinum [14], graphite [15, 16], MnO2 [17], PbO2 [18, 19], RuO2 [20], IrO2 [21], SnO2 [22, 23], and boron-doped diamond [24]. Among these electrodes, PbO2 is one of the best candidates widely used in electrocatalysis due to its high oxygen evolution potential (OEP), good corrosion resistance, low price, and high electrocatalytic activity [10]. However, due to its fragility and ease of deactivation [25], many attempts have been made to improve the performance of the PbO2 electrode. A frequently used method consists in doping the PbO2 layer with some materials, such as cations (Bi3+ [26], Fe3+ [27], Cu2+ [28], Zr4+ [29], Ce3+ [30], and Co2+ [31]), anions (F− [32] and [Fe(CN)6]3− [33]), surfactants (cetyltrimethylammonium bromide [34], sodium dodecyl sulfate [35, 36], sodium dodecyl benzene sulfonate [37], polyvinylidene fluoride [38], polyethylene glycol [39]), ionic liquids [40], carbon nanotubes [41], etc. The results show that the modified PbO2 electrode exhibits excellent electrocatalytic activity and stable performance.

Our investigation focuses on the modification of the ΡbO2 electrode. In this paper, we suggest a new method of modification of ΡbO2 electrode in order to increase the electrochemical activity and the lifetime of ΡbO2 electrode. It is widely known that carbon has a unique structure, extraordinary chemical, mechanical, and electronic properties and has been widely studied in fuel cells, supercapacitors, and biosensors [41, 42]. It is widely known that carbon has extraordinary chemical, a unique structure, extraordinary chemical, electronic, and mechanical properties [43, 44], and has been widely studied in fuel cells, biosensors, and supercapacitors. At the same time, some investigations have demonstrated that Fe doping in ΡbO2 electrodes can achieve better coating quality and higher electrocatalytic performance [45]. But until now, there are no reports on the modification of ΡbO2 electrode by co-doping carbon and iron.

In this work, lead dioxide electrodes undoped and doped with Fe3+ and/or carbon black were prepared by electrodeposition. The morphology, crystalline structure, and electrochemical performances were characterized. Reactive Yellow 14 (RY14) azo dye (C20H19ClN4Na2O11S3, CAS number: 18976-74-4) was chosen as the model pollutant for electrocatalytic oxidation to evaluate electrochemical activity of the electrodes.

2 Experimental

2.1 Materials

Pb(NO3)2, Fe(NO3)3·9H2O, NaOH, and C2H2O4 were purchased from VWR Prolabo Chemicals. Na2SO4 was supplied by Panreac. KNO3 was obtained from Janssen Chimica, Pure (> 99.9%). Carbon black was purchased from Alfa Aesar. Titanium plates with 0.1 mm thickness were purchased from Eszkozok Tools (Zhejiang, China) and used as the substrate. Doubly deionized water was used for the preparation of all solutions and Ti plate washing. All of the experiments were performed at 20 °C.

2.2 Electrode preparation

A titanium plate was used as substrate. It was initially pretreated by polishing with different types of sandpaper (Grit No. 100, 400, 600, 1200) and cleaned with double-distilled water to remove sand particles. Afterward, the plate was immersed in a 40% sodium hydroxide solution (50 °C) for 2 h to remove organic residues from the surface, and then it was placed in a slightly boiling 10% oxalic acid, etched for 2 h and washed with distilled water. Finally, in order to avoid the formation of TiO2, the titanium plate was stored in a 1% oxalic acid solution. This treatment should, therefore, strengthen the bond strength between the electrode surface and the oxide coating, improve the conductivity of the titanium plate, and extend the lifetime of the electrode [42, 43]. PbO2 was deposited galvanostatically on the pretreated titanium substrate. The deposition solution (30 mL) was composed of 0.2 M Pb(NO3)2 and 0.1 M KNO3 solution. The deposition processes were carried out at room temperature between 5 and 10 min, and the current density was controlled at 20 mA cm−2. The electrodes were coated in the absence and presence of different concentrations of Fe3+ (0.05 M, 0.1 M, and 0.2 M) and/or carbon black 0.05 g. The fabricated electrodes were marked as Ti/PbO2, Ti/PbO2-0.05Fe, Ti/PbO2-0.1Fe, Ti/PbO2-0.2Fe, Ti/PbO2-0.05C, and Ti/PbO2-0.1Fe-0.05C for 0 M Fe3+ + 0 g C, 0.05 M Fe3+ + 0 g C, 0.1 M Fe3+ + 0 g C, 0.2 M Fe3+ + 0 g C, 0 M Fe3+ + 0.05 g C, and 0.1 M Fe3+ + 0.05 g C, respectively (Table 1), dependent on the electrode fabricated in different concentrations of Fe(NO3)3∙9H2O and 0.05 g carbon black.

Table 1 Name and composition of the prepared electrodes
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2.3 Electrode characterization

The surface morphology of the Ti/PbO2 electrodes was observed by a scanning electron microscopy (SEM, JEOL JSM-IT 100) at an accelerating voltage of 20 kV.

The composition and crystalline structure of the coatings were characterized by Energy Dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD, LABXXRD-6100 SHIMADZU) using Kα radiation (λ = 1.5406 Å) in the 2θ range of 10°–70°. The X-ray tube was operated at 40 kV with a Cu target.

For electrochemical characterization of the prepared films, three-electrode cell configuration is connected to the PGZ 301 electrochemical workstation driven by VoltaMastrer 4 software. Ti/PbO2, Ti/PbO2-0.05Fe, Ti/PbO2-0.1Fe, Ti/PbO2-0.2Fe, Ti/PbO2-0.05C, and Ti/PbO2-0.1Fe-0.05C were used as working electrode with an active surface of 1 cm2, a Ti plate was served as the auxiliary electrode and saturated calomel electrode (SCE) as the reference electrode. The measure of linear sweep voltammetry (LSV) was performed in 0.1 M Na2SO4 solution at the scan rate of 10 mV s−1 from 0 to 3 V. Cyclic voltammetry (CV) was used to study the electrochemical behavior of the prepared electrodes using a 10–2 M RY14 in 0.1 M Na2SO4 solution as the supporting electrolyte and scan rate of 100 mV s−1 and potential range from 0 to 1700 mV. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a range of 100 kHz–0.1 Hz at a potential of 0 V/SCE with a sine wave of 5 mV amplitude.

2.4 Electrocatalytic oxidation performance of the electrodes

The electrocatalytic oxidation activity of the prepared electrodes was evaluated by electrolysis of RY14. The experiments were carried out by the PGZ 301 electrochemical workstation in a 250 mL undivided electrochemical reactor. Each prepared electrode was used to electrolyze a solution containing 0.1 mM of RY14 dye in 0.1 M Na2SO4. The solution was kept under agitation using a magnetic stirrer. The experiments were carried out at room temperature (20 °C) for 120 min. During the experiments, samples were taken from the electrolytic cell every 20 min for UV–VIS analysis (Analytik Jena, Specord 210 plus). The maximum adsorption wavelength of RY14 dye is 410 nm. The color removal efficiency of RY14 in electrochemical oxidation can be calculated as follows [44]:

$$\mathrm{Color removal efficiency }\left(\mathrm{\%}\right)=\frac{{A}_{0}-{A}_{t}}{{A}_{0}}\times 100 \%,$$
(1)

where A0 and At are the absorbance value at an initial time and time t, respectively.

3 Results and discussion

3.1 Surface morphology of Ti/PbO2 electrodes

Figure 1 shows SEMs of PbO2 prepared from solutions containing 0.2 M Pb(NO3)2 and 0.1 M KNO3 at 20 mA cm−2 on the surface of Ti electrode in the absence and presence of Fe3+ and/or C. Figure 1a shows the morphology of prepared Ti/PbO2 in the absence of Fe3+ and C. As is obvious, the morphology of the surface layer of the Ti/PbO2 electrode is rough with typical pyramidal shapes, as well as some damage and cracks have been observed on the surface, which was similar to the earlier reports [33, 38, 45]. Figure 1b–d presents the morphology of lead dioxide deposited on the surface of Ti electrode from the deposition solution containing different concentrations of Fe3+ ions. As can be seen, at low Fe3+ concentration (0.05 M), the electrode lost its pyramidal shape and a uniform structure with well-distributed globular particles arranged in rice shape is deposited on the electrode surface (Fig. 1b). After adding (0.1 M) Fe3+ to the deposition solution, the modified Ti/PbO2-0.1Fe has a higher adherence to the electrode surface than that of sample prepared in the absence of Fe3+ ions and still had the pyramidal structure, but with smaller size, ordered, uniform, and smoother than the unmodified electrode (Fig. 1c). Further increase in Fe3+ concentration up to 0.2 M resulted in smaller, globular-shaped PbO2 particle structures with obvious boundaries (Fig. 1d). Figure 1e shows the influence of adding 0.05 g of carbon black on the morphology of PbO2 electrodeposited on the surface of the Ti electrode. A rice shape structure like that found in Fig. 1b but with smaller size is observed. Figure 1f shows the influence of the addition of a mixture of 0.1 M Fe3+ and 0.05 g of carbon black on the morphology of the PbO2 electrodeposited on the surface of the Ti electrode. A uniform structure is observed consisting of a pileup-pellet pattern with an undefined boundary and the smallest globular particles [28, 39, 46].

Fig. 1
figure 1

SEM images of a Ti/PbO2, b Ti/PbO2-0.05Fe, c Ti/PbO2-0.1Fe, d Ti/PbO2-0.2Fe, e Ti/PbO2-0.05C, f Ti/PbO2-0.1Fe-0.05C electrodes

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The EDS analysis results for the entire electrodes are shown in Fig. 2. From the EDS spectrums, it could be seen that the elements Pb, O, Fe, and C were detected by EDS. Moreover, with the increase of Fe3+ doping amount, the content of Fe element was gradually increased from 0.41 to 1.89% (atom %). Also, the addition of 0.1 M Fe3+ increases the amount of C doping from 12.87 to 33.28% (atom %). From the above results, it was proved that Fe and C elements were successfully and uniformly doped into the PbO2 electrodes. Moreover, no other elements were detected, indicating that the surface of the electrodes was completely covered by the PbO2 layer.

Fig. 2
figure 2

EDS analyses of the prepared electrodes

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3.2 Structure of Ti/PbO2 electrodes

Figure 3 shows XRD patterns of the different modified Ti/PbO2 electrodes. The crystal structure of the Ti/PbO2 electrode prepared in the absence of Fe3+ consisted of a mixture of two common crystallographic structures, namely the forms α- (orthorhombic) and β-(tetragonal) with a higher content of β-PbO2, which can be identified by its intense peaks, namely, (101 at 31.74°), (211 at 49.03°), (200 at 36.2°), and (110 at 25.4°). However, only a single peak α-PbO2 (111 at 28.32°) is observed. After the introduction of Fe3+ in the electrochemical deposition solution, peaks such as β(110) and α(111) disappeared and no new peaks corresponding to Fe were detected. The predominant phases were always β-PbO2 (β(101) and β(211)). The addition of carbon black alone or with Fe3+ (0.1 M) to the electrodeposition solution shows no new peak. It can be said that the doping of PbO2 with Fe/C has not led to the appearance of a new phase. This means that iron and carbon could exist as an amorphous phase. This result was also obtained by Yang et al. who were able to confirm the presence of β-phase only in the XRD analysis of Bi-PbO2 modified with different concentrations of polyethylene glycol (PEG) [39].

Fig. 3
figure 3

XRD patterns of the Ti/PbO2 electrodes

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Table 2 provides information on the grain size evolution calculated from the X-ray line broadening of the (101) reflections using the Debye–Scherrer equation (Eq. 2) [47]:

$$D= \frac{K \lambda }{\beta \mathrm{cos\theta }},$$
(2)

where D is the crystallite size, λ is the X-ray wavelength, β is the full width at half maximum of the peak, and θ is the diffraction angle.

Table 2 Particle size calculated from XRD data
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It can be seen that doping PbO2 with different concentrations of iron and carbon decreases the size of the electrodeposited particles.

3.3 Linear sweep voltammetry (LSV)

A high oxygen evolution potential (OEP) is highly recommended in the development of electrode material for wastewater treatment [48]. The OEP of the electrodes was determined using LSV. Figure 4 shows the LSV of different Ti/PbO2 electrodes in 0.1 M Na2SO4 solution with a scan rate of 10 mV s−1 between 0 and 3 V/SCE at room temperature (20 °C). Based on the polarization curves, the unmodified Ti/PbO2 electrode had an OEP value of 1.61 V/SCE, which was close to other studies [49, 50]. The OEPs of the modified lead dioxide electrode are 1.60 V/SCE, 1.64 V/SCE, 1.60 V/SCE, 1.61 V/SCE, and 1.57 V/SCE, respectively, for Ti/PbO2-0.05Fe, Ti/PbO2-0.1Fe, Ti/PbO2-0.2Fe, Ti/PbO2-0.05C, and Ti/PbO2-0.1Fe-0.05C, indicating that the addition of 0.1 M Fe3+ has slightly increased the OEP of the electrode. Higher OEP contributed to reducing the occurrence of oxygen evolution and enhanced the formation of hydroxyl radicals [38, 46], resulting in increased oxidation efficiency and decreased energy consumption.

Fig. 4
figure 4

LSV curves of different Ti/PbO2 electrodes in 0.1 M Na2SO4 solution, scan rate: 10 mV s−1

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3.4 Cyclic voltammetry

Cyclic voltammetry was used to test the electrocatalytic activity of Ti/PbO2 electrodes for RY14 oxidation. With the cyclic voltammetry technique, the oxidation of the pollutants is attributed to the direct transfer of electrons from the pollutants onto the electrodes. The cyclic voltammograms of 0.1 M Na2SO4 media with and without 10–2 M RY14 on the different prepared electrodes at a scan rate of 100 mV s−1 within a potential range of 0 to 1.7 V/SCE are presented in Fig. 5. As we can see, in the blank electrolyte (Fig. 5a), there is no anodic peak except the one formed by the water discharge for all electrodes. After RY14 dye was added to the blank solution (Fig. 5b), a broad anodic peak at about 1.45 V/SCE appeared. In comparison to the cyclic voltammograms in the blank solution, it was clear that the anodic peak at 1.45 V/SCE depended on the oxidation of RY14 dye. The oxidation potential of RY14 was lower than the oxygen evolution potential, indicating that the oxidation process was due to direct electron transfer.

Fig. 5
figure 5

Cyclic voltammograms of different Ti/PbO2 electrodes measured in a 0.1 M Na2SO4 and b 0.1 M Na2SO4 containing 10–2 M RY14 dye, scan rate: 100 mV s−1

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3.5 Electrochemical impedance spectroscopy (EIS)

Figure 6 shows the Nyquist plots of freshly prepared Ti/PbO2 electrodes electrodes in 0.1 M Na2SO4 solution over a range of 100 kHz–0.1 Hz with a sine wave of 5 mV amplitude. The Nyquist plots show the presence of two semicircles, one in the high-frequency domain describes the electron transfer process, while the second in the low-frequency domain explains the adsorption of the intermediate. As shown in Fig. 6, the impedance of the Ti/PbO2 anode is lower than that of the modified Ti/PbO2 electrode, implying that the element-doped electrode disadvantages the electrochemical reaction process. The impedance parameters were extracted using the equivalent circuit (EC) inserted in Fig. 6. Rs is the solution resistance, Cdl represents the double-layer capacitance, Rct describes the charge transfer resistance, Cads and Rads are the capacitance and resistance generated by the adsorption of the intermediate on the electrode surface.

Fig. 6
figure 6

Nyquist plots of Ti/PbO2 electrodes and equivalent circuit model (the inset) in 0.1 M Na2SO4 solution

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In addition, the parameters of EIS data are adjusted and rearranged in Table 3. In this study, the polarization resistance (Rp) was used to explore the effect of doped elements on the electrochemical reaction process of Ti/PbO2 electrode. This parameter includes all the effects that occur during an electrochemical process. From Table 3, it can be observed that the anodes doped with 0.05 g carbon and 0.05 M Fe3+ had high Rp, with the values of 637.90 Ω cm2 and 456.90 Ω cm2, respectively. In contrast, the other prepared anodes have an Rp close to the PbO2 electrode, especially for the one doped with 0.1 M Fe3+. Moreover, we notice that the polarization resistance decreases significantly by adding 0.1 M Fe3+ to the prepared carbon-doped anode, which demonstrates the positive effect of Iron in promoting the electrochemical reaction process in the presence of carbon.

Table 3 Impedance parameters of Ti/PbO2 electrodes
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3.6 Electrocatalytic oxidation of RY14 dye

The electrocatalytic activity of the Ti/PbO2 electrodes was evaluated via electrocatalytic elimination of RY14 under galvanostatic condition at a current density of 200 mA cm−2 for 120 min in 0.1 M Na2SO4 solution. Figure 7a shows the percentage of RY14 decolorization as a function of time. It can be seen that the decolorization rate reaches more than 90% using Ti/PbO2 and Ti/PbO2-0.1Fe electrodes after 120 min of electrolysis, while it was more than 70% using Ti/PbO2-0.2Fe and Ti/PbO2-0.1Fe-0.05C electrodes. According to these results, it can be said that the Ti/PbO2 and Ti/PbO2-0.1Fe electrodes showed the highest activity for color removal. However, the Ti/PbO2 electrode does not hold well on the electrode, it deteriorates with time. Τhe oxidation of the complex azo dye molecule results in intermediates with low molecular weight like aliphatic and aromatic compounds. Τhey are formed by displacement of the chromophore functional group and subsequent oxidation of the organic compounds to carbon dioxide and organic acids (carboxylic acids) [51]. Hu et al. (2015) reported the electrolysis of acid red Β azo dye (1 g L−1) using Ce-PbO2/C electrode. They found 60.75% decolorization when the applied voltage is 2 V [52]. However, Andrade et al. (2007) found 90% of decolorization of Blue Reactive 19 (25 mg L−1) dye using Fe-doped PbO2 (5 cm2) electrode when applying current density of 50 mA cm2 [53].

Fig. 7
figure 7

a Performance of Ti/PbO2 electrodes for RY14 degradation, b pesudo-first-order kinetics for RY14 oxidation [Conditions: current density: 200 mA cm−2; V = 250 mL; (RY14) = 0.1 mM; electrolyte: 0.1 M Na2SO4]

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The curves of Ln (C0/C) versus time for different prepared anodes are shown in Fig. 7b. From the good linear correlation for all electrodes, the degradation of RY14 followed the pseudo-first-order kinetics according to Eq. 3.

$$\text{C} = {\text{C}}_{0} {\text{e}}^{-{K}_{app}\text{ t}},$$
(3)

where kapp is the apparent kinetics coefficient.

The pseudo-first-order apparent rate constants (kapp) of 0.027 min−1 and 0.020 min−1 were found for Ti/PbO2 and Ti/PbO2-0.1Fe, respectively (Table 4). This indicates that the oxidation power of these two electrodes is higher compared to the others. The better performance for the degradation of RY14 on Ti/PbO2 and Ti/PbO2-0.1Fe can be ascribed to the high oxygen evolution potential, high specific surface area, and good electrical conductivity. These properties allowed the generation of more HO∙-free radicals which lead to an improvement in the decolorization ability of these anodes [54].

Table 4 The kinetics for the electrochemical degradation of RY14 (electrolysis time: 2 h)
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4 Conclusion

Fe and/or C-doped Lead dioxide electrodes were prepared onto the pretreated Ti plate by anodic oxidation of solutions containing Fe3+ and/or C to Pb2+ in the electrodeposition bath. SEM/EDS and XRD tests show that Fe and/or C doping in lead dioxide films can decrease the crystal size of the anodes and increase their specific surface area. The highest OEP is found for the Ti/PbO2-0.1Fe electrode. In the CV test, the presence of the anodic peak for all the prepared electrodes suggests that the oxidation of the RY14 dye was easily achieved on the surface of those anodes studied. Furthermore, the resistance to polarization is found to decrease significantly by adding 0.1 M Fe3+ to the prepared carbon-doped anode, indicating the positive effect of iron in promoting the electrochemical reaction process in the presence of carbon. The use of pure and Fe3+ (0.1 M)-doped PbO2 electrodes in the decolorization of the RY14 dye proved to be good candidates for the mineralization of the RY14 dye, since more than 90% mineralization was achieved after 2 h of electrolysis.