Introduction

With the rapid development of textile and printing leather industry, a large amount of printing and dyeing wastewater is discharged into the water environment every year [1, 2]. Most dyes are persistent organic pollutants, characterized by high concentration and high toxicity [3, 4]. So, effective control and treatment of the dye wastewater has become an urgent problem to be solved. Methyl orange is a complex azo organic compound, widely used in printing and dyeing, and textile and material coloring as a coloring agent. Methyl orange has good stability and is not easily degraded under natural conditions, so it is necessary to expand its degradation methods [5]. Traditional treatment methods for dye wastewater mainly include coagulation, biological oxidation, adsorption, ion exchange, and chemical oxidation [6,7,8]. However, meeting the more and more strict discharge standard of dye wastewater treatment is difficult for these methods.

Recently, electrochemical advanced oxidation processes (EAOPs) have gained increasing attention as a promising method for wastewater treatment [9,10,11,12,13,14,15,16]. It has the advantages of strong oxidation ability, good environmental compatibility, high energy efficiency, and cost effectiveness. Anodic oxidation (AO) is the simplest EAOP; it includes direct anodic oxidation by electron transfer to the anode (M) surface and indirect oxidation by the powerful physisorbed M(∙OH) at the electrode surface produced by Eq. (1) [17]. And the organics are oxidized to CO2 by the highly reactive M(·OH) (2). The efficiency of AO is highly dependent on the mass transfer of pollutants from the bulk to the electrode surface or its vicinity [18]. Besides, the electrode material nature also has strong influence on both efficiency and selectivity of AO. There are two types of electrode materials used for AO, the active electrodes and the non-active electrodes; the former one has low oxygen evolution potential (OEP), in which the M(∙OH) is transformed into a higher state oxide or superoxide that in combination with the anode surface M acts as a selective mediator in the oxidation of organics, and the latter one has high OEP, in which M(∙OH) are so weakly physisorbed at the electrode surface that can react with organics. As a general rule, the higher the potential for oxygen evolution of the electrode material, the weaker the interaction of M(∙OH) with the electrode surface and the higher the chemical reactivity toward organics oxidation [17].

$$ \mathrm{M}+{\mathrm{H}}_2\mathrm{O}\rightleftarrows \mathrm{M}\left(\cdotp \mathrm{OH}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$
(1)
$$ \mathrm{Organics}+\mathrm{M}\left(\cdotp \mathrm{OH}\right)\rightleftarrows {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O} $$
(2)

The typical active electrodes include platinum (Pt), graphite, ruthenium dioxide (RuO2), and iridium dioxide (IrO2); generally speaking, their potentials for O2 evolution are lower than 1.8 V/SHE. On the other hand, lead dioxide (PbO2), tin dioxide (SnO2), and boron-doped diamond (BDD) can be considered non-active electrodes, whose potentials of O2 evolution are from 1.7 to 2.6 V/SHE. The BDD electrode is the most effective non-active electrode at present [19, 20], which is considered to be the most suitable electrode for AO. However, its application is severely restricted by its high cost. As a promising electrode material, PbO2 has gained more and more attention because of its high electrical conductivity, strong oxidation ability, large surface area, and low cost [21].

Compared with those by other chemical methods, the PbO2 layers prepared by electrodeposition have higher electrochemical activity. Meanwhile, the morphology and crystalline structures of electrodeposited PbO2 are related to many factors, such as the type of substrate, electrodeposition process, pH of the electrodeposition solution, and type of dopants. The common substrates used for the deposition of PbO2 films mainly include Ti, Pb, Pt, Ta, C, and TiO2 [22,23,24,25,26,27,28,29,30,31]. Titanium was used as the substrate in this study due to its natural corrosion resistance; in addition, Ti is also preferred because it strongly passivates under high positive potentials in the case of any local failure of the PbO2 layer during electrolysis.

Recently, a lot of researches afford to increase the chemical stability and electrocatalytic capacity of PbO2 by doping modification. Some metal ions like nickel, iron, neodymium, bismuth, cobalt, and cerium have been used for modification of PbO2 electrode as dopants by the electrodeposition method [4, 32,33,34]; besides, some metal oxides were also doped into PbO2 electrodes such as CeO2, ZrO2, Sm2O3, TiO2, and Ga2O3 [35,36,37,38,39,40,41,42,43,44,45,46,47,48]. Cerium is a promising candidate to modulate the electrocatalytic properties of oxide electrodes due to its high Ce(III)/Ce(IV) redox potential.

The aim of this work focuses on the investigation of electrochemical activity and stability of lead dioxide electrode co-doped with cerium and neodymium, i.e., Ti/Ce-Nd-PbO2. The electrochemical activities of the modified electrode were investigated and compared with Ce-PbO2, Nd-PbO2, and undoped PbO2 electrodes. Different doping ratios of cerium and neodymium were studied and the optimum ratio was determined. Analysis techniques such as SEM, XRD, and XPS were used to characterize the modified electrode. Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and accelerated life test were also used to study the electrochemical properties of Ti/Ce-Nd-PbO2. Moreover, UV-visible and COD were used to investigate dye removal and mineralization. The anodic oxidation of methyl orange as a model dye wastewater was examined to determine the optimum operating parameters of the Ti/Ce-Nd-PbO2 electrodes for degradation of organic pollutants.

Experiment and methodology

Materials

All the reagents and materials used in this work were of analytical grade without further purification. Cerium and neodymium tri-nitrate, lead and copper di-nitrate, sodium sulfate, oxalic acid, nitric acid, sodium hydroxide, lead oxide, sulfuric acid, methyl orange, and sodium fluoride were obtained from Aladdin Reagent Company. Distilled water (DI water) was used throughout the work.

Electrode preparation and characterization

The main preparation procedure is shown in Fig. 1. High-purity (99.7%) titanium sheets (2.0 cm × 3.0 cm × 0.08 cm) were used as the substrate. Firstly, titanium sheets were mechanically polished by abrasive paper, then were rinsed in ethanol and DI water by ultrasonic washing, and after that, were cleaned by degreasing in 40 wt% NaOH solution at 358.15 K for 30 min and etching in boiled 10 wt% oxalic acid for 1 h to attain the uniform roughness on the surface of electrodes. Finally, the titanium sheets were rinsed carefully using DI water with ultrasonic [49].

Fig. 1
figure 1

Preparation procedure of Ti/SnO2-Sb2O3/α-PbO2/β-PbO2 electrode

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After pretreatment, an interlayer of Ti/SnO2-Sb2O3 and α-PbO2 between the Ti substrate and surface active layer was prepared to improve the stability and extend the life of PbO2 electrodes. In this step, dip-coating and thermal deposition of SnCl4∙5H2O, SbCl3, n-butanol, and hydrochloric acid with a molar ratio of Sn:Sb = 8:1 were used. The pretreated titanium sheets were painted, dried, and sintered at 773 K in a muffle furnace. This same procedure was repeated ten times. The SnO2-Sb2O3 interlayer was finally obtained by last baking at 773 K for 1 h. Then, an interlayer of α-PbO2 was electrodeposited using the Ti/SnO2-Sb2O3 as an electrode and a Pt plate as a cathode in alkaline solution (0.1 mol L−1 PbO, 3.5 mol L−1 NaOH) at 313.15 K and a current density of 10 mA cm−2 for 2 h. Finally, the surface active layer β-PbO2 was further electrodeposited on the above-prepared Ti/SnO2-Sb2O3/α-PbO2 substrate in acidic solution that consisted of 0.2 mol L−1 Cu(NO3)2, 40 mmol L−1 NaF, 0.5 mol L−1 Pb(NO3)2, and 0.1 mol L−1 HNO3 for pure PbO2 electrode, whereas 0.3 g L−1 of Ce(NO3)3 and Nd(NO3)3, respectively, was used for Ce- and Nd-doped PbO2 electrodes. Furthermore, the nominal compositions of Ce:Nd = 1:1, 1:2, 2:1, 1:5, 5:1, respectively, were used for the electrodeposition of Ce-Nd co-doped PbO2 electrodes, and the total concentration of Ce and Nd was 0.3 g L−1. In addition, pure PbO2 and single-doped and co-doped PbO2 electrodes were all deposited within 2 h at 333.15 K and 20 mA cm−2 current density.

A scanning electron microscope (SEM; Hitachi-SU8010, Japan) was used to characterize the morphology of the surface. This instrument incorporated an energy-dispersive X-ray analysis (EDX) for elemental analysis. X-ray diffraction (XRD) was used to examine the crystalline structures of the modified PbO2 electrodes, which were recorded on an X-ray diffraction instrument (D8Focus, Bruker AXS, Germany) under Cu Kα radiation (45 kV, 30 mA) with a scanning range from 20° to 80° at the speed of 8°/min. X-Ray photoelectron spectroscopy (XPS, PHI5000VersaProbe, Ulvac-Phi) was used to determine the atomic valence state by using the Mg Ka X-ray source with a base vacuum operated at 300 W.

The electrochemical characterization of the modified PbO2 electrodes was performed by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements using an electrochemical workstation (CHI660E, China) with standard three-electrode configuration. PbO2, platinum, and saturated calomel electrodes (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively. The LSV measurements were made in 0.5 mol L−1 Na2SO4 solution. The curves were recorded from 0 to 2.0 V with a scanning rate of 5 mV/s, and the potential corresponding to the inflection point of polarization curve was defined as the oxygen evolution potential (OEP) of the electrodes. The EIS measurements were made in 0.5 mol L−1 Na2SO4 solution, and the open-circuit potential was used as the measurement potential. The frequencies swept were from 100 KHz to 100 mHz with an applied sine wave of 10-mV amplitude. The accelerated life of the modified PbO2 electrodes was tested in 1.0 mol L−1 H2SO4 at j = 2.0 A cm−2 and 333 K to assess the stability of electrodes.

Experiment method and analysis

An open glass reactor of 100-mL capacity connected with a DC power supply (IT6332L, ITECH, China) was used for the electrochemical oxidation experiments of methyl orange. The different modified PbO2 electrodes were used as anodes while the Ti plate was used as the cathode, and the interelectrode gap was kept at 1.0 cm in our experiments. A 100 mL solution of 60 mg L−1 methyl orange was placed in the reactor under continuous stirring of 400 rpm on a magnetic stirrer [50]. Samples were analyzed after being drawn at certain intervals. The AO process was optimized with the Ce-Nd-PbO2 electrode at different parameters such as current density (j) (5–30 mA cm−2), initial methyl orange concentration (C0) (30–120 mg L−1), and initial solution pH (2–11), respectively, at room temperature. The 0.5 mol L−1 H2SO4 and NaOH solutions were used to adjust the pH of the solution in the experiment.

UV-Vis absorption spectra of methyl orange solution were recorded on UV-Vis spectrophotometer (Lambda 35, PerkinElmer) at 465 nm. And the COD was measured by the COD detector (6B-200, SHENGAOHUA, China). The MO and COD removal can be calculated via Eqs. (3) and (4), respectively.

$$ \mathrm{MO}\ \mathrm{removal}=\frac{A_0-{A}_t}{A_0}\times 100\% $$
(3)

A0 and At are the initial and final absorbances of MO.

$$ \mathrm{COD}\ \mathrm{removal}=\frac{CO{D}_0- CO{D}_t}{CO{D}_0}\times 100\% $$
(4)

COD0 and CODt are the initial and final concentrations of COD (mgO2 L−1).

The average current efficiency (ACE, ζ) was calculated by Eq. (5) from the COD results [51].

$$ \varsigma =\frac{\left({\mathrm{COD}}_0-{\mathrm{COD}}_t\right)\cdot F\cdot V\mathrm{L}}{8I\cdot \varDelta t\cdot 1000}\times 100\left(\%\right) $$
(5)

where COD0 and CODt are the initial and final concentrations of COD (mgO2 L−1), F is the Faraday constant (96,487 C mol−1), VL is the volume of electrolyte solution (L), ∆t is the degradation time (s), and I is the current intensity (A).

A well-fitted pseudo-first-order kinetic of MO dye degradation was determined by using the following expression:

$$ \ln \left(\frac{C_0}{C_t}\right)= kt $$
(6)

where C0 and Ct are the initial and final absorbances of MO, and k is the reaction rate constant.

Results and discussion

Effect of Ce(NO3)3 and Nd(NO3)3 concentrations in deposition bath on electrocatalytic activity of PbO2 electrodes

Initial experiments were carried out to optimize the proportion of Ce and Nd in the electrodeposition bath of Ce-Nd co-doped PbO2 electrodes in terms of MO and COD removal. The total concentration of Ce(NO3)3 and Nd(NO3)3 was 0.3 g L−1, and Ce:Nd = 1:1, 1:2, 2:1, 1:5, and 5:1 were defined as Ce-Nd-PbO2-1, Ce-Nd-PbO2-2, Ce-Nd-PbO2-3, Ce-Nd-PbO2-4, and Ce-Nd-PbO2-5, respectively. The electrolysis processes were performed at the condition of 60 mg L−1 MO, room temperature, 20 mA cm−2 current density, 0.15 mol L−1 Na2SO4 concentration, and pH = 7. Figure 2 shows the effect of different concentrations of Ce(NO3)3 and Nd(NO3)3 in electrodeposition bath on MO removal, as well as the kinetics of degradation. As can be seen in the graph, a linear relationship between removal efficiency as a function of treatment time depicted by MO on different electrodes followed the pseudo-first-order kinetic. Compared with the PbO2 electrode, single-doped and different proportions of Ce-Nd co-doped PbO2 electrodes have been observed to show the higher MO removals in this chart. Furthermore, the Ce-Nd-PbO2-2 electrode, which has a content of 0.1 g L−1 Ce(NO3)3 and 0.2 g L−1 Nd(NO3)3 in the electrodeposition bath, shows the highest MO removal in the shortest time. About 99.8% was formed with Ce-Nd-PbO2-2 electrode after 70 min of electrolysis compared with 98.7%, 99.1%, and 97.1% with Ce, Nd-PbO2, and PbO2 electrodes, respectively. The COD removal and the average current efficiency are shown in Fig. 3; similar to the result of the MO removals, higher COD removals were shown with all of the doped PbO2 electrodes over the undoped PbO2 electrode, and the Ce-Nd-PbO2-2 electrode also shows the highest COD removal, as well as the highest average current efficiency. Almost 100% COD removal was reached with Ce-Nd-PbO2-2 electrode after 180 min of electrolysis, which was higher compared when using the other electrodes, such as 91.8% with the Ce-PbO2 electrode, 90.2% with the Nd-PbO2 electrode, and 83.4% with the undoped PbO2 electrode. For MO removal, COD removal, and the average current efficiency, it was reasonable to conclude that Ce and Nd co-doping under suitable conditions helps to improve the electrode performance, and the Ce-Nd-PbO2–2 electrode that contains 0.1 g L−1 Ce(NO3)3 and 0.2 g L−1 Nd(NO3)3 can be chosen as the best Ce-Nd co-doped PbO2 electrode for methyl orange degradation. The Ce-Nd-PbO2–2 electrode was redefined to Ce-Nd-PbO2 electrode used in the later experiments.

Fig. 2
figure 2

Effect of different concentrations of Ce(NO3)3 and Nd(NO3)3 in electrodeposition bath on MO removal and the kinetics of degradation at 60 mg L−1 MO, room temperature, 20 mA cm−2 current density, 0.15 mol L−1 Na2SO4 concentration, pH = 7, and 70-min treatment time

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Fig. 3
figure 3

Effect of different concentrations of Ce(NO3)3 and Nd(NO3)3 in electrodeposition bath on COD removal and the ζ versus treatment time at 60 mg L−1 MO, room temperature, 20 mA cm−2 current density, 0.15 mol L−1 Na2SO4 concentration, pH = 7, and 180-min treatment time

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Morphologies and crystal structure of PbO2 electrodes

The SEM micrographs of Ti/PbO2, Ti/Ce-PbO2, Ti/Nd-PbO2, and Ti/Ce-Nd-PbO2 electrodes are shown in Fig. 4. PbO2 surface had a pyramidal cluster-like morphology with many cracks. This structure may be caused by the dropping of PbO2 from Ti substrate which may shorten the service life of PbO2 electrodes [45]. The Nd and Ce-PbO2 were found to be quite similar, and their crystal particles were smaller than that of PbO2 electrode. However, the morphology of PbO2 and Ce-Nd-PbO2 electrode was also found to be slightly similar, but the crystal of Ti/Ce-PbO2 electrode was smaller and the structure was more compact than that of no-doped and single-doped PbO2 electrodes. The doping of Ce and Nd into PbO2 significantly affected the film morphology, and the Ce-Nd-PbO2 electrode has a higher catalytic activity because of its larger effective area than the other PbO2 electrodes. Furthermore, the surface of PbO2 electrode that was doped with Ce and Nd became denser and smoother, and that would be favorable to prevent reactive oxygen species from diffusing into the internal layer in the process of electro-oxidation, improving the stability and the service life of electrodes [37].

Fig. 4
figure 4

SEM images of different PbO2 electrodes: a Ti/PbO2, b Ti/Ce-PbO2, c Ti/Nd-PbO2, and d Ti/Ce-Nd-PbO2 electrodes

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Figure 5 shows the EDX results of Ti/PbO2 (Fig. 5a) and Ti/Ce-Nd-PbO2 (Fig. 5b) electrodes. It can be found that the real atomic percentage of Ce in the surface of Ti/Ce-Nd-PbO2 electrode was 1.77%, and the Nd was 1.899% (data not shown).

Fig. 5
figure 5

EDX images of a PbO2 electrode and b Ce-Nd-PbO2 electrode

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Figure 6 shows the XRD patterns of the different modified PbO2 electrodes, which are in good arrangement with the JCPDS file (card number 75-2417). No diffraction peak corresponding to CeO2 or Nd2O3 was observed; this may be due to the low content of Ce and Nd. By the way, the main diffraction peaks of the electrodes were β-PbO2. The peaks of doped PbO2 electrodes are identical to that of pure PbO2 without any shift, meaning that the addition of Ce and Nd would not change the phase of the PbO2 electrode. However, compared to the pure PbO2 electrode, the intensities of some peaks of doped electrodes were significantly changed, such as the typical peak at 36.9° which can be found stronger with the dope of Nd and most peaks of Ce-Nd-PbO2 electrode were significantly smaller, suggesting the existence of Ce and Nd onto modified PbO2 electrode and this may have an effect on the growth and structure of the crystal.

Fig. 6
figure 6

X-Ray diffraction patterns of different modified PbO2 electrodes

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In order to further delve into the chemical composition of the different PbO2 electrodes, XPS of the doped PbO2 electrodes was conducted as shown in Fig. 7. The Pb 4f spectrum (Fig. 7a) presents two well-defined and symmetric peaks centered at 137.5 eV and 142.6 eV, which can be attributed to Pb 4f5/2 and Pb 4f7/2, respectively, in agreement with the spectral values for PbO2. The O1s spectrum (Fig. 7b) fitted into a sharp peak at 529.1 eV, which corresponded to strongly bound (lattice) oxygen. Moreover, the binding energies of Ce 3d5/2 and Ce 3d3/2 are located at 895.2 and 935.2 eV (Fig. 7c), which indicated that Ce ion of CeO2 was successfully introduced onto the surface of PbO2 electrode. The peaks at 978.68 eV and 998.93 eV corresponding to Nd 3d5/2 and Nd 3d3/2, which illustrates that the Nd atoms were also successfully introduced at PbO2 surface, and the Nd 3d5/2 peak at 978.68 ev suggested a good correlation with Nd3+ oxidation state in Nd2O3.

Fig. 7
figure 7

XPS spectra of different modified PbO2 electrodes: a Pb 4f; b O1s; c Ce 3d; d Nd 3d

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Generally speaking, the stability of electrode is an important property in practical application, and it is usually influenced by the dropping of PbO2 from Ti substrate due to the preparation conditions. In addition, the stability of PbO2 electrode can be measured by the accelerated life test. Figure 8 shows the accelerated life of the modified PbO2 electrodes which were tested in 1.0 mol L−1 H2SO4 at j = 2.0 A cm−2 and 333 K. The electrolysis time when the potential reached 10 V was regarded as the service life of the electrode. As can be seen in the chart, the service lifetimes of Ce-PbO2, Nd-PbO2, Ce-Nd-PbO2, and the pure PbO2 electrodes were 87, 82, 95, and 76 h, respectively, which is in good agreement with the previous research.

Fig. 8
figure 8

Accelerated lifetime test in 1.0 mol L−1 H2SO4 at j = 2.0 A cm−2 and 333 K

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Electrochemical measurements

Figure 9 shows the linear polarization curves obtained in 0.15 mol L−1 Na2SO4 using Ti/PbO2, Ti/Ce-PbO2, Ti/Nd-PbO2, and Ti/Ce-Nd-PbO2 electrodes. The intersection of the two almost linear regions at low and high potentials was defined as the onset potential of the electrodes. As observed in the graph, the onset potential of the Ce-Nd-PbO2 electrode was 2.17 V, which is much higher than that of PbO2 (1.6 V), Ce-PbO2 (1.8 V), and Nd-PbO2 (1.65 V). As a general rule, the higher the onset potential of the electrode material, the weaker the interaction of M(∙OH) with the electrode surface and the higher the chemical reactivity toward organics oxidation [17]. On the other hand, high onset potential can effectively reduce energy consumption [52]. Therefore, the Ce-Nd-PbO2 electrode has the highest chemical reactivity toward organics oxidation.

Fig. 9
figure 9

Linear sweep voltammetry of the different modified PbO2 electrodes in 0.15 mol L−1 Na2SO4, scan rate = 100 mV/s

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The electrochemical impedance spectroscopy measurements were undertaken to study the electrochemical properties of the different doped electrodes. Figure 10 presents the Nyquist plots for these doped electrodes, and EIS simulation results were obtained by fitting experimental data using an equivalent circuit model. In this circuit, Rs presents the solution resistance, Q1 (CPE1) is the constant phase element for double-layer, and Rct is the charge transfer resistance. For the Warburg impedance (W), due to mass transfer of ions in surface film, its impedance behavior is displayed in the low frequency.

Fig. 10
figure 10

Electrochemical impedance spectroscopy of different modified PbO2 electrodes in 0.15 mol L−1 Na2SO4

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From the EIS Nyquist plots, we can see that pure PbO2 electrodes display the biggest arc radius, and Ce-Nd-PbO2 electrodes display the smallest arc radius among all the electrodes, suggesting the highest charge transfer resistance across pure PbO2, which signifies that the interfacial charges transfer more rapidly when modified PbO2 with Ce and Nd.

Optimization of process parameters

Effect of initial pH

Some studies have shown the important role that solution pH plays in the anodic oxidation of dye degradation, but the results are so different due to the different nature of pollutants and electrode materials [53,54,55,56,57]. The effect of initial pH values ranging from 2 to 11 on MO degradation is presented in Fig. 11. H2SO4 and NaOH solutions were used for pH adjustments. It can be seen from the figure that the MO removals were increased by increasing the pH from 2.0 to 7.0; then, as the pH increases, it shows a downward trend. So, pH = 5 was chosen as a suitable pH for the further studies.

Fig. 11
figure 11

Effect of initial pH on the MO degradation at 20 mA cm−2 current density, 60 mg L−1 initial concentration, 0.15 mol L−1 Na2SO4 concentration, and 60 min of electrolysis time

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Effect of applied current density

Figure 12 shows the effect of current density from 5 to 30 mA cm−2 on MO removal as a function of treatment time. One can see that the MO removals were dramatically increased as the current density increases from 5 to 20 mA cm−2; however, when the current density increases from 20 to 30 mA cm−2, a slight effect of MO removal was observed. In consideration of electrolyte consumption and MO removal, the 20 mA cm−2 current density was chosen for the further studies.

Fig. 12
figure 12

Effect of initial current density on the MO degradation at initial pH = 5.0, 60 mg L−1 initial concentration, 0.15 mol L−1 Na2SO4 concentration, and 60 min of electrolysis time

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Effect of initial dye concentration

The effect of initial MO concentration on the degradation is shown in Fig. 13. As can be seen from the figure, less time was spent to achieve a high removal as the initial MO concentration increases from 30 to 120 mg L−1. The initial MO concentration of 60 mg L−1 was used in the studies.

Fig. 13
figure 13

Effect of initial concentration of MO on the degradation at initial pH = 5.0, 20 mA cm−2 current density, 0.15 mol L−1 Na2SO4 concentration, and 60 min of electrolysis time

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Conclusions

In this paper, a Ti/SnO2-Sb2O3/α-PbO2/Ce-Nd-β-PbO2 composite electrode was successfully prepared by the electrodeposition method. The proportion of Ce and Nd in the electrodeposition bath of Ce-Nd co-doped PbO2 electrodes was optimized to 0.1 g L−1 Ce(NO3)3 and 0.2 g L−1 Nd(NO3)3 in terms of the MO and COD removals. The structure of Ce-Nd co-doped PbO2 electrode was compared with those of the undoped and single-doped PbO2 electrodes, and the crystal size of Ce-Nd-PbO2 electrode was found to be smaller than the other PbO2 electrodes from the results of SEM and XRD. The images of EDX and XPS have indicated that Ce and Nd were perfectly incorporated onto the surface of the PbO2 electrode. The Ce-Nd-PbO2 electrode showed the highest activity for the methyl orange by electrochemical oxidation. The MO and COD removals after 70 min of electrolysis were 99.8% and 100%, respectively. In addition, the Ce-Nd-PbO2 electrode has the advantage of higher current efficiency and lower energy consumption.

Many studies have proven the doping of Ce or Nd in PbO2 electrode can change the crystal structure and improve the electrochemical activity [4, 22, 33, 35, 36]. In this study, the Ce-Nd co-doped PbO2 electrode has shown better catalytic activity, and the improved performance of the modified electrode may be due to the dopants that changed the crystallite size and slowed down the deposition rate of PbO2 crystal, but the specific mechanism needs further study. In conclusion, the Ce and Nd co-doped PbO2 electrode can be considered a promising electrode for electrochemical oxidation of organic pollutants due to its high electrocatalytic activity and long service life.