Elaboration of Highly Modified Stainless Steel/Lead Dioxide Anodes for Enhanced Electrochemical Degradation of Ampicillin in Water

Abstract

Lead dioxide-based electrodes have shown a great performance in the electrochemical treatment of organic wastewater. In the present study, modified PbO₂ anodes supported on stainless steel (SS) with a titanium oxide interlayer such as SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10% Boron (B) were prepared by the sol–gel spin-coating technique. The morphological and structural properties of the prepared electrodes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that the SS/TiO₂/PbO₂-10% B anode led to a rougher active surface, larger specific surface area, and therefore stronger ability to generate powerful oxidizing agents. The electrochemical impedance spectroscopy (EIS) measurements showed that the modified PbO₂ anodes displayed a lower charge transfer resistance R_ct. The influence of the introduction of a TiO₂ intermediate layer and the boron doping of a PbO₂ active surface layer on the electrochemical degradation of ampicillin (AMP) antibiotic have been investigated by chemical oxygen demand measurements and HPLC analysis. Although HPLC analysis showed that the degradation process of AMP with SS/PbO₂ was slightly faster than the modified PbO₂ anodes, the results revealed that SS/TiO₂/PbO₂-10%B was the most efficient and economical anode toward the pollutant degradation due to its physico-chemical properties. At the end of the electrolysis, the chemical oxygen demand (COD), the average current efficiency (ACE) and the energy consumption (EC) reached, respectively, 69.23%, 60.30% and 0.056 kWh (g COD)⁻¹, making SS/TiO₂/PbO₂-10%B a promising anode for the degradation of ampicillin antibiotic in aqueous solutions.

1. Introduction

The presence of antibiotics in the aqueous ecosystem represents nowadays an emerging concern since it can be a real threat on human health, aquatic life, and plants [1]. The main sources of the pharmaceuticals detected in the environment are domestic sewage, hospital effluents and pharmaceutical manufacturing companies [2]. Therefore, it is necessary to develop an effective treatment to remove these emerging organic pollutants from wastewaters in order to prevent their presence in natural water receptors.

Over the last few decades, advanced oxidation processes (AOPs) were proposed as the most promising technologies for antibiotic removal from the environment since the conventional methods are no more efficient. Anodic oxidation (AO) stands out among the most promising AOPs to treat refractory pharmaceutical compounds owing to its simple, safe, economic and environmentally compatible technology [3,4,5,6].

The AO process is based on the degradation of persistent organic pollutants with highly reactive hydroxyl radicals ($O{H}^{●}$) produced at the anode surface [7,8,9]. The efficiency of the generated hydroxyl radicals depends mainly on the anode materials in terms of composition and morphology [10,11]. Lead dioxide (PbO₂) is among the widely used anodes for wastewater treatment thanks to its high electrical conductivity, good stability, low cost and long service lifetime [8,12,13]. However, the fragile binding force and the low stability of PbO₂ coating may lead to lead ions leaching during the electrolysis process, or it could even peel off of the coating from the substrate surface, affecting therefore the lifetime of the electrode and limiting its practical application [14].

Therefore, further improvements are required to enhance the electrode efficiency for organic pollutants degradation. Over the past decades, research focused on PbO₂ coating modification and its subsequent effects on the active layer performance [15,16,17,18].

Conventional PbO₂ anodes were modified by the introduction of a conductive intermediate layer which contributed to prolonging the service life, minimizing the lead ions leaching and improving the oxidation effectiveness and current efficiency of lead dioxide electrodes [19,20,21]. For example, silicon dioxide (SiO_x) thin film was used by Elaissaoui et al. as an interlayer in order to improve the SS/PbO₂ anode efficiency and stability for dye removal [22]. Duan and co-workers reported that the deposition of a carbon nanotube interlayer increased the service lifetime and promoted the oxidation capacity of pure PbO₂ electrodes for 4-chlorophenol degradation [23].Tin dioxide (SnO₂) is also among the added metal oxides allowed to reach high degradation rates [24,25] as well as manganese dioxide (MnO₂) [26] and titanium dioxide (TiO₂) [27,28,29] interlayers used to improve the electrochemical oxidation treatments of refractory organic wastewater, respectively, by strengthening the bonding between the substrate and the active outer layer which increases, therefore, the anode’s electrical conductivity.

Furthermore, doping the PbO₂ layer has also been extensively adopted as an effective modification method to promote the electrocatalytic characteristics and improve the stability of PbO₂-based electrodes [19]. Several elements such as indium [30], cobalt [31], copper [32] and aluminum [33] were used as dopants to enhance the oxidizing power of lead dioxide anode since dopant introduction provides a larger active surface area, higher oxygen evolution potential, and lower charge transfer resistance [8,15].

However, to the best of our knowledge, there are no reports about PbO₂ anodes modified by boron (B). Only boron-doped diamond (BDD) electrodes were used, in recent years, and considered as an optimal electrode material for the electrochemical oxidation of organic pollutants in the wastewater system thanks to their physical and chemical properties [34,35] and the distribution of sp² carbon impurities on the anode surface that influences the electrocatalytic properties [36].

The aim of the current research is to improve the electrocatalytic activity and stability of the PbO₂ electrode by (i) the introduction of a TiO₂ inner layer between the stainless steel substrate and the outer coating and (ii) the combination of the high activity of boron and the lead oxide in order to develop SS/TiO₂/PbO₂-B anode using sol–gel spin coating. This study aligns with the overall goal of the InTheMED project aiming to develop and to implement low-cost sustainable remediation strategies. The main purposes of this work were (i) to remove residual antibiotics from wastewater, (ii) to contribute to increasing the safe reuse of treated effluent and (iii) to preserve the MED aquifers to mitigate anthropogenic threats in changing climate.

The physicochemical and morphological properties of SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-B electrodes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The understanding of electric properties was performed by the electrochemical impedance spectroscopy measurements (EIS) and compared with those of the conventional SS/PbO₂ anodes. Ampicillin (AMP) was selected as a pharmaceutical pollutant model since it is one of the most commonly detected antibiotics in wastewater in several countries [37]. According to recent studies, AMP was detected in Kenya [38] and Czech Republic wastewater at a concentration of 28.09 ng/L [39], and also, concentrations ranging from 0.012 to 32.57 μg/mL were reported in Pakistan [40]. The electrocatalytic activity of the developed anodes was investigated during the anodic oxidation of AMP in synthetic wastewater. The mineralization of the organic matter and the concentration of ampicillin were, respectively, evaluated by chemical oxygen demand and high-performance liquid chromatography analysis.

2. Materials and Methods

2.1. Chemicals

Titanium isopropoxyde (TTIP, Ti(OCH(CH₃)₂)₄, Sigma-Aldrich, 97% purity) was used as a precursor for TiO₂ interlayer deposition. Lead(II) acetate trihydrate (Pb(C₂H₃O₂)₂, Assay ≥ 99%,379.33 g mol⁻¹), methacrylic acid (MAA: H₂C=C(CH₃)COOH, Assay ≥ 99%, 86.09 g mol⁻¹), hydrochlorhydric acid (HCl, 37%, 36.46 g mol⁻¹) and 1-propanol (CH₃CH₂CH₂OH, Assay ≥ 99.7%, 60.10 g mol⁻¹) were used for PbO₂ deposition. Boric acid (H₃BO₃, Sigma-Aldrich, ≥99.5% purity, 61.83 g mol⁻¹) was used as dopant. Sodium sulfate (Na₂SO₄, 142.04 g mol⁻¹, LOBA chemie) was used as supporting electrolyte during the electrochemical measurements and the degradation experiment. Potassium dihydrogen phosphate (KH₂PO₄, 136.086 g mol⁻¹) and methanol (HPLC grade) were used for chromatographic analysis. Acetone (CH₃COCH₃, 58.01 g mol⁻¹) was used to clean the substrates. Ampicillin antibiotic (C₁₆H₁₈N₃NaO₄S, FLUKA Chemie, 371.39 g mol⁻¹) was used as an organic pollutant.

2.2. Preparation of Modified PbO₂ Electrodes

Stainless steel-AISI 304 plates were used as substrates. The electrodes were, first, polished with abrasive paper with different roughness ranging from 320 to 1200, degreased in acetone for 10 min using an ultrasonic bath (Ultrasonic Batch-FALC), rinsed with distilled water and finally dried under nitrogen flow.

The PbO₂ electrodes were prepared by the sol–gel spin-coating technique. Concerning the titanium oxide interlayer, TiO₂ sol gel solution was prepared by mixing 2 mL of methacrylic acid and 1 ml of titanium isopropoxyde for 5 min; then, 0.5 mL of propanol was added to the mixture and stirred for 10 min. Finally, 0.9 mL of 0.37 M HCl was added, and the solution was stirred for 1 h. The TiO₂ interlayer was spin coated at 1000 rpm during one minute and annealed at 600 °C for 1 h in order to evaporate the solvents.

The PbO₂ solution was synthesized by dissolving 1.28 g of lead acetate in methacrylic acid, 1-propanol and 0.37 M HCl. The solution was stirred for 1 h. For the boron (B)-doped anode, 0.023 g of boric acid (H₃BO₃) was added with 10% mass percentage during the first step of Pb precursor solution formulation. The B doping rate was chosen based on the work of Boukhchina et al. [41]. Small droplets of the un-doped and doped PbO₂ were deposited on a TiO₂ layer that was then spun at 2000 rpm for 60 s. Five thin films of PbO₂ were deposited layer by layer to obtain a final thickness around 1000 nm, and the coating was finally dried at 200 °C for 24 h on a hot plate as described in Figure 1. In order to obtain the optimized thickness of the TiO₂ and PbO₂ layer, the rotation speed, the number of the deposited layers and the annealing temperature were varied, and the thicknesses of the films were measured after each test in order to determine the appropriate parameters [42].

2.3. Electrode Characterizations

2.3.1. Electrode Morphology and Composition

The morphology of the deposited films was examined by scanning electron microscopy (SEM, FEI quantum 400 FEG Electron Microscope). An energy-dispersive X-ray analysis (EDX) is included in this instrument for elemental analysis. The composition and chemical state of the modified PbO₂ anodes were investigated using X-ray photoelectron spectrometry (XPS) analysis. The XPS analysis was performed via a VG Scienta SES 2002 spectrometer equipped with a monochromatized Al Kα X-ray source (1486.6 eV) and hemispherical analyzer. The analyzed surface area was 24 mm². The high-resolution spectra and wide scan were recorded with a pass energy of 100 eV and 500 eV.

2.3.2. Electrochemical Properties

The electrochemical behaviors of PbO₂ anodes were characterized by electrochemical impedance spectroscopy (EIS). EIS measurements were carried out in a conventional three-electrode cell system containing 0.1 M Na₂SO₄ using a VoltaLab PGZ301 potentiostat. PbO₂ anodes were used as the working electrodes, platinum wire was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. Impedance spectra were recorded in a frequency range varying from 100 kHz to 100 mHz. The experimental data were fitted using ZsimpWin 3.2 software.

2.4. Anodic Oxidation of Ampicillin

2.4.1. Electrolysis

The electrochemical degradation of AMP was carried out in a glass reactor containing 105 mg L⁻¹ of ampicillin and 0.1 M of Na₂SO₄ used as a supporting electrolyte at pH = 4. The SS/PbO₂, SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B electrodes were prepared and used as anodes with an effective area of 4 cm². An SS plate with the same size was used as the cathode and was supported vertically and parallel to the working anode. The solution was magnetically stirred by a magnetic stirrer during the electrolysis process, and the anodic oxidation of ampicillin was performed under galvanostatic mode at a constant current density of 50 mA cm⁻² for 5 h (Figure 2).

2.4.2. Analytical Methods and Evaluation of Degradation Efficiency

The concentration of AMP was determined at different instants during the electrolysis by high-performance liquid chromatography (HPLC). HPLC analysis was carried out using an Agilent technology instrument 1260 infinity II equipped with a thermo-scientific LC18 column Zorbax eclipse XDB (250 mm × 4.6 mm× 5 µm) used as stationary phase and coupled with a diode strip detector set at the detection wavelength of ampicillin (λ =204 nm). The mobile phase is a mixture of methanol: 0.05 M KH₂PO₄ (40:60 v/v) flowing through the column at a constant flow rate of 1.0 mL min⁻¹. The mobile phase was filtered through 0.45 μm Millipore filter. The injection volume of AMP was 20 µL.

To assess the mineralization of AMP in solution, the Chemical Oxygen Demand (COD) was determined using the reactor digestion method based on oxidizing the organic matter present in water with an excess of potassium dichromate. The quantity of potassium dichromate used in the reaction is equivalent to the oxygen used to oxidize the organic matter of wastewater. This measurement is carried out according to the French water quality standard protocol (NF T 90-101), and the COD is expressed in mg L⁻¹ [43]. The COD removal rate is calculated from the following equation:

$$\mathrm{COD}\mathrm{removal}\left(\%\right)=\frac{{\mathrm{COD}}_0-{\mathrm{COD}}_{\mathrm{t}}}{{\mathrm{COD}}_0}\times 100$$

(1)

where ${\mathrm{COD}}_0$ and ${\mathrm{COD}}_{\mathrm{t}}$ are, respectively, the COD of initial concentration and the COD at given time t.

The average current efficiency (ACE) representing the proportion of the generated radicals serving to oxidize the AMP is determined from COD values (in ${\mathrm{g}}_{O2}$ L⁻¹) using this expression [44]:

$$\mathrm{ACE}\left(\%\right)=\frac{({\mathrm{COD}}_0-{\mathrm{COD}}_{\mathrm{t}})\times \mathrm{F}\times \mathrm{V}}{8\times \mathrm{I}\times \mathrm{t}}\times 100$$

(2)

where F is the Faraday constant (96,487 C mol⁻¹), V is the solution volume (L), 8 is the oxygen equivalent mass (g eq⁻¹), I is the applied current (A), and t is the electrolysis time (s).

The electric energy needed to degrade AMP at a given time “t” was estimated by calculating the energy consumption per amount of degraded COD ($\mathrm{kWh}{\left(\mathrm{g}\mathrm{COD}\right)}^{-1}$) according to the following formula [45]:

$$\mathrm{EC}\left(\mathrm{kWh}{\left(\mathrm{g}\mathrm{COD}\right)}^{-1}\right)=\frac{\mathrm{I}\times \mathrm{E}\times \mathrm{t}}{∆\mathrm{COD}\times \mathrm{V}}$$

(3)

where I (0.2 A) is the applied current flowing between the anode and cathode, E is the average cell voltage, t is the electrolysis time, ΔCOD is the decay in COD (mg L⁻¹), and V is the volume of the treated solution (L).

3. Results and Discussion

3.1. Morphological and Structural Characterization of the Anodes Surface

3.1.1. SEM and EDX Analysis

Figure 3 shows SEM micrographs of PbO₂ coating before and after the addition of the TiO₂ interlayer, respectively, in Figure 3a,b. The boron doping of the outer layer is represented in Figure 3c. The addition of the TiO₂ interlayer affects significantly the coating morphology. We notice as depicted in Figure 3b that the anode surface with the TiO₂ interlayer became rougher compared to the SS/PbO₂ (Figure 3a) directly coated anode surface, which looks more homogeneous and uniform. In comparison with the undoped PbO₂ coating, the incorporation of boron into the PbO₂ coating (Figure 3c) led to a rougher and a more heterogeneous surface covered with small-sized lead dioxide particles. The rough structure obtained with SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes increases the active surface area which provides more active sites for electrochemical reactions and facilitates, thus, the degradation of the pollutant [13,46].

According to EDX images of SS/TiO₂/PbO₂ anode in a cross-section (Figure 4), the anode was developed on a silicon substrate, since it is easily breakable compared to the SS substrate. The cross-section images allowed us to check the TiO₂ intermediate layer and the PbO₂ outer layer thicknesses corresponding, respectively, to (112 ± 25) nm and (1000 ± 25) nm after heat treatment.

EDX analysis was carried out to determine the chemical composition of the anodes. From the EDX element mappings of Si/TiO₂/PbO₂ (Figure 4) and SS/TiO₂/PbO₂-10%B (Figure 5) electrodes, we notice that lead (Pb), titanium (Ti) and oxygen (O) are the main detected elements. The homogeneous distribution of Ti and O confirms that the intermediate layer of TiO₂ covers the entire surface of the stainless steel substrate and the Pb distributed in the form of small crystals over the anode surface demonstrates the successful deposition of the PbO₂ active layer.

3.1.2. XPS Analysis

To further study the elementary composition and the chemical environment of SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes, the electrodes surfaces were investigated by XPS.

The XPS spectrum presented in Figure 6a reveals that Pb and O are the main elements present on the surface of both electrodes and are associated to the PbO₂ film. The detected carbon (C) is attributed to the anode surface pollution. Since the PbO₂ layer is too thick, the Ti of the intermediate layer does not appear in the XPS spectrum. The boron present in SS/TiO₂/PbO₂-10%B anode is also not observed because of the small relative sensitivity factor (RSF) cross-section of the B1s orbital (B1s = 0.486). In order to quantify the chemical nature of the elements present, high-resolution XPS spectra of Pb and O were measured, and the main results are summarized in

Table 1. XPS data of different chemical states of O and Pb elements on the surface of SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B electrodes.

Electrode	Binding Energy (eV)		O1s Organic (Atom%)	O1s Lattice (Atom%)	O1s Defective (Atom%)	O1s Lattice/O1s Defective (Atom%)	Pb4f7/2/O1s Defective (Atom%)
	O1s Lattice	Pb4f7/2
SS/TiO2/PbO2	529.69	138.87	16.07	4.44	4.06	1.09	3.70
SS/TiO2/PbO2-10%B	529.52	138.71	17.37	4.98	3.63	1.37	4.32

.

It can be seen from Figure 6b that the XPS spectrum of the Pb 4f core-level peak is formed by two contributions: the Pb 4f _5/2 and the Pb 4f _7/2 located, respectively, at 143.48 eV and 138.87 eV with a spin-orbit coupling of 4.9 eV. The Pb 4f _7/2 can be attributed to Pb ²⁺ present in the lead oxide phase (Pb₃O₄) [47]. The XPS spectrum of O1s (Figure 5c) is deconvolved into three peaks relative to the three chemical states of oxygen in the coating. The main peak, centered at an energy of 531.59 eV, corresponds to organic oxygen (O1s _organic), the second peak centered at an energy of 531.10 eV corresponds to defective O/-OH oxygen (O1s _defective), and the third peak at 529.52 eV is relative to the lattice oxygen (O1s _lattice) [14,15]. The O1s _lattice could be assigned to strong oxygen binding (Pb-O bond) derived from the PbO₂ active layer [28,48,49]. The high binding energy of the organic oxygen is attributed to the adsorbed hydroxyl oxygen. The high content of O1s _organic improves the catalytic performance of the electrode by providing more adsorbed reactive oxygen species (ROS), which in turn facilitates the degradation efficiency of the pollutant [50,51]. The percentage of O1s _organic for the SS/TiO₂/PbO₂-10%B anode (17.37%) is higher than that for the SS/TiO₂/PbO₂ anode (16.07%), indicating therefore that the doped PbO₂ coating has a higher electrocatalytic activity toward the degradation of the organic pollutant than the un-doped coating.

Table 1 shows the atomic percentage of different chemical elements present on the surface before and after doping. The obtained results reveal, on the one hand, a slight shift in the binding energies of the O1s _lattice and Pb 4f _7/2 peaks toward low energies. On the other hand, we notice after doping the layers of 10% B an increase in the atomic percentage of the O1s _lattice and a decrease in the atomic percentage of the O1s _defective. We also observe a decrease in the ratio between the atomic percentage of O1s _lattice/O1s _defective and Pb 4f _7/2/O1s _defective. This means that defective (lacunar) oxide structures have been transformed into lattice oxide by the incorporation of boron into the defective structure of PbO₂. Therefore, the changes at the O1s _lattice peak are associated to the presence of boron dopant in lead dioxide lattice, which proves that boron atoms have been introduced into the active layer. Concerning the Pb 4f _7/2 core-level peak, the changes are attributed to the sensitivity of the lead atom to the chemical environment after the insertion of the dopant.

3.2. EIS Measurements and Fitting

EIS measurements were performed to further investigate the effect of the TiO₂ interlayer and B dopant on the electrochemical properties of PbO₂ anodes. The EIS parameters listed in

Table 2. EIS parameters obtained from fitting the electrical equivalent circuit on the experimental data of SS/PbO₂, SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes.

	SS/PbO2	SS/TiO2/PbO2	SS/TiO2/PbO2-10%B
Rs (Ω cm2)	480.6	64.71	104.5
Q coating (S sn cm−2)	5.80 × 10−6	4.59 × 10−6	22.8 × 10−6
n	1	0.59	0.68
R coating (Ω cm2)	68.48	659	435.9
Qdl (S sn cm−2)	16.8 × 10−6	86.6 × 10−6	74.3 × 10−6
n	0.86	0.62	0.65
Rct (Ω cm2)	3.18 × 106	3.76 × 104	5.64 × 104
W	4.01 × 10−6	1.02 × 104	-

were obtained from the collected experimental data fitted with the equivalent circuit (Figure 7) where R_s corresponds to the solution resistance, and (R_coating, Q_coating) and (R_ct, Q_dl) characterize, respectively, the properties of the coating and the charge transfer process at the interface electrode/electrolyte. The Warburg impedance (W) is assigned to the diffusion of charged species [12].

It can be seen from the Nyquist plots displayed in Figure 8 that SS/PbO₂ (Figure 8a) and SS/TiO₂/PbO₂-10%B (Figure 8c) electrodes present straight lines while SS/TiO₂/PbO₂ impedance spectra (Figure 8b) present a straight line with the appearance of a semi-circular shape at a high-frequency range attributed to the fast charge transfer at the anode interface [52]. The EIS responses shown in Table 2 indicate a significant difference in R_ct and R_coating values. The R_ct of SS/TiO₂/PbO₂ (3.76 × 10⁴ Ω cm²) and SS/TiO₂/PbO₂-10%B (5.64 × 10⁴ Ω cm²) anodes are smaller than the R_ct of SS/PbO₂ (3.18 × 10⁶ Ω cm²) anode, and this is mainly explained by the compact structure and the small size of the crystalline particles of the B-doped PbO₂ coating demonstrated earlier with the SEM micrographs as well as by the improvement of the electronic conductivity with the B doping [46,53,54]. Therefore, we can say that the reduced charge transfer resistance induced by the introduction of an intermediate layer and by the B doping allows a faster electrochemical reaction rate [27] and could contribute to a lower power waste and a better current efficiency during the anodic oxidation process [15].

Concerning the R _coating, the resistance increased from 68.48 to 435.9 Ω cm² with, respectively, SS/PbO₂ and SS/TiO₂/PbO₂-10%B anodes. This result implies that the modified lead dioxide coating exhibits a better adhesion to the SS substrate by minimizing the transfer charge through the defects of coating. The intermediate layer contributed to strengthening the bonding between the electrode active layer and the substrate, which is reflected in the obtained values of coating resistance [55]. This high coating resistance leads to a lower punctual dissolution of the active layer and thus a longer lifetime of the developed anode [28].

3.3. Electrochemical Oxidation Performance of Electrodes

3.3.1. AMP Removal

In order to evaluate the electrocatalytic oxidation performance of the prepared anodes, HPLC analysis was carried out to identify AMP concentration during the electrochemical treatment, and COD removal rates were calculated at the end of the treatment since the COD is an indicator of the mineralization rate of organic matter including the initial polluting molecule as well as the by-products generated during the treatment.

As shown in Figure 9, the AMP concentration decreased gradually as the electrochemical oxidation reaction proceeded, suggesting that SS/PbO₂ SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes exhibit efficient anode materials for the electrochemical degradation of AMP. Although the SS/PbO₂ anode showed the fastest degradation kinetics, it can be clearly seen that this anode had the lowest COD removal rate (30.77%) after 5 h of electrolysis compared to SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes which reached, respectively, 61.54% and 69.23%. Therefore, we can conclude that the addition of a TiO₂ intermediate layer and B doping of the PbO₂ active layer resulted in a better degradation of total organic compounds including ampicillin and the by-products that could be generated during the electrochemical treatment.

Since the electrochemical degradation process efficiency is mainly ascribed to the$O{H}^{●}$ radicals produced on the anode surface, the enhanced mineralization rate of the organic matter obtained with SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B anodes could be explained by their strong hydroxyl radical generation ability [30,41]. This high hydroxyl radicals rate is attributed to the new physico-chemical properties of the electrode after the introduction of TiO₂ oxide film and the B doping of the outer layer. The small charge transfer resistance obtained with the modified PbO₂ anodes, demonstrated by impedance spectroscopy measurements, increased the conductivity of the coating and accelerated the charge transfer, which in turn promoted the water oxidation and the resultant $O{H}^{●}$ radicals generation [51]. The highest COD removal rate at the end of the electrolysis is reached by the B-doped PbO₂ electrode, and this is ascribed to the morphological difference at the electrode active layer [30,52]. The compact structure of the SS/TiO₂/PbO₂-10%B anode (Figure 3c) formed by PbO₂ small particles provides a large active surface and therefore more active sites to generate more$O{H}^{●}$ radicals [24]. In addition to the crystal structure modification, B doping increases the conductivity by inducing sp² carbon impurities which influence the mineralization performance of the anode and enhance the electrogeneration of hydroxyl radicals$O{H}^{●}$ formed from water discharge on its surface [36,53].

$$H_{2}O\to O{H}^{●}+H^{+}+e^{-}$$

The Fenton method is among the various advanced oxidation processes used to eliminate ampicillin where the antibiotic is completely eliminated within 2 min [56]. However, comparing to anodic oxidation, the pH solution is a parameter that affects Fenton reaction and may lead to ineffective Fenton oxidation in addition to the problem of iron sludge pollution treatment [57]. Concerning the anodic oxidation method, several anode materials were used for ampicillin degradation such as Ru-Ir-TiO₂ [58], Ti/TiO₂/PbO₂ [27] or boron-doped diamond [59] anodes where the pharmaceutical pollutant was totally eliminated after a short time of electrolysis. Despite the superior performance of these electrodes, they remain relatively expensive compared to the more affordable cost of the stainless steel substrate used in this study.

3.3.2. Average Current Efficiency and Energy Evaluation

An evaluation of the efficiency of the treatment method in terms of average efficiency of the applied current (ACE) and consumed energy (EC) was also carried out in order to estimate the energy cost of the proposed optimized treatment.

The evolution trend of COD removal rate with the different anodes at the end of electrolysis is similar to that of the average current efficiency. As expected, the ACE values of SS/TiO₂/PbO₂ and SS/TiO₂/PbO₂-10%B were 53.60% and 60.30%, respectively, which were higher than those of the traditional SS/PbO₂ anode. These results are explained by the difference in the reactivity of the hydroxyl radicals electrogenerated on the anodes surfaces, which is influenced by the anode material [60].

The evolution of EC presented in Figure 10 shows that the consumed electric energy was about two times lower in the presence of the TiO₂ interlayer and B dopant. The EC values decreased from 0.125 to 0.056 kWh (g COD)⁻¹ with, respectively, SS/PbO₂ and SS/TiO₂/PbO₂-10%B electrodes. This decrease is due essentially to the high COD removal rate achieved with SS/TiO₂/PbO₂-10%B during the last hour of treatment, since the higher the anode material electrocatalytic activity, the faster the electron transfer rate, and consequently, a large number of hydroxyl radicals can be generated. This leads to the lower energy consumption and lower energy cost of the electrochemical treatment process which represents, thus, a great advantage from an economic perspective [61,62].

4. Conclusions

In this paper, a B-doped SS/TiO₂/PbO₂ anode was prepared on a stainless steel substrate by the sol–gel spin-coating method in order to study the electrochemical activity of the electrode toward the electrochemical degradation of a pharmaceutical pollutant.

Based on the morphological and structural characterization, we can say that the introduction of the TiO₂ inner layer and B doping of the PbO₂ outer layer provided a higher specific surface area which led to larger electrochemical active sites and promoted, consequently, the anodic oxidation reaction. In electrochemical measurements, the SS/TiO₂/PbO₂-10%B anode exhibited a smaller charge transfer resistance compared to the traditional SS/PbO₂ anode leading, therefore, to a higher hydroxyl radical generation capacity and a better electrochemical oxidation performance. The electrocatalytic degradation of AMP in aqueous solutions using modified PbO₂ anodes was investigated. The SS/TiO₂/PbO₂-10%B electrode had the highest oxidation ability with a widely higher COD removal efficiency (69.23%) than the SS/PbO₂ electrode (30.77%). Thus, the introduction of the TiO₂ intermediate layer and boron doping of the active layer represent two attractive methods to enhance the electrocatalytic activity and minimize the energy consumption of the PbO₂-based anode during the electrochemical treatment, making the SS/TiO₂/PbO₂-10%B electrode a promising anode for real applications in wastewater organic pollutants removal.