Optimization of Electrocatalytic Chlorazol Yellow Degradation Using PbO₂ Nanostructure Immobilized on Stainless Steel Substrate

Abstract

In this study, an electrochemical approach was utilized to degrade the anionic Chlorazol Yellow (CY) dye in an aqueous solution using a lead oxide-modified stainless steel electrode (denoted as PbO₂-SS). The fabrication of this electrode involved scanning a clean stainless steel (denoted as SS) plate within a range of −1.0 V to +1.0 V against Ag/AgCl (saturated KCl) for three cycles at a scan rate of 0.1 V s⁻¹ in a 0.1 M Pb(NO₃)₂ solution. Analysis via X-ray photoelectron spectroscopy (XPS) confirmed successful fabrication, with Pb⁴⁺ being the predominant species observed in the XPS spectra. Additionally, scanning electron microscopy (SEM) imaging of the fabricated electrode revealed the deposition of PbO₂ in a flower-like, nanostructured form on the SS surface. To provide a cost-effective method for dye treatment, the PbO₂-SS anode was utilized to oxidize chloride ions (Cl⁻) into hypochlorite ions (ClO⁻), which subsequently oxidized CY molecules. Optimization of parameters such as the voltage, supporting electrolytes, and solution pH was conducted to determine the most effective degradation conditions. The method achieved a degradation efficiency of approximately 97% over a wide pH range within 20 min, indicating its applicability across various pH conditions. Consequently, this technique presents a promising approach for the treatment of industrial wastewater.

1. Introduction

A nation’s economic growth is dependent on the growth of its industry [1]. Environmental pollution, however, is brought on by the industry’s wastewater outflow [2,3]. The biggest causes of environmental pollution are dyeing [4,5] and chemical-related sectors such as textiles, chemicals, paper, food processing, pharmaceutical, leather, cosmetics, etc. [6,7,8]. These industrial plants generate enormous amounts of effluent and release the effluent without proper treatment. As a result, the untreated effluent mixes with fresh river water through sewers. The presence of deleterious chemicals and river water tainted with dyes, employed for agricultural purposes, leads to their absorption into the human body via the food chain [9]. This phenomenon is linked to the outbreak of multiple diseases including cancer, diarrhea, dysentery, and skin related human health [10,11,12].

Many techniques have been applied to treat industrial dye wastewater, such as biological treatment [13,14], coagulation [15,16], adsorption [17,18,19,20,21,22], photocatalytic degradation [23,24,25,26,27,28,29,30,31], electro photocatalytic degradation [32,33], electrochemical degradation [34,35,36,37,38,39], Fenton oxidation [40,41], etc. Conventional biological techniques are typically regarded as safer, more eco-friendly, economically viable, effective, and less expensive. However, these techniques are ineffective for complete dye removal from a mixture of different dyes [42,43,44]. Adsorption, a physical approach for removing dye, has significant challenges because it demands long contact times and a significant amount of space as well as expensive adsorbent regeneration procedures [42]. The coagulation process proceeds slowly, during which the interaction between the dye’s opposite charge and the coagulant results in the formation of an insoluble, potentially hazardous sludge, and as a result, increases the Total Dissolved Solids (TDSs) [45]. In addition, this process does not facilitate the complete removal of dye from the environment, which makes it unsuitable for the removal of dye [46]. On the other hand, the photocatalytic degradation process is an efficient, quick, and low-cost approach for dye removal from wastewater that can start working in several hours at room temperature. Furthermore, organic contaminants can be mineralized into comparatively non-hazardous compounds without the development of poisonous products [47,48]. The drawback it carries is its sensitivity towards operational parameters such as the initial dye concentration, pH of the solution, irradiation intensity, and reaction temperature [49,50]. The Fenton process, a conventional technique to treat wastewater, can be operated at ambient temperature and atmospheric pressure. The Fenton reagent’s accessibility, ease of use, and storage make this process feasible. Additionally, there is no environmental risk associated with the Fenton reagent. Although it has numerous benefits, it also has several drawbacks. The Fenton reaction is beneficial only at a very low pH condition [40,51,52]. The Fenton reagent’s activity reduces at higher pH levels due to the presence of comparatively inactive iron oxyhydroxides and the formation of ferric hydroxide precipitate [51]. Hence, there arises a necessity for an innovative strategy that proves efficacious over broad pH conditions while remaining cost-effective.

Recently, the electrochemical advanced oxidation process (EAOP) has garnered considerable attention among researchers due to its remarkable ability to rapidly and efficiently degrade dyes to completion [46,53]. More importantly, EAOP can eliminate dye over a wide range of pH values [52]. Moreover, it requires simple equipment and has an easy operation and the environmental compatibility to attract attention [53,54]. This method uses an anode and a cathode with electrodes submerged in the electrolyte. The oxidation at the anode and reduction at the cathode occur at the electrolyte and electrode interface. The EAOP technique mostly relies on the electrodes producing a lot of hydroxyl radicals through oxygen evolution, water oxidation, or the Fenton process. The hydroxyl radicals produced by EAOP are potent oxidizers that break down the organic material in wastewater, accelerating the process of decomposition [46,55,56]. The in situ generation of hydroxyl radicals has been exploited largely due to their high degradation rate in treating various organic pollutants, which function best under slightly acidic conditions. Several other oxidizers, such as persulphate and sulfate radicals generated by EAOP, have also drawn significant attention in treating organic pollutants [57]. Moreover, the bleaching of organic pollutants with hypochlorite is a popular water treatment method that executes dye degradation even under neutral pH. However, only a limited number of works are reported in the literature concerning wastewater treatment that deals with the in-situ synthesis of hypochlorite by EAOP from chloride ions [58,59,60]. One of the great advantages of dye degradation by electrochemically generated hypochlorite is that it requires simply NaCl; hence, no secondary treatment is needed to isolate any chemical due to the facile anodic process. Recently, concerning the electro-oxidation of chloride ions, numerous electrodes such as IrO₂/Ti [58], a BDD anode [61], graphite [62], TiO₂/ITO [63], Ti/IrO₂–SnO₂–Sb₂O [64], and BDD/CG [65] have been proposed. However, the procedure of electrode development is either complex or the materials are expensive. Regarding this concern, Pb can be an alternative because it is a cheap metal that can be easily immobilized as oxides on a substrate like stainless steel (SS). The oxide of lead, that is, PbO₂, has been widely appreciated as an anode material due to its simple preparation and recycling processes, affordability, exceptional electrocatalytic performance—including high oxygen evolution potential, strong oxidizing power, and excellent electrical conductivity—and remarkable resistance to corrosion [66]. PbO₂ materials also hold significant promise for diverse applications, ranging from sustainable chemical industries and energy storage to cutting-edge advancements in environmental analysis and remediation [36,39,66].

Therefore, the objective of this work was to assess the removal of the anionic Chlorazol Yellow (CY) dye by electrochemical oxidation using a PbO₂-modified stainless steel (PbO₂-SS) electrode to save the ecosystem. Here, SS has been selected for PbO₂ support since SS is very cheap and ubiquitously available. The model anionic CY, an azo dye, was chosen because it is widely used in the textile industry [67,68,69]. In this research, the fundamental variables were examined, including the impact of the electrode material, various supporting electrolytes, applied voltage, solution pH, supporting electrolyte concentration, and selected dye (CY) concentration.

2. Results and Discussion

2.1. Electrode Characterization

The performance of electrode materials in catalytic experiments is related to their elemental composition, oxidation states of the elements, and surface morphology of the material. In this regard, the PbO₂-SS electrode was meticulously analyzed using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). With its intrinsic ability to distinguish between elemental and chemical state information, XPS revealed intricate details about the oxidation states present on the surface. The survey spectrum shown in Figure 1a reveals the major elements present on the surface. The peaks at around 130.00 eV, 294.60 eV, and 530.00 eV correspond to Pb 4f, C 1s, and O 1s, respectively. It confirms the presence of Pb species on the SS surface, with C 1s typically representing adventitious carbon or carbon contamination [70,71,72]. The deconvoluted XPS spectrum of Pb 4f, as shown in Figure 1b, shows two major peaks corresponding to the Pb 4f_7/2 and Pb 4f_5/2 electronic orbitals. Notably, these peaks were further deconvoluted, resulting in four distinct peaks at 136.96 eV, 137.64 eV, 141.81 eV, and 142.63 eV. These peaks can be attributed to the Pb in its +4-oxidation state, a hallmark of PbO₂ [70,72,73]. The presence of dual peaks for both Pb 4f_7/2 and Pb 4f_5/2, as presented in Figure 1b, suggests the existence of two slightly varied chemical environments for the Pb atoms, potentially hinting at interactions with the oxygen [73,74]. The O 1s spectrum was also deconvoluted into two peaks at 529.22 eV and 531.13 eV, as given in Figure 1c. These peaks correspond to different types of oxygen species present on the surface. The peak at 529.22 eV can be assigned to lattice oxygen in PbO₂, representing chemically-bonded oxygen within the crystal lattice [75]. Meanwhile, the peak at 531.13 eV represents adsorbed oxygen on the surface, possibly stemming from ambient environmental interactions [71,72,75,76].

In order to know the surface morphology of the electrode, the SEM images, as shown in Figure 2a,b, of the PbO₂-SS at two different magnifications were taken. According to the SEM images, flower-like nanostructures of PbO₂ particles were found to be formed on the SS surface.

2.2. Effect of PbO₂ on CY Degradation

The selection of anode material significantly controls the electrochemical dye degradation process in terms of efficiency, radical generation, byproduct formation, and operational costs. To investigate the effects of PbO₂ on the electrocatalytic degradation of 15 mg/L CY dye, two electrodes, such as an SS electrode and a PbO₂-SS electrode, were used as working electrodes for a solution with pH~7, applying 2.0 V as the working voltage, where the supporting electrolyte was 2 g/L NaCl. The degradation process was continued for 35 min in both cases. The experimental findings are shown in Figure 3. It can be noted that the degradation of 15 mg/L CY dye for both electrodes increased with the increasing time up to 20 min, and after that point the degradation became static, indicating no degradation occurred for the rest of the time. The percentage degradation of CY dye obtained was 68% and 97% on the SS electrode and PbO₂-SS electrode, respectively. This observation suggests that the presence of PbO₂ particles on the SS substrate significantly accelerated CY degradation under the experimental conditions. Consequently, the PbO₂-SS electrode was adopted in this study due to its efficacy.

2.3. Effect of Applied Voltage

The applied voltage in electrochemical dye degradation is the crucial factor that mainly controls the efficient generation of reactive species for the oxidation of dye molecules. In this regard, the effect of applied voltages, within the range of 0.5 to 2.0 V, at the PbO₂-SS electrode was investigated at pH~7, in the presence of 2 g/L NaCl, for a constant time of 20 min. The experimental results are shown in Figure 4.

It can be seen from the figure that the degradation of 15 mg/L CY dye for all the applied voltages rose with the time up to 20 min, and after this, the degradation process remained constant. Notably, the highest degradation was obtained at ca. 99%, when 2.0 V was applied. Thus, it can be inferred that the optimized voltage is 2.0 V for the electrochemical degradation of 15 mg/L CY dye by the fabricated electrode.

2.4. Effect of Supporting Electrolyte

A supporting electrolyte is used to improve the electrical current transfer in the solution mass. To understand the effect of the supporting electrolyte for the electrochemical degradation of 15 mg/L CY dye by PbO₂-SS, two supporting electrolytes, such as 2 g/L NaCl and 2 g/L Na₂SO₄, were examined at pH~7 and E = 2.0 V, for 35 min of electrolysis.

Figure 5 demonstrates that the degradation of 15 mg/L CY dye increased for both supporting electrolytes within the first 20 min, after which no further degradation was observed. For a 2 g/L NaCl solution, approximately 94% degradation was achieved within 20 min, whereas only 11% degradation was observed for a 2 g/L Na₂SO₄ solution over the same period. This difference is likely due to the chlorine radical $({\mathrm{C}\mathrm{l}}^{⦁})$ being a stronger oxidizing agent than the sulfate radical $({\mathrm{S}\mathrm{O}}_4^{⦁-})$. Furthermore, at high Na₂SO₄ concentrations, excess sulfate ions $({\mathrm{S}\mathrm{O}}_4^{2-})$ may migrate to the anode surface, accumulate, and occupy active sites, thereby deactivating the electrocatalytic properties of the anode. Consequently, dye degradation was restricted. In the case of the NaCl electrolyte, this limitation did not occur, as Cl⁻ ions are readily oxidized on the anode [31]. Thus, it can be inferred that the solution of NaCl is a suitable supporting electrolyte for electrochemical dye degradation by means of the PbO₂-SS electrode.

2.5. Effect of NaCl Concentration

Since chlorine free radicals $({\mathrm{C}\mathrm{l}}^{⦁})$ are the primary species responsible for this dye degradation experiment, it is crucial to use optimal concentrations of NaCl to ensure sufficient generation of these radicals for effective reactions with dye molecules at appropriate rates. Regarding this concern, the effect of NaCl concentration on the electrochemical degradation of 15 mg/L of CY dye by the PbO₂-SS electrode was examined at various concentrations of the NaCl supporting electrolyte, in the range of 0.5–3.0 g/L at around pH 7, with t = 20 min and E = 2.0 V.

It was observed that increasing the concentration of the NaCl electrolyte from 0.5 g/L to 2.0 g/L over 20 min significantly enhanced the degradation of 15 mg/L CY dye, with the removal efficiency rising from 20% to 97%, as depicted in Figure 6. However, further increases in NaCl concentration did not result in additional degradation, identifying 2.0 g/L as the optimal concentration for the supporting electrolyte. This improvement can be attributed to the increased conductivity of the solution with higher NaCl concentrations, which facilitated electron transfer and enhanced the generation rate of ${\mathrm{C}\mathrm{l}}^{⦁}$ radicals at the anode surface, as described by Equation (1).

$${\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}^{⦁}+{\mathrm{e}}^{-}$$

(1)

In the case of the highly concentrated NaCl solution, the excessive formation of ${\mathrm{C}\mathrm{l}}^{⦁}$ radicals undergo recombination to regenerate Cl₂ gas, as per Equation (2).

$${\mathrm{C}\mathrm{l}}^{⦁}+{\mathrm{C}\mathrm{l}}^{⦁}\to {\mathrm{Cl}}_2$$

(2)

At NaCl concentrations exceeding 2.0 g/L, no further increase in the dye degradation rate was also observed in this experiment. Given that ${\mathrm{C}\mathrm{l}}^{⦁}$ is a powerful oxidizing agent, it effectively oxidizes organic substances, including industrial dye waste.

2.6. Effect of CY Concentration

Various concentrations of CY dye (5–30 mg/L) were examined for degradation by the PbO₂-SS electrode under the experimental conditions of pH~7, t = 20 min, E = 2.0 V, and [NaCl] = 2.0 g/L to observe the effect of CY dye concentration. The result is shown in Figure 7.

It can be seen from the figure that dye degradation decreased from 99% to 45% with the elevation of the CY dye concentration from 5 mg/L to 30 mg/L during the 20 min. Thus, a lower concentration of CY dye is more suitable for dye degradation in this study, since the degradation rate is dependent on the amount of ${\mathrm{C}\mathrm{l}}^{⦁}$ radicals generated.

2.7. Effect of pH

In dye degradation, the pH of the solution is a crucial factor, as it can affect the dye degradation rate in various ways. Specifically, the pH of the reaction media influences the electrode surface properties, generation of reactive species, dye stability, and ionization. To investigate the effect of the solution pH on the electrochemical degradation of 15 mg/L CY dye on PbO₂-SS, the pH of the reaction medium was varied from 3 to 11, considering other experimental conditions such as t = 20 min, E = 2.0 V, and [NaCl] = 2.0 g/L were kept fixed.

It was found that 97% dye degradation was obtained for all the pH values within 20 min, confirming that there was no effect for pH variation (shown in Figure 8). This observation also indicates that changes in pH do not have a remarkable effect on the degradation of the CY dye. In other words, the formation of radicals on the PbO₂-SS surface is favored irrespective of the solution pH.

2.8. Mechanism

In the electrocatalytic CY dye degradation process, dye degradation occurred on the electrode surface. The mechanism of CY degradation may be explained in reference to literature [77]. Firstly, electrons transferred from the anode to the cathode, making them positively and negatively charged, respectively. In the bulk solution, the NaCl electrolyte discharged ${\mathrm{C}\mathrm{l}}^{-}$ ions, which then approached the anode surface to react with the Pb(IV) species deposited on the SS sheet, forming a Pb(IV)-Cl(−1) complex. Subsequently, this complex dissociated into Pb(III) and a ${\mathrm{C}\mathrm{l}}^{⦁}$ radical. In the following step, the Pb(III) species oxidized another Cl⁻ ion, and produced a Pb(II) species. The as-generated ${\mathrm{C}\mathrm{l}}^{⦁}$ radicals reacted with water molecules to form hypochlorite ion (ClO⁻) oxidants. Both the ${\mathrm{C}\mathrm{l}\mathrm{O}}^{-}$ ion and $\mathrm{C}{\mathrm{l}}^{⦁}$ free radical are strong oxidizing agents, which might oxidize the dye and produce dye-degraded products. The mechanism of CY degradation at the anode surface then may be summarized as follows.

$$\mathrm{NaCl} \to {\mathrm{Na}}^{+}+{\mathrm{C}\mathrm{l}}^{-}$$

(3)

$$\mathrm{Pb}(\mathrm{IV}) +{\mathrm{C}\mathrm{l}}^{-}\to \text{Pb(IV)-Cl}$$

(4)

$$\text{Pb(IV)-Cl} \to \mathrm{Pb}(\mathrm{III})+\mathrm{C}{\mathrm{l}}^{⦁}$$

(5)

$$\mathrm{Pb}(\mathrm{III})+{\mathrm{C}\mathrm{l}}^{-}\to \mathrm{Pb}(\mathrm{II})+\mathrm{C}{\mathrm{l}}^{⦁}$$

(6)

$$2{\mathrm{Cl}}^{⦁} \to {\mathrm{Cl}}_2$$

(7)

Cl₂ + H₂O → HCl + HClO

(8)

$$(\mathrm{C}{\mathrm{l}}^{⦁},{\mathrm{C}\mathrm{l}\mathrm{O}}^{-})+\mathrm{Dye} \to \text{Dye-degraded} \text{products}$$

(9)

As the $\mathrm{C}{\mathrm{l}}^{⦁}$ and ${\mathrm{C}\mathrm{l}\mathrm{O}}^{-}$ are strong oxidizing agents, they reacted with the azo bond of the CY dye, producing harmless degraded products such as CO₂, SO₂, NO₂, H₂O, etc.

3. Experimental

3.1. Chemicals

In this study, all the chemicals used were of analytical grade and obtained with the highest purity available (Merck, Darmstadt, Germany). The main chemical, Chlorazol Yellow (CY) dye, was supplied by Philip Harris Limited, Shenstone, UK. Other chemicals such as sulfuric acid (H₂SO₄), Lead Nitrate [Pb(NO₃)₂], distilled water, Methylene Blue (MB), Sodium Chloride (NaCl), Sodium sulfate (Na₂SO₄), HCl, and NaOH were used to accomplish this research.

3.2. Instruments

The electrochemical experiments were performed with potentiostats (Wavedrive20; PINE Incorp. Waconia, MN, USA, CHI 600; CH Instruments, Inc., Bee Cave, TX, USA, and Auolabe128 N; Yamazaki Mazak Corporation, Houten, The Netherlands). A KCl-saturated Ag/AgCl electrode was used as a reference electrode, a stainless steel (SS) sheet was used as a counter electrode, a PbO₂-SS electrode was the working electrode, a 100 mL Pyrex glass beaker was used as a cell, and a pH meter (PH800, Qingdao Puhua Heavy Industrial Machinery Co., Ltd., Qingdao, China) was used to set the pH in the solution; a sonication machine was used for a homogeneous mixture, where a magnetic stirrer (hot plate, made in China) with a rotational speed of 100 rpm was used to maintain the electrolytic homogeneity of the solutions, and the UV–visible spectral changes were monitored using the AvaSoft8 Spectrophotometer (Avantes, Apeldoorn, The Netherlands).

3.3. Electrode Fabrication

For electrochemical dye degradation, an SS plate having a geometrical area of 0.8 cm³ (4 cm $\times$ 1 cm $\times$ 0.2 cm) was polished with sandblast paper, and then washed with ample DI water. After washing, SS plates were sonicated for 10 min in an acetone and ethanol mixture to remove abrasive particles. Afterwards, SS plates were drowned into the 0.1 M Pb(NO₃)₂ solution and Pb was deposited as PbO₂ by cycling the voltammogram between the potential range from −1.0 V to +1.0 V for 3 cycles or 6 sweep segments at a scan rate of 0.1 Vs⁻¹. During the negative scan, Pb particles were deposited onto SS due to the reduction reaction; meanwhile, the positive scan oxidized the deposited Pb particles into PbO₂ species.

3.4. Characterization

The surface morphology of the prepared PbO₂-SS electrode was inspected by an FE-SEM instrument (JEOL, Tokyo, Japan). The X-ray photoelectron spectra (XPS) analysis was executed using the Kratos Axis-Ultra delay line detector (Kratos Analytical Ltd., Manchester, UK) spectrometer maintained at 10 kV (radiation source, Al Kα: 1486.6 eV).

3.5. Dye Degradation Experiment

The dye degradation experiments were conducted in two compartment cells and the volume of each compartment was 40 mL connected with a membrane frit. The prepared CY solutions were electrolyzed anodically in the presence of 2 g/L NaCl, where the distance between the anode and cathode was 0.3 cm. The absorbance was measured in 5 min intervals for the analysis of the CY dye concentration at pH~7. A real-time Avates spectrometer was used to monitor the CY concentration at 388 nm. The percent dye degradation (%DD) was calculated by the change in absorbance using the following Equation (10):

$$\%\mathrm{DD}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{i}}-{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_{\mathrm{i}}}}\times 100$$

(10)

where A_i and A_t are the absorbance values of CY initially and at a given time, t, respectively.

4. Conclusions

The electrocatalytic degradation of the anionic CY was successfully accomplished using the PbO₂-SS anode. The convincing high degradation performance was achieved for a NaCl concentration of 2.0 g/L and applied voltage of 2.0 V. Under these optimizing conditions, the degradation of CY dye increased up to 97% within 20 min. Moreover, unlike the Fenton process, the variation of the solution pH over an extensive range, from pH 3 to 11, had no significant inverse effect on the degradation of CY dye. Considering these results, it could be recommended that the PbO₂-SS electrode could be treated as a great catalyst for the electrochemical breakdown of CY azo dye. This electrode, therefore, can be employed for treating environmental organic pollutant-containing wastewater.