Rapid Degradation of Carbamazepine in Wastewater Using Dielectric Barrier Discharge-Assisted Fe³⁺/Sodium Sulfite Oxidation

Abstract

The discharge of medical and domestic wastewater has resulted in increasing levels of pharmaceutical pollutants in water bodies. We combined dielectric barrier discharge (DBD) technology with an Fe³⁺/sodium sulfite oxidation system to address the limitations associated with traditional water treatment technologies in removing carbamazepine, exploring the application efficacy and mechanisms of this approach in carbamazepine degradation. Under optimized experimental conditions, our system achieved a 97% degradation efficiency for carbamazepine within 4 min, significantly outperforming both DBD and sodium sulfite standalone systems. Using response surface methodology to optimize experimental parameters, the effects of sodium sulfite concentration, pH, and Fe³⁺ concentration on degradation efficiency were assessed. Under optimal conditions, the system’s degradation efficiency was 2.5 times higher than that of individual systems. Hydroxyl and sulfate radicals contributed 65% and 85%, respectively, to carbamazepine degradation, while superoxide radicals contributed only 30%. The study demonstrated that this system effectively breaks down the molecular structure of carbamazepine. Eight primary intermediate degradation products were identified, and, as degradation progressed, the concentrations of these intermediates gradually decreased, ultimately achieving a mineralization rate exceeding 85%. This study not only provides an effective technical solution for rapidly treating recalcitrant organic pollutants in water but also offers new insights for environmental protection and the sustainable use of water resources while providing theoretical and experimental data for future related research.

1. Introduction

Water pollution has become a growing global environmental concern, particularly due to the widespread presence of persistent pharmaceutical contaminants in wastewater from both the pharmaceutical and domestic sectors [1]. The discharge of pharmaceuticals and personal care products (PPCPs) into aquatic environments poses significant risks to ecosystems and human health [2,3]. Pharmaceutical wastewater contains substances such as antibiotics, antidepressants, and antiepileptic drugs, many of which persist in the environment and harm aquatic life [4]. Among these, carbamazepine (CBZ), a widely used drug for epilepsy and neuropathic pain, is of particular concern. Its chemical stability and resistance to degradation have led to its frequent detection as a persistent pollutant in water sources [5,6]. CBZ’s complex structure contributes to a long environmental half-life, making traditional water treatment methods insufficient [7,8]. Advanced oxidation processes (AOPs) have emerged as a promising solution for the treatment of persistent organic pollutants like CBZ [9,10]. These methods generate highly reactive species capable of degrading complex organic molecules utilizing various techniques [11], such as chemical oxidation [12,13,14], photolysis [15,16,17], and plasma technology [18,19,20]. Dielectric barrier discharge (DBD) plasma technology, in particular, offers high oxidation efficiency and broad applicability for wastewater treatment. Wang [21] demonstrated the degradation of methyl violet using DBD plasma, achieving near-complete decoloration and substantial chemical oxygen demand (COD) reduction. Shao [22] combined DBD plasma with hydrophilic ceramic foam (CF) for tetracycline hydrochloride (TCH) degradation, achieving over 80% TCH removal through enhanced mass transfer and reactive species involvement. DBD technology excites oxygen and water molecules with high-voltage plasma, generating powerful oxidizing agents like hydroxyl radicals (·OH), ozone (O₃), and singlet oxygen (¹O₂) [23]. Li [24] developed a pulse-powered water-film DBD plasma system for the efficient removal of ibuprofen (IBP), achieving up to 96.1% removal at 95 W input power, driven by reactive species like OH, electrons, and ¹O₂, and identified possible degradation pathways using Liquid Chromatography-Mass Spectrometry (LC-MS) and Density functional theory (DFT) simulations, demonstrating the system’s potential for real wastewater treatment. Tang [25] investigated the degradation of 2,4-dinitrophenol (DNP) in aqueous solutions using gas-phase DBD plasma, finding that under optimal conditions, the degradation efficiency reached 95.0% with a TOC removal of 51%. The study identified ·OH radicals as the key reactive species driving degradation. Despite these advancements, standalone DBD systems may still struggle to achieve sufficient removal efficiencies for persistent pollutants like CBZ. As a result, researchers have increasingly explored combining DBD technology with various oxidants or catalysts to enhance treatment efficiency. Sang [26] studied the coupling of oxidants with DBD plasma to improve the degradation of Orange G (OG) in aqueous solution, finding that the addition of oxidants, particularly Fe (VI), significantly enhanced degradation efficiency, kinetics, and energy yield, with H₂O₂ and O₃ playing key roles in the reaction mechanism. Chen [27] developed a novel cascade reactor combining ozonation and DBD plasma with activated carbon fabric to efficiently remove micro-pollutants (µPs) from secondary municipal wastewater. The system achieved high removal efficiencies (84–98%) and energy savings by utilizing both reactive oxygen species (ROS) and molecular oxidants, with optimal conditions identified for practical applications.

A promising approach involves the Fe³⁺/sodium sulfite system [28,29], which activates sulfite ions (SO₃²⁻) to generate sulfate radicals (·SO₄⁻). Chen [30] developed a novel FeS-activated bisulfite system for the degradation of β-blockers, such as propranolol (PRO), in near-neutral pH water, achieving up to 95% PRO removal under optimized conditions, with ·SO₄⁻ playing a dominant role in the process and demonstrating the potential of this system for the effective degradation of various organic pollutants compared to other oxidants. Moreover, studies have shown that the oxidation potential of SO₄⁻ (2.5–3.1 V) exceeds that of OH, enabling the efficient degradation of various organic pollutants, including CBZ, within a short time frame. Zhang [31] developed a homogeneous Co (II)/persulfate activation system for caffeine (CAF) degradation, exploring the distinct roles of ·OH and SO₄⁻ radicals through product analysis and theoretical calculations, revealing that SO₄⁻ is more kinetically reactive due to its higher oxidation potential, while both radicals primarily attack CAF via free radical addition, providing insights into the mechanisms of organic pollutant degradation in AOPs. Furthermore, the synergistic interaction between SO₄⁻ and OH can accelerate the degradation of pollutants while significantly improving energy utilization and reducing treatment costs [32].

Integrating the Fe³⁺/sodium sulfite system with DBD technology offers a novel approach to enhancing the production of reactive oxygen species (ROS) and improving pollutant degradation. Jiang [33] investigated the role of Fe²⁺ in enhancing ibuprofen (IBP) degradation in a non-thermal plasma (NTP)-activated bisulfite system, achieving a 94.8% removal rate, a 43.1% increase compared to the system without Fe²⁺. Wang [34] developed a water film DBD plasma system activated by sulfite (S (IV)), increasing IBP removal efficiency by 22.3%. S (IV) also provided a cost-effective alternative to persulfate, with reactive species like ·OH and SO₄⁻ playing key roles in the degradation process, while toxicity tests showed significant reduction in toxicity after treatment.

Although DBD plasma and Fe³⁺/sodium sulfite systems have shown promise, few studies have investigated their synergistic effects on CBZ degradation. This research aims to combine DBD plasma technology with the Fe³⁺/sodium sulfite oxidation system to enhance CBZ degradation. By examining the effects of sodium sulfite concentration, Fe³⁺ concentration, and pH, the study will optimize experimental conditions using response surface methodology (RSM). Additionally, it will analyze degradation products and reaction pathways to provide deeper insights into CBZ degradation mechanisms.

This study has the potential to improve the efficiency of treating persistent pollutants while contributing to the development of more effective water treatment technologies. By uncovering the mechanisms of CBZ degradation in the DBD− Fe³⁺/sodium sulfite system, this work may provide valuable insights for future studies focused on similar organic pollutants.

2. Materials and Methods

2.1. Dielectric Barrier Discharge System

The experimental system, depicted in Figure 1, consists of several key components: a reactor, a high-voltage high-frequency power supply, an electromagnetic air pump, a gas mass flow meter, a peristaltic pump, a magnetic stirrer, an oscilloscope, and voltage and current probes. The reactor is constructed from transparent quartz glass, which has a high dielectric constant (ε = 3.70).

The reactor is structured with a lower base and an upper quartz outer tube. The outer tube is equipped with three evenly spaced orifices, each measuring 1 mm in height. This design allows the treated wastewater, which is circulated by the peristaltic pump, to flow out through the orifices and form a thin water film on the inner wall of the tube, thereby increasing the effective reaction area. The outer tube has an inner diameter of 14 mm, an outer diameter of 18 mm, and a height of 220 mm. It is wrapped in tin foil, which is connected to a grounding wire, serving as a grounding electrode.

Inside the reactor, a quartz inner tube, 210 mm long with an inner diameter of 6 mm and a wall thickness of 1 mm, is placed. The bottom of the inner tube is filled with a 15 mm layer of dried fine sand to prevent tip discharge phenomena. Above this sand layer, 100-mesh copper powder is used as the high-voltage discharge electrode. During the filling process, the quartz tube is gently tapped to ensure the copper powder is uniformly compacted, preventing any gaps that could lead to uneven discharge.

To maintain the inner quartz tube in the center of the outer tube, waterproof tape is wrapped around the section where the inner tube aligns with the center hole of the pagoda head, allowing for precise insertion. The discharge gap between the inner and outer quartz tubes is set at 3 mm, facilitating the simultaneous flow of water and air through the discharge area. An intake hole, 4 mm in diameter, is located on the side of the pagoda head, allowing gas to flow between the high-voltage and grounding electrodes for ionization. At the bottom center of the outer quartz tube, a 4 mm outlet hole connects to a beaker via a plastic tube, enabling the recirculation of treated wastewater through the discharge area.

During experiments, the voltage from the power supply is controlled using the oscilloscope to facilitate discharge within the reactor. The gas flow meter and peristaltic pump work together to regulate the gas produced by the air pump and the recirculating water, ensuring a uniform mixture in the electric discharge zone and thus enhancing both reaction efficiency and degradation effectiveness.

2.2. Experimental Reagents and Equipment

The reagents used in this study included: CBZ, humic acid, p-benzoquinone (Maclean Biochemical Technology Co., Ltd., Shanghai, China), anhydrous sodium sulfite, sodium dihydrogen phosphate (Xilong Science Co., Ltd., Shantou, China), sodium hydroxide, sulfuric acid (China National Pharmaceutical Group Co., Ltd., Beijing, China), sodium bicarbonate (China National Pharmaceutical Group Co., Ltd., Beijing, China), and sodium chloride (China National Pharmaceutical Group Co., Ltd., Beijing, China). None of the reagents were purified before use, and all solutions were prepared with deionized water.

The following equipment was used: high-frequency high-voltage power supply (CTP-2000KP; Nanjing Suman Plasma Technology Co., Ltd., Nanjing, China), oscilloscope (RIGOL-MSO4034; Puyuan Fine Electronics Technology Co., Ltd., Suzhou, China), current and voltage probe (RIGOL-RP3500A; Puyuan Fine Electronics Technology Co., Ltd., Suzhou, China), peristaltic pump (BT300-2J; Baoding Lange Constant current Pump Co., Ltd., Baoding, China), electromagnetic air pump (ACO-004; Xingcheng Electromechanical aquarium Supplies Company, Chaozhou, China) and gas mass flowmeter (LZB-3WB; Taizhou Junhai Instrument Co., Ltd., Taizhou, China), Dual Beam UV–Vis Spectrophotometer (TU-1950; Beijing Purkinje GENERAL Instrument Co., Ltd., Beijing China), High Performance Liquid Chromatography-Mass Spectrometry (Qtof6550; Agilent Technologies Inc., Santa Clara, CA, USA).

2.3. Setting of Initial Experimental Conditions

2.3.1. Degradation of CBZ Wastewater by DBD

This experiment utilized a beaker containing 250 mL of CBZ wastewater with an initial pH value of 5.0 and an initial concentration of 10 mg/L. During the experiment, various parameters such as discharge voltage, airflow rate, solution volume, and initial pH value were altered to investigate their impact on the removal efficiency of CBZ. The discharge duration was set to 4 min, with samples collected every 30 s to determine the concentration of CBZ in the solution post treatment and calculate the degradation efficiency. At a CBZ concentration of 10 mg/L and an initial pH value of 5.0, different concentrations of anions and humic acids were added to the CBZ wastewater to examine their effect on the degradation of CBZ.

2.3.2. Degradation of CBZ by DBD in Conjunction with Catalysts

To identify the optimal conditions for the highest degradation efficiency of CBZ in a system combining DBD with the Fe³⁺/sulfite system, we examined the effects of sulfite concentration, pH, and Fe³⁺ concentration. The concentrations of sulfite were set at 0.1 mM, 0.55 mM, and 1.0 mM; pH levels at 3, 5, and 7; and Fe³⁺ concentrations at 0.5 mg/L, 1.0 mg/L, and 1.5 mg/L.

2.3.3. Reactive Species and Degradation Mechanisms

In the DBD system combined with the Fe³⁺/sulfite system, 250 mL of CBZ wastewater with an initial pH of 5.0 and concentration of 10 mg/L was treated under high-voltage discharge for 4 min, with sampling every 30 s. The degradation mechanism and pathways of CBZ were analyzed using UV–Vis, EPR, and LC-MS.

2.4. Analytical Methods

CBZ concentration was measured by HPLC. The mobile phase consisted of 0.5% acetic acid and acetonitrile (55:45 v/v), and the flow rate was 0.8 mL/min. The detection wavelength was set at 285 nm. The degradation efficiency for CBZ was estimated as follows:

$$\mathsf{\eta }(\%)={\displaystyle \frac{{\mathrm{c}}_0-{\mathrm{c}}_{\mathrm{t}}}{{\mathrm{c}}_0}}\times 100$$

(1)

where c₀ was the CBZ initial concentration, and c_t was the CBZ concentration at an arbitrary time. The total organic carbon of the treated CBZ was measured by a total organic carbon analyzer (Vario TOC). The concentration of H₂O₂ in solution was analyzed by the potassium titanium (IV) oxalate method. Analysis of ozone in solution was by indigo disulfonic acid sodium spectrophotometry.

The discharge power was calculated using the Lissajous figure method, and the calculation equation was as follows:

$$p={\displaystyle \frac{1}{T}}{\int }_0^{T}P\left(t\right)dt=f\times {\int }_0^T{u}_1\left(t\right)\times c\times d{u}_2=c\times f\times S$$

(2)

where P (w) was the discharge power, T (ns) was the total time of each discharge, f (kHz) was the discharge frequency, c (0.47 μF) was the measuring capacitance, S was the area of the Lissajous figure, $u_1$ (t) was the instantaneous voltage, and $u_2$ (kV) was the voltage on the capacitance.

The energy efficiency (Y, mg/(kw·h)) is defined as the CBZ degradation divided by the discharge power:

$$\mathrm{Y}={\displaystyle \frac{{36\times c}_0\times V\times \eta }{Pt}}$$

(3)

where $c_0$ (mg/L) was the initial concentration of CBZ, V (mL) was the volume of degradation solution, η (%) was the degradation efficiency of CBZ, P (w) was the discharge power, and t (s) was degradation time.

3. Results and Discussion

3.1. Optimizing CBZ Degradation: Impact of Key Operational Parameters

The experiment investigated the comprehensive effects of environmental conditions such as voltage, gas flow rate, solution volume, and pH value on the degradation efficiency of CBZ, aiming to optimize experimental parameters to enhance treatment efficiency and provide references for subsequent experimental designs.

The results revealed that the discharge voltage is crucial for generating sufficient reactive species to oxidize CBZ [35]. At 21 kV, nearly complete degradation efficiency was observed, which not only ensured effective degradation of CBZ but also considered energy consumption and equipment safety (Figure 2a). This is because a higher discharge voltage increases the energy input to the plasma reactor, facilitating the activation of gas and water molecules in the discharge zone, thereby forming reactive chemical species such as hydroxyl radicals (OH), superoxide (·O₂⁻), high-energy electrons, ozone (O₃), and hydrogen peroxide (H₂O₂), along with physical effects like ultraviolet light and shock waves. These reactive oxygen species can interact with the water film and further react with CBZ, enhancing its degradation efficiency.However, at lower discharge voltages, the discharge phenomenon may be less pronounced or uneven due to the presence of gas and water flow. Ground-state gas and water molecules require sufficient energy to achieve ionization; if the energy is too low, complete ionization cannot occur. As the discharge voltage increases, the electric field strength in the discharge area also rises, enhancing electron energy and creating a more uniform and dense discharge filament, which makes it easier for gas and water molecules to be excited into reactive species. Despite the advantages of higher voltage in promoting the reaction, excessive energy input may lead to energy waste and increase the risk of quartz glass rupture, causing the discharge to transition from uniform discharge to spark discharge. This transition negatively impacts the stability of experimental results and shortens the lifespan of the reaction apparatus. Additionally, under the direct influence of DBD plasma, CBZ molecules undergo ionization and dissociation, particularly at the benzene ring and C=N bonds, which are more prone to cleavage, resulting in primary degradation products. By adjusting the discharge voltage, the degradation efficiency of CBZ can be optimized, leading to more efficient water treatment. Regarding the gas flow rate, in the discharge area most reactive species are generated from the plasma formed by gas breakdown. a higher gas flow rate reduces the residence time of reactive species like ozone in the discharge area, which shortens the contact time with the organic compound CBZ and subsequently lowers its degradation efficiency. The experiments indicated that a flow rate of 0.8 L/min, under which the reaction efficiency was highest, was most suitable, thereby defining the optimal point in the dynamic balance between the generation of reactive species and the interaction with CBZ molecules (Figure 2b). The amount of reactive species generated in the discharge area is fixed. An increase in the volume of circulating water leads to a reduced frequency of carbamazepine molecules passing through the discharge zone per unit time. This decrease in encounters between carbamazepine molecules and reactive species ultimately results in a lower degradation efficiency of carbamazepine. A treatment solution volume of 250 mL ensured efficient utilization of reactive species, achieving maximal degradation of CBZ while considering the energy efficiency of the reaction system (Figure 2c). The pH value significantly influenced the generation and reactivity of active species, with higher degradation efficiency of CBZ under acidic conditions. This was attributed to the increased production of hydroxyl radicals and other reactive species at lower pH values [36], which are key factors for the efficient degradation of CBZ (Figure 2d).

3.2. Investigation of the Optimal System

To further validate the efficacy of combining DBD technology with the Fe³⁺/sulfite system, we explored the impact of various system parameters on the degradation efficiency of CBZ. As depicted in Figure 3, integrating DBD with the Fe³⁺/sulfite system significantly enhanced the degradation of CBZ in solution. After 2 min of treatment, the degradation efficiency of CBZ reached 84.76%, and this efficiency further increased to 97% after 4 min, surpassing the degradation outcomes observed with standalone DBD, DBD/sodium sulfite, and DBD/Fe³⁺ systems. This improvement is attributable to the reactive species generated during the discharge process, which lower the pH of the solution, thereby creating an acidic environment. With an ample supply of oxygen, Fe³⁺ is catalyzed by plasma to react with sulfite, leading to the formation of Fe²⁺ and sulfate radical ions. Subsequently, Fe²⁺ ions participate in Fenton reactions with hydrogen peroxide, producing a plethora of highly oxidizing ·OH radicals [37]. The synergistic effect of ·OH and sulfate radicals facilitates the degradation of CBZ.

As shown in Figure 4, using the integration function in the Origin software (2021 version) to calculate the area of the Q–V Lissajous figures, the areas for the single DBD system and the optimized synergistic system were found to be 16.910 and 14.209, respectively. Given that these areas include 2.5 cycles, the areas per cycle in the Lissajous figures are 6.764 and 5.684, respectively. The output frequency of the power supply is 8.4 kHz, with a sampling capacitance of C = 0.47 μF and a voltage sampling ratio of 1000:1, denoted as K = 1000. The attenuation factors for CH1 and CH2 signal acquisition were Kx = 1 and Ky = 1, respectively. By applying formulas (2) and (3), the calculated output power and energy efficiency for the single DBD and synergistic systems were 26.704 w and 22.440 w, and 1247.706 mg/(kw·h) and 1620.989 mg/(kw·h), respectively. Evidently, the optimized synergistic system exhibits higher energy efficiency, thus more effectively enhancing the degradation of CBZ.

3.3. Investigation of Factors Affecting Degradation Efficiency Under the Optimal System

In this study, we examined in detail the impact of reaction conditions on CBZ degradation efficiency. An increase in sulfite concentration was expected to enhance the generation of reactive species (Figure 5a). However, the results indicated that at a constant concentration of Fe³⁺, higher concentrations of sulfite inhibited CBZ degradation. Conversely, increasing the concentration of Fe³⁺ significantly enhanced the degradation efficiency of CBZ, owing to the fact that, under the influence of DBD plasma, oxygen and water molecules in the air are excited to generate high concentrations of reactive species such as OH, singlet oxygen (¹O₂), and O₂⁻. These species effectively activate Fe³⁺ in the solution, facilitating its conversion to Fe²⁺. In a synergistic system with a fixed concentration of Fe³⁺, excess sulfite can consume reactive sulfur species, thereby inhibiting the degradation of CBZ. However, Fe²⁺ participates in Fenton reactions with H₂O₂ to produce stronger oxidants like ·OH, promoting the degradation of CBZ. This reaction pathway lowers the redox potential of Fe³⁺, enhancing the oxidative capability of the system and laying the groundwork for the activation of sulfite. Fe²⁺ further reacts with sulfate (SO₃²⁻) to generate sulfate radicals (SO₄⁻), which exhibit significant oxidizing power, enabling them to undergo electron transfer reactions with CBZ and disrupt its molecular structure, leading to the formation of oxidative intermediates and final products. Additionally, SO₄⁻ can react with water molecules to generate even stronger oxidants, such as OH, providing multiple reactive oxygen species for the degradation system. This process significantly enhances the overall oxidative capacity of the system and promotes the degradation of CBZ. (Figure 5b). Additionally, the highest degradation efficiency was observed under acidic conditions (pH = 3). This is because the optimal pH for the self-circulating system of Fe²⁺ and Fe³⁺ is between 3 and 5, and sulfite catalysis requires a certain level of acidity to occur. Additionally, under different pH conditions, the form of sulfite varies; at excessively low pH, sulfurous gas may be released, suggesting that the reactivity of the Fe³⁺/sulfite system was maximized at this pH value (Figure 5c). These findings not only reveal the importance of regulating pH values and reactant concentrations to optimize the degradation process of CBZ, but also provide strategies for the effective treatment of recalcitrant organic pollutants.

3.4. Analysis of Response Surface Methodology

Based on the experiments conducted, the primary factors influencing the degradation of CBZ in solution were determined to be the sodium sulfite and Fe³⁺ concentrations and the pH value. To investigate the optimal combination of these three factors for degradation efficiency under the optimal system, an experimental plan with the three factors at three levels was designed using the Box-Behnken Design method in Design-Expert 10 software, as shown in

Table 1. Factor levels for response surface analysis.

Variable	Level
	−1	0	1
A-Sodium metabisulfite concentrationm (M)	0.1	0.55	1
B-pH	3	5	7
C-Fe3+ concentration(mg/L)	0.5	1	1.5

.

To model the degradation efficiency of CBZ (Y) as the response variable, the model is formulated as follows:

$$\mathrm{Y}={\gamma }_0{+\gamma }_{1}A+{\gamma }_{2}B+{\gamma }_{3}C+{\gamma }_{12}AB+{\gamma }_{13}AC+{\gamma }_{23}BC+{\gamma }_{11}{A}^2+{\gamma }_{22}{B}^2+{\gamma }_{33}{C}^2$$

(4)

where Y represents the degradation efficiency of the CBZ solution, γ₀ is the constant term, γ₁, γ₂, and γ₃ are the coefficients for the interaction terms, and γ₁₁, γ₂₂, γ₃₃, γ₁₃, γ₂₃, and γ₁₃ are the coefficients for the quadratic terms.

Table 2. Response surface experimental design and results.

Sample	A	B	C	CBZ Degradation Efficiency	CBZ Degradation Efficiency
	Sodium Metabisulfite Concentration (M)	pH	Fe3+ Concentration (mg/L)	Experimental Value (%)	Predicted Value (%)
1	0.1	3	1	96.000	99.940
2	1	3	1	70.836	70.610
3	0.1	7	1	93.600	93.830
4	1	7	1	60.211	56.270
5	0.1	5	0.5	83.334	81.260
6	1	5	0.5	30.911	33.010
7	0.1	5	1.5	92.869	90.770
8	1	5	1.5	70.057	72.130
9	0.55	3	0.5	68.327	66.460
10	0.55	7	0.5	55.384	57.230
11	0.55	3	1.5	93.609	91.770
12	0.55	7	1.5	78.687	80.550
13	0.55	5	1	66.000	62.800
14	0.55	5	1	60.000	62.800
15	0.55	5	1	62.000	62.800
16	0.55	5	1	63.000	62.800
17	0.55	5	1	63.000	62.800

presents the design and results of the response surface analysis. By utilizing Design-Expert 10 software to perform a multiple regression fit on the CBZ degradation efficiency data contained within the table, a quadratic polynomial regression equation targeting the CBZ degradation efficiency as the response variable can be derived:

Y = 62.8 − 16.72A − 5.11B + 12.16C − 2.06AB + 7.40AC − 0.49BC + 6.33A² + 11.04B² + 0.17C²

(5)

By employing the analysis function in the Design-Expert 10 software to study the constructed model, we predicted the degradation efficiency of CBZ under optimal conditions. The model analysis indicated that with initial experimental conditions set at a sodium sulfite concentration of 0.102 mM, an Fe³⁺ concentration of 1.457 mg/L, and a pH value of 3.32, the predicted maximum degradation efficiency of CBZ reached 100%. To validate the accuracy of the model’s prediction, three parallel degradation experiments were conducted under these optimal conditions. The experimental results showed CBZ degradation efficiencies of 97.2%, 98.6%, and 98.5%, respectively, with an average degradation efficiency of 98.1%. Compared to the model’s predicted maximum degradation efficiency, the experimental results had an error margin of only 1.9%, confirming the model’s reliability and adaptability.

3.5. Exploration of the Optimal Degradation System for CBZ

As illustrated in Figure 6, the degradation mechanism of CBZ in a DBD plasma system enhanced by Fe³⁺ and sulfite was investigated using ROS scavengers to elucidate the roles of specific oxidants. The addition of methyl isopropyl alcohol (MIPA), a hydroxyl radical (OH) scavenger, significantly inhibited CBZ degradation, indicating that ·OH is a primary oxidant in the process. Similarly, the introduction of histidine, which targets singlet oxygen (¹O₂), led to a notable reduction in degradation efficiency, highlighting the involvement of ¹O₂ in CBZ oxidation. p-Benzoquinone (PBQ), a superoxide anion radical (O₂⁻) scavenger, also decreased CBZ degradation but to a lesser extent, suggesting that while ·O₂⁻ contributes to the reaction, it plays a secondary role compared to OH and ¹O₂. These findings indicate a complex degradation pathway in which multiple ROS, particularly ·OH and ¹O₂, act synergistically to enhance CBZ breakdown in the plasma system. The Fe³⁺/SO₃²⁻ combination promotes ROS generation, with Fe³⁺ participating in Fenton-like reactions to produce ·OH and SO₃²−, aiding in the formation of ¹O₂ and ·O₂⁻, resulting in a broad spectrum of oxidative effects. This study demonstrates that DBD plasma coupled with Fe³⁺/SO₃²⁻ significantly enhances CBZ degradation, offering a promising approach for the removal of recalcitrant pollutants in wastewater treatment.

3.6. Analysis of Degradation Pathways for CBZ

Figure 7 displays the UV–Vis spectral changes of the CBZ solution during the degradation process in the DBD system combined with Fe³⁺/sulfite. Scanning within the wavelength range of 200 nm to 500 nm revealed a significant principal absorption peak at 285 nm before the reaction, indicating the presence of benzene rings within the CBZ molecular structure. As the discharge process progressed, the absorption peak at 285 nm gradually diminished, suggesting the destruction of the CBZ molecular structure and the stepwise decomposition of CBZ molecules.

Furthermore, the absorbance in the regions of 200 nm to 260 nm and 315 nm to 385 nm gradually increased with degradation time. This change might indicate the formation of small molecular intermediates containing benzene ring structures and possibly hydroxylated benzene ring products during the CBZ degradation process, leading to an increase in absorbance. These spectral changes provide direct evidence for the formation of intermediates during the CBZ decomposition process, revealing the degradation pathways of CBZ in the DBD system combined with Fe³⁺/sulfite.

To comprehensively elucidate the degradation mechanism of CBZ and predict its degradation pathways, we employed the MULLIKEN analysis function of the GaussianView 6.0 software. Through this approach, a detailed analysis was conducted on the natural charge distribution of atoms within the CBZ molecule and the bond lengths within the molecule [38]. The related results are shown in

Table 3. Main bond length of the CBZ molecule and natural charge distribution of atoms.

Atom	Natural Charge	Atomic Bond	Bond Length Å
N27	0.687	N27-C26	1.374
O30	−0.283	O30-C26	1.231
N25	−0.212	N25-C26	1.395
C26	0.714	N25-C7	1.433
C1	−0.087	N25-C12	1.432
C3	−0.111	C1-C3	1.399
C5	−0.081	C3-C5	1.394
C7	0.113	C5-C7	1.399
C8	0.161	C7-C8	1.413
C9	−0.158	C8-C9	1.409
C11	0.144	C8-C21	1.462
C12	0.178	C21-C23	1.553
C13	−0.077	C23-C11	1.463
C15	−0.119	C11-C12	1.411
C17	−0.090	C12-C13	1.393
C19	−0.161	C13-C15	1.392
C21	−0.347	C15-C17	1.399
C23	−0.342	C17-C19	1.390
		C19-C11	1.410

.

Through this approach, potential bond-breaking sites within the CBZ molecule and sites that might be attacked by reactive species can be theoretically analyzed. This analysis provides important information about the chemical reaction kinetics during the CBZ degradation process, providing a comprehensive understanding of the CBZ degradation mechanism.

Based on the theoretical analysis, in this study, we preliminarily inferred potential intermediate degradation products of CBZ, such as 10,11-epoxycarbamazepine [39]. To further explore and verify other potential intermediate products formed during the CBZ degradation process, LC-MS was employed for detailed analysis. Moreover, by applying LC-MS analysis software (Q-TOF B.08.00), we successfully identified eight degradation products of CBZ. The molecular formulas, molecular weights, and structural formulas of these products are detailed in

Table 4. CBZ and its main degradation products.

Serial Number	Molecular Formula	Molecular Weight	Structural Formula
P-237	C15H12N2O	237
P1-267	C15H10N2O3	267
P2-253	C15H12N2O2	253
P3-284	C15H12N2O4	284
P4-208	C14H9NO	208
P5-196	C13H9NO	196
P6-224	C14H9NO2	224
P7-180	C13H9N	180
P8-271	C15H14N2O3	271

.

In this study, we extensively explored the three main degradation pathways of CBZ during advanced oxidation processes (Figure 8). Initially, owing to the significant electrophilic nature of hydroxyl radicals, they first attack the electron-rich olefinic double bonds in the CBZ molecule, forming epoxycarbamazepine, a specific hydroxylated product [40]. Previous research has confirmed that 10,11-epoxycarbamazepine (CAS: 36507-30-9) is the primary intermediate product formed during advanced oxidation processes [41]. This product further reacts with hydroxyl groups, leading to C-O bond cleavage, followed by a series of chemical transformations, eventually mineralizing into water and carbon dioxide. The second pathway involves the strong oxidizing action of ozone, singlet oxygen, and hydroxyl radicals produced by DBD, which can attack the carbon–carbon double bonds in the intermediate product P253, triggering an epoxide ring-opening reaction to form the alcohol-containing P271. P271 is transformed into P267 [41] through a dehydration cyclization reaction. This intermediate then undergoes a series of transformation and oxidation steps, ultimately converting into water and carbon dioxide. In the third pathway, the olefinic double bonds in CBZ are attacked by active oxygen species such as ·OH, forming intermediates containing carboxyl and aldehyde groups (P284) [42]. Subsequently, through the reaction of the amino and carboxyl groups, the acylamide-containing P267 is formed. P267 then undergoes a series of conversion and reduction steps, ultimately transforming into water and carbon dioxide.

4. Conclusions

In this study, we explore a novel advanced oxidation process combining DBD technology with the Fe³⁺/sodium sulfite oxidation system to degrade CBZ, a recalcitrant organic pollutant in water. The experimental results demonstrate that this composite system effectively generates strong oxidizers such as hydroxyl radicals through the activation of sodium sulfite and its interaction with Fe³⁺, significantly enhancing the efficiency of CBZ degradation. Compared to traditional water treatment techniques and other advanced oxidation processes, the DBD technology integrated with the Fe³⁺/sodium sulfite system not only offers high degradation efficiency but also features ease of operation, minimal secondary environmental pollution, and low energy consumption. The use of RSM further optimizes operational parameters, ensuring optimal experimental conditions and improving the repeatability and reliability of the experiments. Overall, this study provides an effective technological approach for treating waterborne recalcitrant organic pollutants, offering significant environmental protection implications and promising applications.