Enhanced Sulfamethazine Degradation over a Wide pH Range by Cost-Effective Zero-Valent Iron-Based Electro-Fenton/Sulfite Process

Abstract

Sulfamethazine (SMT) is an antibiotic with good antimicrobial effect and is widely used to treat human and livestock diseases. Though the degradation of SMT by the conventional Fenton and electro-Fenton (EF) processes is efficient, it is limited by a narrow pH and iron sludge generation. Herein, we constructed a cost-effective EF system with the synergistic effect of zero-valent iron (Fe⁰) and sulfite (Fe⁰-EF/Sulfite), and key parameters such as applied current, catalyst dosing, sulfite dosage, and initial pH were optimized. Under the optimal conditions (Fe⁰ dosing of 50 mg/L, sulfite dosage of 1.5 mM, current of 40 mA, and pH of 3), the removal efficiency of 10 mg/L SMT reached 100% within 30 min, and the degradation rate constant reached 0.194 min⁻¹. Electron paramagnetic resonance (EPR) analysis and quenching experiments confirmed the generation of various reactive oxygen species (ROS), such as ^•OH, SO₄⁻, O₂⁻, and ¹O₂, which significantly improved the pollutant removal efficiency. Sulfite accelerated iron cycling and inhibited the formation of iron sludge, thus broadening the pH range of the reaction from three to eight and overcoming the limitations of the conventional EF process. The Fe⁰-EF/Sulfite system performs cost-effectively at a wide pH range, providing an efficient and low-carbon solution for environmental pollution remediation with broad application prospects.

1. Introduction

The rapid growth of society and the economy in recent decades has led to an increased use of antibiotics, particularly sulfonamide antibiotics, such as sulfamethazine (SMT). SMT is widely used in both human and livestock applications and is frequently detected in groundwater, wastewater, and surface water [1,2]. The persistent presence of SMT in aquatic systems is a growing concern due to its potential to induce environmental toxicity and contribute to the development of antibiotic resistance, which poses significant risks to both ecological systems and human health [3,4].

Given the widespread use of SMT and its environmental persistence, the need for novel and efficient methods for removing SMT from aquatic environments has become critical [4,5]. Traditional water treatment technologies, such as biological, adsorption, membrane filtration, and photodegradation processes, have been explored for organic pollutant removal. Among these, advanced oxidation processes (AOPs) have attracted significant attention due to their ability to generate reactive oxygen species (ROS), such as hydroxyl radicals (^•OH), superoxide (O₂^•−), singlet oxygen (¹O₂), and sulfate radicals (SO₄^•−), which are highly effective at degrading organic pollutants [6].

Electro-Fenton (EF) is a promising AOP that has been widely studied for the degradation of various organic pollutants [7]. This process involves the in situ generation of hydrogen peroxide (H₂O₂) on the cathode surface through the 2e⁻ oxygen reduction reaction (ORR) (Equation (1)) [8,9]. The introduction of Fe²⁺ catalyzes the Fenton reaction, leading to the production of ^•OH and the non-selective oxidation of organic contaminants (Equation (2)) [10]. However, the narrow pH range (about three) slows the iron ion reduction rate, and difficulties in catalyst recovery limit the broad applicability of the traditional EF process [11,12].

O₂ + 2H⁺ + 2e⁻ → H₂O₂

(1)

Fe²⁺ + H₂O₂ → Fe³⁺ + ^•OH + OH⁻

(2)

Sulfate radical-based AOPs (SR-AOPs) are also a potential technology for environmental remediation, where SO₄^•− has strong oxidative properties that enable it to effectively degrade organic pollutants [13]. In previous studies, SO₄^•− can be obtained by the activation of persulfate (PS) or peroxymonosulfate (PMS) by UV, ultrasound, heat, and transition metals [14]. Recently, sulfite has been applied to transition metal-activated SR-AOPs as an alternative to PS/PMS. Compared with PS/PMS, sulfite is an industrial by-product with the advantages of an abundant source, a low cost, and a low ecotoxicity [15]. The development of AOPs using sulfite can enable waste utilization.

Recently, many studies have been conducted to remove organic pollutants using the Fe²⁺/Fe³⁺ activation of cheap sulfite to produce SO₄^•− [16,17]. The composite process of sulfite and iron-based materials has the advantages of a low cost, non-toxicity, ease of operation, and high efficiency, and is expected to be an excellent method for producing SO₄^•−. However, Fe²⁺/Fe³⁺-based SR-AOPs are limited in wastewater treatment due to a high sludge yield, slow conversion from Fe³⁺ to Fe²⁺, and the consumption of SO₄^•− by excess Fe²⁺ [18]. Zero-valent iron (Fe⁰) has attracted much attention as an environmentally friendly and cost-effective material, especially as an alternative to Fe²⁺ catalysts in the heterogeneous Fenton reactions [19,20]. Fe⁰ could ease the consumption of SO₄^•− through the slow release of Fe²⁺ from Fe⁰, and sulfite could accelerate iron cycling, so that the use of Fe⁰ was considered as a cheap catalyst for sulfite activation [21].

In this study, we used Fe⁰ and sulfite together in the EF process (Fe⁰-EF/Sulfite) to overcome the limitations of the conventional EF process through mechanisms such as iron cyclic regeneration and multiple ROS generation, and to achieve the efficient degradation of SMT over a wide pH range. The objectives of this study were (1) to explore the feasibility and superiority of the Fe⁰-EF/Sulfite system for SMT removal; (2) to evaluate the effects of key parameters, such as pH, sulfite dosage, applied current, and catalyst dosage, on the removal efficiency of SMT; (3) to identify the major ROS involved in the degradation process and elucidate the reaction mechanism.

2. Results and Discussion

2.1. Characterization of Fe⁰

As shown in Figure 1a, the SEM image reflected the morphology of Fe⁰ used in the experiments, showing a spherical or polyhedral morphology with a relatively smooth and dense surface. The Fe⁰ diffraction peaks used in this experiment were consistent with the standard XRD patterns of Fe⁰ (JCPDS 99-0064) with diffraction peaks at 2θ of 44.7°, 65.1°, and 82.3°, which correspond to the (110), (200), and (211) crystal planes, respectively (Figure 1b) [22].

2.2. Elimination of SMT in Different Systems

SMT elimination using different systems (Fe⁰-EF/Sulfite, Fe⁰-EF, AO-H₂O₂, and Fe⁰-E/Sulfite system) was compared. As shown in Figure 2a, after 60 min of reaction, only 10% of SMT was removed by the AO-H₂O₂ system in the absence of Fe⁰ and sulfite, and the removal of SMT was only 42% in the absence of sulfite activation by the Fe⁰-EF system. The removal of SMT in the Fe⁰-EF/Sulfite system was significantly higher, reaching 90% after 60 min of reaction, suggesting that the synergistic effect of Fe⁰ and sulfite enhanced the degradation of SMT. In order to verify the superior performance of the Fe⁰-EF/Sulfite system, we also designed a heterogeneous electrically activated sulfite (Fe⁰-E/sulfite) system using a stainless-steel mesh as the cathode. This system hardly produced H₂O₂ to drive the EF process, and thus after 60 min of the reaction, the Fe⁰-E/sulfite system had a low SMT removal of only 32%. The degradation of SMT by these systems conformed to apparent first-order kinetics, and the degradation rate constants are shown in Figure 2b. The rate constant of Fe⁰-EF/Sulfite (0.0321 min⁻¹) was much higher than that of the other three systems, which again proved its superiority.

The H₂O₂ accumulation of the different systems was evaluated, as shown in Figure 2c, which was ranked as AO-H₂O₂ system (16.87 mg/L) > Fe⁰-EF system (11.09 mg/L) > Fe⁰-EF/Sulfite system (7.16 mg/L). This result proved that the AO-H₂O₂ system generated H₂O₂ directly through the 2e⁻ ORR, while in the latter two processes the presence of Fe²⁺ catalyzed the decomposition of H₂O₂. The anode was usually dominated by the oxygen evolution reaction (OER) [9], which had no negative effect on H₂O₂ accumulation, so the H₂O₂ accumulation was the highest. The Fe⁰-EF system was the second highest due to its poor iron cycling efficiency, which led to the accumulation of Fe³⁺ and a decrease in H₂O₂ utilization [23]. The Fe⁰-EF/Sulfite system had the lowest H₂O₂ accumulation, indicating that this system had the strongest ability to activate H₂O₂ among the three systems. This is because it generated H₂O₂ in situ at the cathode with an efficient and sustained yield, and Fe⁰ provided a stable source of Fe²⁺ (Equation (3)). Maintaining Fe²⁺ concentration through a dual mechanism of sulfite reduction (Equation (4)) and cathodic electrochemical reduction greatly improved the efficiency of H₂O₂ utilization [23].

We also evaluated the variation in leached total iron concentration with time for the different systems, as shown in Figure 2d. The multiple effects of the electrochemical process, sulfite activation, and Fenton reaction were combined in the Fe⁰-EF/Sulfite system. The cathode generated H₂O₂ in situ through the ORR, which combined with the Fe²⁺ released by Fe⁰ to form a highly efficient EF reaction and produce Fe³⁺. The addition of sulfite allowed it to form a complex with Fe³⁺ and inhibited the formation of Fe(OH)₃, which allowed higher concentrations of iron ions to be detected [23]. Sulfite induced the rapid oxidative dissolution of Fe⁰ [24], and the resulting Fe²⁺ could also directly activate sulfite to generate ROS, with the reaction proceeding efficiently and iron dissolution accelerated. The Fe⁰-E/Sulfite system lacked the direct participation of H₂O₂, which relied on Fe⁰ and electrically activated sulfite to generate ROS. The intensity of the reaction was lower and the amount of iron leaching was lower than that of the Fe⁰-EF/Sulfite system. The Fe⁰-EF system had no participation of sulfite in the reaction and relied solely on the EF reaction to generate ^•OH, which resulted in the lowest reaction intensity. The Fe³⁺ produced formed a precipitate of Fe(OH)₃ at pH = 7 which was not present in the dissolved form. So, iron leaching was the lowest in the Fe⁰-EF system.

Fe⁰ → Fe²⁺ + 2e⁻

(3)

Fe³⁺ + HSO₃⁻ → Fe²⁺ + SO₃⁻ + H⁺

(4)

The trends in sulfite concentration with time for different systems are shown in Figure S1. Sulfite was consumed more in the Fe⁰-EF/Sulfite system than in the Fe⁰-E/Sulfite system, supporting that anodic oxidation also played a significant role in sulfite conversion, in addition to the activation of sulfite by Fe³⁺. Figure S2 showed the difference in SMT removal efficiency between the Fe⁰-EF/Sulfite and Fe⁰-EF systems at different SMT initial concentrations (5–20 mg/L). The Fe⁰-EF/Sulfite system exhibited excellent performance: the removal efficiency reached nearly 100% at 5 mg/L for 30 min, and maintained 94% efficiency at 20 mg/L. However, the efficiency of the Fe⁰-EF system was significantly limited, with the removal efficiency reaching 90% at 5 mg/L, and the decay of the efficiency intensified with increasing concentration (down to 64% at 20 mg/L). This phenomenon implied that the introduction of sulfite enhanced the tolerance to high concentration pollutants by accelerating the iron cycling and altering the pathway of free radical generation.

2.3. Impacts of Key Factors

2.3.1. Effect of Current

Figure 3a shows the effect of current on SMT removal in the Fe⁰-EF/Sulfite system. The SMT removal efficiency increased when the current was increased from 5 mA to 80 mA. The rate constant increased from 0.028 min⁻¹ (5 mA) to 0.131 min⁻¹ (80 mA). Figure 3b shows the changes in electric energy consumption (EEC) for the Fe⁰-EF/Sulfite system under different current conditions. The time-varying curves of the EEC showed a significant current intensity dependence: the EEC of the low current group (5–20 mA) grew gently, while the EEC of the high current group (40–80 mA) grew rapidly. At 60 min, the EEC of the 80 mA increased to 194.4 kWh/kg, which was 2.6 times higher than that of the 40 mA (74.6 kWh/kg). However, the rate constant did not increase significantly compared with that of the 40 mA, which was only increased from 0.107 min⁻¹ to 0.131 min⁻¹. The competition of OER at currents higher than 40 mA might cause a decrease in the energy utilization efficiency [25]. Additionally, the excess current promoted H₂O₂ disproportionation at the cathode which reduced the available H₂O₂ for ROS generation, requiring a higher operational energy to compensate [12,25]. Figure S3 demonstrates the H₂O₂ accumulation at different currents. The H₂O₂ accumulation gradually increased with increasing current density from 2.81 mg/L (5 mA) to 13.70 mg/L (80 mA). Considering the degradation efficiency and EEC, the optimal current for the Fe⁰-EF/Sulfite system was 40 mA.

2.3.2. Effect of Catalyst Dosage

Figure 4a shows the effect of Fe⁰ catalyst dosage on SMT removal in the Fe⁰-EF/Sulfite system. In particular, the increase in Fe⁰ dosage from 10 mg/L to 50 mg/L enhanced the SMT removal from 81% to 100% at 30 min. This is because Fe⁰ was a reactive metal and could activate O₂ to generate ROS, such as H₂O₂ and ^•OH (Equation (5)) [20]. The increased Fe⁰ dosage could also increase the release of dissolved Fe²⁺, which improved the catalytic decomposition of H₂O₂ to ^•OH and promoted the SMT removal. However, as the dosage of Fe⁰ continued to increase, the SMT removal efficiency gradually decreased. The rate constant increased from 0.058 min⁻¹ (10 mg/L) to 0.142 min⁻¹ (50 mg/L) with the increasing Fe⁰ dosage then decreased to 0.081 min⁻¹ (200 mg/L), a decrease that could be attributed to excess Fe⁰ [23]. Figure 4b demonstrates the relationship between the amount of iron leaching and the Fe⁰ dosage. At the low dosage stage (10–50 mg/L), the leaching amount increased linearly with the dosage and sulfite could induce the rapid oxidative dissolution of Fe⁰ [24]. When the Fe⁰ dosage was increased to 100 mg/L, the iron leaching showed an increasing and then decreasing trend with the reaction. Excess Fe⁰ dosing produced excess Fe²⁺, which led to the consumption of ^•OH (Equation (6)). The decrease in iron leaching might also be due to the adsorption of iron on the cathode and reduce interfacial reaction efficiency [23]. Excess Fe⁰ disrupted the optimal Fe²⁺/radical stoichiometry, causing a decrease in the SMT removal efficiency. Therefore, the optimal catalyst dosage for the Fe⁰-EF/Sulfite system was 50 mg/L.

Fe⁰ + O₂ + 2H⁺ → Fe²⁺ + H₂O₂

(5)

Fe²⁺ + ^•OH → Fe³⁺ + OH⁻

(6)

2.3.3. Effect of Sulfite Dosage

Similar to the catalyst dosage, the sulfite dosage also affected the degradation of SMT in the Fe⁰-EF/Sulfite system. The effect of sulfite dosage on the removal of SMT from Fe⁰-EF/Sulfite system was shown in Figure 5a. It could be seen that SMT was completely eliminated at the amount of 1.0 mM sulfite for 60 min. There was a significant increase in the degradation rate constant of SMT as the sulfite dose was increased from 0.5 mM to 1.5 mM, with a decrease in the rate constant when the sulfite dose was further increased to 2.0 mM. As a reducing agent, excess sulfite would capture ^•OH, resulting in a decrease of ^•OH. Therefore, considering the efficiency and economy, the optimal dosage of sulfite was 1.5 mM with a maximum rate constant of 0.191 min⁻¹ (Figure 5a). The consumption rate of sulfite showed a tendency of increasing and then decreasing with the increase in its dosage (Figure 5b). Sulfite could rapidly reduce Fe³⁺ to Fe²⁺, thus accelerating the regeneration of Fe²⁺ and maintaining the continuation of the EF reaction. Sulfite could also be directly involved in the oxidative degradation of SMT through the generation of oxysulfur radicals, further accelerating its own consumption, at which time the rate of sulfite consumption rose with the increase in dosage [6,26]. However, when the sulfite was added in excess, it might quench with the free radicals in the system, leading to a decrease in the utilization efficiency of the free radicals [4,6,27]. The dosing of excess sulfite might increase the pH of the system, inhibit the activation of O₂ by Fe⁰ and the corrosion of Fe⁰, and indirectly slow down the consumption of sulfite [28].

2.3.4. Effect of Initial pH

Previous studies have reported that the removal efficiency was pH-dependent in iron-based catalytic sulfite systems, and that pH affected the distribution of sulfite species and the catalytic activity of iron [2,29]. In addition, the EF process was known to be pH-limited, with optimal efficiencies typically occurring under pH 3, which might limit its application in neutral or alkaline environments. Different initial pH values (3, 4, 5, 6, 7, 8, 9, and 10) were selected to investigate the effect of initial solution pH on the catalytic performance of Fe⁰ for different systems. In the Fe⁰-EF/Sulfite system, SMT was effectively removed under both acidic and neutral conditions, but the removal of SMT decreased under alkaline conditions (Figure 6a). The rate constants were 0.194 min⁻¹ and 0.191 min⁻¹ for initial solution pH values of 3 and 4, respectively, and the SMT removal was better under strongly acidic conditions (Figure 6b). Although the removal efficiency of SMT decreased gradually with increasing pH, the degradation rate constant remained between 0.010 and 0.194 min⁻¹. The optimal pH for the Fe⁰-EF/Sulfite system was 3. SMT (pK_a1 = 2.28, pK_a2 = 7.42) transitions from cationic (pH < 2.28) to zwitterionic (pH = 2.28–7.42) and anionic forms (pH > 7.42) [30]. The zwitterionic domain (pH 3–7) enhanced adsorption onto iron surfaces via dual electrostatic interactions, while so many other systems have been more susceptible to the loss of SMT degradation ability under alkaline conditions [23,31,32]. The Fe⁰-EF/Sulfite system could remain active over a wide pH range, probably because it produced a variety of ROS, such as SO₄^•−, in addition to ^•OH, which broadened the range of pH applications. In the Fe⁰-EF system, the initial pH also had an effect on the removal of SMT, and the removal efficiencies of the Fe⁰-EF system were all lower than those of the Fe⁰-EF/Sulfite system (Figure 6c). The maximum degradation rate constant was 0.073 min⁻¹ under strongly acidic conditions (pH = 3) and only 0.005 min⁻¹ under alkaline conditions (pH = 10) (Figure 6d). This phenomenon could be explained by the introduction of sulfite, which broadened the applicable pH range. Under acidic conditions (pH 3–6), sulfite maintained the Fe²⁺ concentration and enhanced the efficiency of the EF process [24]. Under neutral to weakly alkaline conditions (pH 7–9), sulfite could activate produced SO₄^•−, which was alkaline-tolerant. Additionally, sulfite forms soluble complexes with Fe³⁺ to retard the formation of iron oxides and prevent the catalyst from passivation, thus maintaining activity at a higher pH [23].

Figure 7a shows the variation in iron leaching with pH. The higher amount of iron leaching was observed at low pH conditions because the acidic environment promoted the activation of O₂ by Fe⁰ and the corrosion of its surface, while the reductive regeneration of Fe³⁺ maintained the continuous leaching of iron ions [33]. However, leaching decreased with increasing pH because Fe²⁺ and Fe³⁺ were progressively hydrolyzed to produce Fe(OH)₂ and Fe(OH)₃ precipitates, resulting in a decrease in the soluble iron concentration [12]. Iron leaching stagnated under alkaline conditions, and iron was completely precipitated as hydroxide or oxide, and the amount of iron leaching tended to zero. Figure 7b showed the variation in H₂O₂ accumulation with pH. At low pH, the accumulation of H₂O₂ was higher, the cathode efficiently generated H₂O₂ through the ORR, and H₂O₂ was more stable and decomposed at a slower rate under acidic conditions. As pH increases, the accumulation decreased because H₂O₂ decomposed readily to H₂O and O₂ under alkaline conditions [34]. The rate of sulfite consumption was shown in Figure 7c. When the initial pH increased, the amount of iron leaching decreased. The redox reaction of Fe³⁺ with sulfite under acidic conditions proceeded rapidly, and sulfite was consumed in large quantities as a reducing agent [23]. At neutral to weakly alkaline conditions, the sulfite consumption rate decreased, and the elevated pH led to the hydrolysis of Fe³⁺ to produce an Fe(OH)₃ precipitate, which reduced the effective Fe³⁺ concentration, and thus the rate of the sulfite reduction reaction decreased [23,35]. Under alkaline conditions, the Fe⁰ corrosion was inhibited and sulfite consumption was essentially stagnant [36]. An analysis of the effect of pH on the rate of SMT degradation (Figure 6) confirmed that sulfite increased the reaction rate, but did not increase the useful pH range. The reason for this could be the formation of Fe(OH)_x under high pH conditions, which had a low solubility and strong adsorption properties [19]. It was possible that the removal of SMT at high pH was caused by sorption or the formation of coordination compounds and not by degradation [19,30].

2.4. Mechanism of the Fe⁰-EF/Sulfite System

The electron paramagnetic resonance (EPR) technique was used to study the ROS involved in the reaction in the Fe⁰-EF/Sulfite system. The H₂O₂ generated by the ORR occurring at the cathode was decomposed by the Fe²⁺ released by Fe⁰ to generate ^•OH in the solution, while Fe³⁺ was generated. The presence of sulfite accelerated the reduction of Fe³⁺ to Fe²⁺, and Fe²⁺ also reacted with sulfite to form the FeHSO₃⁺ complex (Equation (7)), and FeHSO₃⁺ was oxidized by O₂ to form the FeSO₃⁺ complex (Equation (8)), which promoted iron cycling [16]. Fe³⁺ could activate sulfite, triggering a chain reaction to produce SO₃^•− (Equation (9)), which subsequently reacted with O₂ to produce SO₅^•− (Equation (10)), and SO₅^•− was converted to SO₄^•− (Equations (11) and (12)) by the reaction with sulfite [6,23,28]. As shown in Figure 8a, it was possible to observe ^•OH and SO₄^•− characteristic peaks for the binding of DMPO observed in the Fe⁰-EF/Sulfite system, with signals for DMPO-^•OH and DMPO-SO₄^•− observed at 10 min, 30 min, and 60 min. In addition, as the reaction progressed, the peak intensity of DMPO-^•OH increased, while the peak intensity of DMPO-SO₄^•− decreased slightly, suggesting that SO₄^•− was converted to ^•OH (Equations (13) and (14)). Dong et al. and Song et al. also reported the presence of ^•OH and SO₄^•− conversion in the Fe-Cu/sulfite system and FeC/sulfite system during SMT degradation [9,13]. The SO₄²⁻ concentration in solution was measured over time and a progressive increase in SO₄²⁻ concentration was observed to be directly correlated with the extent of SMT degradation (Figure S4). The initial concentration of SO₄²⁻ was 7053 mg/L and the final detected concentration was 7196 mg/L, confirming the accuracy of the ROS generation mechanism.

Figure 8b shows that DMPO-O₂^•− was initially detected within 10 min with little change in intensity with time and remained stable throughout the reaction. This might be due to the dynamic equilibrium between O₂^•− production and consumption. In the reaction, FeSO₃⁺ could react with H₂O₂ to form FeHSO₃⁺ and O₂^•− (Equation (15)) [23]. While O₂^•− had a short lifetime, it might be rapidly consumed by Fe²⁺/Fe³⁺ or pollutants, leading to the maintenance of dynamic equilibrium in its concentration. In addition, TEMP-¹O₂ signals were observed in the Fe⁰-EF/Sulfite system, which appeared at 10 min and gradually enhanced (Figure 8c). Furthermore, ¹O₂ might be converted from O₂^•− (Equation (16)) and accumulation occurred as the reaction proceeded [37]. As shown in Figure S5a,b, the corresponding peak intensities of ^•OH and ¹O₂ produced by the Fe⁰-EF system were significantly lower than those of the Fe⁰-EF/Sulfite system at 30 min. This difference in ROS production clearly confirmed that the Fe⁰-EF/Sulfite system was capable of generating more ROS, which was effective and advantageous in the treatment of SMT.

To reveal the role of ROS in the Fe⁰-EF/Sulfite system, the effects of scavengers, such as methanol (MeOH), tertiary butyl alcohol (TBA), furfuryl alcohol (FFA), and benzoquinone (BQ), on the SMT removal were investigated through quenching experiments. As shown in Figure 8d, the degradation of SMT was significantly inhibited compared with the blank sample without quencher. In the presence of MeOH, TBA, FFA, and BQ, the removal of SMT decreased from 100% to 22%, 37%, 48%, and 43%, respectively, after 60 min of reaction. This implied the presence of reactive substances such as ^•OH, SO₄^•−, O₂^•−, and ¹O₂ in the system, which was consistent with EPR tests indicating that they were the ROS for SMT degradation in the Fe⁰-EF/Sulfite system. In summary, the schematic diagram of the Fe⁰-EF/Sulfite system is shown in Figure 9. The Fe⁰-EF/Sulfite system improved the efficiency of H₂O₂ utilization, promoted iron cycling, and produced a variety of ROS, which improved the removal efficiency of the organic pollutants.

Fe²⁺ + HSO₃⁻ → FeHSO₃⁺

(7)

4FeHSO₃⁺ + O₂ → 4FeSO₃⁺ + 2H₂O

(8)

FeSO₃⁺ → Fe²⁺ + SO₃^•−

(9)

SO₃^•− + O₂ → SO₅^•−

(10)

SO₅^•− + HSO₃⁻ → SO₄^•− + SO₄²⁻ + H⁺

(11)

SO₅^•− + SO₅^•− → 2SO₄^•− + O₂

(12)

SO₄^•− + H₂O → ^•OH + HSO₄⁻

(13)

SO₄^•− + OH⁻ → ^•OH + SO₄²⁻

(14)

FeSO₃⁺ + H₂O₂ → FeHSO₃⁺ + O₂^•−

(15)

O₂^•− + 2H₂O → ¹O₂ + H₂O₂ + 2OH⁻

(16)

2.5. Environmental Applications

To evaluate the environmental adaptability and application potential of the Fe⁰-EF/Sulfite system, we first investigated its stability during multiple reuse cycles. The Fe⁰-EF/Sulfite system was successfully reused for 10 cycles while maintaining >95% SMT removal efficiency under identical experimental conditions (Figure 10a). These results validated the system’s excellent stability and reusability. To address the potential impact of interferences and assess the applicability closer to real scenarios, we specifically investigated the effect of common inorganic ions (Cl⁻, NO₃⁻, HCO₃⁻, and H₂PO₄⁻) and natural organic matter (NOM, simulated using humic acid (HA)) on the degradation efficiency of SMT [38]. As shown in Figure 10b, NOM was found to have a negligible impact on the Fe⁰-EF/Sulfite system. Furthermore, the presence of Cl⁻, NO₃⁻, HCO₃⁻, and H₂PO₄⁻ did not suppress the degradation efficiency. The Fe⁰-EF/Sulfite system was relatively insensitive to background substances in wastewater due to the formation of a variety of ROS. We also evaluated the degradation performance in real water samples. Table S1 presents the quality parameters of mariculture wastewater. The Fe⁰-EF/Sulfite system exhibited a superior removal efficiency in mariculture wastewater and the SMT removal efficiency could reach 100% after only 30 min (Figure 10c). The system was tested in a complex real matrix mariculture wastewater system for direct application, fully verifying the practical utility of the technology.

3. Materials and Methods

3.1. Chemicals and Reagents

The reagents used were all analytical grade. The following chemicals and reagents were procured from Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China): Fe⁰, sodium hydroxide (NaOH), sodium sulfate (Na₂SO₄), potassium titanium oxalate (K₂TiO(C₂O₄)₂), 1,10-phenanthroline, furfuryl alcohol (FFA), and 5,5-dimethyl-1-pyridine N-oxide (DMPO). Sulfuric acid (H₂SO₄), anhydrous ethanol (EtOH), methanol (MeOH), and dimethyl sulfoxide (DMSO) were purchased from Tianjin Kangkede Technology Co., Ltd. (Tianjin, China). Tert-butyl alcohol (TBA) was purchased from Shanghai Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). SMT, sodium sulfite (Na₂SO₃), 5,5′-dithiobis (2-nitrobenzoic acid) (DTNB), hydroxylamine hydrochloride, benzoquinone (BQ), and 2,2,6,6-tetramethylpiperidine (TEMP) were purchased from Tianjin Solomon Biotechnology Co., Ltd. (Tianjin, China). Carbon black (CB) and polytetrafluoroethylene (PTFE) (60 wt.%) were purchased from Shanghai Hesen Electric Co., Ltd. (Shanghai, China). Dimension stable anode (DSA) was bought from Shaanxi Baoji Zhiming Co., Ltd. (Baoji, China). Carbon felt (CF) was purchased from Shanghai Qijie Carbon Material Co., Ltd. (Shanghai, China).

3.2. Experimental Procedure

All the experiments were carried out in 200 mL of solution equipped with a continuous stirrer, with SMT as the target pollutant and 50 mM Na₂SO₄ as the supporting electrolyte. The CF cathode was modified with CB according to our previous studies [9,39]. The 0.28 g CB was mixed with 2 mL ethanol, 2 mL deionized water, and 0.5 mL PTFE, and then the mixture paste was coated on CF (4 cm × 2.5 cm). After drying in air, the modified CF was annealed at 360 °C under air atmosphere for 30 min. DSA and modified CF cathode were used with 1 cm electrode spacing. The desired concentration of Na₂SO₃ was added to the solution. The initial pH was adjusted to values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 using 0.1 M H₂SO₄ and 0.1 M NaOH. After adjusting the pH, the applied current was set at 10–80 mA, and a certain amount of Fe⁰ was added to the 200 mL working solution. Samples (0.5 mL) were taken at predetermined time intervals, promptly quenched with methanol, and filtered through a 0.22 μm filter. The samples were placed in liquid-phase vials for SMT analysis. All experiments were conducted in duplicate.

Several systems were compared with the Fe⁰-EF/Sulfite system. In the anodic oxidation with H₂O₂ (AO-H₂O₂) system, neither sulfite nor catalysts were introduced, and H₂O₂ was produced by the cathode [40]. In the Fe⁰-EF system, Fe⁰ slowly released Fe²⁺ by corrosive oxidation, which was crucial for facilitating the EF process. In the Fe⁰-E/Sulfite system, a stainless-steel mesh was used as the cathode to prevent H₂O₂ formation [9]. The other operational parameters, such as initial pH, current, and sulfite dosage, were maintained the same as in the Fe⁰-EF/Sulfite system.

Quenching experiments were conducted employing specific chemical scavengers. MeOH was employed as a scavenger for both ^•OH and SO₄^•− with rate constants of k_{MeOH, •OH} = 1.2–2.8 × 10⁹ M⁻¹ s⁻¹ and k_{MeOH, SO4•−} = 1.6–7.7 × 10⁷ M⁻¹ s⁻¹ [9]. Conversely, TBA selectively quenched ^•OH with a rate constant of k_TBA,•OH = 3.8–7.6 × 10⁸ M⁻¹ s⁻¹, while its quenching efficiency towards SO₄^•− was lower, with a rate constant of k_{TBA,SO4•−} = 4.0–9.1 × 10⁵ M⁻¹ s⁻¹ [9]. Additionally, BQ and FFA were employed as quenchers for O₂^•− and ¹O₂, respectively [41].

3.3. Characterization and Analytical Methods

Scanning electron microscopy (SEM) images were obtained with a Zeiss LEO-1530VP to observe the morphology. The structure of the catalyst was characterized using X-ray powder diffraction (XRD) on a Philips-12045B/3 diffractometer equipped with Cu Kα radiation.

SMT concentration was determined using a high-performance liquid chromatograph (HPLC, Ultimate 3000, Thermo Fisher Scientic, Waltham, MA, USA) equipped with a diode array detector (DAD). A C18 column was utilized with a column temperature of 30 °C at a flow rate of 0.5 mL/min, and the wavelength was measured at 270 nm. The mobile phase consisted of a water/acetonitrile mixture (65:35, v/v). The method demonstrated good linearity for SMT over the concentration range of 0.1–100 mg/L, with a correlation coefficient (R²) > 0.999. Electron paramagnetic resonance (EPR) spectra were obtained using a Bruker EMX nano spectrometer to identify radicals. DMPO captured ^•OH, SO₄^•−, and SO₃^•−, while DMPO’s methanol solution captured O₂^•−, and TEMP captured ¹O₂. The concentrations of DMPO and TEMP were 100 mM and 10 mM, respectively. Solution pH was measured with a pH meter (FE20, Mettler Toledo, Columbus, OH, USA). The concentration of SO₄²⁻ was determined by ion chromatography (ICS-900, Thermo Fisher Scientific, Waltham, MA, USA) using KOH as the eluent.

The concentration of H₂O₂ was determined by potassium titanium oxalate spectrophotometry at a wavelength of 400 nm [42]. The sulfite concentration was measured using a modified spectrophotometric method with DTNB at a wavelength of 412 nm [43]. Meanwhile, the concentration of iron ions was determined by 1,10-phenanthroline spectrophotometry at a wavelength of 510 nm [44].

The removal efficiency (R, %) was calculated using the following Equation (17) and the degradation process was conformed to a pseudo-first-order kinetic model (Equation (18)):

R = (C₀ − C_t)/C₀ × 100%

(17)

ln(C_t/C₀) = −kt

(18)

where C_t represents the concentration of SMT at t min (mg/L), C₀ is the initial concentration of SMT (mg/L), k is the pseudo-first-order rate constant (min⁻¹), and t is the reaction time (min).

Electric energy consumption (EEC, kWh/kg) was calculated as follows in Equation (19):

EEC = (1000UIt)/(CVs)

(19)

where U is the voltage (V), I is the current (A), t is the time (h), C is the concentration of SMT removal (mg/L), and Vs is the bulk volume (L).

4. Conclusions

In this study, a sulfite–iron cycling synergistic strategy was proposed to construct a Fe⁰-EF/Sulfite system for degrading SMT. The Fe⁰ could activate O₂ to generate ROS and Fe⁰ corrosion promoted the in situ generation of soluble iron. Sulfite accelerated iron cycling and inhibited the formation of iron sludge, thus broadening the pH range of the reaction and overcoming the limitations of the conventional EF process. The important parameters of the system were optimized, and the Fe⁰-EF/Sulfite system could completely remove 10 mg/L SMT within 30 min at a current of 40 mA, sulfite dosage of 1.5 mM, Fe⁰ dosing of 50 mg/L, and an initial pH of 3. EPR and quenching experiments verified the presence of multiple ROS during the degradation of SMT, namely ^•OH, SO₄^•−, O₂^•−, and ¹O₂. The Fe⁰-EF/Sulfite system achieved the efficient degradation of pollutants and could also drive the ROS chain reaction, which significantly reduced the ecological risk of the degradation intermediates and opened up a new path for the development of green water treatment technology.