Application of Different Waveforms of Pulsed Current in the Classical Electro-Cocatalytic Process for Effective Removal of Sulfamethoxazole: Oxidation Mechanisms

Abstract

In this study, sulfamethoxazole (SMX) was applied as the model pollutant to assess the performance of pulsed current (PC) waveforms in the decontamination efficiency of the PC/peroxymonosulfate (PMS)/Fe(III) process and to investigate underlying oxidation mechanisms. Among the various waveforms tested, the sinusoidal wave (SIN), combined with the Dimensionally Stable Anode (DSA) electrode, demonstrated superior degradation performance, with the order being SIN > ramp > square > direct current (DC). The operational conditions for the SIN/PMS/Fe(III) system were optimized to an initial pH of 3, a voltage of 6 V, 0.6 mmol/L of Fe³⁺, 1.0 mmol/L of PMS, and a frequency of 1 kHz. The results of quenching and probe experiments confirmed the generation of abundant reactive radicals such as ^•OH, SO₄^•−, O₂^•−, Fe(IV), and ¹O₂ in the SIN/PMS/Fe(III) process, which collectively enhanced the degradation of SMX. Additionally, results of high-resolution mass spectrometry analysis were employed to identify the SMX oxidation byproducts, and the toxicity of SMX byproducts was evaluated. Overall, the SIN/PMS/Fe(III) process exhibits effective degradation capacity with high energy efficiency, establishing itself as an effective strategy for the practical treatment of medical wastewater.

1. Introduction

As a broad-spectrum antibiotic, sulfamethoxazole (SMX) plays a vital role in the medical therapy of both human and livestock illness, known for commonly being employed in the treatment of urinary tract infections and intestinal infections caused by Escherichia coli and Proteus [1], thus finding widespread application in medical settings. However, SMX has garnered attention due to its pervasive detection and high concentrations in various environmental matrices, including hospital wastewater, municipal wastewater, and groundwater, across different regions worldwide [2]. The discharge of SMX towards the environment causes a direct threat to non-target organisms and significantly amplifies the danger of disseminating drug-resistant bacteria and resistance genes [3], thereby adversely affecting human and environmental health. Consequently, wastewater from hospitals and pharmaceutical facilities containing SMX represents a formidable challenge to environmental ecology and human well-being. Addressing this challenge necessitates urgent scientific research aimed at developing innovative treatment methods to mitigate the environmental impact of SMX contamination.

Over the past few decades, researchers have been dedicated to developing technologies capable of efficiently removing resistant organic contamination from industrial wastewater. Among these technologies, advanced oxidation processes (AOPs) are expected as the most promising [4,5,6]. Typically, AOPs generate short-lived free radicals (^•OH) with powerful oxidizing capabilities under specific conditions (such as light, heat, electricity, or the presence of catalysts) for the oxidation of large, recalcitrant organic molecules into low-toxicity or non-toxic smaller molecules. Electrochemical Advanced Oxidation Processes (EAOPs), an important subset of AOPs, have gained significant reputation recently due to their environmental friendliness and high efficiency [7,8,9,10]. They have been diffusely utilized in the disposal of various organic pollutant-containing wastewaters. In EAOPs, the electro-Fenton technology leverages the interaction between H₂O₂ and Fe²⁺ to produce reactive radicals, which are employed for the removal of contaminations [11,12]. The choice of electrodes in the electro-Fenton method is crucial for the efficiency of organic pollutant oxidation. Dimensionally Stable Anodes (DSAs), such as Ti/RuO₂–IrO₂, are cost-effective; they involve depositing a subtle layer of metal oxide on a base metal (usually titanium), which reacts synergistically with catalysts or anions in water under the influence of an electric current. Graphite electrodes, used as cathodes, are low-cost, exhibit good electrical conductivity, and have demonstrated excellent properties in previous studies [13,14].

In recent years, pulse electrochemistry (PE) has garnered significant attention as an emerging electrocatalytic method, with many scientists seeking its advantages in terms of lower energy consumption and higher degradation efficiency [15,16]. Current research has demonstrated that PE systems have been used in conjunction with catalysts to determine inorganic metal pollutant concentrations [17], detect biological protein levels in biomedicine [18], and drive the periodic regeneration of catalysts [19]. Notably, studies on the degradation of dye wastewater or antibiotics wastewater using PE have found that the •OH radicals generated in pulse systems exceed those in direct current (DC) systems [20,21,22]. Although the mechanism of PE system degradation has not been thoroughly explained, experimental conclusions have provided a broad scope for further research. The waveform of the pulse current has been scarcely studied; thus, considering the impact of the current waveform on the reaction rate and degradation efficiency, we aim to identify the optimal current waveform and evaluate its advantages compared to DC and other pulse current waveforms, and to identify the reasons for its high efficiency and energy performance. Based on a literature report, the primary advantage of pulsed current lies in maintaining high degradation efficiency in the electro-Fenton process while simultaneously reducing energy consumption [20,23]. Moreover, pulsed current systems have been observed to generate new oxidative degradation pathways, potentially resulting in less toxic or non-toxic byproducts, thus being more environmentally friendly [24]. Under the unique parameter control of pulsed current, it is possible to achieve a degradation efficiency and energy utilization efficiency that direct current cannot attain.

Peroxymonosulfate (PMS), commonly used as an oxidant in the electro-Fenton method [25], is rapidly recognized as a selective, effective, and green option for aqua-purification [26]. The electrochemical activation of PMS can enhance PMS activation and decontamination efficiency through both radical oxidation (e.g., ^•OH) and non-radical oxidation pathways [27]. Compared to other ions that can catalyze PMS, such as Co²⁺, Fe³⁺ has advantages including non-toxicity, availability, and low cost [28]. Additionally, the internal recycling reaction of Fe²⁺/Fe³⁺ within the system accelerates the Fenton reaction, resulting in a higher degradation efficiency [29].

Herein, this experiment focused on the following points: (1) investigating the removal efficiency and reaction rate of SMX, as the primary pollutant, and employing the SIN/PMS/Fe(III) system and comparing it with direct current (DC) and other waveform pulse currents; (2) determining the ideal experimental parameters for SMX removal within the SIN/PMS/Fe(III) process by varying factors such as pH, voltage, and Fe³⁺ concentration; (3) performing quenching and probe experiments to identify and quantify the active radicals produced within the SIN/PMS/Fe(III) process, and comparing these results with those from the DC system to clarify the SIN/PMS/Fe(III) system’s advantages; and (4) identifying the byproducts during degrading SMX employing the SIN/PMS/Fe(III) process, assessing their toxicity, and evaluating the practical applicability of the SIN/PMS/Fe(III) process within various water matrices under the influence of different coexisting ions.

2. Results and Discussion

2.1. Impact of System Parameters on Degradation Performance

In the preliminary investigation of the impacts of various current waveforms on the removal rate of SMX, four types of currents were selected: sinusoidal, square, ramp, and direct current. As depicted in Figure 1a,b, the removal efficiency is as follows: sinusoidal > square > ramp > square > direct current. The characterization reveals that the degradation efficacy of various alternating currents surpasses that of direct current. This can be attributed to the continuous conversion of anode–cathode polarity with the current, akin to the practical effect of the pulse duty cycle in experiments with square and ramp waveforms. The duty cycle denotes the percentage of time during the whole cycle that the pulse is applied. Generally, a higher duty cycle extends the discharge duration and enhances the production of active free radicals through the electrode surface, thereby accelerating the efficiency in degrading the pollutant, whereas extremely high duty cycles can induce concentration polarization, which diminishes the material transfer efficiency and subsequently hinders the degradation efficiency [30]. Consequently, a 50% duty cycle was chosen as the ideal condition to achieve both effective degradation and energy efficiency, and this condition was employed in subsequent experiments.

Sinusoidal current continuously reverses the polarity of the electrode, causing ions between the electrodes to undergo relative oscillations under the electric field in a very short period before reaching the electrode, thereby minimizing the likelihood of side reactions such as water electrolysis [20,23]. This minimizes the electrolysis of water reaction and allows more electrical energy to be utilized in catalyzing the PMS process, resulting in a higher degradation efficiency over a certain period compared to other waveform currents. Additionally, polarity switching helps prevent the accumulation of fouling layers on the electrodes, a common issue during extended operation in a fixed polarity mode that can hinder degradation processes [31]. Furthermore, pulsed current systems have been observed to generate novel oxidative degradation pathways, potentially leading to the formation of additional reactive oxygen species (ROS), which are essential for achieving efficient and environmentally friendly degradation [24].

Therefore, waveforms that can adjust generation time under low energy consumption may offer significant advantages. Furthermore, this article aims to enhance the general understanding in this field regarding the application of pulsed current.

During experimentation, optimization experiments were managed based on the SIN/PMS/Fe(III) system, considering initial pH, voltage, alternating current frequency, and iron ion concentration [32,33].

Figure 2a,d illustrate the removal rate of SMX by SIN/PMS/Fe(III) at different initial pH values. An initial pH of 3 was chosen in accordance with the research literature as the first-rank condition for the EAOP process [34], and it resulted in the complete degradation of pollutants under the optimal conditions in this study. As the initial pH increased, the removal rate of SMX decreased, but it is observed that the degradation increased again until the initial pH reached 9~11. This phenomenon may be related to the hydrolysis and precipitation of Fe(III) in an alkaline environment. Some scholars have also noted that at an initial pH of 3 (acidic conditions), the generated ^•OH exhibits a higher potential (2.7 V), and simultaneously, the generated Fe(OH)²⁺ possesses a certain stability in the system [35]. When the initial pH increases to 9~11, PMS decomposes spontaneously to generate ¹O₂ to promote degradation [36], as shown in Equation (1), and ¹O₂ is also an important component of the active species for SMX degradation.

$${\mathrm{HSO}}_5^{-}+{\mathrm{SO}}_5^{2-}\to {\mathrm{HSO}}_4^{-}+{\mathrm{SO}}_4^{2-}+{}^1{\mathrm{O}}_2$$

(1)

Figure 2b,e illustrate the degradation efficiency of SIN/PMS/Fe(III) on SMX at different voltages. As sinusoidal current was employed, the voltage values represented the effective values of the sinusoidal voltage. When the voltages were 3 V, 4 V, 5 V, and 6 V, the efficiencies of the removal of SMX were 33.8, 64.3, 70.0, and 100, respectively. The voltage increased from 2 V to 6 V, achieving complete degradation under the conditions of 6 V. With increasing voltage, the oxidative performance on the anode also increased, simultaneously activating the catalytic performance of PMS to generate ^•OH, O₂^•−, SO₄^•−, ¹O₂, and other active radicals for SMX degradation [37]. However, simultaneous observations of oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER) phenomena are noted, evidenced by bubble formation on the electrode plate, indicating that a portion of electrical energy was reluctantly utilized in the electrolysis of water processes, as depicted in Equations (2)–(4) [38,39].

$${\mathrm{H}}_2\mathrm{O}\to {}^{•}\mathrm{OH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{•}\mathrm{O}+{2\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(2)

$${}^{•}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OOH}}^{•}\to {\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(3)

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$

(4)

The EE/O value is typically used to indicate the energy required (kWh/m³) to decompose the target pollutant in 1 m³ of contaminated water matrix. The EE/O value was calculated based on the following equation for the degradation curves that fitted the pseudo-first-order kinetic model [40].

$$\mathrm{E}\mathrm{E}/\mathrm{O}={\displaystyle \frac{\mathsf{\phi UIt}}{\mathrm{Vlog}\left(\frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{t}}}\right)}}$$

(5)

where φ, U, and I denote the pulse duty cycle, applied voltage (kV), and current (A) of the power supply, and V, C₀, and C_t refer to the wastewater volume, the initial pollutant concentration, and the specific concentration at reaction time t, respectively.

The relationship between different voltages and the corresponding EE/O values is illustrated in Figure S2. As shown, when the voltage was set to 6 V, the EE/O value decreased while the removal efficiency simultaneously reached 100%. Based on these results, the parameter of 6 V was obviously the optimal voltage condition because the highest energy efficiency (EE) and degradation performance were observed in such conditions, despite minor side reactions likely occurring.

Figure 2c,f present the degradation efficacy of SIN/PMS/Fe(III) on SMX under varying alternating current frequencies. Remarkably, when the frequencies were set at 0.1, 1, 10, 100, and 1000 kHz, the removal efficiency of SMX reached 100%. This outcome can be characterized as the notion that a pulse of higher frequency reduces the on-time within the pulse cycle, enhancing the material transfer process. This enhancement facilitates the utilization of ^•OH radicals while reducing their recombination reactions. Consequently, the drawback of low pulse efficiency is mitigated by using a higher pulse frequency [41,42]. As a result, we selected 1 kHz as the optimal parameter.

In the experimental investigation of the impact of Fe(III) concentration on the removal efficiency of SMX, it increased continuously as the Fe(III) concentration increased from 0.1 mmol/L to 0.6 mmol/L, which is depicted in Figure 3a,b. This trend indicates that the decomposition of PMS catalyzes the production of more active species. Notably, SMX achieved complete removal while the Fe(III) concentration reached 0.6 mmol/L, and the following reactions occurred in the solution as shown in Equations (6)–(8) [43,44].

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+}$$

(6)

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{HSO}}_5^{-}\to \mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{SO}}_4^{•-}+{\mathrm{OH}}^{-}$$

(7)

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{HSO}}_5^{-}\to \mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{SO}}_4^{2-}+{}^{•}\mathrm{OH}$$

(8)

However, in Figure 3c,d, when the Fe(III) concentration exceeded 0.6 mmol/L, SMX degradation was hindered. At Fe(III) concentrations of 0.6, 0.7, 0.8, 0.9, and 1.0 mmol/L, the efficiency of the removal of SMX was 100%, 80.3%, 81.3%, 84.7%, and 83.9%, respectively. This is attributed to the excessive Fe(III) concentration leading to the formation of more Fe(OH)₃ precipitated on the cathode. In addition, Fe(OH)₃ at 1 mmol/L at pH 3 has already shown precipitation, and some OH^- was generated in the surrounding area of the cathode; thus, on the cathode, the Fe(OH)₃ precipitate was clearly deposited [45,46], thereby reducing the active sites for H₂O₂ generation [47], resulting in a decreased SMX removal efficiency.

2.2. Effectiveness of Iron Species and PMS in SIN/PMS/Fe(III) Process

Simultaneously, as shown in Figure 4a, by measuring the concentrations of Fe²⁺ and total iron ions, it can be observed that the average contents of Fe(II) and total iron during the experiment are 4.6 and 251.5 μmol/L, respectively. As the conversion from Fe(II) to Fe(III) has been identified as the rate-limiting step within the process [48], it is demonstrated that sinusoidal waveform currents effectively utilize Fe(II) within the system for catalytic conversion. Additionally, some iron ions adsorbed onto the electrode plate from the outset of the reaction, resulting in the decrease in total iron [44]. Nonetheless, a significant portion of iron ions remained in a free form in the container, participating in catalytic cycles.

In the experiment, the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) method was employed in measuring the content of PMS [49,50] in Figure 4b. It was observed that although PMS acted as a catalyst, its content also decreased over time. This reduction is partly due to electrode adsorption, reducing the PMS content in the solution. Additionally, some of the PMS may react with SO₄²⁻ in the solution, producing weaker oxidizing SO₅^•− radicals (as shown in Equations (9)–(11)), leading to the depletion of PMS [51,52].

$${\mathrm{SO}}_4^{•-}+{\mathrm{HSO}}_5^{-}\to {\mathrm{SO}}_5^{•-}+{\mathrm{SO}}_4^{2-}+{\mathrm{H}}^{+}$$

(9)

$${\mathrm{SO}}_4^{•-}+{\mathrm{SO}}_4^{-}\to {\mathrm{S}}_2{\mathrm{O}}_8^{2-}$$

(10)

$${\mathrm{SO}}_4^{•-}+{\mathrm{OH}}^{-}\to {\mathrm{HSO}}_5^{-}$$

(11)

2.3. Removal Efficiency of SMX in Various Processes

From Figure 5a,b, it is evident that in the absence of PMS and iron ions, and in the absence of electrical current, the concentration of SMX remained virtually unchanged. This observation aligns with the characteristic behavior of SMX as a persistent organic pollutant (POPs), while also indicating the weak adsorption of SMX onto DSA and graphite electrodes. Apart from the SIN/PMS/Fe(III) system, which achieved completely degraded SMX, the SMX removal efficiency in the remaining systems was only around 35%. Specifically, the system with only PMS and Fe(III) added demonstrated that the immediate oxidation of PMS and the adsorption of Fe(III) alone were insufficient in effectively degrading SMX [53]. Within the PMS/Fe(III) system, without the support of electrical current, Fe(III) did not exhibit a crucial catalytic effect on PMS, and their synergistic action also failed to efficiently remove SMX. The term “Switched” refers to experiments where DSA served as the cathode while the graphite electrode served as the anode, without adding any other substances, and only direct current was passed for SMX degradation. Continuous direct current electrolyzed water on the electrode, generating H₂O₂ at the anode for SMX degradation. However, the hydrolysis-generated radicals were also ineffective in degrading SMX. The system with only electrical current demonstrates that hydroxyl radicals are produced on the Ti/RuO₂-IrO₂ anode throughout immediate electron transfer or Equation (12) for SMX degradation [54].

$$\mathrm{M}\left({}^{•}\mathrm{OH}\right)\to \mathrm{M}\mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(12)

2.4. Identification of Reactive Species and Mechanisms of SIN/PMS/Fe(III) Process

2.4.1. EPR Test

For the purpose of examining the positive species produced during various systems, EPR spin trapping experiments were utilized, which employed DMPO and TEMP as probes to detect SO₄^•−/^•OH and ¹O₂, respectively, which is illustrated in Figure 6a. These experiments aimed to elucidate the active radicals generated within specific processes [55]. Prior scholarly investigations have indicated the absence of ^•OH and SO₄^•− generation in systems where only PMS is introduced [30]. Consequently, this serves as a critical baseline reference to discern the quantification of DMPO-^•OH or DMPO-SO₄^•− adducts generated within alternative systems [56]. Notably, the hierarchical arrangement of systems based on ^•OH and SO₄^•− production, delineated from highest to lowest, is SIN/PMS/Fe(III) > DC/PMS/Fe(III) > SIN/PMS > PMS, suggesting the potential for sinusoidal waveform current systems to catalytically yield a higher abundance of active species.

In the context of EPR investigations targeting ¹O₂ with TEMP [57] in Figure 6b, a blank control value was established by initially employing TEMP prepared with ultrapure water. Significantly, the discernible production of the TEMP-¹O₂ adduct within the SIN/PMS/Fe(III) system underscores the presence of ¹O₂ within this specific experimental framework.

2.4.2. Quenching Experiment

In the quenching experiments, the quenching agents used were tert-butyl alcohol (TBA), ethanol, furfuryl alcohol (FFA), 1,4-benzoquinone (P-BQ), L-histidine, and superoxide dismutase (SOD); the reaction rate coefficients of various quenching agents for different reactive radicals are shown in Table S1; and the quenching efficiency is shown in Figure 7a.

TBA is an effective scavenger of ^•OH radicals [58], while ethanol is also an efficient scavenger for both ^•OH and SO₄^•− radicals [59]. Upon the addition of 100 mmol/L TBA, the removal rate of SMX decreased to 80.0%, whereas the addition of 100 mmol/L ethanol reduced the removal rate of SMX to 75.3%, reflecting the possible existence of ^•OH and SO₄^•− radicals within the system. FFA, as a primary scavenger of ¹O₂, exhibits a high reaction rate [60]. Considering FFA’s strong reactivity with ^•OH, its concentration was adjusted to one hundredth the concentration of TBA, 1 mmol/L, to eliminate the influence of ^•OH [61]. From the graph, it can be observed that upon adding 1 mmol/L of FFA, the removal rate of SMX reduced to 45.9%, indicating the generation of ¹O₂ within this system. However, FFA may also react with PMS within the process, potentially interfering with the experimental results; thus, L-histidine was employed to scavenge ¹O₂ for further verification [62]. Upon adding 10 mmol/L of L-histidine, the removal rate of SMX reduced to 38.5%, also identifying the presence of ¹O₂ within the SIN/PMS/Fe(III) system. P-BQ exhibits high reaction rates for scavenging both ^•OH and O₂^•− radicals, and upon adding 5 mmol/L of P-BQ, the removal rate of SMX reduced to 55.0%. Subsequently, the use of SOD, which exhibits high reactivity towards O₂^•− radicals, was employed for further validation [63]. Upon adding 500 U/mL of SOD, the degradation efficiency of SMX reduced to 63.6%, identifying the presence of O₂^•− radicals within the system.

2.4.3. Chemical Probe Experiments

Further evidence supporting the production of ^•OH within the SIN/PMS/Fe(III) process was garnered through the coumarin (COU) fluorescence probing technique [64], utilizing COU, one kind of chemical probe. COU exhibits a notably high reaction constant with ^•OH, yielding the principal byproduct 7-hydroxycoumarin (7-HC) [65], depicted in Equation (13).

$$\mathrm{COU}+2{}^{•}\mathrm{OH}\to 7-\mathrm{HC}+{\mathrm{H}}_2\mathrm{O}$$

(13)

Depicted in Figure 7b, experimental observations revealed a decrement in 7-HC concentration concomitant with an escalation in the concentration of the aforementioned byproducts over a 30 min reaction period, thereby pointing to the unequivocal production of ^•OH during the SIN/PMS/Fe(III) process.

Furthermore, the detection of SO₄^•− generation within the SIN/PMS/Fe(III) process was approached indirectly through the p-benzoquinone (p-BQ) formation assessment, which occurred primarily when p-hydroxybenzoic acid (p-HBA) reacted with SO₄^•− [48]. The findings were derived from the probe experiments. Figure 7c suggests that the SIN/PMS/Fe(III) process produced SO₄^•− while the concentration was 6.67 µmol/L. Comparatively, earlier research indicated that the SO₄^•− concentration in the DC/PMS/Fe(III) process was 5.61 µmol/L [30]. This demonstrates that the SIN system produced 18 ± 1% more SO₄^•− than the DC system, highlighting one reason for the excellent degradation of the SIN/PMS/Fe(III) process.

Recent research has identified Fe(IV) as an available species in the Fe(II)/PMS process [66,67]. During this experiment, methyl phenyl sulfoxide (PMSO) was employed as a probe for Fe(IV), with PMSO₂ detected as a product using HPLC. The production of Fe(IV) in the SIN/PMS/Fe(III) process occurred through the reaction between Fe(II) and PMS depicted in Equation (14).

$$\mathrm{HS}{\mathrm{O}}_5^{-}+{\mathrm{Fe}}^{2+}\to {\mathrm{Fe}}^4{\mathrm{O}}^{2+}+{\mathrm{SO}}_4^{2-}+{\mathrm{H}}^{+}$$

(14)

Figure 7c shows a rapid increase in PMSO₂ concentration during the SIN/PMS/Fe(III) process. Additionally, the entire formation of PMSO₂ in the SIN/PMS/Fe(III) process was 46.18 µmol/L. By comparison, previous studies reported a PMSO₂ concentration of 44.06 µmol/L in a DC system [61], indicating that Fe(IV) oxidized PMSO to produce PMSO₂ more efficiently and generated Fe(IV) in greater quantities in the SIN/PMS/Fe(III) process than that while using direct current.

In the experiment, Nitroblue Tetrazolium (NBT) was employed as the chemical probe to measure the superoxide radicals (O₂^•−) that may be generated by the reaction [68], depicted in Figure 7d. The absorbance of NBT ranged from 295 nm to 560 nm, and the formazan (MF) and diformazan (DF) formed by the reaction between NBT and O₂^•−, whose generation depicted in Equations (15) and (16) reduced the original absorbance of NBT.

$$\mathrm{NBT}+2{\mathrm{O}}_2^{•-}\to \mathrm{MF}$$

(15)

$$\mathrm{NBT}+4{\mathrm{O}}_2^{•-}\to \mathrm{DF}$$

(16)

Therefore, the full-spectrum scanning method was used for measurement. From the results, it can be observed that the absorbance of NBT decreased progressively from 0 min to 30 min, indicating that the production of O₂^•− accumulated over time in the experiment. Compared with previous studies [69,70], it was found that a significantly higher amount of O₂^•− is produced in the SIN/PMS/Fe(III) process, which may also be the answer to the great degradation efficiency of the SIN/PMS/Fe(III) system. In addition, O₂^•− is generally believed to have been produced in the following ways (Equations (17) and (18)):

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{SO}}_4^{2-}\to {\mathrm{HSO}}_4^{-}+{\mathrm{HO}}_2^{-}$$

(17)

$${\mathrm{HO}}_2^{-}\to {\mathrm{O}}_2^{•-}+{\mathrm{H}}^{+}$$

(18)

2.5. Degradation Pathways of SMX and Toxicity Evaluation

Figure 8 illustrates the byproducts and pathways from the degradation of SMX employing the SIN/PMS/Fe(III) process. Through UHPLC-QTOF-MS analysis, eleven byproducts of SMX were identified and are detailed in Table S2, listing their molecular formulas, m/z values, and structures.

Furthermore, Figure 9 depicts the outcomes of toxicity assessment conducted employing the Toxicity Estimation Software Tool (T.E.S.T.) version 4.1 [71,72,73,74]. This implement utilizes a quantitative structure–activity relationship (QSAR) prediction matrix to evaluate the developmental toxicity, mutagenicity, acute toxicity (LD50), and bioaccumulation factor of SMX, as well as its byproducts. As shown in Figure 9a, the LD50 values for byproducts P284, P258, P266, P142, P99, P114, P255, P300, P503, P519, and P239 were 5384.89 mg/kg, 3158.52 mg/kg, 2663.45 mg/kg, 503.46 mg/kg, 383.09 mg/kg, 263.03 mg/kg, 33.88 mg/kg, 3256.46 mg/kg, 3215.08 mg/kg, 2764.26 mg/kg, and 6856.51 mg/kg, respectively. Byproducts exhibiting LD50 values lower than 1000 mg/kg were designated as possessing toxic properties. In Figure 9b, all byproducts exhibited lower bioaccumulation factors compared to SMX, except for P284, P284, and P239, indicating a potential decrease in bioaccumulation during degradation. Figure 9c illustrates that SMX is categorized as a “developmental toxicant” based on its developmental toxicity value of 0.85. Most intermediates also demonstrated developmental toxicity, with values greater than 0.5. The results for P142 and P99, which had developmental toxicity values less than 0.5, indicated that these intermediates were “developmental non-toxicants”. As depicted in Figure 9d, only P99 showed a mutagenicity value higher than 0.50, indicating that it was mutagenicity-positive. In contrast, both SMX and the other byproducts tested negative for mutagenicity. As shown in Figure 9, most byproducts had lower LD50 values than SMX, with only P284, P239, and another P284 showing a higher bioaccumulation potential. P142 and P99 exhibited developmental toxicity, and P99 was mutagenicity-positive. Although byproducts may show increased toxicity qualitatively, the general toxicity of the sample decreased due the low abundances of these byproducts. In addition, the decrease in total organic carbon (TOC) after the degradation process indicated that mineralization could be achieved in the current system with an extended reaction time to remove the toxicity of SMX byproducts.

2.6. Practical Application of the SIN/PMS/Fe(III) System

Anions in natural water matrices such as Cl⁻, NO₃⁻, and H₂PO₄⁻ are widely found, and their presence may impact the removal of various contaminants. Hence, the impacts of varying concentrations of Cl⁻, NO₃⁻, H₂PO₄⁻, and humic acid (HA) on SMX removal employing the SIN/PMS/Fe(III) process were explored.

Figure 10a depicts the influences of Cl⁻ on the SMX removal. The removal rate of SMX notably rose with higher concentrations of Cl⁻. This appearance can be attributed to the oxidation of Cl⁻ to generate Cl₂, which subsequently reacts with water to produce free chlorine species (HOCl, ClO⁻) [75,76,77,78] through the following reactions (Equations (19) and (20)), thereby accelerating the degradation of SMX.

$$2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2+2{\mathrm{e}}^{-}$$

(19)

$${\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}$$

(20)

On the other hand, Cl^• can be generated from Cl⁻ reacting with ^•OH and SO₄^•− through the following reactions (Equations (21)–(23)). Subsequently, Cl2^•− can be further generated from Cl^• reacting with Cl⁻ (Equation (24)) [79,80], which has been shown to boost the removal of other organic contamination, including Cl^• [77].

$${\mathrm{Cl}}^{-}+{}^{•}\mathrm{OH}\to {\mathrm{ClOH}}^{•-}$$

(21)

$${\mathrm{ClOH}}^{•-}+{\mathrm{H}}^{+}\to {\mathrm{Cl}}^{•}+{\mathrm{H}}_2\mathrm{O}$$

(22)

$${\mathrm{Cl}}^{-}+{\mathrm{SO}}_4^{•-}\to {\mathrm{Cl}}^{•}+{\mathrm{SO}}_4^{2\mathrm{―}}$$

(23)

$${\mathrm{Cl}}^{•}{+\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2^{•-}$$

(24)

Figure 10b depicts the impact of NO₃⁻ on SMX degradation. At concentrations of 1 mmol/L and 5 mmol/L, NO₃⁻ revealed a minimal influence on degradation efficiency. This is likely due to the excess NO₃⁻ ions potentially scavenging a fraction of both ^•OH and SO₄^•− radicals, thereby marginally reducing the reaction rate of the system [81]. Figure 10c depicts the impact of H₂PO₄⁻ on SMX decontamination. Within 30 min each, the removal rate of SMX reduced from 100.0% to 59.6% and 50.3% while the concentration of H₂PO₄⁻ rose from 0 to 1.0 and 5.0 mmol/L, respectively. These phenomena showed that H₂PO₄⁻ may not facilitate the activation of PMS, as observed in preceding experiments [82,83]. Moreover, the energy barrier for PMS activation by H₂PO₄⁻ appears to be too high to overcome, indicating its ineffectiveness in activating PMS. Figure 10d illustrates the influence of HA on SMX removal. Despite HA exhibiting a prohibitive effect on SMX degradation, the degradation efficiency reached 100.0% after 30 min in the experiment. This slight prohibitive effect is attributed to the carboxyl and hydroxyl groups within HA molecules potentially quenching some active species in the system, thereby reducing SMX degradation [81,84,85].

Figure 10e demonstrates how real-world water matrices impact SMX removal within the SIN/PMS/Fe(III) process. The data reveal a modest reduction in the efficiency of SMX removal when tap water and lake water are utilized as matrices in comparison with ultrapure water. This decline is attributed to the presence of diverse anions and humic acids (HAs) in natural water samples, which hinder the degradation process [60].

Moreover, Figure 10f shows the outcomes of the cyclic testing for SMX degradation. Under consistent parameters in experiments, the reusability of the SIN/PMS/Fe(III) process was intuitionistic to evaluate. Remarkably, with six consecutive cycles, this system has maintained the full degradation efficiency of SMX within a 30 min reaction period, underscoring its exceptional reusability. In conclusion, these findings affirm the robustness of the SIN/PMS/Fe(III) process for effectively treating SMX in real-world water matrices, ensuring a high sustained degradation efficiency and system stability.

3. Material and Methods

3.1. Chemicals and Reagents

All the chemicals utilized in this study are listed in Text S1 of the Supplementary Materials.

3.2. Degradation Experiment

The degradation experiment of SMX was operated in an open container of a glass cylindrical vessel, with a diameter of 75 mm and height of 80 mm, and it had three openings on its cover. Two of these openings were used for placing electrodes, while the third was utilized for adding and sampling. The anode used for degradation was DSA with dimensions of 3.0 cm × 3.0 cm × 0.1 cm, and the cathode was a graphite electrode with dimensions of 3.0 cm × 3.0 cm × 0.1 cm. After each experiment, ultrasonic oscillation rinsing was performed. The power supply used for this experiment was a dual-channel function/arbitrary waveform generator (DG1032Z, RIGOL, Shanghai, China), capable of providing current with different waveforms in the 0.1–100 kHz frequency range and 0–10 V voltage range. To ensure stable operation and meet the desired current requirements, a power amplifier (PA1011, RIGOL, Shanghai, China) was also employed for experiments.

The degradation process primarily goes as follows: Firstly, the pollutant SMX, PMS, Na₂SO₄, Fe₂(SO₄)₃, and ultrapure water are added into the reaction vessel, resulting in a final volume of 150 mL, with a concentration of SMX of 8 μmol/L, PMS of 0.001 mol/L, Fe³⁺ of 0.6 mol/L, and Na₂SO₄ of 0.02 mol/L. Secondly, the dropwise addition of 100 mmol/L of H₂SO₄ or NaOH is performed to coordinate the pH to stabilize at 3. The reaction time is set to 30 min, and samples of 1 mL of the solution while reacting are gathered at 0, 5, 10, 15, 20, 25, and 30 min for the detection of SMX concentration. Thirdly, the samples are placed in sample vials with a volume of 2 mL, and 40 μL of Na₂S₂O₃ is added as a terminating agent in advance. A control group is set up for error analysis in this experiment, and all data displayed in the figures and tables of the paper are covered as the mean and standard deviation derived from replicate samples.

3.3. Analytical Methods

Equipped with an Eclipse XDB C-18 column (5 µm, 250 × 4.6 mm, Agilent, Santa Clara, CA, USA), high-powered liquid chromatography (HPLC, Agilent 1260, Agilent, Santa Clara, CA, USA) was utilized to measure the concentrations of SMX and the rest of the model compounds as well. The specifics of the method are shown in Text S2 and Table S1.

Ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS, 6545 QTOF system, Agilent, Santa Clara, CA, USA) is capable of identifying the byproducts from SMX degradation, and an electrospray ionization (ESI) source was operated in positive mode. More method specifics are shown in Text S3 as well. UV-vis spectroscopy (UV-L5S, Shanghai, China) was utilized to measure iron species and PMS concentrations with more method details, respectively, outlined in Text S4 and Text S5.

Electron Paramagnetic Resonance (EPR) spectroscopy spectra were obtained using an EPR200-Plus instrument (CIQTEK, Hefei, China), and the methodology is outlined in Text S6. Reactive oxygen species (ROS) concentrations were quantified using chemical probe experiments, as detailed in Text S7.

3.4. Theoretical Calculation

The Gaussian 09 package and Multiwfn software were employed to represent density functional theory (DFT) calculations, which underpin the analyses presented throughout this study. The 6-31g(d,p) basis set was employed to optimize the geometry of the condensed Fukui index and the Highest Occupied Molecular Orbital (HOMO) of SMX and its byproducts, with detailed methodologies provided in Text S8. The T.E.S.T. was used to estimate acute toxicity, developmental toxicity, mutagenicity, and bioaccumulation factors of SMX and byproducts from its degradation, leveraging foreseen QSAR models.

4. Conclusions

In this work, the SIN/PMS/Fe(III) system has been established with an exceptionally high removal efficiency of SMX, reaching a complete degradation rate (100%). Through optimization of the system’s experimental parameters, it was determined that effective SMX degradation could be accomplished under the following conditions: pH of 3, voltage of 6 V, and concentrations of PMS and Fe³⁺ of 1.0 mmol/L and 0.6 mmol/L, respectively. In addition, the concentrations of Fe³⁺ and PMS did not fully deplete after the reaction, indicating the system’s potential for sustained reactivity. Results of EPR, quenching, and probe experiments further demonstrated that the SIN/PMS/Fe(III) process generated higher levels of ^•OH, SO₄^•−, ¹O₂, Fe(IV), and O₂^•− radicals in comparison with the DC system, which likely accounts for its high efficiency. Toxicity assessments of byproducts from SMX degradation employing the T.E.S.T. with the QSAR model indicated slightly increased toxicity of the byproducts. This study also investigated, within the SIN/PMS/Fe(III) system, the impacts of various anions and water matrices on SMX removal, revealing negligible effects from these factors and emphasizing the system’s resilience. This study reveals the significance of new knowledge that the choice of current waveforms of pulsed current may lead to elevated energy efficiency and degradation performance. Overall, these results underscore the SIN/PMS/Fe(III) process’s high efficiency and successful capabilities of degradation, indicating its potential as a dependable approach for effectively treating authentic wastewater.