Kinetic Analysis and Transformation Pathways of Sulfamethoxazole Degradation in Water and Wastewater Under Electron Beam Irradiation

Abstract

Sulfamethoxazole (SMX), a widely used antibiotic, persists in aquatic environments due to its resistance to conventional wastewater treatments. This work examined the breakdown of SMX in both purified water and urban wastewater through the application of electron beam irradiation (EBI). Experiments were conducted across doses of 0.5–3.0 kGy and varying pHs (2.70, 6.13, 9.00 and 11.10) and initial concentrations (5, 10, 15, 20 and 30 mg/L), and the role of reactive species was investigated with the help of scavengers. The results showed that SMX degradation followed pseudo-first-order kinetics and was most efficient at lower pH and concentrations. The scavenger experiments confirmed hydroxyl radicals as the dominant oxidizing agents responsible for SMX degradation, while wastewater constituents slightly inhibited the process. Nevertheless, over 99% SMX degradation was achieved at higher doses (1.5–3.0 kGy). TOC analysis revealed the partial mineralization of SMX, indicating the persistence of intermediate by-products despite high degradation efficiency. LC-MS analysis revealed multiple transformation products including hydroxylated sulfonamides and nitro-substituted derivatives, reflecting diverse degradation pathways. These results demonstrate that EBI is a highly effective laboratory-scale method for degrading SMX from water and wastewater, with promising potential for practical application.

1. Introduction

The presence of pharmaceutical contaminants in aquatic environments has become a significant environmental concern, particularly with antibiotics like sulfamethoxazole (SMX) frequently detected in water bodies worldwide [1,2,3]. SMX, a sulfonamide antibiotic extensively used to treat bacterial infections in humans and animals, is known for its resistance to conventional wastewater treatment processes, leading to its persistent occurrence in surface waters, groundwater, and even drinking water sources [4]. This persistence poses considerable risks, including the promotion of antibiotic-resistant bacteria and adverse effects on aquatic ecosystems [5,6,7,8]. Research indicates that SMX persists in aquatic environments, appearing in wastewater discharge (100–2500 ng/L), natural surface waters (60–150 ng/L), and even treated drinking water (up to 12 ng/L), demonstrating its widespread environmental presence [9].

Traditional wastewater treatment methods, such as biological degradation and physicochemical processes, have demonstrated limited effectiveness in degrading SMX from water matrices [3,10]. To address these challenges, advanced oxidation processes (AOPs) such as ozonation, photocatalysis, and Fenton-based oxidation have been explored for SMX degradation, leveraging reactive oxygen species (ROS), particularly hydroxyl radicals (•OH), to break down refractory organic compounds [11,12,13]. Despite demonstrating varying degrees of success, these methods often suffer from limitations such as dependence on specific pH conditions and the formation of toxic transformation byproducts [14]. These drawbacks highlight the need for more efficient, scalable, and environmentally sustainable treatment alternatives.

Ionizing radiation, particularly electron beam irradiation (EBI), has emerged as a promising tool to treat water contaminated with pharmaceuticals [15]. EBI works by producing highly reactive species such as hydroxyl radicals ($•\mathrm{O}\mathrm{H}$), hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$), and hydrogen atoms ($•\mathrm{H}$), which can rapidly degrade complex molecules like SMX [16,17]. E-beam irradiation offers rapid and non-selective degradation, operates at ambient conditions, and does not require chemical additives, making it a green and energy-efficient treatment option [18].

Recent studies have investigated the degradation of sulfonamide antibiotics, dyes and pesticides using nuclear-based methods such as gamma irradiation and EBI [19,20], demonstrating their versatility for diverse contaminants. For instance, research has demonstrated that sulfonamides like sulfamerazine (SMR), sulfadiazine (SDZ), and sulfapyridine (SPD) can be effectively degraded by EBI, following pseudo-first-order kinetics, with nearly complete degradation achieved at a dose of 5 kGy [21]. Similarly, another investigation focused on SMX degradation in aqueous solutions revealed that gamma irradiation could achieve complete decomposition at a dose of 1.5 kGy. The study emphasized the importance of reactive radicals and evaluated the effects of various additives on the degradation process. For sulfamethoxazole specifically, a few reports suggest that EBI is effective at doses between 3 and 10 kGy [22,23]. These studies also identified transformation products and proposed possible reaction pathways, although many were carried out in distilled or deionized water. While these studies provide valuable insights into the application of ionizing radiation for sulfonamide degradation, the primary focus is on model aqueous solutions or specific sulfonamides other than SMX. In addition, detailed evaluations of how reactive species influence SMX degradation via electron beam treatment are still scarce.

In this context, this study aims to address these gaps by systematically investigating the degradation of SMX in both water and wastewater matrices under electron beam irradiation. The research focused on elucidating the kinetics of the degradation process, identifying the resulting degradation products, and assessing the effect of reactive species. By comparing the degradation behavior in pure water and complex wastewater environments, this research seeks to provide a more comprehensive understanding of EBI's effectiveness and applicability for SMX degradation in real-world scenarios. This study not only contributes to the existing body of knowledge on advanced oxidation processes for antibiotic degradation but also offers practical insights into the potential implementation of EBI in wastewater treatment facilities to mitigate the environmental impact of pharmaceutical contaminants.

2. Materials and Methods

2.1. Chemicals

The study used sulfamethoxazole (SMX) (>99% purity) as the target compound. High-performance liquid chromatography (HPLC)-grade acetonitrile (≥99.9%) and formic acid (≥96%) were employed for analytical purposes. Chemical reagents, including tert-butanol (≥99.5%), potassium dichromate (99%), silver nitrate (99%), perchloric acid (95%), hydrochloric acid (35%), and sodium hydroxide (≥98%), were utilized for various experimental procedures. All chemicals were procured from Sigma-Aldrich (Sigma-Aldrich, Hamburg, Germany), with the exception of dinitrogen oxide (98%) and high-purity argon gas (grade 5.0), which were obtained from Air Products Sp. z o.o. (Air Products Sp. z o.o., Siewierz, Poland). Distilled water used for solution preparation was produced using a Thermo-Fisher distillation unit supplied by Merck (Merck, Darmstadt, Germany). Wastewater samples were collected from a municipal wastewater treatment plant located in Warsaw, Poland.

2.2. Irradiation Process

The SMX degradation experiments were performed using an ILU6 electron beam accelerator with an energy of 1.65 MeV, operating at 2 Hz pulse frequency, 400 µs pulse width, and 50 mA beam current. These investigations were conducted at the Institute of Nuclear Chemistry and Technology (INCT) in Warsaw, Poland. For dose measurement, a low-range dichromate dosimeter was used consisting of 0.5 mM silver dichromate dissolved in 0.1 M perchloric acid solution [24]. The dichromate dosimeter solution was prepared by dissolving 0.5 mM potassium dichromate and 1 mM silver nitrate in deionized water. For irradiation experiments, 30 mL aliquots of SMX solution were sealed in low-density polyethylene (LDPE) sleeve bags. These samples were then exposed to progressively increasing radiation doses of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 kGy using the electron beam system. To achieve a cumulative dose of 3.0 kGy, six sequential fractions of 0.5 kGy were applied, allowing for uniform exposure and the potential to observe intermediate degradation stages. Each 0.5 kGy increment corresponded to 12.5 seconds of exposure. The solutions were stored at 5 °C before analysis to minimize any potential degradation or microbial activity that could alter the chemical composition of the samples.

2.3. SMX Degradation

A 30 mL solution of sulfamethoxazole (SMX), prepared at concentrations ranging from 5 to 30 mg/L in distilled water, as well as a 10 mg/L SMX solution spiked with wastewater, was subjected to electron beam irradiation to investigate the degradation behavior in both matrices. Note that this concentration (10 mg/L) exceeds typical environmental SMX levels (ng/L–µg/L) but was selected to standardize the evaluation of EBI efficacy under controlled conditions. To explore the influence of pH on SMX degradation, aqueous solutions containing 30 mg/L of SMX were prepared and adjusted to specific pH values 2.70, 6.13, 9.00, and 11.10 using 0.1 M HCl or 0.1 M NaOH to span a wide range of acidic-to-alkaline conditions. These solutions were then irradiated at doses ranging from 0.5 to 3.0 kGy to evaluate degradation efficiency under different pH conditions.

To investigate the contribution of specific reactive species in SMX degradation, selective scavengers were introduced. Nitrous oxide (N₂O) was employed to eliminate solvated electrons, while a 0.5 M solution of tert-butanol in the presence of argon (Ar) was used to quench hydroxyl radicals [25]. Additionally, hydrogen radical effects were assessed by irradiating samples containing 0.5 M tert-butanol under argon saturation at pH 2 [26]. Prior to irradiation, N₂O and Ar gases were bubbled through the samples for 30 min to remove dissolved oxygen and establish the desired reactive environments.

The experimental error was evaluated using SMX (10 mg/L) under irradiation at 0.5 kGy. Due to the short exposure time, the error was the highest at this dose, with a relative standard deviation of 3.47% across three replicates. At higher doses, nearly all of the SMX (over 99.1%) was decomposed.

2.4. Analytical Methods

2.4.1. Product Identification and Qualitative/Quantitative Analysis

The UV-Vis absorption spectra of sulfamethoxazole (SMX) were measured using a JASCO V-670 spectrophotometer (JASCO, Warsaw, Poland), scanning wavelengths from 190 to 400 nm. The analysis revealed a characteristic absorption peak at 265 nm, corresponding to SMX’s maximum absorbance.

SMX concentrations were quantified using a Shimadzu VP-class HPLC system (Agilent 1200, Agilent Technologies, Santa Clara, CA, USA) equipped with a diode array detector (DAD). Separation was achieved using a Gemini C18 column (5 µm, 250 × 4.6 mm, 110 Å; Phenomenex, Warsaw, Poland) maintained at 40 °C. The mobile phase consisted of a 50:50 (v/v) mixture of acetonitrile and distilled water, delivered at a flow rate of 1 mL/min with an injection volume of 50 µL.

The transformation products formed during electron beam (EB) irradiation were characterized using an LC-MS system (Q Exactive, Thermo Scientific, Tianjin, China) coupled with an Agilent 1200 HPLC system (USA). Separation was achieved on a reversed-phase C18 column (150 mm × 4.6 mm, 5 µm particle size) at a constant temperature of 30 °C. Detection was performed using a diode array detector (DAD) with the wavelength set to 275 nm for optimal SMX byproduct monitoring. A gradient elution was employed at a flow rate of 0.2 mL/min, starting with 90% water (containing 0.1% formic acid) and 10% acetonitrile. Over 30 min, the acetonitrile concentration was increased to 100%, held for 2 min, and then returned to the initial conditions over another 2 min.

TOC measurements were conducted using the Nanocolor VIS II spectrophotometer (AQUA LAB, Warsaw, Poland) with TOC kit 2–30 mg/L from Aqua tests (Warsaw, Poland), designed for quick and accurate photometric determination to assess SMX mineralization.

The pH of the samples was determined using an Elmetron CX-461 multimeter (Elmetron, Zabrze, Poland).

2.4.2. Ion Chromatography

Inorganic anions were quantified using a Thermo Scientific Dionex ICS-5000 ion chromatograph (Dionex Corporation, Sunnyvale, CA, USA), equipped with a Dionex AERS 500 4 mm suppressor and suppressed conductivity detection operated at 25 mA. Separation was carried out on a Dionex Ion Pac™ AS23 analytical column with an AG23 guard column (4 × 250 mm) using an eluent composed of 4.5 mM sodium carbonate and 0.8 mM sodium bicarbonate (Fluka Chemie AG, Taufkirchen, Germany), delivered at 1.0 mL min⁻¹. The column temperature was maintained at 30 °C, and the injection volume was 25 µL. Standard calibration solutions were prepared by the appropriate dilution of Dionex Seven Anion Standard II (Dionex Corporation, Sunnyvale, CA, USA) in deionized water (18 MQ-cm, Milli-Q RG, Millipore Co., Sunnyvale, CA USA). Prior to each analysis, the system was allowed to equilibrate for 30–45 min.

3. Results and Discussion

3.1. Dose Influence

The degradation efficiency of antibiotics is highly dependent on the absorbed dose. Studies consistently show that degradation rates decline over time due to competition between parent antibiotic molecules and organic intermediates for reactive species (e.g., hydroxyl radicals) [27,28]. As these intermediates accumulate, fewer radicals remain available for attacking the parent compounds, slowing the process. However, despite this competition, antibiotic degradation remains the dominant reaction pathway. Figure 1 shows the adsorption spectra of 10 mg/L SMX irradiated by an electron beam under several doses (0–3.0 kGy). Under electron beam irradiation, the degradation of SMX was found to be dose-dependent. UV absorbance at 265 nm decreased progressively with increasing radiation dose, indicating the breakdown of SMX molecules. The degradation efficiency (%) of SMX was calculated using the relation in Equation (1).

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)=\left(1-{\displaystyle \raisebox{1ex}{$\mathrm{C}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{C}}_{\mathrm{o}}$}\right.}\right)\times 100$$

(1)

where C_o is the initial SMX concentration and C is the concentration after irradiation.

At a lower dose (0.5 kGy), a significant reduction in absorbance was observed, suggesting partial degradation (90.8%). As the dose increased to 1.0–2.0 kGy, the degradation became more pronounced, reflecting major structural modifications. At higher doses (2.5–3.0 kGy), the degradation efficiency exceeded 99.9%, implying near-complete degradation of SMX. However, the degradation rate plateaued slightly at these doses, likely due to the accumulation of intermediate products competing for reactive species.

The radiation chemical yield (G) represents the number of particles including molecules, ions, and radicals formed or decomposed when absorbing 100 eV of energy. The G-value in $\mathsf{\mu }\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{J}$ is given as in Equation (2).

$$\mathrm{G}-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}={\displaystyle \frac{\left[\mathrm{R}\right]}{\mathrm{D}}}\times 1.0\times {10}^6 \mathsf{\mu }\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{J}$$

(2)

where D is the absorbed dose (Gy) and R is the change in the reactant concentration (mol/L). Previous studies have reported that the absorbed dose increases while G-value decreases [29], thereby following the same trend as in Figure 2. The aforementioned trend might be the result of the recombination reactions between the reactive radicals produced during the radiolysis of water, as described in Equations (3)–(6).

$$•\mathrm{O}\mathrm{H}+•\mathrm{H}\to {\mathrm{H}}_2\mathrm{O} (\mathrm{k}=7.0\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(3)

$$•\mathrm{O}\mathrm{H}+•\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2{\mathrm{O}}_2 (\mathrm{k}=5.5\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(4)

$$•\mathrm{O}\mathrm{H}+{\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}\to {\mathrm{O}\mathrm{H}}^{-} (\mathrm{k}=3.0\times {10}^{10} \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(5)

$${\mathrm{H}}_2\mathrm{O}+•\mathrm{H}+{\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}\to {\mathrm{H}}_2+{\mathrm{O}\mathrm{H}}^{-} (\mathrm{k}=2.5\times {10}^{10} \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(6)

3.2. Effect of Initial Concentration

This study examined the electron beam (EB)-assisted degradation of sulfamethoxazole (SMX) in aqueous solutions under varying conditions. Experiments were conducted with initial SMX concentrations ranging from 5 to 30 mg/L while applying radiation doses between 0 and 3.0 kGy. All trials were performed at a pH of 6.74 to evaluate the degradation efficiency across different treatment scenarios. Figure 3a shows an obvious initial concentration dependence on the radiolytic degradation of SMX. At 0.5 kGy, the degradation efficiencies are, respectively, 94.4%, 90.8%, 90.5%, 85.1%, and 79.0% for 5, 10, 15, 20, and 30 mg/L. This trend suggests that as the initial concentration increases, the degradation efficiency decreases. This is likely due to the limited amount of reactive species generated by the electron beam, which are more effective when fewer SMX molecules are present. The reactive species become less effective at higher concentrations due to increased competition from SMX molecules for available radicals [29]. However, at higher radiation doses, such as 2.5 or 3.0 kGy, the difference in degradation efficiency between different concentrations becomes minimal as all approaches complete degradation at around 99%, indicating that sufficient doses can overcome the limiting effect of high initial concentrations. Overall, lower initial concentrations of SMX result in more efficient degradation at lower doses, while higher concentrations require higher doses to achieve similar levels of degradation.

Figure 3b shows the decrease in degradation rate constants (k) as the initial concentration increases, with values of 4.9, 4.4, 3.6, 3.9, and 3.4 kGy⁻¹ corresponding to initial SMX concentrations of 5, 10, 15, 20, and 30 mg/L, respectively. This behavior aligns well with the pseudo-first-order reaction kinetic model, as described by Equations (7) and (8) [15,30].

$$\mathrm{C}={\mathrm{C}}_{\mathrm{O}}{\mathrm{e}}^{-\mathrm{k}\mathrm{D}}$$

(7)

$$-\ln {\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_{\mathrm{o}}}}=\mathrm{k}\mathrm{D}$$

(8)

where C_o is the initial SMX concentration (mg/L); C is the residual SMX concentration after irradiation (mg/L); D is the absorbed dose (kGy); and k is the dose constant (kGy⁻¹).

The linear fits of $-\ln {\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_{\mathrm{o}}}}$ against absorbed dose (kGy) yield high correlation coefficients (R² > 0.96), confirming that the process adheres to pseudo-first-order behavior. The high linearity of the fitted curves across all concentrations (R² = 0.994 for 10 mg/L and R² = 0.999 for 30 mg/L) demonstrates the robustness of the model and supports findings from previous studies on electron beam and radiolytic degradation of antibiotics [29].

3.3. Effect of Initial pH on SMX Degradation

The efficiency of antibiotic degradation through water radiolysis is strongly influenced by the types and behaviors of reactive species generated, which in turn depend on the solution’s pH. SMX’s ionization depends on pH due to its two pKa values (1.7 and 5.6) [31]. Below pH 1.7, it exists in a protonated form; between pH 1.7 and 5.6, it is mostly neutral; and above pH 5.6, it becomes anionic. The protonated and neutral forms favor degradation by interacting more effectively with reactive species like $•\mathrm{O}\mathrm{H}$, while the anionic form at higher pH can repel these species, reducing degradation efficiency. The major reactive species generated from electron beam water radiolysis are stated in Equation (9) [21].

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}\left(0.28\right)+•\mathrm{H}\left(0.062\right)+•\mathrm{O}\mathrm{H}\left(0.28\right)+{\mathrm{H}}_2{\mathrm{O}}_2\left(0.072\right)+{\mathrm{H}}_2\left(0.047\right)+{\mathrm{H}}_3{\mathrm{O}}^{+}\left(0.28\right)$$

(9)

The experimental results indicated that acidic conditions tend to enhance antibiotic degradation. This improvement is attributed to the interaction between hydrogen ions (H⁺) and hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$) producing hydrogen radicals ($•\mathrm{H}$) (Equation (10)). This reaction, which occurs with a rate constant of approximately $2.3\times {10}^{10} \mathrm{L}/\left(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\right)$, plays a key role in suppressing the recombination of ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ and $•\mathrm{O}\mathrm{H}$ to form ${\mathrm{O}\mathrm{H}}^{-}$ (Equation (5)) allowing more $•\mathrm{O}\mathrm{H}$ in the reaction [32]. This trend is clearly supported by the graph in Figure 4a, where SMX degradation is most effective at lower pH values (pH 2.70 and 6.13), as shown by the steeper decline in C/Co with increasing dose. At 0.5 kGy, SMX degradation reached 66.3% at pH 2.70, significantly higher than at pH 9.00 (64.4%) and pH 11.10 (61.5%), and at 1.5 kGy, degradation reached 98.3% at pH 2.70, while it was slightly lower at higher pH values: 98.1% (pH 6.13), 97.5% (pH 9.00), and 95.9% (pH 11.10). Furthermore, the kinetic analysis in Figure 4b demonstrates that the degradation rate constant is highest at pH 2.70 (k = 2.6) and gradually decreases with increasing pH, confirming the enhanced efficiency of degradation under acidic conditions.

Conversely, under alkaline conditions, the increased concentration of hydroxide ions (${\mathrm{O}\mathrm{H}}^{-}$) promotes their reaction with hydroxyl radicals ($•\mathrm{O}\mathrm{H}$), forming less reactive species such as O⁻ and H₂O (Equation (11)). This reaction, with a rate constant of $1.3\times {10}^{10} \mathrm{L}/\left(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\right)$ [33], results in a reduction in •OH availability [32], ultimately decreasing the degradation efficiency of antibiotics. This is corroborated by the graphs, where SMX degradation at pH 9.00 and 11.10 is less efficient and the corresponding rate constants (2.4 and 2.2) are lower than those observed at acidic pH, indicating reduced degradation kinetics under alkaline conditions.

$${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}+{\mathrm{H}}^{+}\to •\mathrm{H} (\mathrm{k}=2.3\times {10}^{10} \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(10)

$$•\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O} (\mathrm{k}=1.3\times {10}^{10} \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(11)

3.4. Effect of Reactive Species

The radiolysis of water leads to the formation of several reactive species, as shown in Equation (9). Of these, hydroxyl radicals ($•\mathrm{O}\mathrm{H}$), hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$), and hydrogen atoms ($•\mathrm{H}$) are the primary agents involved in the oxidative or reductive degradation of organic contaminants. To assess the role of $•\mathrm{O}\mathrm{H}$ in the degradation of SMX, experiments were conducted under N₂O-saturated conditions, where ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ are scavenged, enhancing $•\mathrm{O}\mathrm{H}$ generation (Equation (12)) [34]. The individual contribution of ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ was examined using a 0.5 M tert-butanol solution saturated with argon at pH 6.74, wherein tert-butanol selectively reacts with $•\mathrm{O}\mathrm{H}$ (Equation (13)), allowing the effects of ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ to be isolated. To study the influence of $•\mathrm{H}$, the same system was acidified to below pH 2 using 0.1 M HCl, facilitating the conversion of ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ into $•\mathrm{H}$ (Equation (10)) [16], while $•\mathrm{O}\mathrm{H}$ is scavenged in the same time.

$${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}+{\mathrm{N}}_2\mathrm{O}+{\mathrm{H}}_{2}0\to •\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{N}}_2$$

(12)

$${\left({\mathrm{C}\mathrm{H}}_3\right)}_3\mathrm{C}\mathrm{O}\mathrm{H}+•\mathrm{O}\mathrm{H}\to •{\mathrm{C}\mathrm{H}}_2\mathrm{C}{\left({\mathrm{C}\mathrm{H}}_3\right)}_2\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(13)

Figure 5a illustrates SMX (10 mg/L) degradation as a function of absorbed dose (0–3 kGy) in four different systems: (1) SMX, (2) SMX in N₂O-saturated water, (3) SMX in tert-butanol/Ar-saturated solution, and (4) SMX in a tert-butanol/Ar system acidified to pH < 2. In the N₂O-saturated system, SMX degradation was significantly enhanced. A degradation efficiency of 96.6% was achieved at just 0.5 kGy, and nearly complete degradation (>99.8%) was observed beyond 1 kGy. This demonstrates the dominant oxidative role of $•\mathrm{O}\mathrm{H}$ radicals in the degradation process. When $•\mathrm{O}\mathrm{H}$ radicals were selectively scavenged, the degradation efficiency dropped substantially, with only 18.6% degradation at 0.5 kGy and 93.6% at 3 kGy. This reduction confirms the limited contribution of ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ to SMX degradation when $•\mathrm{O}\mathrm{H}$ is unavailable, emphasizing the predominance of $•\mathrm{O}\mathrm{H}$ in driving degradation. Under the fourth system, SMX degradation compared to the neutral tert-butanol system was enhanced, reaching 96.9% at 3 kGy. These findings suggest that $•\mathrm{H}$ radicals significantly contribute to the degradation of SMX, although still less effectively than $•\mathrm{O}\mathrm{H}$. A similar result was obtained for the degradation of sulfadiazine (SDZ) in aqueous solution under electron beam [35].

The kinetic analysis further supports the dominant role of hydroxyl radicals ($•\mathrm{O}\mathrm{H}$) in SMX degradation under electron beam irradiation. As shown in Figure 5b, the degradation followed pseudo-first-order kinetics across all systems, with distinct differences in reaction rates depending on the reactive species involved. The system enhanced with N₂O, which promotes $•\mathrm{O}\mathrm{H}$ generation, exhibited the highest rate constant (k = 5.2 kGy⁻¹) and a strong correlation (R² = 0.945). In contrast, when $•\mathrm{O}\mathrm{H}$ was scavenged by tert-butanol, the degradation proceeded much more slowly. The rate constants for hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-})$ and hydrogen radicals ($•\mathrm{H}$) were significantly lower, measured at 0.6 kGy⁻¹ and 0.9 kGy⁻¹, respectively (both with R² = 0.960). A clear reactivity hierarchy was observed ($•\mathrm{O}\mathrm{H}>•\mathrm{H}>{\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$), indicating that $•\mathrm{O}\mathrm{H}$ is the most effective species for SMX degradation, while $•\mathrm{H}$ shows moderate activity and ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ contributes the least.

3.5. SMX Degradation in Wastewater

The composition of the water matrix plays a critical role in determining the efficiency of advanced oxidation and reduction processes. Unlike distilled water, real wastewater contains various inorganic constituents such as some common anions (${\mathrm{C}\mathrm{O}}_3^{2-},{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-},{\mathrm{N}\mathrm{O}}_3^{-},{\mathrm{S}\mathrm{O}}_4^{2-},{\mathrm{C}\mathrm{l}}^{-}, \mathrm{a}\mathrm{n}\mathrm{d} {\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}$) [30] and organic constituents that can influence the generation, stability, and reactivity of radiolytically produced species such as hydroxyl radicals ($•\mathrm{O}\mathrm{H}$), hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$), and hydrogen atoms ($•\mathrm{H}$). However, this study used spiked wastewater to simulate contamination. These constituents may act as scavengers or promoters, thus altering the overall degradation kinetics of sulfamethoxazole (SMX) [36].

To evaluate the impact of water matrix effects, SMX degradation under electron beam irradiation was investigated in distilled water and secondary wastewater effluent (initial concentration: 10 mg/L) across an absorbed dose range of 0–3.0 kGy (Figure 6). At a low dose of 0.5 kGy, SMX in distilled water achieved 90.8% degradation, while the corresponding efficiency in wastewater was lower, at 82.0%. As the dose increased, the gap between the two systems narrowed, with both matrices reaching more than 99% degradation by 2 kGy. However, the consistently higher degradation observed in distilled water at every dose point indicates the inhibitory influence of wastewater constituents. In this work, ${\mathrm{N}\mathrm{O}}_3^{-},{\mathrm{S}\mathrm{O}}_4^{2-} \mathrm{a}\mathrm{n}\mathrm{d} {\mathrm{C}\mathrm{l}}^{-}$ were identified in wastewater using ion chromatography. Their concentration in wastewater was around 13.4mg/L, 95.9 mg/L, and 354 mg/L, respectively. These anions may have influenced the degradation of sulfamethoxazole (SMX) by scavenging reactive species, as described by Equations (14)–(19):

$${\mathrm{N}\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}\to \mathrm{H}\mathrm{N}{\mathrm{O}}_3 (\mathrm{k}=4.4~6.0\times {10}^8 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(14)

$$\mathrm{H}\mathrm{N}{\mathrm{O}}_3+•\mathrm{O}\mathrm{H}\to {\mathrm{N}\mathrm{O}}_3^{•}+{\mathrm{H}}_2\mathrm{O} (\mathrm{k}=0.88~1.2\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(15)

$${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}+{\mathrm{N}\mathrm{O}}_3^{-}\to {\mathrm{N}\mathrm{O}}_3^{2-} (\mathrm{k}=9.4\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(16)

$${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}+{\mathrm{S}\mathrm{O}}_4^{2-}\to {\mathrm{S}\mathrm{O}}_4^{3-} (\mathrm{k}=1.0\times {10}^6 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(17)

$${\mathrm{C}\mathrm{l}}^{-}+•\mathrm{O}\mathrm{H}\to \mathrm{C}\mathrm{l}\mathrm{O}{\mathrm{H}}^{•-} (\mathrm{k}=4.3\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(18)

$$\mathrm{C}\mathrm{l}\mathrm{O}{\mathrm{H}}^{•-}\to {\mathrm{C}\mathrm{l}}^{-}+•\mathrm{O}\mathrm{H} (\mathrm{k}=6.1\times {10}^9 \mathrm{L}/(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s}\left)\right)$$

(19)

The concentration of ${\mathrm{C}\mathrm{l}}^{-}$ in wastewater was around 354 mg/L, while studies have demonstrated that above 20 mg/L [36] of chloride ions in natural water, $•\mathrm{O}\mathrm{H}$ and ${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$ are removed during radiolysis [37].

3.6. Mineralization of Sulfamethoxazole in Water and Wastewater

To evaluate the extent of mineralization, TOC analysis was performed on the SMX solutions in both distilled water and municipal wastewater after electron beam treatment. The normalized TOC values ([TOC]/[TOC]_o) presented on Figure 7 show only partial reduction in both distilled water and wastewater. In distilled water, TOC decreased to a minimum of 0.73 at 1.5 kGy, then slightly increased at higher doses, indicating the formation of intermediate organic compounds. In wastewater, TOC values remained relatively high (0.88–0.95), suggesting that matrix components inhibited mineralization. These results confirm that while SMX was effectively degraded ($>99\%)$, complete mineralization was not achieved [30], and some by-products likely remained in the solution.

3.7. LC-MS Results

Figure 8 shows the proposed degradation pathway of SMX under electron beam irradiation involving multiple transformation reactions, predominantly initiated by reactive species such as hydroxyl radicals ($•\mathrm{O}\mathrm{H}$), hydrated electrons (${\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{-}$), and hydrogen atoms ($•\mathrm{H}$). These species promote a range of reactions including hydroxylation, bond cleavage, and oxidative degradation, which result in the formation of various intermediate products. The red-colored compound (A) represents the parent compound (SMX), the blue compounds are degradation products identified in this study, and the black-colored species are reference intermediates reported in the literature listed in Table 1.

The degradation initiates with an attack on the isoxazole and sulfonamide moieties of SMX, both of which are sensitive to electrophilic and nucleophilic radical species. One key pathway involves cleavage of the N-C bond and loss of the heterocyclic ring, resulting in the formation of compound J, a sulfanilamide derivative. Compound J undergoes further oxidation to form compound K, where the amine group is oxidized to a sulfonate. This transformation aligns with known pathways in the literature and supports the notion of progressive oxidation. Compound K can be mineralized into inorganic sulfate ions (I) and aniline-type species (L).

Another degradation route involves protonation in the nitrogen atom of the isoxazole ring, leading to the formation of intermediate G [38], a methylated imidazole derivative. The addition of a hydroxyl radical to the benzene ring moiety can also result in hydroxylated sulfonamide products, such as compound E [39]. Compound E undergoes further oxidation to form compound F.

From SMX (A), another pathway leads to compound B through nitration. Compound B transforms into compound D, a nitro-substituted derivative, which undergoes ring transformation to form compound C [25]. These steps indicate a series of oxidative rearrangements involving both nitration and ring modification. Additional observed products include compound H, which retains the sulfonamide group but with a nitro substituent.

Table 1. The main intermediates during SMX electron beam irradiation.

Compounds	Retention Time	Main Fragments (m/z)	Proposed Structure
A (SMX)	19.10	254
B	16.40	282
C	30.32	226
D	17.31	229
E	0.89	271
F	12.11	286
G	7.24	98
H	12.01	187
I	3.98	96
J	10.42	173	[40]
K	8.63	172	[40]
L	9.14	94	[40]

Table 1. The main intermediates during SMX electron beam irradiation.

Compounds	Retention Time	Main Fragments (m/z)	Proposed Structure
A (SMX)	19.10	254
B	16.40	282
C	30.32	226
D	17.31	229
E	0.89	271
F	12.11	286
G	7.24	98
H	12.01	187
I	3.98	96
J	10.42	173	[40]
K	8.63	172	[40]
L	9.14	94	[40]

4. Conclusions

This study demonstrated that electron beam irradiation (EBI) is a highly effective method for degrading sulfamethoxazole (SMX) in both distilled water and spiked wastewater matrices. The degradation efficiency of SMX was shown to depend on several key parameters, including radiation dose, initial concentration, pH, and the presence of reactive species. Hydroxyl radicals ($•\mathrm{O}\mathrm{H}$) were identified as the dominant reactive species responsible for the degradation process, with hydrated electrons and hydrogen atoms contributing to a lesser extent. The kinetic analysis confirmed that SMX degradation follows pseudo-first-order kinetics under varying experimental conditions. The TOC results showed only partial mineralization, particularly in wastewater matrices. Furthermore, the formation of intermediate products was elucidated through LC-MS, offering insights into the transformation pathways of SMX under EBI.

The results also highlighted that the water matrix significantly influences degradation efficiency, with complex wastewater components such as chloride ions, nitrate ions, and sulfate ions acting as scavengers for reactive species and thus reducing SMX degradation rates. Nevertheless, high degradation efficiencies (>99%) were still achievable at sufficient radiation doses (1.5–3.0 kGy), even in wastewater environments.

Although this study shows that EBI is highly effective at removing sulfamethoxazole (SMX) in both water and wastewater under laboratory conditions, applying this method in real treatment plants involves important practical challenges. These include high energy use, expensive equipment, and the need for strict safety rules to handle radiation [18]. Until recently, most EBI studies were only conducted in the laboratory, and its use in real wastewater treatment was uncertain. However, for the first time, electron beam technology has been used at full scale to treat dyeing wastewater in Jiangmen City, Guangdong Province, China, with a daily capacity of 30,000 m³/day. This achievement proves that EBI can be used in real-world situations when the system is properly designed. Still, issues such as incomplete mineralization [30], long-term performance, and the treatment of other types of contaminants must be addressed. Continued work is needed to improve system design, attain lower costs, and combine EBI with other advanced oxidation processes for safer and more efficient water treatment.