aState Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China
bShanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China
cState Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
dShanghai Academy of Environmental Sciences, Shanghai 200233, China
Despite the extensive application of advanced oxidation processes (AOPs) in water treatment, the efficiency of AOPs in eliminating various emerging contaminants such as halogenated antibiotics is constrained by a number of factors. Halogen moieties exhibit strong resistance to oxidative radicals, affecting the dehalogenation and detoxification efficiencies. To address these limitations of AOPs, advanced reduction processes (ARPs) have been proposed. Herein, a novel nucleophilic reductant—namely, the carbon dioxide radical anion ($\mathrm{CO}_{2}^{·-}$) —is introduced for the simultaneous degradation, dehalogenation, and detoxification of florfenicol (FF), a typical halogenated antibiotic. The results demonstrate that FF is completely eliminated by $ \mathrm{CO}_{2}^{·-}$, with approximately 100% of Cl− and 46% of F− released after 120 min of treatment. Simultaneous detoxification is observed, which exhibits a linear response to the release of free inorganic halogen ions (R2 = 0.97, p < 0.01). The formation of halogen-free products is the primary reason for the superior detoxification performance of this method, in comparison with conventional hydroxyl-radical-based AOPs. Products identification and density functional theory (DFT) calculations reveal the underlying dehalogenation mechanism, in which the chlorine moiety of FF is more susceptible than other moieties to nucleophilic attack by $ \mathrm{CO}_{2}^{·-}$. Moreover, $ \mathrm{CO}_{2}^{·-}$- based ARPs exhibit superior dehalogenation efficiencies (> 75%) in degrading a series of halogenated antibiotics, including chloramphenicol (CAP), thiamphenicol (THA), diclofenac (DLF), triclosan (TCS), and ciprofloxacin (CIP). The system shows high tolerance to the pH of the solution and the presence of natural water constituents, and demonstrates an excellent degradation performance in actual groundwater, indicating the strong application potential of $ \mathrm{CO}_{2}^{·-}$-based ARPs in real life. Overall, this study elucidates the feasibility of $ \mathrm{CO}_{2}^{·-}$ for the simultaneous degradation, dehalogenation, and detoxification of halogenated antibiotics and provides a promising method for their regulation during water or wastewater treatment.
The overuse of antibiotics has had detrimental effects on aquatic ecosystems and human health [1], [2]. Compared with other types of antibiotics, halogenated antibiotics exhibit a higher capability for bioaccumulation and environmental persistence and have more severe toxic effects, including carcinogenicity, growth abnormality, reproductive dysfunction, and immune-toxic instability [3], [4]. Even though several halogenated antibiotics have been banned or severely restricted for decades in some developed countries [5], [6], [7], they are still widely detected in aquatic environments, with concentrations ranging from nanograms to milligrams per liter [1], [8].
To date, various methods for the decomposition of halogenated antibiotics have been tested in laboratory experiments [9], [10], [11]. For example, advanced oxidation processes (AOPs) have been extensively utilized for removing halogenated antibiotics by generating strong oxidizing species, such as hydroxyl (OH) and sulfate ($\mathrm{SO}_{4}^{·-}$) radicals [12]. Unfortunately, the dehalogenation efficacy of AOPs is always restricted, as the strong electron-withdrawing capability of the C–X bond (where X = halogen atom) makes it difficult for halogen moieties to be destroyed under oxidative conditions (Table S1 in Appendix A) [13], [14], [15], which limits the AOP detoxification of halogen moieties. Although some halogen ions are released by AOPs, they may still react with ·OH or $\mathrm{SO}_{4}^{·-}$ to produce reactive halogen radicals (Cl·, Br·, $ \mathrm{Cl}_{2}^{·-} $, etc.)—a process that can lead to the regeneration of halogen-containing byproducts, which may be more toxic than the parent compounds [3], [16], [17].
Recently, advanced reduction processes (ARPs) have emerged as an alternative method for treating halogenated antibiotics [18]. Compared with the oxidizing reactive species of AOPs, halogenated moieties are more susceptible to nucleophilic attack by reductive radicals, such as the sulfite radical ($\mathrm{SO}_{3}^{·-}$), hydrated electron (eaq−), and atomic hydrogen (H·) [19], [20], [21], resulting in better dehalogenation efficiency against halogenated compounds [22]. Furthermore, these reductive radicals remain inert toward halogen atoms [21], thereby fundamentally avoiding the formation of reactive halogen radicals. Thus far, the efficient dehalogenation of halogenated antibiotics has been reported for several types of ARPs, including electron beam irradiation (eaq−), ultraviolet (UV)/iodide (eaq−), UV/sulfite ($\mathrm{SO}_{3}^{·-}$ and eaq−), and electrochemical reduction treatment (H·) [23], [24], [25], [26]. Nevertheless, some of these processes require high capital investment and significant energy consumption [24], [25], [27], while others present environmental risks due to residual iodide or the formation of sulfur-containing byproducts [23], [26].
Carbon dioxide radical anions ($\mathrm{CO}_{2}^{·-}$)- based ARPs, which can address the abovementioned limitations of capital- and energy-intensive processes, are promising for the removal and detoxification of aquatic halogenated contaminants [28], [29], [30]. $ \mathrm{CO}_{2}^{·-}$ can be obtained from a hydrogen abstraction reaction between low-molecular-weight organic acids and the oxidative radical species of AOPs (e.g., ·OH and $\mathrm{SO}_{4}^{·-}$), so many cost-effective routes are feasible for the generation of $\mathrm{CO}_{2}^{·-}$ [28], [31]. In addition, $\mathrm{CO}_{2}^{·-}$ is a green single-electron carrier and can be converted into CO2 during the reduction process, thus blocking the potential risks associated with residual iodide and other byproducts [32]. $\mathrm{CO}_{2}^{·-}$-based ARPs have been used to degrade alkyl and aromatic halogenated contaminants [28], [29], [33], demonstrating a relatively higher rate constant toward perfluorobutanesulfonate than eaq− (E0 = −2.9 V) [33]. Recently, Hendy et al. [30] observed that $\mathrm{CO}_{2}^{·-}$ exhibits notable dehalogenation selectivity in the reductive activation of halogenated compounds, suggesting the possibility of using $\mathrm{CO}_{2}^{·-}$- based ARPs for the efficient dehalogenation of halogenated antibiotics. However, the effectiveness of this hypothesis has not yet been systematically evaluated, raising the fundamental question of whether or not simultaneous dehalogenation and detoxification can be achieved when $\mathrm{CO}_{2}^{·-}$ participates in the removal of halogenated antibiotics. Moreover, the underlying reductive dehalogenation mechanisms mediated by $\mathrm{CO}_{2}^{·-}$ remain ambiguous. Previous studies have utilized laser flash photolysis technology to confirm that single-electron transfer is the dominant reaction pathway between $\mathrm{CO}_{2}^{·-}$ and organic compounds [34], and dehalogenation pathways have been demonstrated using mass spectrometry [33]. However, mass spectrometry exhibits significant limitations in elucidating reaction mechanisms, as it fails to capture unstable intermediates or short-lived transient species [35], leading to an unclear understanding of the dehalogenation mechanisms between $\mathrm{CO}_{2}^{·-}$ and organic compounds.
To address these concerns, the present work investigates the degradation, dehalogenation, and detoxification performance of $\mathrm{CO}_{2}^{·-}$- based ARPs using florfenicol (FF) as the targeted compound. FF, which possesses two Cl atoms and one F atom, is extensively used in animal husbandry and is frequently detected in aquatic ecosystems with concentrations as high as 11 mg·L−1 [36], [37], [38]. Furthermore, FF exhibits chronic toxicity to aquatic organisms, even when present in nanograms-per-liter levels [39]. To elucidate the reductive dehalogenation mechanisms mediated by $\mathrm{CO}_{2}^{·-}$, density functional theory (DFT) calculations are employed to overcome the limitations of experimental methods, and are shown to have significant advantages in terms of describing the reaction kinetics and mechanisms involving radical species [35]. In addition, the applicability of $\mathrm{CO}_{2}^{·-}$- based ARPs is assessed, including the dehalogenation of a series of typical halogenated antibiotics, the degradation performance for various natural water constituents, and performance testing for actual groundwater. The results obtained in this study will help scholars and engineers better understand the feasibility and mechanism of the $\mathrm{CO}_{2}^{·-}$-mediated elimination of emerging contaminants.
2. Materials and methods
2.1. Chemicals and reagents
Table S2 in Appendix A presents the chemicals and reagents used in this study. Ultra-pure water (18.2 MΩ·cm) derived from a Milli-Q system (ELGA; Marlow, UK) was used to prepare all solutions.
2.2. Experimental procedures
$\mathrm{CO}_{2}^{·-}$ was produced by the reaction of HCOO− and ·OH, which is derived from the photolysis of H2O2 (Eqs. (1), (2)).
where h is Planck’s Constant and v is the frequency of the photon.
Batch degradation experiments were performed in 50 mL quartz colorimetric tubes containing halogenated antibiotics (20 mg·L−1), 10 mmol·L−1 H2O2, 100 mmol·L−1 HCOO−, and phosphate buffer solution (pH 7.0). Nitrogen (N2) purging at a flow rate of 0.3 L·min−1 was applied to eliminate any interference from oxygen (O2). The quartz colorimetric tubes were subsequently placed in a photoreactor equipped with a 10 W low-pressure mercury lamp (GPH212T5L/4; Heraeus, Germany) to initiate the reaction process. At specific time intervals, 0.5 mL solution samples were collected and quenched by Na2S2O3 for further analysis. For the degradation experiments in actual groundwater, samples were obtained from a well approximately 15 m deep below the surface (Songjiang, Shanghai, China). The main characteristics of the actual groundwater are presented in Table S3 in Appendix A. In addition, radical probe tests and electron paramagnetic resonance (EPR) spectroscopy were employed to identify the predominant radical species. Detailed information regarding the experimental procedures is presented in Sections S1 and S2 and Fig. S1 in Appendix A. All kinetic experiments were carried out in triplicates, and the results are presented as mean values with standard deviation.
2.3. Analysis methods
The concentrations of FF, chloramphenicol (CAP), thiamphenicol (THA), diclofenac (DLF), triclosan (TCS), ciprofloxacin (CIP), nitrobenzene (NB), and benzoic acid (BA) were determined using a high-performance liquid chromatography system (LC-20AT; Shimazu, Japan). Carbon tetrachloride (CT) was measured by gas chromatography (Agilent 7890A; Agilent, USA) using the method presented in our previous work [40]. The concentrations of inorganic Cl− and F− were quantified by ion chromatography (Eco IC; Metrohm, Switzerland); detailed analytical parameters are described in Section S3 in Appendix A.
2.4. Toxicity evaluation of FF and its intermediates
The toxic effects of FF and its intermediates were assessed using Chlorella vulgaris (C. vulgaris) as the indicator [11], [41]. In brief, C. vulgaris was cultivated in an Erlenmeyer flask filled with liquid blue-green (BG)11 medium, grown at 25 °C in a light incubator with a photoperiod of 12 h light and 12 h dark. Aqueous solutions containing FF and its intermediates were added into 96 well assays containing algae solutions for 72 h inculcation. The microalgal biomass, expressed as the optical density at 650 nm (OD650), was measured using a microplate reader (Infinite M200 Pro; Tecan, Switzerland). The relative growth inhibition for C. vulgaris (as calculated using Eq. (3)) was used to indicate the toxicity of FF and its intermediates.
where ODcontrol and ODtest represent the optical density values in the control and treatment groups, respectively. More details on the cultivation of C. vulgaris and the relative growth inhibition tests are described in Section S4 in Appendix A.
2.5. Identification of FF degradation intermediates
After the solid-phase extraction (SPE) process, the degradation intermediates of FF were identified using ultraperformance liquid chromatography coupled with a Q Exactive Focus Orbitrap high-resolution mass spectrometer (UPLC-HR-MS; Orbitrap Exploris 240; Thermo Fisher Scientific, Germany). In short, each SPE cartridge (Oasis HLB, 3 mL, 60 mg; Waters, USA) was conditioned with 10 mL of methanol and ultrapure water. The reaction mixture (50 mL) was loaded onto the cartridges at a flow rate of 3 mL·min−1. Then, 6 mL of ultrapure water was used to remove the residue of inorganic salts. Afterward, the cartridges were dried for 1 h using a vacuum pump and eluted using 6 mL of methanol. The extracts were concentrated under a gentle flow of high-purity nitrogen, reconstituted with methanol to a volume of 1.0 mL, filtered through 0.22 μm nylon membrane filters, and stored at 4 °C before the intermediates identification.
The pretreated solution samples were analyzed using the UPLC-HR-MS system. More specifically, a 10 µL sample was injected into an ACQUITY CSH C18 column (100 mm × 2.1 mm × 1.7 µm; Waters) maintained at 40 °C. The mobile phases used for the analysis consisted of Milli-Q water containing 0.1% formic acid (pH 2.5) (A) and methanol (B). The gradient program started at 10% B for 2 min, followed by a linear increase to 80% B for 6 min; it was then held for 2 min, returned to 10%, and maintained for 3 min until the next injection. The flow rate of the mobile phase was set to be 0.3 mL·min−1. A detailed description of the mass spectrometer is presented in Section S5 in Appendix A. The structural elucidation of FF and its intermediates was based on the following criteria: ① the error between the measured mass and the theoretical accurate mass (< 10 parts per million (ppm)); ② the fragmentation pattern of the molecular ions; and ③ the isotopic distribution of chlorine substituents.
2.6. Quantum chemical calculations
DFT calculations were performed to reveal the underlying mechanisms of the initial dehalogenation processes. Condensed Fukui functions were calculated to recognize the potential reactive sites of FF, which were conducted based on Hirshfeld charges using the Multiwfn 3.8 software package [42]. Since $\mathrm{CO}_{2}^{·-}$ is a nucleophilic reductant that prefers to react with the electron-withdrawing moieties of the target compounds [30], nucleophilic condensed Fukui functions (f+) were used to quantitatively evaluate the electron-accepting ability of each atom. A larger f+ value indicated higher probability of a nucleophilic attack by $\mathrm{CO}_{2}^{·-}$, and vice versa [42], [43]. Subsequently, geometrical optimization and frequency analysis of FF were executed using the Gaussian 09 software package at the B3LYP 6-31+G (d,p) level, including a solvent model density (SMD) model to account for the effect of water [44]. The optimized structures were subsequently confirmed by vibrational frequencies, while the transition states (TS) were characterized with one imaginary vibrational frequency, as previously documented in our study [45]. The intrinsic reaction coordinate was determined to verify that the transition states were correctly connected with the reactants and products. Afterward, the single point energies of all the optimized structures were calculated at the M06-2X 6-31+G (d,p) level to enhance the accuracy of the calculated energies [46].
3. Results and discussion
3.1. FF degradation kinetics and detoxification performance
As shown in Fig. 1(a), FF was not removed under dark conditions, indicating that FF is relatively stable and inert to direct attack by H2O2 or HCOO−. A slight FF decay was observed under UV irradiation with or without HCOO− (0.055 min−1), which can probably be attributed to the direct photolysis of FF under UV irradiation [47]. Under UV/H2O2 treatment, the ·OH radical was generated from the activation of H2O2 (Fig. S1) and reacted rapidly with FF, accelerating the elimination of FF (0.083 min−1) [48]. With the addition of excess HCOO− (100 mmol·L−1) to the UV/H2O2 process, nearly all the ·OH radicals (99.97%) were converted into $\mathrm{CO}_{2}^{·-}$ (Section S1); the FF-removal rate was significantly increased, with an observed rate constant (kobs; Fig. S2 in Appendix A) approximately 2.3 times higher (0.189 min−1) than that of the ·OH treatment process (0.083 min−1).
The acute toxicity of the reaction mixture at different time intervals was also evaluated according to the relative growth inhibition of C. vulgaris (Fig. 1(b)). Sole UV irradiation was not effective in eliminating the toxicity, as the relative growth inhibition decreased from 94.9% to 79.6% within 120 min. The introduction of ·OH was beneficial for the detoxification of FF; after 120 min, the relative growth inhibition decreased steadily to 40.8%. When the dominant radicals were changed from ·OH to $\mathrm{CO}_{2}^{·-}$, the detoxification performance was further strengthened, and the residue of relative growth inhibition was 24.7% after 120 min of treatment. The variation in the detoxification performance was attributed to the differences in FF degradation intermediates and pathways under distinct reaction conditions.
3.2. FF degradation intermediates and their ecotoxicity assessment
As presented in Table 1, a total of 13 FF intermediates were identified during the treatment using $\mathrm{CO}_{2}^{·-}$- based ARPs. Of these, ten (accounting for 76.9% of all the identified intermediates) were chlorine-free compounds, indicating that the dehalogenation process induced by the nucleophilic attack of $\mathrm{CO}_{2}^{·-}$ plays a dominant role in degrading the FF. This behavior is considerably different from that during the ·OH-based AOPs. Table S4 in Appendix A summarizes the FF intermediates during ·OH-based AOPs, as reported in previous studies [47], [48], [49]. The results of our research showed that 13 out of 19 identified intermediates (accounting for 68.4%) were chlorine-containing intermediates (Fig. S3 in Appendix A), suggesting that the dominant reaction pathway under OH differs from that under $\mathrm{CO}_{2}^{·-}$ treatment.
The distinct degradation intermediates further contributed to the variation in toxicity, which was estimated using the Estimation Program Interface (EPI) Suite software package [50]. As presented in Table 1, the predicted acute and chronic toxicities of all ten chlorine-free intermediates under $\mathrm{CO}_{2}^{·-}$ treatment were significantly lower (t-test, p < 0.05) than those of FF or the three chlorine-containing reductive products (RP; RP1, RP4, and RP5), indicating that the toxicity of FF can be efficiently eliminated by the dehalogenation process [51]. In contrast, the major intermediates observed during the ·OH treatment process were chlorine-containing compounds, most of which exhibited similar or higher toxicities compared with that of FF (Table S4). Furthermore, the FF dechlorination processes demonstrated distinct toxicity effects under $\mathrm{CO}_{2}^{·-}$ and ·OH treatment. As shown in Fig. S4 in Appendix A, the $\mathrm{CO}_{2}^{·-}$-mediated hydrodechlorination reactions significantly reduced the toxicity of FF, while the ·OH-mediated hydroxylation dechlorination processes generated intermediates with higher acute and chronic toxicity. This is a possible reason for the relatively weaker detoxification performance of ·OH-based AOPs compared with that of $\mathrm{CO}_{2}^{·-}$-based ARPs.
3.3. FF degradation pathway
According to the time-dependent evolution profiles of the intermediates (Fig. S5 in Appendix A), four degradation pathways of FF under $\mathrm{CO}_{2}^{·-}$ treatment were proposed. As shown in Fig. 2, Pathway I corresponds to the hydrodechlorination reaction, in which the C–Cl bond of FF is initially attacked by $\mathrm{CO}_{2}^{·-}$. One chlorine is replaced by a hydrogen atom, thus contributing to the generation of the dechlorination product RP1. Afterward, another chlorine is detached to form the chlorine-free intermediate RP2, which is finally transformed into a halogen-free intermediate (RP3) via the defluorination process. Pathway II illustrates an attack on the C–S bond and the subsequent dehalogenation process. The absence of the sulfomethyl moiety contributes to the accumulation of RP4, while a further hydrodechlorination reaction leads to the generation of dehalogenation intermediates (RP5, RP6, and RP7). Pathway III is related to reductive dechlorination and the aldolization reaction (RP8), while the subsequent attack on the C–S and C–C bonds results in the formation of RP9, RP10, and RP11. In Pathway IV, the C–N bond of FF is attacked, resulting in the generation of florfenicol amine (RP12). RP13 is assigned as the intermediate (RP12) that undergoes detachment of the sulfomethyl moiety. It is noteworthy that three of the four degradation pathways are related to the destruction of halogenated moieties, indicating that the dehalogenation process is the key mechanism in the transformation of FF under $\mathrm{CO}_{2}^{·-}$ treatment.
The crucial role of the dehalogenation reaction is further supported by the results from a quantitative analysis of free Cl− and F− at different time intervals. As shown in Fig. 3(a), after 120 min of treatment, the concentrations of free Cl− and F− increased to 3.92 and 0.48 mg·L−1, respectively, accounting for approximately 100% and 46% of the theoretical contents of Cl− (3.96 mg·L−1) and F− (1.05 mg·L−1) in the reaction mixture. This finding indicates complete dechlorination and partial defluorination during the $\mathrm{CO}_{2}^{·-}$-based ARPs.
Based on the evolution of the released halogen ions, the molar concentration of the halide ions was plotted against the degradation of FF (Fig. 3(b)). The average dehalogenation efficiency (ADE) was calculated according to Eq. (4) [24].
where [FF]0 is the initial concentration of FF (µmol·L−1), and [Cl−]t, [F−]t, and [FF]t represent the concentrations of Cl−, F−, and FF at certain time periods (µmol·L−1).
According to Eq. (4), the ADE was estimated to be 0.566, indicating that 56.6% of the halogen moieties were converted to inorganic halogen ions in the initial degradation of FF (0–30 min); this finding further supports the conclusion that the dehalogenation process is a major pathway for the degradation of FF during $\mathrm{CO}_{2}^{·-}$-based ARPs.
Furthermore, the relationship between dehalogenation and detoxification was analyzed according to a Pearson correlation analysis. As shown in Fig. 3(c), the relative growth inhibition for C. vulgaris showed a significantly negative correlation with the dehalogenation efficiency (R2= 0.97, p < 0.01), suggesting that the eliminated toxicity was largely derived from the release of inorganic halogen ions. This was consistent with the toxicity predicted by the EPI Suite software package, demonstrating that the dehalogenation reaction plays a considerable role in the detoxification of FF.
3.4. FF initial dehalogenation mechanisms
Kinetics and toxicity investigations emphasized that the $\mathrm{CO}_{2}^{·-}$- mediated dehalogenation reactions were primarily responsible for the detoxification of FF. DFT calculations were then used to further understand the underlying mechanisms of the dehalogenation process at the molecular level. Fig. S6 and Table S5 in Appendix A show that 1Cl (0.1254) and 2Cl (0.0722) hold relatively higher f+ values for a strong electron-withdrawing capability, suggesting that they are most likely to be attacked by $\mathrm{CO}_{2}^{·-}$. Furthermore, the f+ values of 19C (0.0545), 16C (0.0492), and 12C (0.0480) are higher than those of the other atoms, indicating that the C–S and C–N bonds are vulnerable to nucleophilic attack by $\mathrm{CO}_{2}^{·-}$. In contrast, the f+ values of 4F (0.0115) and 13C (0.0059) are approximately one order of magnitude lower than those of the chlorine atoms, implying that the C–F bond is not prone to $\mathrm{CO}_{2}^{·-}$ attack, and the defluorination process is less likely to occur than the dechlorination process.
In addition to the condensed Fukui functions, the evolution of the Gibbs free energy was used to demonstrate the thermodynamic feasibility of the $\mathrm{CO}_{2}^{·-}$-mediated initial dehalogenation of FF. Fig. 4 shows that two different stages are involved in the dechlorination reaction of FF: electron transfer from $\mathrm{CO}_{2}^{·-}$ to FF (Stage I) and the formation of HCO3− and the release of chloride (Stage II). The energy barriers for the dechlorination reaction are easy to overcome under ambient conditions, indicating that such a process is thermodynamically favorable under the action of $\mathrm{CO}_{2}^{·-}$-based ARPs.
The $\mathrm{CO}_{2}^{·-}$-initiated defluorination process is similar to the dechlorination process (Fig. S7 in Appendix A), indicating that defluorination is also thermodynamically favorable. Nevertheless, the efficiency of the defluorination reaction was lower than that of the dechlorination reaction. Further studies should be carried out to elucidate these differences.
3.5. Application assessment of $\mathrm{CO}_{2}^{·-}$-based ARPs
3.5.1. Efficient dehalogenation for various halogenated antibiotics
In addition to their activity against FF, the $\mathrm{CO}_{2}^{·-}$-based ARPs exhibited attractive degradation capabilities against a variety of halogenated antibiotics. As illustrated in Fig. 5(a), all five selected halogenated antibiotics were significantly destroyed under the $\mathrm{CO}_{2}^{·-}$-based ARPs, and four of them were completely degraded after treatment for 120 min, suggesting the universality of $\mathrm{CO}_{2}^{·-}$ for degrading halogenated antibiotics. Moreover, Fig. 5(b) demonstrates that the organic halogen moieties can be destroyed to form inorganic halogen ions; the final dehalogenation percentages of the halogenated antibiotics were more than 75% after treatment for 120 min, indicating the potential of $\mathrm{CO}_{2}^{·-}$-based ARPs for the simultaneous degradation and dehalogenation of halogenated antibiotics.
3.5.2. FF degradation under various natural water constituents
The impacts of various natural water constituents (pH, Cl−, SO42−, NO3−, HCO3−, and humic acid) on the degradation of FF were examined. As depicted in Fig. S8 in Appendix A, FF can be degraded completely by $\mathrm{CO}_{2}^{·-}$-based ARPs over a broad range of pH (4–10), with a better degradation rate being recorded under acidic conditions. The presence of inorganic anions (0–100 mmol·L−1) and humic acid (0–10 mg·L−1) had a negligible effect on the degradation of FF (Figs. S9 and S10 in Appendix A). In addition, efficient degradation of FF was achieved in actual groundwater (Fig. S11 in Appendix A), demonstrating the superior anti-interference capability of $\mathrm{CO}_{2}^{·-}$-based ARPs.
4. Conclusions
In this work, based on the formation of $\mathrm{CO}_{2}^{·-}$, a novel and highly efficient degradation strategy was developed for the simultaneous degradation, dehalogenation, and detoxification of FF, a typical halogenated antibiotic. The results demonstrated that FF was eliminated completely by $\mathrm{CO}_{2}^{·-}$, with approximately 100% of Cl− and 46% of F− released after 120 min of treatment. The $\mathrm{CO}_{2}^{·-}$-induced reductive dehalogenation significantly eliminated FF toxicity (R2 = 0.97, p < 0.01), with the formation of halogen-free products being the dominant reason for the better detoxification performance in comparison with conventical ·OH-based AOPs. Based on the identification of products and DFT calculations, the $\mathrm{CO}_{2}^{·-}$- induced FF dehalogenation mechanisms were comprehensively elaborated, revealing that the chlorine moiety of FF is most likely to come under nucleophilic attack by $\mathrm{CO}_{2}^{·-}$. In addition, the $\mathrm{CO}_{2}^{·-}$- based ARPs exhibited superior dehalogenation efficiencies (> 75%) in degrading a series of halogenated antibiotics, including CAP, THA, DLF, TCS, and CIP. Solution pH and the presence of natural water constituents exhibited a negligible effect on the degradation of FF. Furthermore, efficient degradation of FF was achieved in actual groundwater, demonstrating the great application potential of $\mathrm{CO}_{2}^{·-}$-based ARPs. In summary, $\mathrm{CO}_{2}^{·-}$ was found to be efficient for the simultaneous degradation, dehalogenation, and detoxification of halogenated antibiotics, providing a promising method for regulating halogenated antibiotics under oxygen-free conditions.
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (22176059, 21777042, and 22076045); the authors would also like to acknowledge support from the Science and Technology Commission of Shanghai Municipality’s Yangfan Special Project (23YF1408400) and the Fundamental Research Funds for the Central Universities.
Compliance with ethics guidelines
Yanzhou Ding, Xia Yu, Shuguang Lyu, Huajun Zhen, Wentao Zhao, Cheng Peng, Jiaxi Wang, Yiwen Zhu, Chengfei Zhu, Lei Zhou, and Qian Sui declare that they have no conflict of interest or financial conflicts to disclose.
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