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• Antibiotic pharmaceutical wastewater harbors substantial amounts of antibiotic residues, posing challenges for degradation using conventional biochemical methods [1]. The release of antibiotics contributes to the emergence of resistance genes in the environment, fostering increased resistance among environmental microorganisms [2, 3]. Advanced oxidation processes (AOPs) emerge as a leading approach for the remediation of organic pollutant-contaminated wastewater [4]. The pH range specified for the treatment effluent of pharmaceutical wastewater falls between 6 and 9 [5]. It is well-known that the reactivity of dominant oxidation species in AOPs is influenced by solution pH, for example, sulfate radicals (SO₄^•‒) can exhibit higher reactivity than hydroxyl radicals (^•OH) at near-neutral pH levels [6, 7]. Besides, nonradical species such as singlet oxygen (¹O₂) and high-valent metals are widely acknowledged for their low pH dependency and high selectivity [8, 9]. Hence, developing AOPs capable of efficiently treating antibiotic pharmaceutical wastewater by producing reactive oxygen species (ROS) under near-neutral pH conditions is crucial.

  Persulfate-based AOPs have garnered significant attention for their facile activation leading to the production of various ROSs (e.g., SO₄^•‒, ^•OH, and ¹O₂) across a broad pH range [10, 11]. Nevertheless, its practical application is constrained by its high cost [12]. Recent studies have shown that sulfite-AOPs can generate sulfite radicals (SO₃^•‒), which can subsequently transform into a range of oxidizing radicals, such as SO₄^•‒, ^•OH, and persulfate radicals (SO₅^•‒), under aerobic conditions [13]. Furthermore, sulfite exhibits biological toxicity levels an order of magnitude lower than peroxomonosulfate, peroxodisulfate (PDS), and H₂O₂, making it a promising candidate for replacing these conventional peroxides [14]. Calcium sulfite (CaSO₃) is a by-product of the predominant wet desulfurization process currently in use, characterized by cost-effectiveness and adherence to the waste reuse principle [15]. Serving as a sulfite precursor, CaSO₃, with reduced solubility, enables the gradual release of SO₃²⁻ in contrast to sodium sulfite (Na₂SO₃) and sodium bisulfite (NaHSO₃), thereby effectively inhibiting the scavenging of SO₄^•‒ by excess sulfite and enhancing the utilization of SO₄^•‒ [16]. To date, a variety of approaches have been utilized to activate sulfite, including metal ions (Fe³⁺, Mn²⁺, and Cu²⁺) and transition metal oxides (Co₃O₄, Fe₂O₃, and Cu₂O) [17-22]. However, the significant cost, limited pH range, and the potential leaching of toxic metal ions from these activators continue to present major obstacles to their extended application [23]. Therefore, it is imperative to develop an optimal heterogeneous catalyst for activating CaSO₃, enabling the efficient generation of ROS under near-neutral pH conditions.

  Molybdenum-based materials have attracted considerable interest within the realm of AOPs [24]. Molybdenum disulfide (MoS₂) is frequently utilized as a catalyst and cocatalyst in Fenton and Fenton-like reactions [25]. However, the substantial drawback in using MoS₂ stems from the potential release of H₂S upon the oxidation of sulfur atoms [26]. Molybdenum carbide (Mo₂C) possesses a d-band structure akin to that of platinum, allowing the extension of the distance between Mo-Mo bonds on Mo₂C to uncover additional active Mo sites, thereby enhancing its catalytic efficacy [27, 28]. Prior researches have indicated that β-Mo₂C can activate peracetic acid and function as a cocatalyst in PDS-based AOPs within a neutral pH range [29, 30]. Significantly, Mo₂C has been proven to benefit the adsorption capabilities for dissolved oxygen (DO), and the transition between various valent states of exposed Mo could improve the transformation from O₂^•‒ to ¹O₂ [31]. Moreover, DO also plays a crucial role in the conversion of SO₃^•‒ to SO₄^•‒, SO₅^•‒, and ^•OH in sulfite-AOPs [32]. Given the aforementioned points, it is reasonable to posit that Mo₂C could serve as a promising activator for sulfite, exhibiting efficiency under near-neutral conditions and potentially mitigating ion leaching. However, there is a knowledge gap concerning the Mo₂C-activated sulfite process, particularly in the activation of CaSO₃.

  The primary aim of this study is to establish an effective β-Mo₂C (hereafter referred to as Mo₂C) activated CaSO₃ system under near-neutral conditions and to elucidate the corresponding reaction mechanism, thereby validating our hypothesis. Within this investigation, metronidazole (MTZ), a prevalent and refractory antibiotic commonly found in pharmaceutical wastewater with concentration at milligram level, was chosen as the focal contaminant [33, 34]. The efficacy of MTZ removal using Mo₂C/CaSO₃ under varying conditions was investigated, with particular attention to the impact of buffer solutions at different pH levels. The major ROS responsible for degrading MTZ within the Mo₂C/CaSO₃ were identified, and their respective impacts on MTZ removal were quantified. The reaction mechanisms between CaSO₃ and Mo₂C were meticulously verified using a combination of experimental and theoretical calculation methods. This research is poised to complement the non-homogeneous activation method of sulfites by Mo₂C, thereby advancing the development of a molybdenum-based materials/sulfites process for the treatment of pharmaceutical wastewater.

  The chemicals utilized in this work are detailed in Text S1 (Supporting information). All catalytic tests were conducted in a 100 mL beaker at room temperature with continuous stirring. A 50 mmol/L phosphate buffer solution was employed to maintain a constant pH ranging from 3.0 to 9.0. The probe method was introduced in Text S2 (Supporting information). The experimental procedures are outlined in Text S3 (Supporting information). High-performance liquid chromatograph (HPLC) and ultraviolet spectrophotometer were applied to determine the concentration of target organic compound, X-ray diffractometer (XRD), X-ray photoelectron spectrometer (XPS), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM) were used to characterize the properties of Mo₂C. Electron spin resonance spectroscopy (ESR) and probe tests were employed to identify ROS. A modified 5, 5-dithiobis-(2-nitrobenzoic acid) method was applied to quantify the concentration of S(Ⅳ). Electrochemical analysis was performed to investigate the electron transfer between Mo₂C and CaSO₃. The microscopic mechanism of activation was elucidated through density functional theory (DFT) calculation. The details of these methods were introduced in Texts S4-S7, and Tables S1 and S2 (Supporting information).

  The catalytic efficacy of Mo₂C towards sulfite was assessed through the degradation of MTZ in a buffered system to mitigate the influence of pH variations. Various MTZ removal processes and pseudo first-order kinetic analyses within Mo₂C and CaSO₃ are outlined and illustrated in Fig. 1a. Mo₂C/CaSO₃ showed efficient and rapid degradation for MTZ over a 60 min period, with a pseudo-first-order rate constant (k_obs) of 0.02424 min⁻¹. Conversely, MTZ removal was insignificant when utilizing only CaSO₃ or Mo₂C independently. Furthermore, 62.24% of S(Ⅳ) was depleted within Mo₂C/CaSO₃ after 60 min (Fig. S1 in Supporting information), indicating an activation ratio (molar ratio of applied Mo₂C to consumed CaSO₃) of 1:1.95 for Mo₂C (1.2 g/L) activated CaSO₃, thereby suggesting that MTZ elimination predominantly resulted from CaSO₃ activation. To investigate the contribution of released Mo ions during the catalytic process, the liquid supernatant of Mo₂C was collected after 60 min of immersion to activate CaSO₃ for MTZ degradation. Fig. S2 (Supporting information) demonstrates that free Mo ions in conjunction with CaSO₃ were ineffective in MTZ elimination, implying that CaSO₃ activation primarily relied on the bulk phase of Mo₂C.

  Figure 1

  

  Figure 1.  (a) MTZ removal and corresponding pseudo first-order kinetic by diverse processes. (b) Comparison of MTZ degradation by Mo₂C activate different sulfites. (c) CaSO₃ activation by different Mo−based materials for MTZ elimination. Effect of CaSO₃ concentration (d), Mo₂C dosage (e), and buffered pH solution (f) on MTZ decomposition. Conditions: [CaSO₃]₀ = [Na₂SO₃]₀ = [NaHSO₃]₀ = 1 mmol/L, [Mo₂C]₀ = [Mo]₀ = [MoS₂]₀ = [MoO₂]₀ = 1.2 g/L, pH 6.

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  The degradation performance of MTZ in different sulfite-AOPs were compared, such as CaSO₃, Na₂SO₃, and NaHSO₃ (Fig. 1b). CaSO₃/Mo₂C presented the highest MTZ degradation efficiency (85.65%) at 60 min, surpassing that of Na₂SO₃ (54.24%) and NaHSO₃ (65.20%). The solubility of Na₂SO₃ and NaHSO₃ is reported to be 30.7 g/100 g H₂O and 29.0 g/100 g H₂O at 25 ℃, respectively, whereas the solubility for CaSO₃ is only 0.007 g/100 g H₂O. Higher dosage of aqueous SO₃²⁻ released from Na₂SO₃ would consume SO₄^•‒ generated from sulfite activation, hence affecting the MTZ removal. Conversely, the sustainably released SO₃²⁻ from CaSO₃ could avoid the self-quenching of SO₄^•‒ [35]. Additionally, the decomposition of MTZ in NaHSO₃ system was higher than that of Na₂SO₃, which could be attributed to the easier activation of dissociated HSO₃⁻ compared to SO₃²⁻.

  Three other typical Mo−based materials, namely molybdenum powder (Mo), MoS₂, and molybdenum dioxide (MoO₂), were chosen to assess their effectiveness in CaSO₃ activation. In Fig. 1c, minimal MTZ removal was observed in MoO₂/CaSO₃ and MoS₂/CaSO₃, with only 40.51% of MTZ being degraded in Mo/CaSO₃. This suggests that Mo was capable of activating a portion of CaSO₃, whereas MoO₂ and MoS₂ exhibited limited activation potential. The superior degradation of MTZ by Mo₂C/CaSO₃ is likely linked to the structure of Mo₂C, where carbon is incorporated into Mo lattices, resulting in increased exposure of active Mo sites, thus enhancing the activation of CaSO₃ [36].

  The impact of varying doses of CaSO₃ and Mo₂C on MTZ degradation in Mo₂C/CaSO₃ system was systematically explored. In Fig. 1d, as the CaSO₃ concentration increased from 0.5 mmol/L to 1 mmol/L, the removal efficiency of MTZ surged significantly from 70.15% to 85.65%, remained relatively stable upon further increase to 3 mmol/L, and then declined at 6 mmol/L. This noteworthy shift can be attributed to the excess presence of S(Ⅳ), which may engage in competitive interactions with SO₃^•‒ for DO, thereby impeding the conversion of SO₃^•‒ to SO₅^•‒. Additionally, the surplus S(Ⅳ) could partake in reactions with SO₄^•‒ and compete with contaminants for ROS. In Fig. 1e, an increase in catalyst dosage from 0.6 g/L to 1.2 g/L corresponded to a rise in MTZ degradation efficiency from 74.63% to 85.65%. This improvement is likely due to the higher catalyst dosage offering more active sites for CaSO₃ activation, consequently enhancing MTZ removal. Notably, inhibition of MTZ decomposition was observed at a Mo₂C dose of 1.8 g/L, possibly due to the accelerated depletion of CaSO₃ by excessive Mo₂C. Furthermore, faster MTZ degradation rate could be achieved by reducing the initial MTZ concentration and elevating the reaction temperature within Mo₂C/CaSO₃ (Figs. S3 and S4 in Supporting information).

  The effect of reaction pH in buffer solution (3.0–9.0) was systematically evaluated. In Fig. 1f, MTZ removal almost negligible under pH 3.0–5.0 and 9.0. The degradation ratio of MTZ were 85.65%, 66.38%, and 34.41% under pH conditions of 6.0, 7.0, and 8.0, respectively, and the corresponding rate constant were presented in Fig. S5 (Supporting information), indicating that the most favorable pH condition for MTZ removal was 6. To evaluate the interplay between CaSO₃ and Mo₂C under diverse pH conditions, the depletion of CaSO₃ at various pH levels was initially determined. The highest consumption of CaSO₃ was observed at pH 6 (Fig. S6 in Supporting information), suggesting the most efficient mass transfer rate between CaSO₃ and Mo₂C occurred at pH 6, aligning with the observed impact of pH conditions on MTZ degradation. Subsequently, a comparison of the oxidation and adsorption effects for MTZ at different pH circumstances for Mo₂C/CaSO₃ was undertaken. Fig. S7 (Supporting information) revealed that Mo₂C alone exhibited minimal MTZ adsorption across a range of pH levels, indicating that the degradation of MTZ under varying pH conditions was primarily driven by oxidation facilitated by the activation of CaSO₃.

  Electrochemical test was employed to further explore the influence of pH on Mo₂C/CaSO₃ process. Nyquist plots were examined across varying pH levels (Fig. S8 in Supporting information), highlighting the lowest charge transfer resistance at pH 6, thereby confirming its role in facilitating electron transfer between Mo₂C and CaSO₃. Linear sweep voltammetry (LSV) was employed to evaluate the redox potentials at different pH values (Fig. S9 in Supporting information). The introduction of MTZ into the Mo₂C/CaSO₃ system at pH levels of 6, 7, and 8 led to an augmentation in redox potential, providing evidence of electron exchange between the Mo₂C/CaSO₃ framework and MTZ, being in agreement with the degradation pattern of MTZ across varied pH. In addition, the morphological distribution of CaSO₃ varied at different pH levels (Fig. S10 in Supporting information). Under a pH of 6, CaSO₃ predominantly existed in HSO₃⁻, which is particularly favorable for enhancing sulfite activation [37], consequently facilitating MTZ degradation.

  Pre-mixing CaSO₃ and Mo₂C represents an effective approach for elucidating the effect of electron transfer [38]. In Fig. S11 (Supporting information), MTZ removal exhibited a significant decline with prolonged mixing duration, with efficiency nearly negligible after pre-mixing for 20 min. Consequently, no discernible electron-mediated effects were observed, and there were no indications of the presence of Mo-SO₃ complexes within this system. It is plausible that Mo₂C activated CaSO₃ to generate radical/non-radical species responsible for MTZ removal. Subsequently, we will delve deeper into the mechanism underlying CaSO₃ activation by Mo₂C.

  ROS quenching experiments was used to determine the type of ROS produced for MTZ removal during Mo₂C/CaSO₃. Various quenchers were employed, including methanol (MeOH, a quencher of ^•OH and SO₄^•‒), tert‑butanol (TBA, a quencher of ^•OH), furfuryl alcohol (FFA, a quencher of ¹O₂), l-histidine (l-His, a quencher of ¹O₂), and p-benzoquinone (BQ, a quencher of superoxide anion (O₂^•‒)) [39-41]. In Fig. 2a, adding 100 mmol/L TBA presented a modest inhibition on MTZ elimination (from 85.65% to 66.92%), suggesting that ^•OH was not the primary ROS. A distinct phenomenon arose where MTZ removal was inhibited from 85.65% to 18.75% upon introducing 100 mmol/L MeOH, indicating a substantial contribution of SO₄^•‒ to MTZ elimination. Furthermore, the suppression of MTZ degradation by FFA (from 85.65% to 30.91%) mirrored that of l-His (from 85.65% to 26.36%) (Figs. 2b and c), implying a crucial role of ¹O₂. The inhibitory effects of BQ (Fig. 2d) and FFA were comparable, and considering the very low redox potential of O₂^•‒, O₂^•‒ was deemed a primary source of ¹O₂ [41]. The quenching experiments revealed that the predominant ROS produced by Mo₂C/CaSO₃ at pH 6 were SO₄^•‒ and ¹O₂, with a minor presence of ^•OH.

  Figure 2

  

  Figure 2.  Quenching experiments at pH 6: (a) MeOH and TBA, (b) FFA, (c) l-His, (d) BQ. Effect of passing N₂ on MTZ degradation (e). DMPO-trapped ESR (f, g). TEMP-trapped ESR (h). Decomposition of NB, BA and FFA within Mo₂C/CaSO₃ (i). Steady-state concentrations and contribution of ROS (j). Conditions: [CaSO₃]₀ = 1 mmol/L, [Mo₂C]₀ = 1.2 g/L, [MTZ]₀ = 5 mg/L, pH 6.

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  A significant decrease in MTZ removal was occurred with the passage of N₂ (Fig. 2e). This could mean that O₂^•‒ is the main source of ¹O₂, and O₂ is involved in the transformation between ROS. HO· can produce ¹O₂ through disproportionation (Eq. 1) [42, 43]. Whereas, at pH 6, TBA introduction did not visibly inhibit MTZ decomposition, suggesting that ¹O₂ is not produced by this pathway. Given that the transition between different valence states of Mo on the Mo-based materials can benefit the transformation of O₂^•‒ to ¹O₂ [31], it is plausible that ¹O₂ could be originated from the oxidation of O₂^•‒ through the Mo sites of Mo₂C.

  $ 4 \cdot \mathrm{OH} \rightarrow{ }^1 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} $

  (1)

  Further, ESR were used to determine ROS generated in Mo₂C/CaSO₃ using DMPO and TEMP as traps. A gradual increased in the signal intensity of DMPO-SO₄^•‒ and DMPO-^•OH were occurred with increasing time (Fig. 2f), verifying the presences of SO₄^•‒ and ^•OH. At 5 min of the reaction, the signal of SO₄^•‒ was significantly increased by the passage of N₂, which could be probably due to the inhibition of SO₃^•‒ decomposition in the absence of O₂, affecting the conversion of SO₄^•‒ to SO₃^•‒. An escalation in the intensity of DMPO-SO₃^•‒ adduct signals was noted as the reaction proceeded (Fig. 2g), validating the formation of SO₃^•‒. As the low redox potential of SO₃^•‒ [44], SO₃^•‒ cannot attack MTZ directly. Besides, SO₃^•‒ could be quickly reacted DO to form SO₅^•‒. However, Fig. 2a shows an absolute restraint of MTZ elimination with 100 mmol/L MeOH (MeOH is fairly inert toward SO₅^•‒, k_MeOH-SO < 10³ L s⁻¹ mol⁻¹), suggesting a weak function of SO₅^•‒ for removing MTZ. The signal of ¹O₂ was gradually enhanced with time (Fig. 2h), verifying the generation of ¹O₂. The addition of BQ to quench O₂^•‒ significantly suppressed the signal of ¹O₂, verifying that O₂^•‒ was the source of ¹O₂. Combined with quenching and ESR results, it could be concluded that the main ROS were SO₄^•‒ (not SO₃^•‒ and SO₅^•‒) and ¹O₂, ^•OH is also produced during the process of Mo₂C/CaSO₃ under pH 6.

  To further ascertain the relative contribution of SO₄^•‒, ^•OH and ¹O₂ in Mo₂C/CaSO₃ process, competitive kinetic test was conducted utilizing nitrobenzene (NB), benzoic acid (BA), and FFA as molecular probes. NB, being unreactive to most ROS, has rapid reactivity with ^•OH [45], BA is susceptible to oxidative attack by ^•OH and SO₄^•‒ [46], and FFA is prone to attack by ¹O₂ [47]. The experimental results demonstrated a notably accelerated degradation rate of FFA (7.92 × 10⁻⁴ s⁻¹) compared to NB (5.48 × 10⁻⁵ s⁻¹) and BA (1.09 × 10⁻⁴ s⁻¹) (Fig. 2i), thereby affirming the significant involvement of ¹O₂ in the degradation mechanism. According to the kinetic expression (Eqs. S1−S9 in Text S2 in Supporting information), the steady-state concentrations for ¹O₂, SO₄^•‒ and ^•OH were calculated to be 1.06 × 10⁻¹² mol/L, 4.89 × 10⁻¹⁴ mol/L, and 1.17 × 10⁻¹⁴ mol/L (Fig. 2j), and their contributions were 37.6%, 44.7%, 11.6%, respectively (the illustration of Fig. 2j). This suggested that the main ROS were ¹O₂ and SO₄^•‒ in Mo₂C/CaSO₃, being in line with the scavenging and ESR results.

  Since the production and contribution of ROS is affected by pH, we further explored ROS production under pH 7 and 8 conditions by quenching experiments. At pH 7, the addition of TBA more severely inhibited the degradation of MTZ compared to the condition of pH 6 (Fig. S12a in Supporting information), indicating that ^•OH acted a vital role in this scenario. The quenching of FFA and BQ at pH 7 were similar to those of pH 6 (Figs. S12b and c in Supporting information). When the pH was elevated to 7, the increased contribution of ^•OH and decreased MTZ degradation may be due to the conversion of SO₄^•‒ to ^•OH. At pH 8, the similar effects of ¹O₂ and SO₄^•‒ were appeared as pH 6 (Figs. S12d-f in Supporting information). More OH⁻ can act as scavengers for ^•OH radicals at alkaline pH, leading to an obvious decrease in the ^•OH contribution and decreasing MTZ removal. Combined with previous results, it can be inferred that is more readily activated by Mo₂C at pH 6, minimizing the impact of OH⁻ and promoting the formation of SO₄^•‒.

  Next, a series of electrochemical tests were conducted to explore the activation mechanism of CaSO₃ by Mo₂C under pH 6. In Fig. 3a, the open-circuit potential (OCP) of Mo₂C remained stable for 300 s, and decreased sharply with the addition of CaSO₃, and increased slightly with the addition of MTZ. The results of OCP curves demonstrated that adding CaSO₃ resulted in the transferred of electrons to the surface of Mo₂C, and electrons were transferred from Mo₂C to MTZ after the introduction of MTZ. After successive additions of CaSO₃ and MTZ to the system containing Mo₂C, the Nyquist radius decreased significantly (Fig. 3b), indicating the reaction of Mo₂C/CaSO₃/MTZ had the smallest charge transfer resistance and stronger electron transfer capability. In Fig. 3c, the current change in the cyclic voltammetry (CV) curve was significantly enhanced after the introduction of CaSO₃ and MTZ, suggesting that Mo₂C/CaSO₃ had a heightened specific capacitance and accelerated electron cycling ability. The distinct electrochemical responses observed across various systems highlighted the pivotal role of Mo₂C in directly mediating the activation of CaSO₃ and the degradation of MTZ.

  Figure 3

  

  Figure 3.  Electrochemical tests of Mo₂C/CaSO₃: (a) OCP curve, (b) Nyquist plots, (c) CV curves. XPS profiles of pristine and used Mo₂C: (d) C 1s, (e) O 1s, and (f) Mo 3d.

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  Full-size XPS spectra of fresh and used Mo₂C are presented in Fig. S13 (Supporting information), revealing no significant species changes in the species of C, Mo, and O before and after the reaction. The high-resolution C 1s spectra at 282.5,283.8,284.8, and 288.3 eV exhibited four distinct peaks corresponding to C−Mo, C−C, C=O, and O−C=O, respectively (Fig. 3d) [48, 49]. The percentage of C−C augmented from 41.4% to 61.6%, implying the potential involvement of the C structure in Mo₂C during the reaction. A distinct C−Mo peak emerged post-reaction, suggesting intensified electron transfer between C and Mo, with C atoms potentially playing a role in expediting electron transfer [29]. The O 1s spectra of both pristine and utilized Mo₂C were resolved into two distinct peaks at 529.8 and 530.8 eV (Fig. 3e) [50], corresponding to Mo-O and −OH, separately [49, 51]. After reaction, the declined content of Mo-O could be ascribed to the reduction of Mo(Ⅵ) to Mo(Ⅳ) or Mo(Ⅱ). The increased content of −OH could be ascribed to O₂ adsorption on the catalyst surface [52]. As shown in Fig. 3f, the Mo 3d spectra of pristine and used Mo₂C were resolved into two distinct peaks at 231.8 and 234.9 eV, corresponding to Mo(Ⅵ) states [53]. Additional peaks were indicative of Mo(Ⅱ) (at 227.7 and 230.6 eV) and Mo(Ⅳ) (at 232.0 eV and 233.6 eV) states [48]. The alterations in Mo ion content are detailed in Table S3 (Supporting information). The reaction led to a decrease in Mo(Ⅳ) content from 77.45% to 28.88%, an increase in Mo(Ⅳ) content from 7.40% to 41.80%, and a rise in Mo(Ⅱ) content from 15.15% to 29.32%. The decrease in Mo(Ⅵ) content could primarily stem from two factors: first, the leaching of Mo(Ⅵ) from the Mo₂C surface under mildly acidic conditions; second, the conversion of Mo(Ⅵ) to Mo(Ⅳ) by successive redox reactions. Moreover, the rise in Mo(Ⅳ) content could be attributed to the interaction of Mo(Ⅱ) with O₂, generating O₂^•‒, and the subsequent reduction of Mo(Ⅵ) by O₂ in the activation process. It could be suggested that the conversion of Mo(Ⅵ) to Mo(Ⅳ)/Mo(Ⅱ) on Mo₂C served as the driving force for the activation of CaSO₃. Furthermore, the change between different Mo valence states promoted the production of O₂^•‒, consequently augmenting the production of ¹O₂.

  Microscopic reaction specifics involving Mo₂C and CaSO₃ were investigated through DFT calculations. TEM and HRTEM findings reveal a lattice stripe spacing of 0.219 nm on Mo₂C (Fig. S14 in Supporting information), affirming the dominance of the (101) crystallographic facet in Mo₂C. Therefore, the crystal cell was generated by sectioning the 101 crystal faces of Mo₂C, and the adsorption energies (E_ads) of different ions and molecules (HSO₃⁻, SO₃²⁻, and O₂) on Mo₂C were computed, with detailed outcomes presented in Table S4 (Supporting information). Initially, we modeled the concurrent adsorption of O₂ and HSO₃⁻ on the Mo₂C surface. Fig. 4a and Fig. S15 (Supporting information) depict the adsorption of HSO₃⁻ at the Mo site and C site, respectively. The E_ads at the Mo site was lower than that at the C site, both below zero, suggesting potential adsorption of HSO₃⁻ at both sites, with a higher likelihood at the Mo site. Additionally, the E_ads of HSO₃⁻ when adsorbed solely at the Mo site was computed (Fig. 4b). As the E_ads of HSO₃⁻ alone exceeded zero, indicating that without O₂ involvement, the adsorption process could not occur spontaneously. This observation further underscored the pivotal role of O₂ in HSO₃⁻ activation process and validated the findings of the earlier N₂ passage experiment. Furthermore, we simulated the initial adsorption of O₂ followed by the subsequent adsorption of HSO₃⁻ on the Mo₂C surface (Fig. S16 in Supporting information). An E_ads of HSO₃⁻ exceeding zero indicated the non-spontaneous nature of this reaction, further confirming that the prerequisite for HSO₃⁻ activation was the simultaneous adsorption of HSO₃⁻ and O₂ on Mo₂C surface. Fig. 4c presents the calculated E_ads when SO₃²⁻ and O₂ were concurrently adsorbed on the Mo site. The findings revealed that the E_ads of HSO₃⁻ was lower than that of SO₃²⁻, with both values being negative, suggesting that HSO₃⁻ was more inclined to be adsorbed on the Mo₂C surface than SO₃²⁻ for activation. This validated the rationale behind the heightened activation and MTZ degradation of S(Ⅳ) under pH 6.

  Figure 4

  

  Figure 4.  Initial and optimized adsorption configurations: (a) HSO₃⁻ and O₂ on Mo₂C surface, (b) HSO₃⁻ on Mo₂C surface, (c) SO₃²⁻ and O₂ on Mo₂C surface. (d) Proposed reaction mechanism for Mo₂C/CaSO₃.

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  Based on the results obtained from ROS determination, elemental valence analysis, and DFT calculations, a mechanism for the activation of CaSO₃ by Mo₂C was proposed (Fig. 4d). Firstly, CaSO₃ was decomposed into Ca²⁺ and HSO₃⁻ (Eq. 2) at pH 6, and then HSO₃⁻ was adsorbed on the surface of Mo₂C in the presence of O₂. Mo(Ⅵ) underwent a series of reduction/self-disproportionation reactions with S(Ⅳ), leading to the transformation into other Mo species (Mo(Ⅳ), Mo(Ⅱ)), and resulting in the generation of SO₃^•‒ ((3), (4)). The interaction of SO₃^•‒ with O₂ produced SO₅^•‒ (Eq. 5), which then underwent a cascade of reactions to form SO₄^•‒, SO₃^•‒, O₂^•‒ (Eqs. 6-10). Notably, the oxidation of O₂^•‒ by Mo(Ⅵ) resulted in the production of ¹O₂ (Eq. 11). Among these intermediate species, SO₄^•‒ and ¹O₂ were identified as the primary ROS responsible for the oxidation of MTZ (Eq. 12).

  $ \mathrm{CaSO}_3+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{Ca}^{2+}+\mathrm{HSO}_3^{-} $

  (2)

  $ \mathrm{Mo}(\mathrm{VI})+\mathrm{HSO}_3^{-} \rightarrow \mathrm{Mo}(\mathrm{IV})+\mathrm{SO}_3 { }^{\cdot-}+\mathrm{H}^{+} $

  (3)

  $ \mathrm{Mo}(\mathrm{IV})+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Mo}(\mathrm{II})+\mathrm{H}_2 \mathrm{O}_2 $

  (4)

  $ \mathrm{SO}_3^{\cdot-}+\mathrm{O}_2 \rightarrow \mathrm{SO}_5^{\cdot-} $

  (5)

  $ \mathrm{SO}_5^{\cdot-}+\mathrm{HSO}_3^{-} \rightarrow \mathrm{H}^{+}+\mathrm{SO}_4^{2-}+\mathrm{SO}_4^{\cdot-} $

  (6)

  $ \mathrm{SO}_5^{\cdot-}+\mathrm{SO}_5^{\cdot-} \rightarrow \mathrm{SO}_4^{\cdot-}+\mathrm{O}_2 $

  (7)

  $ \mathrm{SO}_4^{\cdot-}+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OH}+\mathrm{SO}_4^{2-}+\mathrm{H}^{+} $

  (8)

  $ \mathrm{SO}_4^{\cdot-}+\mathrm{H}_2 \mathrm{O} / \mathrm{OH}^{-} \rightarrow 2 ^{\cdot} \mathrm{OH}+\mathrm{SO}_3^{\cdot-} $

  (9)

  $ \mathrm{Mo}(\mathrm{II})+\mathrm{O}_2 \rightarrow \mathrm{Mo}(\mathrm{IV})+\mathrm{O}_2^{\cdot-} $

  (10)

  $ \mathrm{Mo}(\mathrm{VI})+\mathrm{O}_2^{\cdot-} \rightarrow{ }^1 \mathrm{O}_2+\mathrm{Mo}(\mathrm{IV}) $

  (11)

  $ \mathrm{SO}_4^{\cdot-} /^{\cdot} \mathrm{OH} /^1 \mathrm{O}_2+\mathrm{MTZ} \rightarrow \text { product }+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} $

  (12)

  DFT calculations was used to investigate the electron distribution and Fukui indices of MTZ at the molecular/atomic level. Fig. 5a shows the optimized molecular structure of MTZ. The molecular electrostatic potential, illustrating the electron density on the surface of MTZ molecule (Fig. 5b), used red and blue colors to represent positive and negative values, with color intensity indicating the magnitude of the electron density. Regions displaying higher positive values were identified as more reactive sites susceptible to ROS attack, suggesting that the N−CH₃CH₂OH group in MTZ molecule was particularly vulnerable to ROS attack. Fukui indices of MTZ, including electrophilic reactivity (f⁻), nucleophilic reactivity (f⁺), and radical attack propensity (f⁰), are displayed in Figs. 5c-e. The O atom with the greatest f⁻, f⁺, and f⁰ values was occurred at O12 (Table S5 in Supporting information), indicating that O12 atom was most susceptible to attack by free radicals. Fig. 5f presents the highest occupied molecular orbital (HOMO) (E_HOMO = −5.06 eV) and lowest unoccupied molecular orbital (LUMO) (E_LUMO = −3.06 eV) of MTZ. Regions rich in electrons were indicated in orange, and regions lacking electrons were shown in blue. Notably, N−containing heterocycles and −NO₂ groups exhibited the uppermost sensitivity towards attacks from ¹O₂ and SO₄^•‒ [54, 55].

  Figure 5

  

  Figure 5.  (a) Optimized molecular structure. (b) Molecular electrostatic potential. (c–e) Calculated Fukui index. (f) HOMO and LUMO distributions for MTZ. (g) Possible degradation pathways of MTZ.

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  Drawing upon computational analyses and mass spectrometry results, two conceivable degradation pathways of MTZ were postulated (Fig. 5g). In pathway 1, the initial degradation step involved denitration, yielding 3-(2-hydroxyethyl)−2-methyl-3H-imidazole-4-ol (P1, m/z = 143) [56], subsequently oxidized to form the intermediate (2-methyl-5-nitro-imidazole-1-yl)-acetic acid (P2, m/z = 157) [57]. ROS targeted the N-acetic acid moiety in P2, leading to its cleavage and the formation of 2-methyl-3H-imidazole-4-ol (P5, m/z = 99) [58]. In pathway 2, MTZ underwent oxidation by ROS, resulting in the formation of 2-methyl-5-nitro-imidazole-1-yl (P3, m/z = 185) [59]. Subsequently, after the hydroxylation of the methyl group and N−C, P3 was converted to 2-methyl-5-nitro-1H-imidazole (P4, m/z = 128) [60]. P4 underwent denitrification and oxidation to produce P5, which was further oxidized to yield oxalate acid (P6, m/z = 91) and 2-propene-1-ol (P7, m/z = 59) small molecule compounds. Ultimately, these compounds were eventually mineralized to CO₂, H₂O, and NO₃⁻.

  Inorganic ions, including nitrate (NO₃⁻), bicarbonate (HCO₃⁻), chloride (Cl⁻), sulfate (SO₄²⁻), and carbonate (CO₃²⁻), as well as humic acid (HA), are commonly found in aqueous solutions and can impact the elimination of organic contaminants by AOPs [61]. Effect of different water matrix on MTZ degradation is illustrated in Fig. S17 (Supporting information). NO₃⁻ and HCO₃⁻ showed insignificant inhibition effect (Figs. S17a and b). The decontamination of MTZ declined from 85.65% to 25.07% with an increase in Cl⁻ concentration from 0 to 10 mmol/L (Fig. S17c). Cl⁻ is recognized as a prominent radical remover, susceptible to oxidation through ROS (^•OH, and SO₄^•‒) to form the fewer reactive Cl₂^•‒ (Eqs. S10–S13 in Supporting information) [62]. Thus, the degradation was inhibited under high amount of Cl⁻. The impact of SO₄²⁻ was nearly negligible (Fig. S17d), while CO₃²⁻ exhibited a mild inhibitory effect with an increasing concentration of CO₃²⁻ (Fig. S17e). This observation can be attributed to the reaction of CO₃²⁻ with ^•OH, resulting in the formation of CO₃^•‒, characterized by an inferior redox potential and greater selectivity [63]. The degradation of MTZ by HA was marginally inhibited with an increasing HA concentration (Fig. S17f), possibly attributed to the scavenging of ROS by HA [64].

  To comprehensively illustrate the wide-ranging reactivity of Mo₂C/CaSO₃, the degradation effects on other common emerging pollutants were investigated, including naproxen (NAP), tetracycline (TC), enrofloxacin (ENR), dexamethasone (DXMS), alizarin red (AR), methyl orange (MO), congo red (CR), and methylene blue (MB) (Fig. S18 in Supporting information). Mo₂C/CaSO₃ demonstrated significantly greater degradation efficiency towards antibiotics (NAP, ENR, DXMS, and TC) compared to azo dyes (AR, MO, MB, and CR). The limited degradation of azo dyes can be attributed to the higher energy demand for azo bond cleavage (75.7 kcal/mol) compared to the energy released from ¹O₂ to O₂ (36.7 kcal/mol) and lower than SO₄^•‒ to SO₄²⁻ (148.7 kcal/mol) [65]. Conversely, the steady-state concentration of ¹O₂ was significantly higher than that of SO₄^•‒ in the Mo₂C/CaSO₃. NAP, TC, ENR, and DXMS are recognized as being vulnerable to attack by ¹O₂, SO₄^•‒, and ^•OH [30, 66-69], which constituted the primary ROS generated in our developed Mo₂C/CaSO₃.

  Successive CaSO₃ addition test was conducted to confirm the enduring catalytic capability of Mo₂C. In Fig. S19a (Supporting information), MTZ concentration was raised to 25 mg/L, with Mo₂C amount maintained at 1.2 g/L, and 1 mmol/L CaSO₃ was added hourly. Following five rounds of CaSO₃ addition, the removal efficiency reached 82.15% within 5 h, surpassing the efficacy of the system with five CaSO₃ additions without Mo₂C, and a single CaSO₃ addition with Mo₂C. The cyclic performance of Mo₂C was assessed (Fig. S19b in Supporting information), revealing that the degradation rate of MTZ could achieve 47.3% after 5 cycles. The decreased degradation efficiency could be related to the loss of Mo active sites derived from the oxidation of Mo₂C surface [70]. In Fig. S20 (Supporting information), XRD characteristic peaks of the initial, single-use, and five-use Mo₂C samples were all matched β-Mo₂C (PDF #35-0787), suggesting no notable alteration in the crystal structure of Mo₂C and showcasing it's a certain stability.

  Overall, we systematically evaluated the feasibility of CaSO₃ activation via Mo₂C catalysis for MTZ removal. The reaction process achieved the best degradation effect under near-neutral buffered pH conditions (85.65%), which was favorable for eliminating antibiotics from pharmaceutical effluent. The main ROSs generated in Mo₂C/CaSO₃ were ¹O₂ and SO₄^•‒, and their contributions for MTZ elimination were 44.7% and 37.6%, respectively. Furthermore, the promoted generation of ¹O₂ could be attributed to the conversion between different Mo valence states. The reaction of Mo₂C activated CaSO₃ was a heterogenous catalytic process, and DO could improve the adsorption of HSO₃⁻ on the surface of Mo₂C at pH 6, then benefiting the activation of CaSO₃. Mo₂C/CaSO₃ system exhibited good recycling performance and resistance to water matrix interference, and can effectively degrade multiple typical antibiotics. In this work, we proposed an oxidation method using a desulfurization byproduct CaSO₃ as oxidant and Mo-based materials as catalyst, broadening the understanding and application of S(Ⅳ)-AOPs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Mimi Wu: Writing – original draft, Investigation, Formal analysis, Data curation. Shoufeng Tang: Writing – review & editing, Writing – original draft, Supervision, Methodology, Formal analysis, Conceptualization. Zhibin Wang: Resources, Investigation. Qingrui Zhang: Resources, Investigation. Deling Yuan: Writing – review & editing, Supervision, Resources, Project administration, Formal analysis, Conceptualization.

  Acknowledgments

  The authors express their gratitude for the support received from the National Natural Science Foundation of China (No. 51908485), the Central Guidance on Local Science and Technology Development Fund of Hebei Province (Nos. 246Z3603G and 226Z3603G).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110613.

  
  
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• 

  Figure 1  (a) MTZ removal and corresponding pseudo first-order kinetic by diverse processes. (b) Comparison of MTZ degradation by Mo₂C activate different sulfites. (c) CaSO₃ activation by different Mo−based materials for MTZ elimination. Effect of CaSO₃ concentration (d), Mo₂C dosage (e), and buffered pH solution (f) on MTZ decomposition. Conditions: [CaSO₃]₀ = [Na₂SO₃]₀ = [NaHSO₃]₀ = 1 mmol/L, [Mo₂C]₀ = [Mo]₀ = [MoS₂]₀ = [MoO₂]₀ = 1.2 g/L, pH 6.

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  Figure 2  Quenching experiments at pH 6: (a) MeOH and TBA, (b) FFA, (c) l-His, (d) BQ. Effect of passing N₂ on MTZ degradation (e). DMPO-trapped ESR (f, g). TEMP-trapped ESR (h). Decomposition of NB, BA and FFA within Mo₂C/CaSO₃ (i). Steady-state concentrations and contribution of ROS (j). Conditions: [CaSO₃]₀ = 1 mmol/L, [Mo₂C]₀ = 1.2 g/L, [MTZ]₀ = 5 mg/L, pH 6.

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  Figure 3  Electrochemical tests of Mo₂C/CaSO₃: (a) OCP curve, (b) Nyquist plots, (c) CV curves. XPS profiles of pristine and used Mo₂C: (d) C 1s, (e) O 1s, and (f) Mo 3d.

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  Figure 4  Initial and optimized adsorption configurations: (a) HSO₃⁻ and O₂ on Mo₂C surface, (b) HSO₃⁻ on Mo₂C surface, (c) SO₃²⁻ and O₂ on Mo₂C surface. (d) Proposed reaction mechanism for Mo₂C/CaSO₃.

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  Figure 5  (a) Optimized molecular structure. (b) Molecular electrostatic potential. (c–e) Calculated Fukui index. (f) HOMO and LUMO distributions for MTZ. (g) Possible degradation pathways of MTZ.

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