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• The rapid development of aquaculture is caused by the increased demand for seafood [1,2]. In high-density aquaculture plants, antibiotics are extensively used to deal with bacterial infections and increase seafood production. However, a large portion of antibiotics cannot be absorbed by aquaculture organisms and released into aquaculture water, leading to the accumulation by degrees in the aquatic environment, thereby posing a potential threat to the ecological environment and human health [3-6]. Biological treatment technology is the conventional approach employed to dispose aquaculture wastewater owing to its low cost and energy consumption, but it cannot degrade antibiotics effectively [7-9]. Therefore, developing effective processes are important to degrade antibiotics from aquaculture wastewater.

  Piezoelectricity, as a kind of typical physical phenomenon, is mediated by a transformation of mechanical energy to polarized charges, which has been broadly applied in wastewater treatment [10-17]. However, most reported studies achieved a piezoelectric effect from ultrasonic vibration with a relatively high frequency (kHz), leading to high energy consumption to restrict the potential application. Therefore, it is urgent to capture a low-frequency force for piezoelectric effect motivation. Recently, Zhu's group novelty constructed a self-powered reactor to simulate pipeline drainage for organic pollutants removal [18,19] and disinfection [20]. Ao et al. also studied the important role of hydrodynamic factors in piezo-photocatalytic process with ZnO nanorod array [21]. In the field of aquaculture, aeration is always used to improve the content of dissolved oxygen (DO), thereby prolonging the lifetime of aquaculture organisms [22,23]. Such an aeration process provides a possibility to induce piezoelectric effect by means of turbulence and water bubbles rupture. It is also worth studying the contribution of DO offered by aeration for reactive oxygen species (ROS) generation in piezo-catalytic degradation reactions.

  In this work, an aeration device was introduced to simulate the aeration process in aquaculture, which triggered the piezoelectric effect to degrade antibiotic tinidazole (TNZ) using an odd-layered MoS₂. TNZ is one of the typical and most frequently detected antibiotics in aquatic environment, which is hardly decomposed via conventional wastewater treatment technologies due to its high bioaccumulation potential and low biodegradability [24,25]. The results showed that MoS₂ exhibits a higher removal efficiency of TNZ under aeration of DO than that under N₂ atmosphere and quiescent conditions. The intrinsic mechanisms of the piezo-catalytic degradation were revealed by scavenger experiments and electron paramagnetic resonance analysis. Moreover, a liquid chromatography quadrupole time-of-light tandem mass spectrometry (LC-TOF-MS) was further used to identify the degradation intermediates and pathways of TNZ degradation. The turbulence and water bubbles driven by aeration worked as the piezoelectric force to trigger piezo-catalysis, and also provided sufficient DO to generate ROS for the decomposition of organic contaminants. The strategy based on piezo-catalysis using MoS₂ under aeration to solve aquaculture water pollution is described as Fig. 1.

  Figure 1

  

  Figure 1.  Schematic diagram of piezo-catalytic TNZ degradation using MoS₂ under aeration.

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  All reagents, preparation, and characterization of MoS₂, along with analysis methods are shown in Text S1 (Supporting information). As shown in Figs. S1-S3 (Supporting information), the as-synthesized MoS₂ was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). As depicted in Fig. S1, the peaks at 13.8°, 32.4°, and 57.4° can be indexed to (002), (100), and (110) crystal planes of MoS₂ (JCPDS No. 37–1492) [26], respectively, demonstrating a successful preparation of MoS₂. The TEM image of MoS₂ in Figs. S2a and b exhibits an odd-layers structure of MoS₂, in which the lattice spacing of 0.63 nm is ascribed to the (002) crystallographic plane [27]. The XPS spectra in Fig. S3a present that there are two peaks at 228.2 eV and 231.3 eV assigned to Mo 3d_5/2 and Mo 3d_3/2 (Fig. S3b in Supporting information) [28,29], while S 2p_3/2 and S 2p_1/2 are located at 161.1 eV and 162.4 eV (Fig. S3b) [30]. Besides, the piezoelectric property of as-prepared MoS₂ is verified by piezoelectric force microscope (PFM), piezoelectric current density, and electrochemical impedance spectroscopy (EIS). From Fig. S4 (Supporting information), MoS₂ showed a polarized rotation of 180° in the phase-voltage loop and the amplitude-voltage loop exhibited a typical butterfly-shape curve, suggesting an excellent piezoelectric polarization of MoS₂ [31]. Fig. S5a (Supporting information) presents that MoS₂ exhibited a prompt piezoelectric current response during consecutive on-off aeration cycles, while a semicircular Nyquist diagram of the MoS₂ was smaller than that obtained under static conditions, suggesting a smaller resistance (Fig. S5b in Supporting information) [32]. All proofs above set a solid foundation for a well-synthesized MoS₂ with strong piezoelectric response.

  The water bubbles rupture driven by aerator in Fig. S6 (Supporting information) was employed as the piezo-driving force to study the influence of turbulence driven by aeration and DO in MoS₂/aeration system, control experiments included MoS₂ (Quiescent), MoS₂/aeration, and MoS₂/N₂ system were compared. As depicted in Fig. 2a, the removal rate of TNZ was achieved at 89.6% in the MoS₂/aeration system, while it decreased to 59.9% in the MoS₂/N₂ system. In contrast to MoS₂ (Quiescence) system (2.1% removal rate of TNZ), the degradation activity of MoS₂/N₂ system may due to the water bubbles rupture generated by aeration resulting in a piezoelectric polarization of MoS₂ to produce polarized carriers such as electron (e⁻) and hole (h⁺) to directly remove TNZ. Moreover, the first-order kinetics model of TNZ degradation was employed and shown in Fig. 2b. The reaction rate constant k of the MoS₂/aeration system (k = 0.15 min⁻¹) is 2.4 times higher than the sum of MoS₂/N₂ system (k = 0.061 min⁻¹) and MoS₂ (Quiescent) system (k = 0.0014 min⁻¹), revealing the significant roles in the co-presence of aeration and DO for TNZ degradation.

  Figure 2

  

  Figure 2.  (a) Degradation efficiencies and (b) pseudo-first-order kinetics of TNZ degradation under various oxidation systems. (c) Quenching experiments in MoS₂/aeration system for TNZ degradation. EPR spectra for the detection of (d) HO^•, (e) O₂^•−, and (f) ¹O₂ under different catalytic systems. Experimental conditions: [TNZ] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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  To ascertain the ROS of TNZ degradation with MoS₂, quenching experiments were performed by using isopropanol (IPA), p-benzoquinone (p-BQ), l-histidine (l-His), potassium dichromate (K₂Cr₂O₇), and potassium iodide (KI) as scavengers [33,34]. Fig. 2c shows that the degradation rate of TNZ was not obviously suppressed with the addition of IPA (decreased to 79.2%), while a 66.1% removal rate was achieved in the presence of K₂Cr₂O₇, which indicated a minor contribution of HO^• and e⁻ on TNZ degradation. Besides, the introduction of p-BQ, KI, and l-His restrained the efficiency to 38.3%, 45.4%, and 49.1%, confirming that O₂^•−, h⁺, and ¹O₂ played dramatic roles in TNZ degradation. For a deep understanding, electron paramagnetic resonance (EPR) tests were used to identify the generation of ROS in the piezo-catalytic process. As depicted in Figs. 2d-f, the signals of DMPO-HO^•, DMPO-O₂^•−, and TEMP-¹O₂ were detected in the MoS₂/aeration process, while no EPR peaks were observed in MoS₂ (Quiescence) system, indicating the aeration is an essential factor for piezo-catalysis. Remarkably, compared with the MoS₂/aeration system, no signal of DMPO-O₂^•− could be seen under N₂ atmosphere in the absence of DO, suggesting the generation of O₂^•− is from DO in water [35]. In the meanwhile, there was also no TEMPO-¹O₂ signal observed in the MoS₂/N₂ system (Fig. 2f), which further confirmed that DO was also crucial in ¹O₂ formation. Based on these results, it can be assumed that the piezoelectrically polarized e⁻ reacted with DO to convert into O₂^•−, then follows the generation of ¹O₂.

  The influence of operation parameters in the MoS₂/aeration system for TNZ degradation was conducted by adjusting MoS₂ dosage (0.1–1 g/L), initial TNZ concentration (5–20 mg/L), and aeration flow rate (0–3 L/min). Fig. 3a shows the degradation efficiencies of TNZ improved with increasing dosage of MoS₂, owing to more active sites available in MoS₂. In addition, the removal rate of TNZ was inversely proportional to the enhancement of initial TNZ concentration, which was because of competitive reactions of ROS by numerous TNZ molecules in such a high-concentration solution. Interestingly, increasing the aeration flow rate allowed MoS₂ to boost TNZ degradation efficiency gradually. It could be believed that drastic water bubbles rupture was able to provide a stronger aeration force with MoS₂ to strengthen the piezoelectric polarization for a higher piezo-potential, thus facilitating a more efficient charges separation for ROS generation. Cycles experiment, TEM, and XRD were carried out to evaluate the stability of MoS₂. From Figs. S7-S9 (Supporting information), the catalytic performance only decreased by 6.1% after four cycles and there was no change in morphology of TEM image and crystallinity. Furthermore, TNZ was treated by the MoS₂/aeration system in different water matrices to assess the effectiveness of MoS₂/aeration system in natural samples (Text S2 and Fig. S10 in Supporting information). The above results imply that MoS₂/aeration system shows a high potential for TNZ degradation in real water.

  Figure 3

  

  Figure 3.  (a) Effect of operational paraments in MoS₂/aeration system for TNZ degradation. (b) Proposed degradation pathways and (c) TOC removal of TNZ in MoS₂/N₂ and MoS₂/aeration system.

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  According to the LC-TOF-MS data (Fig. S11 in Supporting information), five degradation intermediates of TNZ degradation were identified. As shown in Fig. 3b, the procedure of degradation possibly started with the hydroxylation on the N-heterocyclic ring to generate P1, then P2 was evolved by further hydroxylation from P1 [36]. Meanwhile, nitro group of P1 was also likely to be attacked by hydroxyl group to produce P3. Afterwards, the opening of the N-heterocyclic ring and the loss of methyl group could eventually lead to P4 and P5 [37]. These degradation intermediates were finally mineralized to CO₂ and H₂O, and the TOC removal efficiency of 54.1% was realized, while the MoS₂/aeration system under N₂ atmosphere only exhibited 16.2% of TOC removal, further demonstrating that the presence of DO not only promotes the generation of ROS to facilitate the degradation of TNZ, but also enhances the mineralization rate of TNZ removal (Fig. 3c). Besides, the bioaccumulation factor, developmental toxicity, and mutagenicity of TNZ and its degradation products were analyzed by Toxicity Estimation Software Tool (T.E.S.T). The results and discussion were given in Table S1 and Text S3 (Supporting information).

  In order to clarify the performance of piezoelectric effect more intuitive, we performed the piezo-decolorization process of methylene blue (MB) and shown in Video S1 (Supporting information) and Fig. 4. Under the turbulence and water bubbles rupture driven by aeration, the color of MB faded from dark blue to light blue in the given time intervals, demonstrating that the aeration-driven piezo-catalysis can be broadly applied in wastewater remediation.

  Figure 4

  

  Figure 4.  Screenshots of the video in given times during the piezo-decolorization process of MB. Experimental conditions: [MB] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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  In addition to TNZ, other various antibiotics including ornidazole (ORZ), tinidazole (TNZ), metronidazole (MTZ), secnidazole (SNZ), ciprofloxacin (CIP), sulfamethazine (SMT), sulfamethoxazole (SMX), oxytetracycline (OTC), tetracycline (TC), and doxycycline (DTC) were also used to investigate the piezo-catalytic ability in the MoS₂/aeration system. As shown in Fig. 5, the removal efficiencies reached 93%, 89.5%, 82.1%, 95.2%, 83.1%, 95.1%, 91.6%, 88.1%, 85.2%, and 86.6%, respectively in MoS₂/aeration system, which convincingly proved that the MoS₂ possesses wide applicability for the elimination of antibiotics under aeration-driven piezo-catalysis in the presence of DO.

  Figure 5

  

  Figure 5.  Structures of the different organic pollutants: (a) ORZ, (b) TNZ, (c) MTZ, (d) SNZ, (e) CIP, (f) SMT, (g) SMX, (h) OTC, (i) TC, (j) DTC. (k) Degradation efficiency of the above ten pollutants in the MoS₂/aeration system. Experimental conditions: [pollutants] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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  Combined with quenching experiments and EPR tests, the reaction mechanism of TNZ degradation in MoS₂/aeration system is proposed as equations ((1), (2), (3), (4), (5), (6), (7)). Under aeration, the piezoelectrically polarized charge carriers (e⁻ and h⁺) are concurrently separated on the surface of MoS₂ (Eq. 1). The piezo-excited e⁻ reacts with DO to produce O₂^•− (Eq. 2), while HO^• is generated by the reaction between remained holes and H₂O (Eq. 3). Then, the holes may oxidize O₂^•− to form ¹O₂ (Eq. 4), and this process is demonstrated by the evident suppression of TNZ removal in the addition of KI (h⁺ scavenger) in scavenger experiments (Fig. 2c). Afterwards, the generated HO^• recombined with O₂^•− to generate ¹O₂ (Eq. 5). Moreover, superfluous O₂^•− will couple with itself in water to evolve ¹O₂ (Eq. 6). Finally, O₂^•−, HO^•, and ¹O₂ are responsible for TNZ degradation (Eq. 7).

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  In summary, an aeration-driven piezo-catalysis on MoS₂ for TNZ degradation in the presence of DO was investigated. During this piezo-catalytic system, the turbulence driven by aeration not only triggers MoS₂ to generate a piezoelectric potential to drive the transfer of charges, but also provides sufficient DO to produce ROS including O₂^•−, HO^•, and ¹O₂. Remarkably, DO plays a critical role in generating O₂^•− and ¹O₂ in the piezo-catalytic reaction for attaining high-efficient TNZ degradation and TOC removal within 15 min. Furthermore, the system has great stability and applicability for ten types of antibiotic degradation as well. This work offers a novel way to achieve water pollutant treatment by piezo-catalysis using aeration.

  Declaration of competing interest

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

  Acknowledgments

  The study was financially supported by Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ESK202102), the Science and Technology Program of Guangzhou (No. 202201020545), and the Special Fund for Basic Scientific Research Business of Central Public Research Institutes (No. PM-zx703-202204-117).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108229.

  
  
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  Figure 1  Schematic diagram of piezo-catalytic TNZ degradation using MoS₂ under aeration.

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  Figure 2  (a) Degradation efficiencies and (b) pseudo-first-order kinetics of TNZ degradation under various oxidation systems. (c) Quenching experiments in MoS₂/aeration system for TNZ degradation. EPR spectra for the detection of (d) HO^•, (e) O₂^•−, and (f) ¹O₂ under different catalytic systems. Experimental conditions: [TNZ] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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  Figure 3  (a) Effect of operational paraments in MoS₂/aeration system for TNZ degradation. (b) Proposed degradation pathways and (c) TOC removal of TNZ in MoS₂/N₂ and MoS₂/aeration system.

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  Figure 4  Screenshots of the video in given times during the piezo-decolorization process of MB. Experimental conditions: [MB] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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  Figure 5  Structures of the different organic pollutants: (a) ORZ, (b) TNZ, (c) MTZ, (d) SNZ, (e) CIP, (f) SMT, (g) SMX, (h) OTC, (i) TC, (j) DTC. (k) Degradation efficiency of the above ten pollutants in the MoS₂/aeration system. Experimental conditions: [pollutants] = 10 mg/L, [MoS₂] = 0.5 g/L, aeration flow rate = 3 L/min, and T = 25 ℃.

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