1. Introduction
Piezoelectric catalysis technology utilizes mechanical stress to induce an electric field in piezoelectric materials, triggering the generation of active species and enhancing material conversion efficiency [1]. Compared to other catalytic technologies, the uniqueness of piezoelectric catalysis lies in its primary reliance on mechanical energy, such as ultrasound, mechanical stirring, or water wave vibration. This approach reduces dependence on other energy sources, thereby improving energy utilization efficiency. As a result, piezoelectric catalysis is regarded as a new technology that can effectively address environmental issues [2, 3]. Currently, the combination of piezoelectric catalysis with advanced oxidation processes (AOPs) for water treatment is considered a promising approach. This combination not only enhances catalytic effectiveness but also expands the application scope of piezoelectric catalysis, making it more competitive in actual water treatment processes [4].
Experimental studies have demonstrated that piezoelectric catalysis exhibits excellent performance in degrading organic pollutants. Piezoelectric BaTiO3, an environmentally friendly lead-free piezoelectric ceramic, has the advantages of low cost and mass production [5, 6]. Additionally, BaTiO3 maintains stable performance even under harsh environmental conditions, and its superior chemical stability enables BaTiO3 to resist corrosion from manycommon chemicals [7].However, the saturation polarization field of BaTiO3 is low, making it susceptible to polarization reversal and resulting in failure [8]. Furthermore, its Curie temperature (Tc) is too high, and the variation in dielectric constant near Tc and the high sintering temperature significantly weaken the practical application performance of BaTiO3 [9]. Therefore, this study aimed to adjust commercial BaTiO3 to address these shortcomings, striving to increase the saturation polarization field and lower the Curie temperature. Through these adjustments, it is hoped that the overall performance of BaTiO3 can be enhanced to better meet the needs of practical applications.
Metronidazole (MNZ) has the chemical formula C6H9N3O3S and is a widely used antibiotic and antiprotozoal drug that often enters water bodies through sewage discharge or human activities, leading to water pollution. It may disrupt the ecological balance of water bodies, influencing the structure and function of microbial communities [10]. Additionally, research has indicated that long-term use of MNZ may be associated with an increased risk of certain types of cancer [11]. Therefore, in this study, MNZ was chosen as the target pollutant to evaluate the potential application effects of the optimized BaTiO3.
In this study, BaTiO3 was selected as a piezoelectric catalytic model, combined with peroxydisulfate (PDS) activation for the degradation of MNZ. This study initially explores the enhanced removal of MNZ by designing experiments to elucidate the mechanism of piezoelectrically induced PDS activation and further evaluating the potential practical applicability of this method. Experimental exploration of piezoelectrically activated PDS holds the promise of not only providing a more effective and environmentally friendly method for PDS activation and the efficient removal of organic pollutants but also potentially opening up new avenues for utilizing piezoelectric catalysis in wastewater treatment.
2. Materials and Methods
2.1. Materials
Metronidazole (MNZ, ≥99.0%), BaTiO3(≥99.9%), tert-butanol (TBA, ≥99.5%), methanol (MA, ≥99.9%), EDTA (≥99.0%), AgNO3 (≥ 99.8%), p-benzoquinone (P-BQ, ≥98.5%), anthocyanin (5% ~ 25%), 2,2,6,6-tetramethyl-4-piperidinol (TEMP), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and dimethyl sulfoxide (DMSO) were purchased from Aladdin Chemical Reagent Co., Ltd., China.
2.2. Synthesis and Characterization of Catalysts
The BaTiO3 nanoparticles used in this study were obtained by annealing commercial BaTiO3 at 800°C, with a purity of 99.9%, and the particle size is less than 100 nanometers. The phase structure and crystallinity of the prepared BaTiO3 were determined by X-ray diffraction (XRD, Xcalibur E, Oxford Instruments). The surface elemental composition and chemical state of BaTiO3 were analyzed using X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos, UK). The microstructure of BaTiO3 was observed using scanning electron microscopy (SEM, Aztec X-Max20, Oxford Instruments). The degradation transformation products (TP) of MNZ were analyzed using LCMS. (TSQ Quantum Ultra, American Thermo Fisher).
2.3. Experimental Procedure and Analysis Methods
A 40 kHz ultrasonic cleaning machine (100 W, Jielimei, Kunshan, China) was employed to apply mechanical force for the deformation of BaTiO3, inducing the piezoelectric effect [12]. The beaker, serving as a reaction vessel, was positioned within the ultrasonic cleaning machine, as illustrated in Fig. S1. The reaction was triggered by activating the ultrasonic cleaning machine to apply mechanical force. To prevent excessive temperature rise due to prolonged sonication, ice packs were placed in the ultrasonic cleaning machine to maintain the temperature at approximately 23°C, with fluctuations kept under 3°C.
The efficiency of activated PDS (1 g/L) using BaTiO3 (0.6 g/L) piezoelectricity was assessed through the degradation of MNZ (20 mg/L). BaTiO3 was dispersed in the MNZ solution and agitated to reach full adsorption-desorption equilibrium. The stirred mixture underwent ultrasonic treatment in the ultrasonic cleaner, followed by centrifugation to obtain a clear solution [13]. Pollutant concentrations in the solution were measured using a chromatographic analyzer. The removal efficiency was determined by tracking the relative concentration (C/C0) over time, where C0 represented the initial concentration and C was the remaining concentration. The degradation kinetics of MNZ were assessed using the pseudo first-order kinetic model ln(C/C0) = −kt, with C and C0 denoting MNZ concentrations at time (t) and t = 0, respectively, where k represents the first-order rate constant. Triplicate experiments were carried out to calculate average values and standard deviations [14].
The piezoelectric current in the BaTiO3-activated PDS system driven by ultrasonic force was measured using an electrochemical workstation equipped with a three-electrode system. The experimental conditions included a gas flow rate of 300 ml/min and a sodium sulfate electrolyte concentration of 1 M [15]. The production of reactive species was examined and detected using the electron spin resonance (ESR) technique (Germany Bruker ESR 5000) at room temperature.
2.4. Density Functional Theory (DFT) Calculation Method
The charge transfer between BaTiO3 and PDS were analyzed by using Quantum ESPRESSO software [16, 17]. The ultrasoft pseudo-potential was adopted to simulate the interactions [18]. A vacuum spacing of 20 A was employed to eliminate the interactions between layers. The cutoff energy was set to be 40 Ry and a Monkhorst–Pack K-point of 6×6×3 was used for geometry optimization and property calculations. The visualization of optimized geometries were obtained by XCrySDen [19], and VESTA [20].
DFT calculations of MNZ were conducted by using ORCA [21, 22]. B3LYP functional with the 6–31G* level was used to optimize geometry. The condensed Fukui function was calculated based on Hirshfeld charges using the Multiwfn program [23, 24]. The VMD16 obtained the visualization of charge density and geometries [25]. Solvation effects were introduced to all calculations.
3. Characterization of Catalyst and Enhanced MNZ Removal by BaTiO3 Piezo-Activated PDS
3.1. Characterization of Catalyst
Fig. 1(a) depicts the crystal phase of the synthesized BaTiO3 as determined by XRD analysis. The peaks observed at 22.237°, 31.495°, 38.894°, 45.372°, and 56.276° correspond to the (100), (101), (111), (200), and (211) crystal planes of BaTiO3, respectively. These peaks align with the standard pattern for tetragonal BaTiO3 (PDF card No. 01-076-0744) [26]. In Fig. 1(b), SEM images showcase the characteristics of the BaTiO3 particles obtained prior to the reaction. These particles predominantly display a relatively narrow size distribution and a spherical grain morphology [27]. From the XPS full spectrum as shown in Fig. 1(c), it is clear that the material contains four elements: Ba, O, Ti, and C. Fig. 1(d) shows the XPS spectra of Ba 3d, O 1s, and Ti 2P in the BaTiO3 compound. The binding energies of 776.25 eV and 791.6 eV can be attributed to Ba 3d5/2 and Ba 3d3/2, respectively. The binding energy of Ti 2p3/2 is located at 455.75 eV and 461.65 eV. The binding energy of O 1s is mainly at 527 eV, attributed to O 1s1/2 [28].
3.2. Enhanced MNZ Removal by BaTiO3 Piezo-Activated PDS
In Fig. 2(a), the elimination profiles across different reaction systems are depicted, describing the MNZ degradation activity (C/C0) over reaction time. with the MNZ degradation activity (C/C0) plotted against reaction time. The specific BaTiO3 dosage and PDS concentration that were optimized are provided in Fig. S2.
In all three systems, a minor amount of MNZ is adsorbed initially, reaching absorption-desorption equilibrium within 10 minutes. In the PDS/ultrasound system, MNZ degradation efficiency was approximately 12%, indicating limited PDS activation by low-frequency ultrasound radiation. When subjected to the BaTiO3/PDS/stirring process, about 16% degradation efficiency suggests that the stirring force alone is insufficient to trigger BaTiO3 via the piezoelectric effect to induce an internal electric field for catalyzing the reaction. However, upon replacing stirring with ultrasonic treatment in the BaTiO3/PDS/ultrasound system, remarkable MNZ degradation performance was observed. Within 30 minutes, the MNZ removal rate surged to 92%, marking an 80% increase compared to the PDS/ultrasound oxidation method and a 76% increase compared to the BaTiO3/PDS/stirring oxidation method. This suggests that the piezoelectric effect of BaTiO3 can efficiently stimulate PDS and boost the reaction activity when subjected to the mechanical force generated by the ultrasonic cleaning machine.
By analyzing the logarithm of MNZ concentration changes over a 30-minute degradation process, the apparent pseudo first-order reaction rate constants (k) are determined to be 0.00632, 0.06426, and 0.00484 min−1 for the BaTiO3/PDS/stirring (1), BaTiO3/PDS/ultrasound (2), and PDS/ultrasound (3) processes, as depicted in Fig. 2(b). When comparing the rate constants of the three processes, the rate constant (k(2)) for the BaTiO3 piezoelectric-catalyzed activation of PDS is approximately six times greater than the combined value of k(1) and k(3). This observation implies that the impact of piezoelectrically activated PDS surpasses the combined effect of the other two processes, indicating a distinct synergistic effect. When contrasted with the PDS/ultrasound and BaTiO3/PDS/stirring configurations, these findings provide robust evidence of the superior performance enhancement of MNZ in the BaTiO3 piezoelectric-catalyzed activation of PDS system.
From a practical application standpoint, the durability of the piezoelectric catalyst is a critical consideration. The reusability of BaTiO3 is assessed through cycling degradation experiments involving the centrifugation and collection of the catalyst suspension. From Fig. 2(c), After 4 cycles, the piezocatalyst BaTiO3 maintains outstanding degradation activity, achieving approximately 87% MNZ removal efficiency within 30 minutes. The post-reaction catalyst was subsequently analyzed once more using SEM and XRD. Fig. 2(d) and (e) demonstrate that there are no alterations in the diffraction patterns, size distribution, and spherical grain morphology post-reaction. These findings indicate the stability and high activity of BaTiO3. As shown in Fig. 2(f), compared to the XPS spectrum before the reaction, the spectrum after the reaction shows a slight shift in peak positions. Based on relevant literature, this shift is mainly attributed to the application of mechanical force on BaTiO3, which leads to surface distortion and consequently affects the electronic structure of the material, causing the peak position shifts [29].
The degradation transformation products (TP) of MNZ were analyzed using LCMS. The corresponding MS spectra of MNZ are shown in Fig. S3. Analyze the product system and prove that MNZ is decomposed into various small molecular substances. To test the practical usage, as shown in the Fig. S4, three common pollutants were treated using BaTiO3 piezoelectric-activated PDS system. From Fig. S5, MNZ was treated using BaTiO3 piezo-activated PDS in different real water matrices that are respectively from purified water, river, rain and tap water. The collective findings suggest that the piezoelectric properties of BaTiO3 can efficiently activate PDS, thereby enhancing the degradation efficiency of typical water contaminants. The initial pH and finial pH of different systems are shown in table S1. Given the variability of natural water conditions across different wastewater sources, the pH level of the solution and the presence of anions are crucial factors to consider. The impact of pH and anions on the reaction was studied by adjusting the solution’s pH level, and the outcomes are illustrated in Fig. S6. The findings indicate that the BaTiO3 piezocatalytically activated PDS process is relatively insensitive to variations in pH and anions, suggesting its potential for effective application in real wastewater treatment scenarios.
3.3. Understanding the Mechanism of Piezocatalytic PDS Activation
3.3.1. The function of piezoelectric charges
As is commonly understood, the generation of holes and electrons through electron transfer is considered the primary factor in triggering PDS activation and the production of various reactive oxygen species (ROS) [30]. As shown in the EIS curve of BaTiO3 (Fig. 3(a)), the charge transfer resistance of the electrochemical system under ultrasound is significantly reduced compared to the case without ultrasound, indicating a higher efficiency of piezoelectric charge transfer. Improved charge transfer enables a greater number of piezoelectric charges to facilitate electrochemical reactions [31]. Additionally, as a characteristic of electron transfer, the piezoelectric current response is directly observed when a force is applied, as shown in Fig. 3(b). This suggests that under the influence of ultrasound, a certain amount of piezoelectric charge is generated and accumulated on the surface of BaTiO3, exhibiting the piezoelectric effect [32].
3.3.2. Involved reactive species
In order to clarify the impact of reactive species on the degradation of MNZ within the BaTiO3/PDS/ultrasound system, a set of quenching experiments were carried out. These experiments involved the use of various scavengers to eliminate specific reactive species and assess their individual contributions. As shown in Fig. 4(a), The degradation efficiency of MNZ was not notably hindered when employing MA, TBA, and AgNO3 as scavengers for ·SO4−, ·OH, and electrons, respectively [33, 34], suggesting that the impact of ·SO4−, ·OH, and electrons on the degradation efficiency of MNZ is negligible. The addition of scavengers EDTA and P-BQ, which scavenge holes and ·O2− [35], slightly impeded the degradation of MNZ. Importantly, as a scavenger of singlet oxygen (1O2) [36], The presence of anthocyanin resulted in the most significant quenching of MNZ degradation. These findings indicate that singlet oxygen (1O2) primarily drives the reaction, with other reactive species, such as electrons and superoxide radicals, potentially participating in MNZ removal as oxidants or in the formation of 1O2 as precursors.
The generated ROS were additionally verified through ESR trapping. Just as in Fig. 4(b) and (c), DMPO-·OH adducts and DMPO-·O2− adducts were detected when DMPO was employed as a spin trapping reagent, suggesting the generation of ·OH and ·O2− in the BaTiO3 piezo-activated PDS system. The absence of DMPO-·SO4− adducts could be attributed to the swift transformation of ·SO4− into ·OH. 1O2, a potent electrophile, has the ability to oxidize TEMP to form the spin-adduct TEMP. A characteristic 1:1:1 triplet signal of the TEMP-1O2 can be observed in Fig. 4(d), confirming the production of 1O2 in the BaTiO3 piezo-activated PDS system. This species plays a crucial role in the degradation of MNZ [37].
3.3.3. DFT calculation and degradation pathways
The adsorption and charge transfer process of PDS on BaTiO3 was further investigated using DFT calculations. The optimized structures and deformation charge densities of the different systems are illustrated in Fig. 5. (Isosurface level is set as 6.0 × 10−3 e/Bohr3. Charge density accumulation and depletion depicted in yellow and blue.)
The adsorption sites of PDS on BaTiO3 were investigated, and the optimized adsorption structures are depicted in Fig. 5(a) and (b). The BaTiO3 substrate and the PDS substrate are shown in Fig. S7. The electron transfer process in the catalytic system is further examined using the deformation charge density, with the isosurface level set at 6.0 × 10−3 e/Bohr3 for comparison. In Fig. 5(a) and (b), the leaf area in the system indicates electron transfer occurring from BaTiO3 to PDS. Additional quantitative analysis of the charge transfer is conducted using Bader charge analysis (Table S2–S4) [38]. Clearly, electrons can be transferred from Ba in BaTiO3 to PDS. The simulation findings indicate that the inclusion of BaTiO3 promotes the charge transfer mechanism, enhancing the efficiency of PDS activation.
The optimized molecular structure of MNZ is shown in Fig. 6(a). The Fukui index suggests potential active sites (Table S5). The atom serial numbers in Table S5 corresponded to those labeled in Fig. 6(a). The corresponding visualized Fukui function orbital weights were plotted, where a higher electron density on an atom indicates a greater likelihood of reaction occurrence. The larger f − is the more susceptible to electrophilic attack, the larger f + is the more susceptible to nucleophilic attack, and the larger f0 is the more susceptible to radical attack [39]. Fig. 6(b) shows that f − is mainly around the O atoms. Fig. 6(c) shows that f + is primarily concentrated at the −NO2 end. Fig. 6(d) shows that the f 0 is primarily concentrated at the N and O atoms. Fig. 6(d) indicates that the value of f 0 for each atom of MNZ is similar to its counterparts f + and f −, suggesting a tendency to be susceptible to radical and non-radical attacks. Based on the simulation results, the most vulnerable active sites in MNZ molecular structure are identified as 9(C), 11(C), 2(O), and 3(O) in the nitro group. Furthermore, the bonds N=O and N–C are prone to breakage. This aligns with the experimental finding that •O2−, •OH, and 1O2 play a role in the degradation of MNZ.
The transformation products (TP) of MNZ degradation were analyzed by LCMS. The corresponding LCMS spectra of MNZ are shown in Fig. S3, and the main TP is listed in Table S6. MNZ degradation is proposed to be three pathways in Fig. 7. MNZ probably undergoes an oxidation reaction to form the intermediate TP190 (Pathway 1). In Pathway 2, TP114 is formed because of the attack of ROS on the lateral nitro group of MNZ. TP114 is then further oxidized to form TP163. In Pathway 3, TP23 is derived from the breakage of the N-O bond in the nitro group and pumping hydrogen reaction. TP114 is then further oxidized to form TP163 because of the attack of ROS on the C-N bond. The subsequent oxidation of heterocyclic intermediates yielded aliphatic intermediates such as TP74. Some intermediate products can be further oxidized and finally mineralized to produce CO2, H2O, and NO3−.
3.3.4. Proposed mechanism for piezocatalytic PDS activation
Drawing from the aforementioned discussion and prior studies [40, 41], a conceivable PDS activation mechanism is suggested and elucidated through the following reactions and Scheme 1. As shown in Eq. (1), when subjected to the mechanical force exerted by the ultrasonic cleaning machine, BaTiO3 undergoes deformation, leading to the generation of piezoelectric charges (comprising holes and electrons) due to the movement of localized electrons [42]. The holes created by piezoelectric BaTiO3 readily react with water to generate ·OH, the reaction could be expressed as Eq. (2). Oxygen also plays a specific role, as shown in Fig. S8 and Eq. (3), by attracting holes to generate 1O2 [43]. This further enhances the oxidation capability of the system. At the same time, PDS can be activated by electrons and ·O2− to produce ·SO4−, as shown in Eq. (4), (5). In contrast to ·OH, ·SO4− exhibits a longer half-life and can readily transform into ·OH in the presence of water or OH−, the reaction could be expressed as Eq. (6), (7). This transformation process allows ·SO4− to participate more fully in subsequent oxidation reactions. Crucially, electrons can interact with oxygen to generate ·O2−, and the combination of ·OH and ·O2− can also produce 1O2 [44]. The diversity of this electron transfer process provides flexibility and efficiency to the reaction system, as shown in Eqs. (8), (9), making it possible to generate reactive oxygen species through different pathways.
In summary, as the primary species in the BaTiO3 piezocatalytic activated PDS system, singlet oxygen can be produced through multiple pathways involving electron transfer processes. These reactions not only elucidate the generation mechanisms of various radicals in the system but also indicate the potential roles of other species in the direct oxidation of MNZ. Additional species may participate in the direct oxidation of MNZ, serving as precursors or intermediates crucial for alternative routes to 1O2 formation. In the BaTiO3 piezocatalytic activated PDS system, 1O2 plays the most significant role, followed by ·O2−, and then ·SO4− and ·OH.
4. Conclusions
The piezoelectric properties of BaTiO3 effectively activate PDS for improved MNZ degradation. Approximately 92% of MNZ was eliminated within 30 minutes, demonstrating a notable enhancement compared to piezocatalysis, PDS/ultrasound, or BaTiO3/PDS/stirring alone. The degradation rate constant reached 0.06426 min−1, representing a nearly sixfold increase compared to the combined effects of the individual components, showcasing a clear synergistic impact. Through scavenger tests and ESR determinations, it was determined that 1O2 is the primary reactive species responsible for the degradation of MNZ. The mechanisms underlying this process were elucidated through the examination of reactive species and DFT calculations. Local charges accumulated on the surface of BaTiO3 disrupt the peroxide bond (O-O) in PDS, continuously generating·SO4−, which further convert into ·OH, ·O2−, and 1O2 to oxidize and remove MNZ in water. The practical applicability potential was showcased by examining the stability of the piezocatalyst. The objective of this study is to investigate the activation of PDS through the electron transfer process of piezoelectricity, offering a novel approach to enhance PDS activation and broaden the utilization of piezoelectricity in wastewater treatment.
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