Preparation of Ag₃PO₄/CoWO₄ S-scheme heterojunction and study on sonocatalytic degradation of tetracycline

Graphical abstract

Highlights

• •Ag₃PO₄/CoWO₄ nanocomposite was prepared by a facile precipitation method.
• •Nanocomposite had been characterized by various techniques.
• •Sonocatalytic activity of Ag₃PO₄/CoWO₄ were studied through TC degradation.
• •The sonocatalytic degradation mechanism of TC by Ag₃PO₄/CoWO₄ was proposed.
• •Ag₃PO₄/CoWO₄ composite showed good stability and reusability in four reuses.

Abstract

In this study, 0.6Ag₃PO₄/CoWO₄ composites were synthesized by hydrothermal method. The prepared materials were systematically characterized by techniques of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption, and UV–vis diffuse reflectance spectrum (DRS). Furthermore, the sonocatalytic degradation performance of 0.6Ag₃PO₄/CoWO₄ composites towards tetracycline (TC) was investigated under ultrasonic radiation. The results showed that, combined with potassium persulfate (K₂S₂O₈), the 0.6Ag₃PO₄/CoWO₄ composites achieved a high sonocatalytic degradation efficiency of 97.89 % within 10 min, which was much better than bare Ag₃PO₄ or CoWO₄. By measuring the electrochemical properties, it was proposed that the degradation mechanism of 0.6Ag₃PO₄/CoWO₄ is the formation of S-scheme heterojunction, which increases the separation efficiency of electron-hole pairs (e^--h⁺) and generates more electrons and holes, thereby enhancing the degradation activity. The scavenger experiments confirmed that hole (h⁺) was the primary active substance in degrading TC, and free radicals (•OH) and superoxide anion radical (•O₂^–) were auxiliary active substances. The results indicated that 0.6Ag₃PO₄/CoWO₄ nanocomposites could be used as an efficient and reliable sonocatalyst for wastewater treatment.

1. Introduction

As one of the significant discoveries of the last century, antibiotics have considerably changed the treatment of a series of infectious diseases. It has been widely used in bacterial infections [1] of humans and animals as well as in agricultural and aquacultural areas [2]. However, only a tiny part of antibiotics can be metabolized or absorbed in humans or animals during the process of application. Most antibiotics (about 40–90 %) are discharged into the water environment or soil in the form of original drugs or primary metabolites [3]. With the deposition of antibiotics in different areas of the environment (including surface water, groundwater, drinking water, municipal sewage, soil, plants and sludge) [4], which leads to the spread of antibiotic-resistant bacteria (ARBs) and antibiotic-resistant genes (ARGs), with severe impacts on agriculture, aquaculture, humans and livestock [5]. TC is the second most produced and used antibiotic due to its broad-spectrum antibacterial activity and low production cost [6], [7]. It is not easily to be destroyed [8], [9] and enriched over time in various aquatic environments for its benzene skeleton structure and high hydrophilicity. Since TC is biologically toxic and carcinogenic [10], [11], it is potentially harmful to human health [12], aquatic ecosystems, and microbial populations [13] if it is accumulated in large quantities in the environment for a long time.

Traditional sewage treatment plants and biological treatment technologies are not sufficient to remove antibiotics from wastewater [14], so an advanced technology with simple operation, high efficiency, good effect, low price, and eco-friendliness is sought to alleviate the problem of environmental pollution. Advanced oxidation processes (AOPs) degrade pollutants by generating free radicals with strong oxidizing ability [15], which have the advantages of simple operation [16], fast degradation rate, good degradation effect [17], low toxicity of degradation products [18], and non-selectivity in the oxidation of contaminants [19]. As a kind of AOPs, sonocatalytic is a chemical effect of ultrasonic waves caused by acoustic cavitation [15], which refers to the propagation of ultrasonic waves in a liquid that could change local pressures over time and space and lead to the formation of bubbles. The radii of the bubbles expand, contract and/or collapse as these pressures change [20]. The collapse of transient bubble leads to the appearance of sonoluminescence (SL) and “hot spots”, which can generate high temperature and high pressure, exceeding 5000 K and 1000 atm, respectively [21], [22]. Under such extreme conditions, water thermally decomposes to form free radicals, such as •OH, •H, •O₂^–, which are highly reactive and non-selective, attack organic molecules to produce CO₂, H₂O, and inorganic ions, or less toxic intermediates [23], [24]. Since ultrasonic degradation combines sonic decomposition and photocatalysis, it is easier to increase the degradation of organic matter than AOPs alone [25], and has the advantages of strong penetrating ability, simple operation, and no secondary pollution [26], [27]. However, the degradation of pollutants by a simple sonocatalytic process usually requires long reaction times, limited degradation efficiency, and high energy consumption [25], [28]. So, some scientists have combined ultrasound with other AOPs technologies to further improve the performance of catalysts for the removal of pollutants. Aydin Hassani et al. found that the combination of ultrasound and electrochemical processes have synergistic effects that lead to enhanced mineralization of organic pollutants [29], such as the combination of Electro-Peroxone (EP) and ultrasound effectively enhanced acid orange 7 degradation [30]. At the same time, studies have shown that solid catalysts in ultrasonic systems could provide more nucleation sites for bubble cavitation and more charge carriers for the formation of free radicals, thereby enhancing the ultrasonic degradation effect [31], [32]. For example, some nanoparticles act as sonocatalysts, which can reduce the energy and time required for the degradation of organic pollutants [33]. According to recent studies, many semiconductor materials, such as TiO₂ [34], CdS [35], ZnO [31], N-TiO₂/Ti₃C₂ [36], CoFe₂O₄-rGO [37], Fe₃O₄/SnO₂/NGP [38] etc., have been used as sonocatalysts.

The nano-semiconductor material cobalt tungstate (CoWO₄) has excellent catalytic and electrochemical properties [39], [40]. At the same time, it has the advantages of a simple preparation method [41], high chemical stability [42], and environmental friendliness [43]. It is used in the fields of photovoltaic electrochemical cell luminescent materials [44], supercapacitors [41], conventional oxidation catalysts, and environmental purification photocatalysts [45], [46]. Although CoWO₄ has a good application in catalysis, it has a low utilization rate of sunlight, slow degradation efficiency, and low separation efficiency of e^--h⁺, which may limit its catalytic degradation ability. These disadvantages can be ameliorated by modifying it [47]. For example, the CoWO₄/g-C₃N₄ heterostructure prepared by Prabavathi et al. has better photocatalytic activity [48]; Cui et al. [49] constructed a Z-scheme-based CoWO₄/CdS, which greatly enhanced the activity for hydrogen production and dye degradation. Recently, silver-based photocatalysts, such as Ag₃PO₄, Ag₂WO₄ [50], Ag₂CO₃ [51], AgX (X = Cl, Br, I) and Ag₂MoO₄ [52] have received extensive attention in the field of photocatalytic degradation of organic pollutants. Among them, Ag₃PO₄ has a strong oxidizing ability under visible light excitation and nearly 90 % quantum utilization efficiency [53], [54], which is considered as a promising visible-light-driven photocatalyst. However, the stability of the Ag₃PO₄ photocatalytic system has always been a significant problem. First, the slight solubility of Ag₃PO₄ in aqueous solution will reduce its stability under working conditions [55]; second, during the degradation of Ag₃PO₄, Ag⁺ in the lattice is reduced to metallic Ag by photogenerated electrons [56], which leads to the structural destruction of Ag₃PO₄, which affects the absorption of visible light by Ag₃PO₄ and reduces its photocatalytic activity [55]. This problem can be addressed by techniques such as structure and morphology control, metal doping, composites carbon-based materials [57], and coupling with other semiconductors [58]. Therefore, this study, for the first time, considered the application of Ag₃PO₄ to the sonocatalytic degradation of pharmaceutical wastewater. Compositing it with CoWO₄ to form a heterojunction and accelerate electron migration, thereby reducing the recombination rate of photogenerated e^--h⁺ [59] and the probability of electron reduction of Ag⁺ [54], making the composites exhibit more excellent sonocatalytic activity and stability.

In this paper, Ag₃PO₄ was used for the first time in the application of sonocatalysis, and the Ag₃PO₄/CoWO₄ composites sonocatalyst was produced by combining with CoWO₄ by hydrothermal method. The prepared composites sonocatalyst was characterized, and the physical and chemical properties of the modified catalysts were analyzed. The sonocatalytic performance of Ag₃PO₄/CoWO₄ was studied by degrading TC, and the influence of different factors on the degradation performance during the degradation process was investigated. In addition, added K₂S₂O₈ to improve the catalytic activity further. At the same time, the reuse experiment was carried out to investigate the stability and reusability of 0.6Ag₃PO₄/CoWO₄. Furthermore, the sonocatalytic mechanism of 0.6Ag₃PO₄/CoWO₄ was discussed by radical trapping experiments and the Mott-Schottky equation. At present, there’s no report about Ag₃PO₄/CoWO₄ composites, and this is the first time that Ag₃PO₄/CoWO₄ composites have been synthesized and used in the field of sonocatalytic degradation of organic pollutants.

2. Experiment

2.1. Materials

Cobalt nitrate hexahydrate (Co (NO₃)₂·6H₂O) was analytical reagent (AR) and purchased from Tianjin Damao Chemical Reagent Factory. Sodium tungstate dihydrate (Na₂WO₄·2H₂O, AR) was purchased from Tianjin No.4 Chemical Reagent Factory. Sodium phosphate (Na₃PO₄·12H₂O, AR) was purchased from Tianjin Yongda Chemical Reagent Co., ltd. Silver nitrate (AgNO₃, 98 %) was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., ltd. Mannitol (d-Man, AR) was purchased from Tianjin Bodi Chemical Co., ltd. K₂S₂O₈ (AR) was purchased from Tianjin Hengxing Chemical Reagent Manufacturing Co., ltd. Ammonium oxalate monohydrate (AO, AR) and TC (96 %) was purchased from Maya Reagent Co., ltd. Deionized water was used in the experiments.

2.2. Preparation of Ag₃PO₄ and Ag₃PO₄/CoWO₄

CoWO₄ was prepared by a simple hydrothermal method [60]. The preparation method of Ag₃PO₄/CoWO₄ composites material was as follows: 0.6156 g of CoWO₄ was ultrasonically dissolved in 20 ml of deionized water, and 0.6115 g of AgNO₃ was weighed and dissolved in 20 ml of deionized water. Then 0.45614 g of 20 ml Na₃PO₄·12H₂O deionized aqueous solution was added dropwise to the mixed solution, and the magnetic stirring was continued for 1 h. The obtained precipitate was vacuum filtered, washed several times with deionized water, dried at 60 °C for 15 h and then ground. Ag₃PO₄/CoWO₄ composites material was obtained finally. Ag₃PO₄/CoWO₄ composites with different molar ratios (0.3, 0.6, 0.9, 1.2) were obtained by adjusting the addition amounts of AgNO₃ and Na₃PO₄·12H₂O. Pure Ag₃PO₄ was prepared by using a similar process without the addition of CoWO₄.

2.3. Characterization of Ag₃PO₄/CoWO₄

The morphology of the samples was observed by TESCAN MIRA4 SEM. The elemental composition and content of the samples were determined by Thermo Scientific K-alpha XPS at 12 kV monochromatic Al Kα radiation. The crystal structures of the samples was determined by using a MiniFlex 600 XRD under Cu Kα radiation with a 2θ scan rate of 10°/min and a scan range from 10° to 80°. The UV absorption spectrum and UV DRS were measured by using a Shimadzu UV-3600 UV–vis-NIR spectrophotometer with BaSO₄ as a reference and a wavelength range of 200–800 nm. The Brunauer-Emmet-Teller (BET) and pore distribution of the samples were determined by using a Mack TriStar II 3020 N₂ adsorption/desorption analyzer at 60 °C.

2.4. Sonocatalytic degradation experiment of TC

In the ultrasonic degradation experiment, the initial conditions were set follows: 20 mg Ag₃PO₄/CoWO₄ powder was added to 20 ml of TC solution, the TC concentration was 45 mg/L, the ultrasonic power was 500 W, and the ultrasonic time was 2 h. Interrelated factors were adjusted in a single factor test. Added Ag₃PO₄/CoWO₄ powder to the TC solution before sonication, then stirred magnetically for 30 min in the darkness to ensure that the adsorption/desorption equilibrium was reached. Afterwards, according to the above conditions, samples were irradiated by using an ultrasonic bath (KQ5200, 40 kHz, 500 W, Kunshan Ultrasonic Instrument Co., ltd., China) at 313 K, during which light was avoided. After ultrasonication, the solution was filtered with a 0.22 μm microporous membrane, and the absorbance at 354.5 nm of the obtained solution was measured with a UV–vis spectrophotometer. The degradation rate was calculated according to the Eq. (1).

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}(\%)=[(A_0-A_t)/A_0]\times 100\%$$ (1)

where $A_0$ is the initial absorbance of TC, and $A_t$ is the absorbance of TC when the ultrasonic time is t.

To examine the cycling stability of 0.6Ag₃PO₄/CoWO₄, the samples used in the sonocatalysis reaction were recovered and used repeatedly for four cycles. To detect the active species in the sonocatalysis process, d-Man [61], AO [62], and N₂ [63] were used as •OH, h⁺, •O₂^– scavengers for active substance studies.

3. Results and discussion

3.1. Characterization of nanocomposites

The crystal structure and phase purity of the as-prepared composites were determined by using XRD patterns. Fig. 1 showed the standard patterns of CoWO₄, Ag₃PO₄ and the XRD patterns of Ag₃PO₄/CoWO₄ with different molar ratios, respectively. According to Fig. 1, the peak of 2θ values was 20.8°, 29.7°, 33.3°, 36.6°, 42.5°, 47.8°, 52.7°, 55.1°, 57.3°, 61.7°, 65.9°, 70.0°, 71.9°, 73.9° and 77.7°, corresponded to the (1 1 0), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2), (3 2 0), (3 2 1), (4 0 0), (3 3 0), (4 2 0), (4 2 1), (3 3 2) and (4 2 2) plane of Ag₃PO₄(JCPDS No. 06–0505) [56]. The diffraction peaks of 2θ characteristic peaks at 23.8°, 24.6°, 30.6°, 36.3°, 38.5°, 41.4°, 52.0°, 54.0°, 61.8°, 65.1°, 68.6° were similar to (0 1 1), (1 1 0), ( −1 1 1), (0 0 2), (2 0 0), ( −1 2 1), (1 3 0), ( −2 2 1), ( −1 1 3), ( −1 3 2) and (0 4 1) plane of CoWO₄ (JCPDS-72–0479) [64]. For the spectrum of Ag₃PO₄/CoWO₄ composites in Fig. 1, all the diffraction peaks could be marked as characteristic peaks of Ag₃PO₄ or CoWO₄. The intensity of the (2 0 0), (2 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 0), (3 2 1), (4 0 0), (4 2 1) peak increased significantly with the increasing of Ag₃PO₄ composites ratio, indicating that the successful assembly of these two materials did not cause obvious change or destruction of the crystal structure. And it could be seen that the shape of the diffraction peaks was very sharp and the intensity is high, indicating that the synthesized Ag₃PO₄/CoWO₄ had a high crystallinity.

Fig. 1.

XRD patterns of standard spectra of CoWO₄, Ag₃PO₄ and Ag₃PO₄/CoWO₄ with different molar ratios.

The surface morphologies of the prepared bare CoWO₄, Ag₃PO₄, and 0.6Ag₃PO₄/CoWO₄ were investigated by SEM. As shown in Fig. 2a that CoWO₄ is a spherical nanoparticle with large particle size, about 100–200 nm, and the agglomeration phenomenon is obvious. Fig. 2b is the SEM image of Ag₃PO₄, and its nanoparticles are also spherical, with a particle size slightly smaller than that of CoWO₄, about 50–80 nm, with slight agglomeration. Fig. 2c is the SEM of the composite material. It could be seen that Ag₃PO₄ particles are uniformly wrapped on the surface of CoWO₄, forming a spherical shape. Compared with the pure material, the dispersion degree is slightly improved and the agglomeration phenomenon is reduced. The elemental purity of 0.6Ag₃PO₄/CoWO₄ was determined by the EDX system. As the EDX peaks shown in Fig. 2d 0.6Ag₃PO₄/CoWO₄ is composed of Ag, P, Co, W, and O elements. In addition, no impurity peaks appear, indicated that the obtained 0.6Ag₃PO₄/CoWO₄ is of high purity, and the relative element/weight ratio was shown in the table (inset Fig. 2d). Fig. 2e showed the specified SEM images of 0.6Ag₃PO₄/CoWO₄ nanocomposites and corresponding elemental mapping. It was further explained that the Ag, P, Co, W, and O elements in the composite are evenly distributed, indicated that Ag₃PO₄ and CoWO₄ are thoroughly combined.

Fig. 2.

The SEM images of CoWO₄ (a), Ag₃PO₄ (b) and 0.6Ag₃PO₄/CoWO₄ (c) and EDX spectra of 0.6Ag₃PO₄/CoWO₄ (d) and specified SEM images of 0.6Ag₃PO₄/CoWO₄ nanocomposites and elemental mapping (e).

In order to shed more light on inner structure of the sample, TEM analysis was taken for 0.6Ag₃PO₄/CoWO₄. As shown in Fig. 3a that Ag₃PO₄ particles were uniformly fixed on the surface of CoWO₄ particles, and the particle size of CoWO₄ was reduced to 20–50 nm, while that of Ag₃PO₄ was reduced to 2–5 nm. The HRTEM in Fig. 3b showed clear lattice fringe of the composites, with band spacing of 0.269 nm and 0.36 nm corresponded to plane (2 1 0) of Ag₃PO₄ [56] and plane (0 1 1) of CoWO₄ [43], respectively.

Fig. 3.

The TEM images (a) and HRTEM images (b) of 0.6Ag₃PO₄/CoWO_4.

The chemical state and composition of 0.6Ag₃PO₄/CoWO₄ were further affirmed by high-resolution XPS. Fig. 3a showed the XPS scanning spectra of CoWO₄, Ag₃PO₄, and 0.6Ag₃PO₄/CoWO₄ composites. It could be seen that the composites sonocatalysis contained Ag, P, Co, W, and O elements. The high-resolution XPS spectra of Ag 3d, P 2p, Co 2p, and W 4f were shown in Fig. 4b-e. Two prominent peaks of Ag 3d are observed at 367.48 eV and 373.48 eV in Fig. 4b, which were attributed to Ag 3d_5/2 and Ag 3d_3/2 photon electrons, respectively, indicated that Ag exists in the form of Ag⁺. It could be seen from Fig. 4c that the P⁵⁺ ion in PO₄³⁻ had a single peak [56] at 132.38 eV, and combined with Fig. 4a, we could infer the formation of Ag₃PO₄. Two prominent peaks of Co 2p were observed at 781.18 eV and 797.88 eV in Fig. 4d, which were attributed to Co 2p_3/2 and Co 2p_1/2 photons, respectively, indicated that Co exists in the form of Co²⁺. At the binding energies of 787.78 eV and 803.65 eV, two satellite peaks on the catalyst surface, corresponded to the adsorption of hydroxide and cobalt salt [65]. At 35.78 eV and 37.98 eV in Fig. 4e, two prominent peaks of W 4f were observed, ascribed to W 4f_7/2 and W 4f_5/2 photons, respectively, indicated that W in WO₄^2- exists as + 6 ion [66]; and combined with Fig. 4a, we could infer the formation of CoWO₄. Compared with the pure material, the binding energy of the composite had shifted, in which the binding energy of Ag and P elements were reduced, while the binding energy of Co and W elements were enhanced, indicated that there was a strong interaction between CoWO₄ and Ag₃PO₄, which might be related to the formation of S-scheme heterojunctions. In conclusion, the XPS results indicated that 0.6Ag₃PO₄/CoWO₄ was successfully synthesized.

Fig. 4.

XPS scanning spectra of CoWO₄, Ag₃PO₄ and 0.6Ag₃PO₄/CoWO₄ composites and high-resolution XPS measurement spectrum of 0.6Ag₃PO₄/CoWO₄ nanocomposites.

It is well known that specific surface area and pore size are crucial for catalytic activity [67]. Therefore, N₂ adsorption/desorption isotherms were used to study the specific surface area and pore size distribution of the samples. The results were shown in Table 1. The corresponding N₂ adsorption/desorption isotherms and the pore distribution were shown in Fig. 5. It was indicated that the prepared sonocatalyst has scheme IV isotherm. From the BET surface area analysis, it could be seen that the BET of Ag₃PO₄ and CoWO₄ were comparatively smaller, 35.78 and 18.20 m²/g, respectively, while the BET increases to 40.48 m²/g when the 0.6Ag₃PO₄/CoWO₄ composites is formed. Used the Barrett-Joyner-Harenda (BJH) diagram to give the pore size of the composites, the average pore size of 0.6Ag₃PO₄/CoWO₄ was 0.10 cm³/g, which is larger than that of Ag₃PO₄ (0.06 cm³/g) and CoWO₄ (0.08 cm³/g). It could be found that the addition of Ag₃PO₄ could increase the specific surface area and pore size. Due to the large specific surface area and abundant pore structure, the adsorption performance of the catalyst can be enhanced and more active catalytic sites can be provided [60]. Therefore, the large specific surface area and pore size of 0.6Ag₃PO₄/CoWO₄ are beneficial for improving its sonocatalytic activity [35].

Table 1.

BET and BJH parameters of CoWO₄, Ag₃PO₄ and 0.6Ag₃PO₄/CoWO₄ composites.

Sample	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
CoWO4	35.78	0.08	9.22
Ag3PO4	18.20	0.06	12.27
0.6CoWO4/ Ag3PO4	40.48	0.10	9.43

Fig. 5.

N₂ adsorption/desorption curves (a) and pore distribution diagrams (b) of CoWO₄, Ag₃PO₄ and 0.6Ag₃PO₄/CoWO₄ composites.

The light absorption capacity of the catalyst has a significant influence on its catalytic activity [64]. Therefore, to study the absorption of the sample in the UV–visible region, its UV–visible absorption spectrum was scanned, as shown in Fig. 6a. It could be seen that, Ag₃PO₄, CoWO₄, and 0.6Ag₃PO₄/CoWO₄ all have excellent UV–vis responses. Among them, the absorption limit of Ag₃PO₄ was 580 nm, and the peak shape was wider, indicated that Ag₃PO₄ has strong visible light absorption. There are two prominent absorption peaks in the absorption curve of CoWO₄, which were located in the ultraviolet area of 225–375 nm and the visible area of 550–600 nm, and an inconspicuous peak was formed at about 525 nm. The peak shape in the ultraviolet area was broader, and the peak shape distribution in the visible area was narrower. It shows that CoWO₄ has a high absorption and utilization efficiency for ultraviolet light, but relatively low utilization for visible light. Compared with pure CoWO₄, the absorbance of 0.6Ag₃PO₄/CoWO₄ composites decreased after adding Ag₃PO₄, but the absorbance appeared red-shifted in the visible region, indicated that 0.6Ag₃PO₄/CoWO₄ has better photoresponse [56]. The band gap energy (E_g) of each sample was calculated with the following Eq. (2):

$$\mathrm{\alpha }\mathrm{h}\mathrm{\nu }=A{\left(h\mathrm{\nu }-{\mathrm{E}}_g\right)}^{2/n}$$ (2)

where α represents the absorption coefficient, hν represents the photon energy, E_g represents the band gap, and A represents the proportionality constant [68]. The value of n depends on the transition scheme of the semiconductor (n = 1 when the transition is allowed directly, n = 4 when the transition is allowed indirectly [69]). Using the light absorption data and Tauc equation, the band gap values of pristine Ag₃PO₄, CoWO₄ and 0.6Ag₃PO₄/CoWO₄ were 2.55 eV, 2.64 eV and 2.60 eV respectively (Fig. 6b).

Fig. 6.

UV–vis-DRS spectra (a) and curves of (αhν)² and hν (b) of CoWO₄, Ag₃PO₄ and 0.6Ag₃PO₄/CoWO₄ composites.

3.2. Influence of operating parameters on sonocatalytic performance

3.2.1. Influence of different Ag₃PO₄ compound ratios

By changing the addition amount of AgNO₃ and Na₃PO₄·12H₂O in the preparation process, the composites with compound ratios of 0.3,0.6,0.9, and 1.2 were synthesized, and TC was degraded under the same ultrasonic conditions. As shown in Fig. 7, with the increasing compound amount of Ag₃PO₄, the degradation of TC by the composites showed a trend of increasing firstly (from 0.3 to 0.6) and then decreasing (from 0.6 to 1.2). It showed that the molar ratio of Ag₃PO₄ in Ag₃PO₄/CoWO₄ has a great contribution to the catalytic activity of the composites, because the photogenerated holes in the Ag₃PO₄ conduction band represent a robust oxidizing ability, when there was an excess of Ag₃PO₄ in the composites, not only the light absorption intensity of the composites being reduced, but also the photo corrosion of Ag₃PO₄ being enhanced, thereby reducing the sonocatalysis activity of the composites [54]. When the compound molar ratio of Ag₃PO₄ to CoWO₄ was 0.6, the sonocatalytic degradation rate was the highest, so the subsequent degradation experiments used a composites material with a molar ratio of 0.6.

Fig. 7.

Effect of Ag₃PO₄ compound ratio on degradation rate. (Catalyst addition = 1 g/L, Initial concentration of TC = 45 mg/L, Ultrasonic power = 500 W, Ultrasonic time = 2 h).

3.2.2. Influence of the amount of catalyst added

By changing the addition amount of 0.6Ag₃PO₄/CoWO₄ catalyst, the effect of the additional amount on the degradation rate was studied. The results were presented in Fig. 8(a). It could be seen that when the addition amount was changed from 0.5 to 2 g/L, the degradation rate increased (from 0.5 to 1 g/L) firstly and then remained almost unchanged. The degradation rate was the highest (71 %) when the dosage was 1 g/L. The reason for the improved degradation efficiency is that an appropriate amount of catalyst can provide sufficient surface area and active sites for sonocatalysis [70], while when the dose continues to increase, the solid particles will have a masking effect on the penetration ability of the ultrasonic wave, thereby reducing the surface reaction sites of •OH radical generation. Moreover, the ultrasound might be disseminated by the additional catalyst in the solution, which could inhibit heat and energy conduction near the surface of the catalyst [71].

Fig. 8.

Effect of catalyst addition amount (a) (Initial concentration of TC = 45 mg/L, Ultrasonic power = 500 W, Ultrasonic time = 2 h), initial tetracycline concentration (b) (Catalyst addition = 1 g/L, Ultrasonic power = 500 W, Ultrasonic time = 2 h), ultrasonic power (c) (Catalyst addition = 1 g/L, Initial concentration of TC = 15 mg/L, Ultrasonic time = 2 h) on degradation rate.

3.2.3. Effect of initial TC concentration

The effect of different TC concentrations (15, 25, 35, 45, and 55 mg/L) on the degradation effect was investigated, and the results are shown in Fig. 8(b). It could be seen that the degradation rate decreased continuously when the concentration increased (from 15 mg/L to 25 mg/), and the degradation efficiency was the highest when the concentration was 25 mg/L. The reason is that at low concentration, the catalyst can be fully contacted with the solution, and a large number of reactive oxygen species in the solution can react with TC molecules, resulting in a higher degradation rate [72]. However, when the initial solution concentration of TC increases and the catalyst was quantitative, the generated active sites and the surface area of bubbles used for the reaction cannot meet the progress of the catalytic reaction, resulting in a decrease in the degradation rate of TC [73]. By calculating the degradation quality, it was found that although the degradation rate decreased when the initial concentration of TC increased, the degradation quality showed an increasing state, indicated that the 0.6Ag₃PO₄/CoWO₄ composites still had an excellent sonocatalytic degradation effect for high concentrations of TC.

3.2.4. Influence of ultrasonic power

The effect of ultrasonic power on the degradation rate was shown in Fig. 8(c). When the ultrasonic power was small, the degradation rate was lower. This is because with the increase of ultrasonic power, not only the energy of cavitation is enhanced, but also the threshold of cavitation is lowered. Therefore, more cavitation microbubbles will be generated during the ultrasonic process, and accordingly more active free radicals will be generated, which significantly improves the catalytic activity [74].

3.2.5. Effect of different ultrasound time

Fig. 9a showed the degradation of TC at different time without catalyst and with Ag₃PO₄, CoWO₄, and 0.6Ag₃PO₄/CoWO₄ catalysts, respectively. It could be seen that when the degradation time was 120 min, the self-degradation of TC was only 34.52 % without the addition of catalyst. Among the three catalysts, the degradation effect of 0.6Ag₃PO₄/CoWO₄ was the best, reaching 73.45 %, which was 1.6 times that of CoWO₄ (45.11 %) and 1.5 times that of Ag₃PO₄ (48.41 %), respectively.

Fig. 9.

Degradation of TC in different degradation systems (a) and plots of -ln (C_t/C₀) vs ultrasonic time (b) (Catalyst addition = 1 g/L, Initial concentration of TC = 15 mg/L, Ultrasonic power = 500 W).

The kinetic behavior of TC degradation under different catalysts was investigated by using pseudo-first-order equations, and the results were shown in Fig. 9b. The sonocatalytic rate constant could be estimated by Eq. (3).

$$-ln(C_t/C_0)=kt$$ (3)

where $C_0$ is the initial drug concentration, $C_t$ is the drug concentration at time t, t is the irradiation time, and k is the rate constant. The first-order rate constants k of Ag₃PO₄, CoWO₄ and 0.6Ag₃PO₄/CoWO₄ were 0.00404 min⁻¹, 0.00409 min⁻¹ and 0.01454 min⁻¹, respectively. It could be seen that k value of 0.6Ag₃PO₄/CoWO₄ was the largest, which was 3.55 times that of Ag₃PO₄ and CoWO₄. The synergy factor (SF) was obtained by Eq. (4) [75], [76], it was found that SF > 1 (SF = 1.79), indicated that Ag₃PO₄ compound CoWO₄ has a positive synergistic effect on the degradation of TC.

$$\mathit{SF}=\frac{k_{\left(0.6A{g}_{3}P{O}_4/CoW{O}_4\right)}}{k_{\left(A{g}_{3}P{O}_4\right)}+k_{\left(\mathit{CoW}{O}_4\right)}}$$ (4)

To sum up, when the addition amount of sonocatalyst was 1 g/L, the initial concentration of TC was 15 mg/L, the ultrasonic power was 500 W, and the ultrasonic time was 90 min, the degradation rate of TC by the 0.6Ag₃PO₄/CoWO₄ composites was 75.3 %.

3.2.6. Effect of adding K₂S₂O₈

According to literature reports, SO₄^-• can be generated by ultrasonic activation after adding persulfate during the degradation process [77]. Compared with •OH, SO₄^-• has greater redox capacity and longer half-life, which may help to enhance the degradation effect [78], [79]. Therefore, K₂S₂O₈ in different concentrations was added during the degradation process, and their influence on the degradation effect was studied. The results were presented in Fig. 10. It could be observed that adding K₂S₂O₈ can significantly improve the degradation rate and shorten the degradation time. When 10 mg of K₂S₂O₈ was added, the degradation rate of TC could reach 97.89 % in 10 min. It showed that K₂S₂O₈ was an ideal additive in sonocatalytic degradation reaction, which had a synergistic effect with the 0.6Ag₃PO₄/CoWO₄ composites in the degradation process.

Fig. 10.

Effect of adding K₂S₂O₈ on the degradation rate.

3.3. Mechanism of sonocatalysis

In order to study the main active substances in the degradation of TC by 0.6Ag₃PO₄/CoWO₄ composites, d-Man (•OH scavengers), AO (h⁺ scavengers), N₂ (•O₂^– scavengers) were added as scavengers to the TC degradation process. The results were presented in Fig. 11. It could be observed that the degradation rate of TC decreased when d-Man and N₂ were added, but did not fluctuate significantly. In contrast, the degradation rate decreased significantly when AO was added. It showed that h⁺ was the primary active substance, and •OH and •O₂^– were auxiliary active substances.

Fig. 11.

Degradation rate of TC after adding different scavengers.

The semiconductor band gap positions have an essential impact on their catalytic performance, as they determine the interfacial charge transfer behavior. Therefore, the band structures and semiconductor type of Ag₃PO₄ and CoWO₄ were analyzed by using the Mott-Schottky dot plots, as shown in Fig. 12. Flat band potentials (E_f) can be estimated from intercepts of the X-axis and are close to Fermi Levels (E_F) [80]. The E_f of Ag₃PO₄ and CoWO₄ were 1.45 V and −0.55 V versus saturated calomel electrode (SCE), respectively [81]. According to the Nernst equation, the potentials are 1.69 V and −0.31 V (vs NHE), respectively. For n-type semiconductors, the position of CB is 0.1–0.3 V above the E_f, and for p-type semiconductors, the position of VB is 0.1–0.3 V above the E_f [82]. It could be known from Fig. 6b that the E_g of Ag₃PO₄ and CoWO₄ were 2.55 eV and 2.64 eV, respectively. Since the energy difference of 1 eV in solid-state physics is the same as the difference in energy of 1 V in the field of electrochemistry. According to the empirical formula E_VB = E_CB + E_g [83], the calculated VB values of Ag₃PO₄ and CoWO₄ were 1.89 V and 2.13 V, respectively, and the CB values were −0.66 V and −0.51 V.

Fig. 12.

Mott-Schottky dot plots of Ag₃PO₄ (a) and CoWO₄ (b).

Based on the above discussion about the band structure, the left side of Fig. 13 showed the band arrangement diagram of the composite 0.6Ag₃PO₄/CoWO₄ before contact. As observed, the valence band (VB) and conduction band (CB) of Ag₃PO₄ and CoWO₄ generated h⁺ and e^-, respectively, because of the appropriate band structure. As shown on the right of Fig. 13, when Ag₃PO₄ formed heterojunctions with CoWO₄, e^- on CoWO₄ CB spontaneously transfered to VB on Ag₃PO₄. Depletion layer was generated on the CoWO₄ side and accumulation layer was formed on the Ag₃PO₄ side, forming the internal electric field from CoWO₄ to Ag₃PO₄. Since e^- was acquired by Ag₃PO₄, the binding energy of Ag and P in Ag₃PO₄ decreased. Conversely, the binding energy of Co and W in CoWO₄ increased due to the loss of e^- in CoWO₄, which was consistent with the XPS results [84].Then the e^- in the CB of CoWO₄ were recombined with the h⁺ in the VB of Ag₃PO₄ through conventional charge separation [85]. Therefore, the e^- of Ag₃PO₄ on relatively negative CB and the h⁺ of CoWO₄ on relatively positive VB are maintained, facilitating to produce more radicals •O₂^– and •OH [86]. Then the e^- on the CB (Ag₃PO₄) reacted with the adsorbed O₂ to generate •O₂^–, because the CB of Ag₃PO₄ more negative than the potential of O₂/•O₂^– (E⁰(O₂/•O₂^–) = −0.33 V). Meanwhile, a part of the h⁺ on VB (CoWO₄) could directly catalyze TC, and the other part h⁺ could oxidize OH^– to •OH (because its oxidation potential is higher than OH^–/•OH (1.99 V). According to the above mechanism, Ag₃PO₄ and CoWO₄ produced interfacial contact, which effectively prevented e^--h⁺ pairs from recombining, improves its separation efficiency, and generated more e^- and h⁺, thereby enhancing the degradation activity. Possible reactions were listed in Eq. (5)-(8). These results indicated that the heterojunction formed by Ag₃PO₄ and CoWO₄ could effectively improve the ultrasonic catalytic performance.

$$0.6A{g}_{3}P{O}_4/CoW{O}_4+Ultrasound\to e_{(CB)}^{-}+h_{(VB)}^{+}$$ (5)

$${\text{e}}^{-}+{\text{O}}_2\to ·{\text{O}}_2^{-}$$ (6)

$$h^{+}+H_{2}O/O{H}^{-}\to ·OH$$ (7)

$$h^{+},·OH,·O_2^{-}+TC\to Degradationproducts$$ (8)

Fig. 13.

Schematic diagram of the sonocatalysis mechanism.

After the addition of K₂S₂O₈ Eq. (9), S₂O₈^2- could react with the e^- reaction on Ag₃PO₄ VB to produce SO₄^-• [87]. SO₄^-• is a strong oxidant and can effectively degrade many organic pollutants Eq. (10) [79]. In addition, SO₄^-• can also be converted to •OH by reacting with H₂O in solution, which can degrade TC and further improve the catalytic activity [88] Eq. (8). In this paper, the reaction time was reduced, and the degradation effect was improved by adding K₂S₂O₈.

$$S_2{O}_8^{2-}+2e^{-}\to 2S{O}_4^{-}·$$ (9)

$$2S{O}_4^{-}·+H_{2}O/O{H}^{-}\to H^{+}+S{O}_4^{2-}+·OH$$ (10)

3.4. Stability and cycling

The stability and reproducibility of sonocatalytic materials is critical for their applications [56]. Therefore, the 0.6Ag₃PO₄/CoWO₄ composites was recycled 4 times. All nanocomposites were collected after each sonication, washed and dried at 60 °C for 2 h. The results were shown in Fig. 14(a). It could be observed that the degradation rate was only reduced by 3 % after 4 cycles. This result indicated that the 0.6Ag₃PO₄/CoWO₄ composites had excellent reproducibility. As shown in Fig. 14(b), the XPS patterns of the recycled composites and the newly prepared composites were the same. The decrease in TC degradation rate might be related to the decline of Ag ions. Therefore, after cycling experiments, we used ICP to measure the leaching of individual ions in the solution. The results showed that the leaching rate of Ag ions was slightly higher, about 2.65 %, and the leaching rates of Co and W were lower, 0.0008 % and 1.77 %, respectively. It showed that Ag ions were still consumed in the experimental process, but had little effect on the degradation rate. Both XPS and ICP results proved good stability of 0.6Ag₃PO₄/CoWO₄ composites.

Fig. 14.

Recovery efficiency of Ag₃PO₄/CoWO₄ composites (a) and XPS spectra before and after sonocatalytic degradation (b).

3.5. Comparison of catalytic activity with published catalysts

Due to the harmful effects of antibiotic wastewater pollution on the environment and human beings, many research have been devoted to developing materials that can degrade TC wastewater in recent years. Table 2 reports a comprehensive comparison of this study, together with other sonocatalysts for TC degradation activity published recently. It could be observed in the table that it usually takes a long time for each material to degrade TC. For example, Qiao et al. [9] used the ternary SrTiO₃/Ag₂S/CoWO₄ composites as a sonocatalyst to degrade TC solution in 300 min. In contrast, the 0.6Ag₃PO₄/CoWO₄ composites in this study can achieve a degradation rate of 75.3 % in only 120 min. In addition, Reza Darvishi Cheshmeh et al. [89] found that adding some free radical enhancers (hydrogen peroxide, periodate, persulfate, and percarbonate, etc.) during the reaction can increase the degradation effect. For example, Hoseini et al. [90] added H₂O₂ in TiO₂ ultrasonic degradation of TC, and the degradation rate could reach 100 % within 75 min. Compared with that, this work achieved a degradation rate of 97.89 % within 10 min by adding K₂S₂O₈, which considerably shortened the reaction time and improved the degradation effect. It indicated that combining K₂S₂O₈ with 0.6Ag₃PO₄/CoWO₄ is a promising TC degradation scheme. These results provide valuable references for the practical application of 0.6Ag₃PO₄/CoWO₄.

Table 2.

The comparative effect of the composites material in this study and other reported catalysts in the degradation of tetracycline.

Catalyst	Source and method of degradation	TC concentration (mg/L)	Catalyst dosage (g/L)	pH	Degradation Efficiency and Time	Reference
SrTiO3/Ag2S/CoWO4	Ultrasonic	10	1.0	6	86.47 % in 300 min	[9]
BiOI	Ultrasonic	10	0.5	–	95.5 % in 70 min	[91]
BiOBr/MgFe2O4	Ultrasonic	10	2.0	7	91.10 % in 30 min	[84]
ZnO/NC	Ultrasonic+0.01 M PMS	50	0.5	4	96.4 % in 15 min	[89]
TiO2	Ultrasonic+100 mg/L H2O2	75	0.25	4	100 % in 75 min	[90]
FeII-MIL-88B/GO/P25	Ultrasonic+20 mM H2O2	10	0.3	5	83.3 % in 7 min	[92]
0.6Ag3PO4/CoWO4	Ultrasonic	25	1.0	–	75.3 % in 120 min	This work
0.6Ag3PO4/CoWO4	Ultrasonic+10 mg K₂S₂O₈	25	1.0	–	97.89 % in 10 min	This work

4. Conclusion

The 0.6Ag₃PO₄/CoWO₄ composite sonocatalyst was prepared for the first time by a simple hydrothermal method and characterized by SEM, TEM, XRD, XPS, BET and UV–vis DRS. 0.6Ag₃PO₄/CoWO₄ had an average particle size of 20 ∼ 50 nm, a narrow band gap of 2.60 eV and a large specific surface area. Compared with Ag₃PO₄ and CoWO₄, the 0.6Ag₃PO₄/CoWO₄ composites had a better sonocatalytic degradation effect on TC solution, and the degradation rate could get 75.3 % within 90 min. After adding 10 mg of K₂S₂O₈, the degradation effect could reach 97.89 % in 10 min. The mechanism was that Ag₃PO₄ and CoWO₄ form an S-scheme heterojunction, which accelerated the separation efficiency of e^--h⁺, and the generated h⁺, •OH, and •O₂^– effectively mineralized TC. In addition, the 0.6Ag₃PO₄/CoWO₄ composites still had good recyclability and high stability after 4 cycles. This study provides a feasible idea for the modification of CoWO₄, and provides a direction for preparing a new high-efficiency sonocatalyst and the solution to wastewater treatment problems.

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.

Data availability

Data will be made available on request.