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Preparation of zinc tungstate nanomaterial and its sonocatalytic degradation of meloxicam as a novel sonocatalyst in aqueous solution
Ultrasonics - Sonochemistry 61 (2020) 104815
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Preparation of zinc tungstate nanomaterial and its sonocatalytic degradation of meloxicam as a novel sonocatalyst in aqueous solution Liang Xua,b, Xin Wangb, Ming-Ling Xub, Bin Liub, Xiao-Fang Wangb, Si-Huan Wangb, Ting Suna, a b
T
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Department of Chemistry, College of Science, Northeastern University, Shenyang 110819, China College of Pharmacy, Liaoning University, Shenyang 110036, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Meloxicam Sonocatalytical degradation Zinc tungstate Water treatment Ultrasonic irradiation
Zinc tungstate (ZnWO4) was previously used as a photocatalyst. In this paper, for the first time as an sonocatalyst, the performance of ZnWO4 for sonocatalytic degradation of meloxicam (MEL) under ultrasonic irradiation were studied. Firstly, ZnWO4 nanomaterials were synthesized at different acidity (pH = 5, 6, 7, 8, 9) via the hydrothermal method. Utilizing SEM, XRD and EDS techniques to characterize composition and morphology of each product, the same crystal forms, but different morphologies (nano-sheet, nano-microspheres or nano-rod) of ZnWO4 could be obtained. Secondly, the sonocatalytic activities of ZnWO4 on degradation of MEL were studied. It was found that the degradation ratio varied with the synthetic pH values, with ZnWO4 under synthetic pH = 6 exhibiting the best sonocatalytic performance (75.7%). While being synthesized at this pH value, ZnWO4 nano-microspheres had the largest BET surface area (27.068 m2/g), the smallest particle size (40–60 nm) so as to provide more active sites on its surface, which were able to produce more reactive oxygen species (ROS) and holes under ultrasonic irradiation. These ROS and holes had a positive effect on the degradation of MEL into CO2, H2O and inorganic. Thirdly, various influential factors including ultrasonic power intensity, ultrasonic time, catalyst addition dosage, initial concentration of MEL solution and reusability of catalyst were also explored. Under the condition of 10 mg/L MEL concentration, 20 mg catalyst dosage, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, the degradation ratio on MEL reached 75.7%. Finally, the presence of hydroxyl radical (%OH) and singlet molecular oxygen (1O2) in the reaction was confirmed by adding ROS scavenger. The experimental results suggested that ZnWO4 nanoparticle could be used not only as an effective photocatalyst, but also, under the condition of ultrasonic irradiation, a promising sonocatalyst for degradation of organic pollutants in aqueous media.
1. Introduction Since the beginning of the 21st century, many publications have revealed the presence of pharmaceutical drugs and dyestuff in surface and sewages waters [1–3]. As one of these micro-pollutants, meloxicam (MEL, as shown in Table S1) was found to have considerable level in wastewaters [4,5], which might exist potential toxicological risks on living organisms even at low concentrations [1]. Hence, the elimination of micro-pollutants such as MEL from environment has aroused people's concern. Advanced oxidation processes (AOPs) are a collection of methods to mineralize a variety of recalcitrant organics through the generation of reactive oxygen species (ROS) and holes (h+) [6,7]. Among the AOPs, the ultrasonic (US) process has been widely investigated [8]. Sonocatalytic degradation of organic pollutants can be described as the ultrasonic cavitation effect happened in aqueous
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medium, which leads to the sonoluminescence, hot spot accompanied with high temperature (5000 K) and high pressure (1000 atm). As is known, hot spots can induce the dissociation of water into ROS including hydroxyl radical (%OH), hydroperoxyl radicals (%OOH), singlet molecular oxygen (1O2) and superoxide radical (%O2−), which can degrade and destroy organic pollutants [9,10]. Sonoluminescence can generate light with a comparatively wide wavelength range (200–500 nm) and an average photon energy of 6 eV, which can excite the aromatic organic pollutants to become unstable [11]. However, ultrasound alone is highly energy intensive and insufficient to totally degrade pollutants in water [12,13]. A lot of sonodegradation studies have shown that involvement of sonocatalyst can give full play to the superiority of ultrasound, which can produce a synergistic effect and enhance the degradation efficiency of organic pollutants [8,14]. Thus, the screening of novel sonocatalysts has become one of the research
Corresponding author. E-mail address: [email protected] (T. Sun).
https://doi.org/10.1016/j.ultsonch.2019.104815 Received 24 April 2019; Received in revised form 10 September 2019; Accepted 29 September 2019 Available online 11 October 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
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100 mL and maintained at 453 K for 24 h. At last, the composition was cooled down to room temperature naturally, filtered and dried at 353 K. Distilled water and anhydrous ethanol were used for washing the white precipitate several times. After fully grinding, the ZnWO4 powder was obtained and stored. In the process of synthesis, all other things being equal, the mixed solution was adjusted for pH by HCl and NaOH (pH = 5, 7, 8, 9), and the ZnWO4 products under different synthesized pH were obtained.
hotspots [15]. Because of connections in the excitation mechanisms between the sonocatalyst and photocatalyst, many papers have studied the sonocatalytical properties of photocatalysts and obtained good results, such as TiO2, polyaluminum chloride, Al2O3 and Bi2WO6 [16,17]. Zinc tungstate (ZnWO4) is a monoclinic tungstate metal oxide with d10s2 - d0 electronic configuration [18]. Like Bi2WO6, ZnWO4 is also a kind of tungstate semiconductor material that has unique optical and chemical properties, e.g. high average refractive index, high X-ray absorption coefficient, high light yield, short decay time, and low afterglow to luminescence and so on [19,20]. Hence, ZnWO4 has been explored for a number of applications, such as photocatalysis [21], optical fibres [22], microwave applications [23] and scintillators [24]. In addition, its unique chemical properties and narrow band gap (3.0–4.2ev) [19] show that it may be a promising sonocatalyst. In this paper, ultrasonic assisted hydrothermal method was used to prepare ZnWO4 under different acidic conditions (pH = 5, 6, 7, 8, 9). The prepared ZnWO4 particles were characterized by scanning electron microscopy (SEM), X-ray powder diffractometer (XRD), energy dispersive X-ray spectroscopy (EDS) and nitrogen adsorption – desorption isotherm measurements. Then, the degradation ratios of ZnWO4 on MEL under different synthetic acidity conditions were compared. Thirdly, several factors on degradation ratios of MEL such as ultrasonic power intensity, ultrasonic time, synthetic pH, catalyst addition amount, initial concentration of the solution and reusability of catalyst were investigated. Finally, the mechanism of sonodegradation of MEL was proposed, the selectivity of catalyst’s degradation efficiency on different pollutants (The names, structural formula, molecular weight and maximum absorption wavelength of these organic compounds are provided in Table S1) was compared, comparison of sonocatalytic with photocatalytic and sonophotocatalytic efficiencies of ZnWO4 was displayed. The present study was intended to propose some new ideas for the application of sonocatalytic activities of ZnWO4 for treatment of micro-pollutants in ecosystem, especially in water.
The crystalline nature of ZnWO4 powder was determined by an Xray diffractometer (D8 Advance, Bruker Axs Company, Germany) with Cu Kα irradiation (λ = 1.5405). The phase purity and homogeneity pattern of ZnWO4 were refined using a Phillips Powder (PW) diffractometer 1710 in a wide range of Bragg angles (10° ≤ 2θ ≤ 80°). The average grain sizes (D) were calculated as reported [21]. The Scanning Electron Microscope (SEM, ULTRA PLUS, Zeiss microscopy, Germany) with energy dispersive X-ray (EDS) system was used to determine the micromorphology and composition of ZnWO4. BET surface areas, pore volumes and pore size distribution were determined by nitrogen adsorption and desorption isotherm at 77 K on a Micrometrics Autosorb iQ-C (American, Quanta chrome Instruments). Prior to analysis, samples were degassed at 373 K to remove any physically adsorbed gas for 6 h. The pore size distribution of the samples was determined by the BJH (Barret – Joyner – Halenda) model from the data of adsorption and desorption branch of the nitrogen isotherms. Specific surface area was determined by the BET method. The controllable serial-ultrasonics (US) apparatus process was performed in a double-jacketed stainless steel reactor (L × W × H: 30 × 24 × 15 cm) with four US transducers (KQ5200DE, Kunshan Ultrasonic Instrument Company, China). The sonicator provided the variable output power intensity of 0.111–0.278 W/cm2 and the fixed frequency of 40 kHz.
2. Experimental
2.4. Evaluation of sonocatalytic performance
2.1. Materials and reagents
2.4.1. Measurements of sonocatalytic activity of the prepared ZnWO4 composites As one of widely used drugs which have been detected in environment, MEL was selected as a model microcontaminants to evaluate the sonocatalytic efficiency of ZnWO4. The entire catalytic sonication process took place at 298 K in dark to prevent the influence of daily or ambient light. Generally, 20 mg of catalysts and 20 mL of MEL solution (10 mg/L, dissolved with 0.1 M NaOH) were mixed together in a beaker. The suspension was stirred for 30 min to ensure the adsorption/ desorption equilibrium of MEL before ultrasonic irradiation. The adsorption ratio of MEL on ZnWO4 was calculated by the absorbance of the solution at its λmax = 360 nm. The formula was: adsorption ratio (%) = [(A0 − At)/A0] × 100. A0 was the initial absorbance of MEL solution; At was the absorbance of MEL solution after stirring for 30 min. High speed freezing centrifuge (Heraeus X1R, Thermo Scientific) and 0.22 μm glass microfiber filters were used to separate ZnWO4 powder from MEL solution. UV–vis absorption measurements were recorded on a UV-2550 spectrophotometer (Shimadzu Company, Japan) with 1.0 cm quartz cells. The effects of catalytic activity of ZnWO4 on MEL were inspected by the change in absorbance of MEL with UV–vis spectrometer. The degradation ratio of MEL was calculated by the absorbance of the solution at its λmax = 360 nm. The formula was: degradation ratio (%) = [(A0 − At)/A0] × 100. A0 was the initial absorbance of MEL solution; At was the absorbance of MEL solution under different experimental conditions [8].
2.3. Characterization of ZnWO4 powder
Sodium tungstate dehydrate (Na2WO4·2H2O) and Zinc nitrate hexahydrate ((Zn(NO3)2·6H2O) were both purchased from Tianjin Damao Chemical Reagent Factory (China). Ethanol absolute (CH3CH2OH), concentrated Hydrochloric acid (HCl), Amaranth (AMN), mannitol (DMan), l-histidine (L-His), vitamin C (VC), Ciprofloxacin Hydrochloride (CIP), Levofloxacin (LEV), Methylene blue (MB) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co. Ltd. Samples were weighed accurately on a microbalance (Sartorius BP211D, Germany) with a resolution of 0.01 mg. MEL (98% purity, Sinopharm Chemical Regent Co, Ltd, China) (as shown in Table S1) was used to undergo the sonocatalytic degradation experiment under ultrasonic irradiation. Double distilled water was used for all solution preparation. 2.2. Preparation of ZnWO4 powder ZnWO4 powders were synthesized by hydrothermal method according to literature [25] with a slight modification. Typically, 5 mmol Zn(NO3)2·6H2O and 5 mmol Na2WO4·2H2O were put into two 250 mL conical flasks with 30 mL of deionized water under continuous magnetic stirring for 10 min at room temperature, respectively. Then, the pH value of the mixed solution was adjusted to 6.0 with dilute HCl (0.5 M) and NaOH solution (0.5 M). Afterwards, the mixture was sonicated in a bath type sonicator (KQ5200DE, Kunshan Ultrasonic Instrument Company, China) with a frequency of 40 kHz for 30 min [26]. After being sonicated for 30 min, the obtained suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of
2.4.2. Measurements of sonocatalytic effect The sonocatalytic effect on degradation ratio about three major influencing aspects was examined: Firstly, ultrasonic operational 2
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parameters (ultrasonic power intensity, ultrasonic time); secondly, catalyst parameters (synthetic pH, catalyst addition amount, reusability); thirdly, solution parameters (initial concentration of solution). The experimental results were shown from Figs. 3–6.
2.4.3. Determination of reactive oxygen species (ROS) In order to qualitatively detect the kinds of ROS produced by the synergistic effects of ultrasound and ZnWO4, three kind of radical scavengers (D-Man, L-His and VC) were used. The experiments were performed as below. Step one, five MEL stock solutions were added into five conical flasks after dilution, respectively and labeled from 1 to 5. Step two, D-Man, L-His and VC stock solutions were added into conical flasks 1 to 3, respectively. Step three, ZnWO4 was added to the conical flasks 1 to 4, and the fifth was added nothing for controlling. Afterwards, these conical flasks were located directly in the ultrasonic apparatus (output power intensity of 0.278 W/cm2) away from light. After 2.0 h, all samples were centrifuged and detected the absorbance at 360 nm by a UV–vis spectrophotometer. The result was shown in Fig. 7B and the mechanism was discussed in Fig. 8. In another set of silent conditions, all the optimized conditions were repeated (MEL and catalyst concentration, pH, temperature, etc.), US energy was replaced by mechanical stirring. After 2.0 h, all samples were centrifuged and detected the absorbance at 360 nm by a UV–vis spectrophotometer. The result was shown in Fig. 7A.
Fig.2. XRD patterns of ZnWO4 sample prepared at different synthetic pH.
3. Results and discussion 3.1. Structural analysis of the synthesized ZnWO4 nanomaterials The results of SEM analysis were shown in Fig. 1(a-e). It could be seen that the products synthesized under different pH conditions (pH = 5, 6, 7, 8, 9) had various morphologies (nano-sheet, nano-microspheres or nano-rod). As can be seen in EDS spectra (Fig. 1f), the surface distribution of each element in the ZnWO4 nanoparticles was clear for the elements O, W, Zn, and the percentage of O, Zn, and W element content was 20.51%, 60.76%, 18.73% respectively, which proved that the synthetic ZnWO4 was very close to the composition of the standard ZnWO4 sample and was consistent with XRD. Surface area was calculated by Brunauer-Emmett-Teller (BET) equation. It could also be seen from Table S2. that the crystal form had the largest specific surface area at the synthesis pH of 6, and the specific surface area decreased in other crystal structures, the order of which was 6 > 7 > 8 > 9 > 5. Fig. 2 showed the XRD pattern of the ZnWO4 samples and the comparative standard card of ZnWO4 (JCPDS No. 89–7624). The calculated cell parameters of a, b, c, α, β and γ (shown in Table S3) of the samples and the monoclinic ZnWO4 (standard card numbers 89–7624) were almost exactly the same. And the diffraction peaks corresponding
2.5. Photocatalytic and sonophotocatalytic experiments of ZnWO4 In the photocatalytic experiment, adding amount of catalyst, MEL solution concentration was the same with sonocatalytic condition. Before illumination, the suspension was stirred for 30 min in darkness to establish an adsorption–desorption equilibrium. Sunlight was simulated by a 500 W xenon lamp (λ > 300 nm, Princeth technology co., LTD, Beijing) and the average light intensity was 200 mW/cm2. For the sonophotocatalytic test, the solution was given both ultrasonic and sunlight irradiations for 2.0 h. During the irradiation, 2.0 mL of suspension was sampled from the reactor at a fixed time interval with a 0.22 μm membrane filter syringe to remove the residual particles. UV–vis absorption of MEL at various time was measured and the degradation ratio of MEL was calculated in Fig. 9B.
Fig. 1. SEM (a-e) and EDS (f) images of ZnWO4 samples prepared at different synthetic pH: a: pH = 5, b: pH = 6, c: pH = 7, d: pH = 8, e: pH = 9, f: pH = 6. 3
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number of cavitation bubbles [28]. Then the sonoluminescence (about 6 eV energy) [11] caused by the acoustic cavitation was able to induce the excitation of ZnWO4 (with energy band gap about 3.0–4.2 eV). The electrons (e−) on valence band (VB) of ZnWO4 could transit to conduction band (CB) and the h+ was generated on VB at the same time, forming e− - h+ pairs [29]. The generated h+ could directly degrade MEL [30]. And the generated e− reacted with oxygen dissolved in aqueous solution, producing %O2−. Furthermore, a variety of ROS (HO2%, %OH and %O etc.) produced by a series of subsequent chemical reactions markedly accelerated the degradation of MEL. The results of this experiment verified our previous assumption that ZnWO4 could play a catalytic role under ultrasonic irradiation, and its good ultrasonic catalytic performance indicated that it could be used to catalyze the ultrasonic degradation of MEL. Fig. 3. UV–vis spectra of MEL solutions at the different conditions. (a: Original c: MEL + Ultrasound; d: MEL solution; b: MEL + ZnWO4; MEL + ZnWO4 + Ultrasound) ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, Irradiation time = 120 min, Ultrasonic power intensity = 0.278 W/cm2.
3.3. Effects of different synthetic pH values A comparative study was carried out with ZnWO4 powders obtained at different synthetic pH values for degradation of MEL and the results are shown in Fig. 4a. In this set of experiments, the initial MEL concentration, sonocatalyst dosage, ultrasonic power intensity and reaction time were constant at 10 mg/L, 1.00 g/L, 120 min and 0.278 W/cm2, respectively. As can be seen from Fig. 4a, the synthesis of ZnWO4 at five acidity levels all exhibited good sonocatalytic activity. Their sonocatalytical degradation ratios of MEL were all above 40%. When the pH value of synthesis was 6, the degradation ratio reached the highest of 75.7%. Under the same reaction conditions, the synthetic pH effect was: 6 (75.7%) > 5 (49.9%) > 8 (45.5%) > 9 (44.8%) > 7 (43.4%). This showed that activities for degrading MEL were varied with morphologies of samples. It could be seen from XRD, SEM and BET that ZnWO4 at synthetic pH = 6 possessed nano-microspheres which could provide the smallest particle size (40–60 nm) and the largest specific surface area (27.068 m2/g) among the obtained samples. Lops Carmine et al. hypothesized that more defects and active sites on the larger surface were more available to produce cavitation bubble [31]. The nano- microspheres obtained at pH = 6 could easily adsorb and trap gas pockets on their surface which facilitating the vapor microbubbles nucleation in aqueous solution, and consequently increasing OH· radicals amount on the particles’ surface. These ROS were beneficial to the degradation of MEL. Therefore, our latter studies focused on the development of ZnWO4 synthesized at the pH of 6.
to (1 0 0), (0 1 1), (1 1 0), (1 1 1), (0 2 1), (1 2 1), (−2 0 2) and (−1 3 2) crystal planes were in good agreement with standard card number 89–7624, which indicated that these synthesized ZnWO4 phases have relatively high purity and high crystallinity. Additionally, we observed that the XRD peaks were becoming narrow and sharp along with reducing pH value.
3.2. Confirmation the sonocatalytic activity of ZnWO4 The UV–vis spectra of MEL solutions at the different conditions were shown in Fig. 3. It could be observed that the MEL had two maximum absorption peaks at 269 nm and 360 nm (curve a), which was attributed to the characteristic absorption bands of aromatic compounds and the conjugated benzothiazine structure of MEL, respectively. As shown in curve b, a small decrease in the absorption peak indicated that MEL could be slightly adsorbed by ZnWO4 in the case of stirring for 0.5 h. The solution had a pH value of 12.9, on the surface of ZnWO4 particles there were superfluous negative charges which would lead to repulsion to the MEL anion in solution, resulting in negligible adsorption. MEL under simple ultrasound (shown in curve c) exhibited a small degradation ratio (about 10.1%). It should be the oxidative decomposition of MEL by free radicals generated during the ultrasonic process [27]. In order to further enhance the catalytic degradation effect of ultrasound, ZnWO4 (synthetic pH = 6) was added. When the ultrasound reaction was 2.0 h, it was found that the two peaks of MEL were significantly decreased, and the degradation ratio of MEL was 75.7%. After adding the sonocatalyst, the presence of heterogeneous ZnWO4 could provide additional nucleation sites that enhanced the
3.4. Influence of ultrasonic power intensity As an important instrumental parameter which affecting the cavitation activity and the efficiency of sonocatalytic degradation,
Fig. 4. Influence of different synthetic pH values (a) and ultrasonic power intensity (b) on the degradation ratio of MEL. ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, Irradiation time = 120 min. a: Ultrasonic power intensity = 0.278 W/cm2. b: synthetic pH value of ZnWO4 = 6.) 4
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3.6. Influence of adding amount of ZnWO4
ultrasonic power intensity applied for this experiment varied from 0.139 W/cm2 to 0.278 W/cm2 at 0.028 W/cm2 intervals. In the experiments, the addition amount of ZnWO4 was 1.00 g/L, the ultrasonic irradiation time was 120 min, the concentration of MEL was 10 mg/L. As shown in Fig. 4b, the degradation ratio was 28.9% at an ultrasonic power intensity of 0.139 W/cm2. As the power increased, the degradation ratio gradually increased. The experiments showed that the higher the ultrasonic power intensity was, the better the effect of the ultrasonic degradation ratio was. For a given ultrasound frequency, an enhancement in the acoustic power could increase acoustic amplitude, which favors more active cavitation bubbles and also the size and potential energy of the individual bubbles [32]. This potential energy, during bubble collapse, could be converted into chemical reactions (i.e., formation of ROS), heat, light and sound emission [33]. Furthermore, the increase in US power intensity might influence local turbulences in the fluid inside the US reactor, which might improve the mass transfer rates of MEL and the amount of active sites available on the ZnWO4 surface [34]. Hence, the increase in ultrasonic power intensity was beneficial for degradation of the drug.
Nanomaterials possessed the quality of adsorption action due to their physicochemical properties such as surface area, phase structure, particle size and interface charge [35]. Hence, when the concentration of a catalyst or drug changed, we measured the subsequent changes in adsorption ratio. In this section, the effect of catalyst amount on degradation was investigated and the adding amount of ZnWO4 was carried out in the range of 0.5–1.5 g/L (interval 0.25 g/L). The ultrasonic irradiation time and output power were kept fixed. After external diffusion and drug adsorption–desorption processes, adsorption degradation ratio was measured. It could be seen from Fig. 6a that the adsorption degradation ratio (between 0.86% and 1.69%) of MEL did not increase obviously with the increase of catalyst concentration. This result indicates that the adsorption capacity of ZnWO4 towards MEL was so weak that could be neglected. However, after 2 h of ultrasonic catalysis, the R-degradation ratio (47.43% to 75.67%) were significantly higher than the adsorption degradation ratio. With the increase of catalyst concentration, both ratio increased firstly (sonocatalyst dosage range from 0.50 g/L to 1.00 g/L) and then decreased (sonocatalyst dosage range from 1.00 g/L to 1.50 g/L). It indicated that a small amount of catalyst could be well dispersed in the solution, and the active site on the surface of ZnWO4 also increased as the amount of catalyst dosage increased [36]. When the adding amount of ZnWO4 exceeded 1.0 g/L, the degradation ratio only slightly increased, indicating that the excessive adding of the catalyst weakened the strength and cavitation of the ultrasonic wave, and the surface the catalyst itself existed as agglomerate, and the contact area with the drug was reduced. The active site and the catalysis of the catalyst were attenuated [16].
3.5. Influence of ultrasonic irradiation time and the reaction kinetics To investigate the influence of ultrasonic irradiation time on the sonocatalytic degradation ratio of MEL, the experiments were carried out at constant condition as following: sonocatalyst dosage (1 g/L), ultrasonic power intensity (0.278 W/cm2) and drug concentration (10 mg/L). It could be seen from Fig. 5a that as the duration of ultrasonic irradiation time increased, the degradation ratio of MEL increased. The degradation efficiency of MEL reached 76.9% at 120 min. It was speculated from the results that under the action of ultrasound, more free radicals were generated in the solution, which caused oxidative decomposition of MEL. In order to explore the reaction system and ratios constant, the kinetics of sonic-catalyzed degradation were also investigated. The firstorder dynamics mode was often used to evaluate the kinetic behavior when the concentration of the drug was low [9]. The curve of the firstorder kinetic reaction was calculated based on ln (C0/Ct) versus ultrasound irradiation time (t). Where Ct was the concentration of MEL after certain time (t) under the ultrasonic irradiation, and the C0 was the initial concentration of MEL solution. As can be seen from Fig. 5b, the curve was approximately linear (R = 0.9934) which indicated that the sonic-catalyzed degradation reaction conformed to the first-order reaction kinetics ln (C0/Ct) = 0.0136 t − 0.2303, and the rate constant was 0.0136 min−1.
3.7. Influence of the initial concentration of MEL Some literature revealed that initial organic concentration might influence degradation [37]. In the experiment, the effects of different MEL concentrations (6–18 mg/L) on degradation ratio were investigated and all the other conditions kept unchanged. It could be seen from Fig. 6b the degradation ratio was firstly increased at low initial concentrations (less than 10 mg/L) then decreased at higher initial concentrations (range at 10–18 mg/L) with the increase of MEL concentration. For a given experimental conditions (20 mg catalyst dosage, 1.00 g/L ZnWO4, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, 40 kHz frequency), the appropriate initial concentration (10.0 mg/L) could attain a high degradation ratio (75.9%) of MEL. In addition, the degradation amount of MEL increased with the augment of initial concentration. Because the higher initial concentration could provide the driving force to overcome the mass transfer resistance
Fig. 5. The influence of ultrasonic irradiation time on the degradation ratio of MEL (a) and kinetics of the sonocatalytic degradation of MEL (b). Initial MEL concentration = 10.0 mg/L, catalyst dosage = 20 mg. 5
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Fig. 6. The influence of sonocatalyst dosage (a), initial concentration of MEL (b) and reusability of the ZnWO4 (c) on the degradation ratio of MEL. (Ultrasonic power intensity = 0.278 W/cm2).
generated ROS, thus the absorbance of MEL was obviously decreased, and the corresponding degradation ratio was estimated to be 70%. After adding D-Man, the degradation reached to 30%, which indicated the existence of %OH. After adding L-His, the absorbance value of MEL was 0.42, indicating the existence of 1O2 in addition to %OH. When VC was added to block all ROS, the absorbance value was 0.48. This experiment showed that at least two ROS (%OH, 1O2) were produced in the process of ultrasonic catalysis of ZnWO4.
of drug [38], more drugs were allowed to reach the surface of the acoustic catalyst and thus be adsorbed and degraded. 3.8. Reusability The sonocatalysts reusability after four sonocatalytic cycles were tested, where, 10.0 mg/L of MEL initial concentration, 20.0 mg of sonocatalyst, 0.278 W/cm2 of ultrasonic power intensity and irradiation time of 120 min were used as experimental conditions. After each cycle, the sonocatalyst was washed with distilled water and anhydrous ethanol alternately, then dried at 80 °C for successive reuse. The results were shown in Fig. 6c. It could be seen that the reused catalyst showed a little decrease in the degradation efficiency of the catalytic activity from run1 (76.1%) to run4 (53.7%). These observations indicated that the structure of ZnWO4 was relatively stable under the reaction conditions and might not be seriously affected by the reactants environment and ultrasound, hence its acoustic catalytic activity was relatively stable [39]. Considering the above results, it could be concluded that ZnWO4 could be safely used for four cycles.
3.10. Possible mechanism in the process of sonocatalytic degradation In this study, ultrasonic was used to replace light to excite ZnWO4. It is well known that sonocatalytic degradation undergoes a number of important processes. Firstly, under the irradiation of ultrasonic, tiny bubbles in the solution would be produced. After nucleation, they collapsed. This process was called ultrasonic cavitation effect [40](Step I in Fig. 8) [37]. Secondly, the cavitation effect would generate sonoluminescence and hot spot (local high temperature (5000 K) and high pressure (1000 atm) regions). Sonoluminescence could result in the formation of the light flash of average photon energy of 6 eV to excite ZnWO4 [40]. Thus, the e− on valence band of ZnWO4 could be transitted to conduction band and the h+ are generated on VB at the same time, forming e− – h+ pairs. The nano-microspheres structure of obtained ZnWO4 (pH = 6) possessed the largest BET surface area (27.068 m2/g) which could provide more active sites on its surface, thus, more e− and h+ could be produced. Meanwhile, in the extreme environment of high temperature and high pressure, the water vapor in the cavitation bubble was cracked into free radicals such as HO2% and % O, some of which would be combined to form H2O2. However, through H2O2, we could get more %OH and other ROS with strong oxidability (Step П in Fig. 8). The detailed producing of h+ and ROS were shown in the following:
3.9. Detection of reactive oxygen species (ROS) It has been proved in the literature that in the process of ultrasonic catalytic degradation, a large amount of ROS will be produced which could oxidize and decompose compounds into carbon dioxide and water [40,41]. In order to quantitatively study the species of ROS, three ROS scavengers were added: D-Man (%OH scavenger), L-His (1O2 and % OH scavengers) and VC (all ROS scavengers). Fig. 7 presented the UV–Vis absorption spectra of MEL at different conditions. It could be seen from Fig. 7A that under stirring, there was a slight decrease in absorption peak for MEL + Vc/D-Man/L-His solutions compared to origin MEL solution. As depicted in Fig. 7B, under the synergistic effect of ultrasound and ZnWO4, MEL was oxidized and decomposed by all
Fig. 7. UV–vis spectra of MEL solutions at the different conditions. ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, US/stirring time = 120 min, Ultrasonic power intensity (0.278 W/cm2) intensity) = 0.278 W/cm2.) 6
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Fig. 8. Proposed mechanism for sonocatalytic degradation of MEL in presence of ZnWO4.
Catalyst +))) → Catalyst(h+) + Catalyst(e−) h + H2O → HO% + H +
the highest.
+
3.12. Comparison of sonocatalytic with photocatalytic and sonophotocatalytic efficiencies of ZnWO4
e− + O2 →%O2− H2O + ))) → HO% + H+
Fig. 9b showed the photocatalytic/sonocatalytic/sonphotocatalytic degradation ratios of MEL under irradiation with ZnWO4 synthesized at pH of 6. As can be seen from Fig. 9b, the degradation ratio of MEL increased with reaction time for all the three experimental methods and the order was sonphotocatalytic activity > sonocatalytic activity ≫ photocatalytic activity. The sonocatalytic effect of ZnWO4 was significant but photocatalytic activity of ZnWO4 under simulated sunlight was much weaker (ranging from 5.0% to 9.0%). ZnWO4 in photocatalytic experiment mainly absorbed high energy ultraviolet light (254 nm) [44,45], which accounted for less than 5% of the output intensity of simulated sunlight. Less light intensity excited ZnWO4 and less photo-induced electrons and holes was obtained [46]. Therefore, the photocatalytic activity of ZnWO4 under simulated sunlight was weak. In the sonophotocatalytic experiment, the degradation ratio of MEL was a little higher than sonocatalytic experiment, but there was no obvious synergistic effect.
O2 + ))) →%O %O2− + H+ → HO2% HO% + %O → HO2% 2HO2% → O2 + H2O2 2H2O2 + %O2− → %2OH + 2OH− + O2 Thirdly, all kinds of ROS and h+ with strong oxidation ability degraded the surrounding MEL molecules, eventurally, to CO2, H2O and inorganic molecules, resulted in a decrease in the peak of UV–visible absorption of MEL (Step Ш in Fig. 8). 3.11. The contrastive degradation ratio of different compounds In this section, ZnWO4 were used to investigate its sonocatalytic degradation performance on several kinds of organic compounds (their names, structures and detection wavelength are shown in Table S1). These organic pollutants selected as degradation models were identified in varying concentrations in natural waters [42,43]. In the experiment, the dosage of ZnWO4 was 20.0 mg (1.0 g/L), the compounds concentration was 20 mg/L. The adsorption/degradation ratio of each compounds are shown in Fig. 9a. It could be seen from Fig. 9a that ZnWO4 could degrade these compounds under US irradiation to some extent. The order of percentage of adsorption degradation was: MB > CIP > LEV > AMA > MEL. The results showed that the cationic MB/CIP in aqueous solution with smaller molecular weight was more easily adsorbed by ZnWO4, the adsorption capacity of ZnWO4 for LEV was in the middle, and ZnWO4 has the weakest adsorption capacity for anionic AMA/MEL. Hence, simply from the point of view of adsorption, the catalyst ZnWO4 is more likely to adsorb positively charged compounds on the surface in the solution. Meanwhile, the order of degradation percentage was: MB > MEL > CIP > AMA > LEV. Due to the different structure of compounds, ROS produced in catalytic process have different destructive effects on different structures. As far as MEL was concerned, its benzothiazine ring was more susceptible to ROS’s attack and degradation. Therefore, the degradation ratio of MEL was
4. Conclusions In conclusion, ZnWO4 were synthesized by hydrothermal method and for the first time used as sonocatalyst. ZnWO4 nano/micro particles were obtained by changing five acidity conditions and characterized by XRD, SEM, EDS and nitrogen adsorption–desorption isotherm measurements. Interestingly, the sonocatalytic activity of ZnWO4 on degradation of MEL varied with the synthetic pH values, with ZnWO4 under pH = 6 exhibiting the best sonocatalytic performance. Under the condition of 10 mg/L MEL concentration, 20 mg catalyst dosage, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, the degradation ratio on MEL have reached 75.7%. At this synthesized pH value, nano-microspheres structure of ZnWO4 displayed the smallest particle size (40–60 nm), the largest BET surface area (27.068 m2/g) and more active site to generate ROS and h+, which were responsible for the excellent sonocatalytic performances. In addition, measurements of sonocatalytic effect indicated that the degradation efficiency of MEL increased with the increase of US power and time. Results of dynamics studies demonstrated that the sonocatalytic degradation conforms to the first-order reaction kinetic equation and the rate constant is 0.0136 min−1. The mechanism of the sonocatalytic process was 7
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Fig. 9. The contrastive degradation ratio of different organic dyes (a) and the contrastive degradation ratio of different catalytic way (b).
suggested with regard to sequence of events: nucleation, sonoluminescence, hot spot and generation of ROS/h+. Finally, sonphotocatalytic effect of ZnWO4 was slightly better than that of sonocatalytic effect and sonocatalytic effect was much higher than that of photocatalytic effect under simulated sunlight. This result suggested a new way for the harmless treatment of drugs using ZnWO4 as an sonocatalyst in aqueous media.
[13]
[14]
[15]
Acknowledgements
[16]
The authors greatly acknowledge the research project of education department of Liaoning province (LQN201715) for financial supports.
[17]
Appendix A. Supplementary data [18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.104815.
[19]
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Preparation of zinc tungstate nanomaterial and its sonocatalytic degradation of meloxicam as a novel sonocatalyst in aqueous solution Liang Xua,b, Xin Wangb, Ming-Ling Xub, Bin Liub, Xiao-Fang Wangb, Si-Huan Wangb, Ting Suna, a b
T
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Department of Chemistry, College of Science, Northeastern University, Shenyang 110819, China College of Pharmacy, Liaoning University, Shenyang 110036, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Meloxicam Sonocatalytical degradation Zinc tungstate Water treatment Ultrasonic irradiation
Zinc tungstate (ZnWO4) was previously used as a photocatalyst. In this paper, for the first time as an sonocatalyst, the performance of ZnWO4 for sonocatalytic degradation of meloxicam (MEL) under ultrasonic irradiation were studied. Firstly, ZnWO4 nanomaterials were synthesized at different acidity (pH = 5, 6, 7, 8, 9) via the hydrothermal method. Utilizing SEM, XRD and EDS techniques to characterize composition and morphology of each product, the same crystal forms, but different morphologies (nano-sheet, nano-microspheres or nano-rod) of ZnWO4 could be obtained. Secondly, the sonocatalytic activities of ZnWO4 on degradation of MEL were studied. It was found that the degradation ratio varied with the synthetic pH values, with ZnWO4 under synthetic pH = 6 exhibiting the best sonocatalytic performance (75.7%). While being synthesized at this pH value, ZnWO4 nano-microspheres had the largest BET surface area (27.068 m2/g), the smallest particle size (40–60 nm) so as to provide more active sites on its surface, which were able to produce more reactive oxygen species (ROS) and holes under ultrasonic irradiation. These ROS and holes had a positive effect on the degradation of MEL into CO2, H2O and inorganic. Thirdly, various influential factors including ultrasonic power intensity, ultrasonic time, catalyst addition dosage, initial concentration of MEL solution and reusability of catalyst were also explored. Under the condition of 10 mg/L MEL concentration, 20 mg catalyst dosage, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, the degradation ratio on MEL reached 75.7%. Finally, the presence of hydroxyl radical (%OH) and singlet molecular oxygen (1O2) in the reaction was confirmed by adding ROS scavenger. The experimental results suggested that ZnWO4 nanoparticle could be used not only as an effective photocatalyst, but also, under the condition of ultrasonic irradiation, a promising sonocatalyst for degradation of organic pollutants in aqueous media.
1. Introduction Since the beginning of the 21st century, many publications have revealed the presence of pharmaceutical drugs and dyestuff in surface and sewages waters [1–3]. As one of these micro-pollutants, meloxicam (MEL, as shown in Table S1) was found to have considerable level in wastewaters [4,5], which might exist potential toxicological risks on living organisms even at low concentrations [1]. Hence, the elimination of micro-pollutants such as MEL from environment has aroused people's concern. Advanced oxidation processes (AOPs) are a collection of methods to mineralize a variety of recalcitrant organics through the generation of reactive oxygen species (ROS) and holes (h+) [6,7]. Among the AOPs, the ultrasonic (US) process has been widely investigated [8]. Sonocatalytic degradation of organic pollutants can be described as the ultrasonic cavitation effect happened in aqueous
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medium, which leads to the sonoluminescence, hot spot accompanied with high temperature (5000 K) and high pressure (1000 atm). As is known, hot spots can induce the dissociation of water into ROS including hydroxyl radical (%OH), hydroperoxyl radicals (%OOH), singlet molecular oxygen (1O2) and superoxide radical (%O2−), which can degrade and destroy organic pollutants [9,10]. Sonoluminescence can generate light with a comparatively wide wavelength range (200–500 nm) and an average photon energy of 6 eV, which can excite the aromatic organic pollutants to become unstable [11]. However, ultrasound alone is highly energy intensive and insufficient to totally degrade pollutants in water [12,13]. A lot of sonodegradation studies have shown that involvement of sonocatalyst can give full play to the superiority of ultrasound, which can produce a synergistic effect and enhance the degradation efficiency of organic pollutants [8,14]. Thus, the screening of novel sonocatalysts has become one of the research
Corresponding author. E-mail address: [email protected] (T. Sun).
https://doi.org/10.1016/j.ultsonch.2019.104815 Received 24 April 2019; Received in revised form 10 September 2019; Accepted 29 September 2019 Available online 11 October 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
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100 mL and maintained at 453 K for 24 h. At last, the composition was cooled down to room temperature naturally, filtered and dried at 353 K. Distilled water and anhydrous ethanol were used for washing the white precipitate several times. After fully grinding, the ZnWO4 powder was obtained and stored. In the process of synthesis, all other things being equal, the mixed solution was adjusted for pH by HCl and NaOH (pH = 5, 7, 8, 9), and the ZnWO4 products under different synthesized pH were obtained.
hotspots [15]. Because of connections in the excitation mechanisms between the sonocatalyst and photocatalyst, many papers have studied the sonocatalytical properties of photocatalysts and obtained good results, such as TiO2, polyaluminum chloride, Al2O3 and Bi2WO6 [16,17]. Zinc tungstate (ZnWO4) is a monoclinic tungstate metal oxide with d10s2 - d0 electronic configuration [18]. Like Bi2WO6, ZnWO4 is also a kind of tungstate semiconductor material that has unique optical and chemical properties, e.g. high average refractive index, high X-ray absorption coefficient, high light yield, short decay time, and low afterglow to luminescence and so on [19,20]. Hence, ZnWO4 has been explored for a number of applications, such as photocatalysis [21], optical fibres [22], microwave applications [23] and scintillators [24]. In addition, its unique chemical properties and narrow band gap (3.0–4.2ev) [19] show that it may be a promising sonocatalyst. In this paper, ultrasonic assisted hydrothermal method was used to prepare ZnWO4 under different acidic conditions (pH = 5, 6, 7, 8, 9). The prepared ZnWO4 particles were characterized by scanning electron microscopy (SEM), X-ray powder diffractometer (XRD), energy dispersive X-ray spectroscopy (EDS) and nitrogen adsorption – desorption isotherm measurements. Then, the degradation ratios of ZnWO4 on MEL under different synthetic acidity conditions were compared. Thirdly, several factors on degradation ratios of MEL such as ultrasonic power intensity, ultrasonic time, synthetic pH, catalyst addition amount, initial concentration of the solution and reusability of catalyst were investigated. Finally, the mechanism of sonodegradation of MEL was proposed, the selectivity of catalyst’s degradation efficiency on different pollutants (The names, structural formula, molecular weight and maximum absorption wavelength of these organic compounds are provided in Table S1) was compared, comparison of sonocatalytic with photocatalytic and sonophotocatalytic efficiencies of ZnWO4 was displayed. The present study was intended to propose some new ideas for the application of sonocatalytic activities of ZnWO4 for treatment of micro-pollutants in ecosystem, especially in water.
The crystalline nature of ZnWO4 powder was determined by an Xray diffractometer (D8 Advance, Bruker Axs Company, Germany) with Cu Kα irradiation (λ = 1.5405). The phase purity and homogeneity pattern of ZnWO4 were refined using a Phillips Powder (PW) diffractometer 1710 in a wide range of Bragg angles (10° ≤ 2θ ≤ 80°). The average grain sizes (D) were calculated as reported [21]. The Scanning Electron Microscope (SEM, ULTRA PLUS, Zeiss microscopy, Germany) with energy dispersive X-ray (EDS) system was used to determine the micromorphology and composition of ZnWO4. BET surface areas, pore volumes and pore size distribution were determined by nitrogen adsorption and desorption isotherm at 77 K on a Micrometrics Autosorb iQ-C (American, Quanta chrome Instruments). Prior to analysis, samples were degassed at 373 K to remove any physically adsorbed gas for 6 h. The pore size distribution of the samples was determined by the BJH (Barret – Joyner – Halenda) model from the data of adsorption and desorption branch of the nitrogen isotherms. Specific surface area was determined by the BET method. The controllable serial-ultrasonics (US) apparatus process was performed in a double-jacketed stainless steel reactor (L × W × H: 30 × 24 × 15 cm) with four US transducers (KQ5200DE, Kunshan Ultrasonic Instrument Company, China). The sonicator provided the variable output power intensity of 0.111–0.278 W/cm2 and the fixed frequency of 40 kHz.
2. Experimental
2.4. Evaluation of sonocatalytic performance
2.1. Materials and reagents
2.4.1. Measurements of sonocatalytic activity of the prepared ZnWO4 composites As one of widely used drugs which have been detected in environment, MEL was selected as a model microcontaminants to evaluate the sonocatalytic efficiency of ZnWO4. The entire catalytic sonication process took place at 298 K in dark to prevent the influence of daily or ambient light. Generally, 20 mg of catalysts and 20 mL of MEL solution (10 mg/L, dissolved with 0.1 M NaOH) were mixed together in a beaker. The suspension was stirred for 30 min to ensure the adsorption/ desorption equilibrium of MEL before ultrasonic irradiation. The adsorption ratio of MEL on ZnWO4 was calculated by the absorbance of the solution at its λmax = 360 nm. The formula was: adsorption ratio (%) = [(A0 − At)/A0] × 100. A0 was the initial absorbance of MEL solution; At was the absorbance of MEL solution after stirring for 30 min. High speed freezing centrifuge (Heraeus X1R, Thermo Scientific) and 0.22 μm glass microfiber filters were used to separate ZnWO4 powder from MEL solution. UV–vis absorption measurements were recorded on a UV-2550 spectrophotometer (Shimadzu Company, Japan) with 1.0 cm quartz cells. The effects of catalytic activity of ZnWO4 on MEL were inspected by the change in absorbance of MEL with UV–vis spectrometer. The degradation ratio of MEL was calculated by the absorbance of the solution at its λmax = 360 nm. The formula was: degradation ratio (%) = [(A0 − At)/A0] × 100. A0 was the initial absorbance of MEL solution; At was the absorbance of MEL solution under different experimental conditions [8].
2.3. Characterization of ZnWO4 powder
Sodium tungstate dehydrate (Na2WO4·2H2O) and Zinc nitrate hexahydrate ((Zn(NO3)2·6H2O) were both purchased from Tianjin Damao Chemical Reagent Factory (China). Ethanol absolute (CH3CH2OH), concentrated Hydrochloric acid (HCl), Amaranth (AMN), mannitol (DMan), l-histidine (L-His), vitamin C (VC), Ciprofloxacin Hydrochloride (CIP), Levofloxacin (LEV), Methylene blue (MB) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co. Ltd. Samples were weighed accurately on a microbalance (Sartorius BP211D, Germany) with a resolution of 0.01 mg. MEL (98% purity, Sinopharm Chemical Regent Co, Ltd, China) (as shown in Table S1) was used to undergo the sonocatalytic degradation experiment under ultrasonic irradiation. Double distilled water was used for all solution preparation. 2.2. Preparation of ZnWO4 powder ZnWO4 powders were synthesized by hydrothermal method according to literature [25] with a slight modification. Typically, 5 mmol Zn(NO3)2·6H2O and 5 mmol Na2WO4·2H2O were put into two 250 mL conical flasks with 30 mL of deionized water under continuous magnetic stirring for 10 min at room temperature, respectively. Then, the pH value of the mixed solution was adjusted to 6.0 with dilute HCl (0.5 M) and NaOH solution (0.5 M). Afterwards, the mixture was sonicated in a bath type sonicator (KQ5200DE, Kunshan Ultrasonic Instrument Company, China) with a frequency of 40 kHz for 30 min [26]. After being sonicated for 30 min, the obtained suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of
2.4.2. Measurements of sonocatalytic effect The sonocatalytic effect on degradation ratio about three major influencing aspects was examined: Firstly, ultrasonic operational 2
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parameters (ultrasonic power intensity, ultrasonic time); secondly, catalyst parameters (synthetic pH, catalyst addition amount, reusability); thirdly, solution parameters (initial concentration of solution). The experimental results were shown from Figs. 3–6.
2.4.3. Determination of reactive oxygen species (ROS) In order to qualitatively detect the kinds of ROS produced by the synergistic effects of ultrasound and ZnWO4, three kind of radical scavengers (D-Man, L-His and VC) were used. The experiments were performed as below. Step one, five MEL stock solutions were added into five conical flasks after dilution, respectively and labeled from 1 to 5. Step two, D-Man, L-His and VC stock solutions were added into conical flasks 1 to 3, respectively. Step three, ZnWO4 was added to the conical flasks 1 to 4, and the fifth was added nothing for controlling. Afterwards, these conical flasks were located directly in the ultrasonic apparatus (output power intensity of 0.278 W/cm2) away from light. After 2.0 h, all samples were centrifuged and detected the absorbance at 360 nm by a UV–vis spectrophotometer. The result was shown in Fig. 7B and the mechanism was discussed in Fig. 8. In another set of silent conditions, all the optimized conditions were repeated (MEL and catalyst concentration, pH, temperature, etc.), US energy was replaced by mechanical stirring. After 2.0 h, all samples were centrifuged and detected the absorbance at 360 nm by a UV–vis spectrophotometer. The result was shown in Fig. 7A.
Fig.2. XRD patterns of ZnWO4 sample prepared at different synthetic pH.
3. Results and discussion 3.1. Structural analysis of the synthesized ZnWO4 nanomaterials The results of SEM analysis were shown in Fig. 1(a-e). It could be seen that the products synthesized under different pH conditions (pH = 5, 6, 7, 8, 9) had various morphologies (nano-sheet, nano-microspheres or nano-rod). As can be seen in EDS spectra (Fig. 1f), the surface distribution of each element in the ZnWO4 nanoparticles was clear for the elements O, W, Zn, and the percentage of O, Zn, and W element content was 20.51%, 60.76%, 18.73% respectively, which proved that the synthetic ZnWO4 was very close to the composition of the standard ZnWO4 sample and was consistent with XRD. Surface area was calculated by Brunauer-Emmett-Teller (BET) equation. It could also be seen from Table S2. that the crystal form had the largest specific surface area at the synthesis pH of 6, and the specific surface area decreased in other crystal structures, the order of which was 6 > 7 > 8 > 9 > 5. Fig. 2 showed the XRD pattern of the ZnWO4 samples and the comparative standard card of ZnWO4 (JCPDS No. 89–7624). The calculated cell parameters of a, b, c, α, β and γ (shown in Table S3) of the samples and the monoclinic ZnWO4 (standard card numbers 89–7624) were almost exactly the same. And the diffraction peaks corresponding
2.5. Photocatalytic and sonophotocatalytic experiments of ZnWO4 In the photocatalytic experiment, adding amount of catalyst, MEL solution concentration was the same with sonocatalytic condition. Before illumination, the suspension was stirred for 30 min in darkness to establish an adsorption–desorption equilibrium. Sunlight was simulated by a 500 W xenon lamp (λ > 300 nm, Princeth technology co., LTD, Beijing) and the average light intensity was 200 mW/cm2. For the sonophotocatalytic test, the solution was given both ultrasonic and sunlight irradiations for 2.0 h. During the irradiation, 2.0 mL of suspension was sampled from the reactor at a fixed time interval with a 0.22 μm membrane filter syringe to remove the residual particles. UV–vis absorption of MEL at various time was measured and the degradation ratio of MEL was calculated in Fig. 9B.
Fig. 1. SEM (a-e) and EDS (f) images of ZnWO4 samples prepared at different synthetic pH: a: pH = 5, b: pH = 6, c: pH = 7, d: pH = 8, e: pH = 9, f: pH = 6. 3
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number of cavitation bubbles [28]. Then the sonoluminescence (about 6 eV energy) [11] caused by the acoustic cavitation was able to induce the excitation of ZnWO4 (with energy band gap about 3.0–4.2 eV). The electrons (e−) on valence band (VB) of ZnWO4 could transit to conduction band (CB) and the h+ was generated on VB at the same time, forming e− - h+ pairs [29]. The generated h+ could directly degrade MEL [30]. And the generated e− reacted with oxygen dissolved in aqueous solution, producing %O2−. Furthermore, a variety of ROS (HO2%, %OH and %O etc.) produced by a series of subsequent chemical reactions markedly accelerated the degradation of MEL. The results of this experiment verified our previous assumption that ZnWO4 could play a catalytic role under ultrasonic irradiation, and its good ultrasonic catalytic performance indicated that it could be used to catalyze the ultrasonic degradation of MEL. Fig. 3. UV–vis spectra of MEL solutions at the different conditions. (a: Original c: MEL + Ultrasound; d: MEL solution; b: MEL + ZnWO4; MEL + ZnWO4 + Ultrasound) ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, Irradiation time = 120 min, Ultrasonic power intensity = 0.278 W/cm2.
3.3. Effects of different synthetic pH values A comparative study was carried out with ZnWO4 powders obtained at different synthetic pH values for degradation of MEL and the results are shown in Fig. 4a. In this set of experiments, the initial MEL concentration, sonocatalyst dosage, ultrasonic power intensity and reaction time were constant at 10 mg/L, 1.00 g/L, 120 min and 0.278 W/cm2, respectively. As can be seen from Fig. 4a, the synthesis of ZnWO4 at five acidity levels all exhibited good sonocatalytic activity. Their sonocatalytical degradation ratios of MEL were all above 40%. When the pH value of synthesis was 6, the degradation ratio reached the highest of 75.7%. Under the same reaction conditions, the synthetic pH effect was: 6 (75.7%) > 5 (49.9%) > 8 (45.5%) > 9 (44.8%) > 7 (43.4%). This showed that activities for degrading MEL were varied with morphologies of samples. It could be seen from XRD, SEM and BET that ZnWO4 at synthetic pH = 6 possessed nano-microspheres which could provide the smallest particle size (40–60 nm) and the largest specific surface area (27.068 m2/g) among the obtained samples. Lops Carmine et al. hypothesized that more defects and active sites on the larger surface were more available to produce cavitation bubble [31]. The nano- microspheres obtained at pH = 6 could easily adsorb and trap gas pockets on their surface which facilitating the vapor microbubbles nucleation in aqueous solution, and consequently increasing OH· radicals amount on the particles’ surface. These ROS were beneficial to the degradation of MEL. Therefore, our latter studies focused on the development of ZnWO4 synthesized at the pH of 6.
to (1 0 0), (0 1 1), (1 1 0), (1 1 1), (0 2 1), (1 2 1), (−2 0 2) and (−1 3 2) crystal planes were in good agreement with standard card number 89–7624, which indicated that these synthesized ZnWO4 phases have relatively high purity and high crystallinity. Additionally, we observed that the XRD peaks were becoming narrow and sharp along with reducing pH value.
3.2. Confirmation the sonocatalytic activity of ZnWO4 The UV–vis spectra of MEL solutions at the different conditions were shown in Fig. 3. It could be observed that the MEL had two maximum absorption peaks at 269 nm and 360 nm (curve a), which was attributed to the characteristic absorption bands of aromatic compounds and the conjugated benzothiazine structure of MEL, respectively. As shown in curve b, a small decrease in the absorption peak indicated that MEL could be slightly adsorbed by ZnWO4 in the case of stirring for 0.5 h. The solution had a pH value of 12.9, on the surface of ZnWO4 particles there were superfluous negative charges which would lead to repulsion to the MEL anion in solution, resulting in negligible adsorption. MEL under simple ultrasound (shown in curve c) exhibited a small degradation ratio (about 10.1%). It should be the oxidative decomposition of MEL by free radicals generated during the ultrasonic process [27]. In order to further enhance the catalytic degradation effect of ultrasound, ZnWO4 (synthetic pH = 6) was added. When the ultrasound reaction was 2.0 h, it was found that the two peaks of MEL were significantly decreased, and the degradation ratio of MEL was 75.7%. After adding the sonocatalyst, the presence of heterogeneous ZnWO4 could provide additional nucleation sites that enhanced the
3.4. Influence of ultrasonic power intensity As an important instrumental parameter which affecting the cavitation activity and the efficiency of sonocatalytic degradation,
Fig. 4. Influence of different synthetic pH values (a) and ultrasonic power intensity (b) on the degradation ratio of MEL. ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, Irradiation time = 120 min. a: Ultrasonic power intensity = 0.278 W/cm2. b: synthetic pH value of ZnWO4 = 6.) 4
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3.6. Influence of adding amount of ZnWO4
ultrasonic power intensity applied for this experiment varied from 0.139 W/cm2 to 0.278 W/cm2 at 0.028 W/cm2 intervals. In the experiments, the addition amount of ZnWO4 was 1.00 g/L, the ultrasonic irradiation time was 120 min, the concentration of MEL was 10 mg/L. As shown in Fig. 4b, the degradation ratio was 28.9% at an ultrasonic power intensity of 0.139 W/cm2. As the power increased, the degradation ratio gradually increased. The experiments showed that the higher the ultrasonic power intensity was, the better the effect of the ultrasonic degradation ratio was. For a given ultrasound frequency, an enhancement in the acoustic power could increase acoustic amplitude, which favors more active cavitation bubbles and also the size and potential energy of the individual bubbles [32]. This potential energy, during bubble collapse, could be converted into chemical reactions (i.e., formation of ROS), heat, light and sound emission [33]. Furthermore, the increase in US power intensity might influence local turbulences in the fluid inside the US reactor, which might improve the mass transfer rates of MEL and the amount of active sites available on the ZnWO4 surface [34]. Hence, the increase in ultrasonic power intensity was beneficial for degradation of the drug.
Nanomaterials possessed the quality of adsorption action due to their physicochemical properties such as surface area, phase structure, particle size and interface charge [35]. Hence, when the concentration of a catalyst or drug changed, we measured the subsequent changes in adsorption ratio. In this section, the effect of catalyst amount on degradation was investigated and the adding amount of ZnWO4 was carried out in the range of 0.5–1.5 g/L (interval 0.25 g/L). The ultrasonic irradiation time and output power were kept fixed. After external diffusion and drug adsorption–desorption processes, adsorption degradation ratio was measured. It could be seen from Fig. 6a that the adsorption degradation ratio (between 0.86% and 1.69%) of MEL did not increase obviously with the increase of catalyst concentration. This result indicates that the adsorption capacity of ZnWO4 towards MEL was so weak that could be neglected. However, after 2 h of ultrasonic catalysis, the R-degradation ratio (47.43% to 75.67%) were significantly higher than the adsorption degradation ratio. With the increase of catalyst concentration, both ratio increased firstly (sonocatalyst dosage range from 0.50 g/L to 1.00 g/L) and then decreased (sonocatalyst dosage range from 1.00 g/L to 1.50 g/L). It indicated that a small amount of catalyst could be well dispersed in the solution, and the active site on the surface of ZnWO4 also increased as the amount of catalyst dosage increased [36]. When the adding amount of ZnWO4 exceeded 1.0 g/L, the degradation ratio only slightly increased, indicating that the excessive adding of the catalyst weakened the strength and cavitation of the ultrasonic wave, and the surface the catalyst itself existed as agglomerate, and the contact area with the drug was reduced. The active site and the catalysis of the catalyst were attenuated [16].
3.5. Influence of ultrasonic irradiation time and the reaction kinetics To investigate the influence of ultrasonic irradiation time on the sonocatalytic degradation ratio of MEL, the experiments were carried out at constant condition as following: sonocatalyst dosage (1 g/L), ultrasonic power intensity (0.278 W/cm2) and drug concentration (10 mg/L). It could be seen from Fig. 5a that as the duration of ultrasonic irradiation time increased, the degradation ratio of MEL increased. The degradation efficiency of MEL reached 76.9% at 120 min. It was speculated from the results that under the action of ultrasound, more free radicals were generated in the solution, which caused oxidative decomposition of MEL. In order to explore the reaction system and ratios constant, the kinetics of sonic-catalyzed degradation were also investigated. The firstorder dynamics mode was often used to evaluate the kinetic behavior when the concentration of the drug was low [9]. The curve of the firstorder kinetic reaction was calculated based on ln (C0/Ct) versus ultrasound irradiation time (t). Where Ct was the concentration of MEL after certain time (t) under the ultrasonic irradiation, and the C0 was the initial concentration of MEL solution. As can be seen from Fig. 5b, the curve was approximately linear (R = 0.9934) which indicated that the sonic-catalyzed degradation reaction conformed to the first-order reaction kinetics ln (C0/Ct) = 0.0136 t − 0.2303, and the rate constant was 0.0136 min−1.
3.7. Influence of the initial concentration of MEL Some literature revealed that initial organic concentration might influence degradation [37]. In the experiment, the effects of different MEL concentrations (6–18 mg/L) on degradation ratio were investigated and all the other conditions kept unchanged. It could be seen from Fig. 6b the degradation ratio was firstly increased at low initial concentrations (less than 10 mg/L) then decreased at higher initial concentrations (range at 10–18 mg/L) with the increase of MEL concentration. For a given experimental conditions (20 mg catalyst dosage, 1.00 g/L ZnWO4, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, 40 kHz frequency), the appropriate initial concentration (10.0 mg/L) could attain a high degradation ratio (75.9%) of MEL. In addition, the degradation amount of MEL increased with the augment of initial concentration. Because the higher initial concentration could provide the driving force to overcome the mass transfer resistance
Fig. 5. The influence of ultrasonic irradiation time on the degradation ratio of MEL (a) and kinetics of the sonocatalytic degradation of MEL (b). Initial MEL concentration = 10.0 mg/L, catalyst dosage = 20 mg. 5
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Fig. 6. The influence of sonocatalyst dosage (a), initial concentration of MEL (b) and reusability of the ZnWO4 (c) on the degradation ratio of MEL. (Ultrasonic power intensity = 0.278 W/cm2).
generated ROS, thus the absorbance of MEL was obviously decreased, and the corresponding degradation ratio was estimated to be 70%. After adding D-Man, the degradation reached to 30%, which indicated the existence of %OH. After adding L-His, the absorbance value of MEL was 0.42, indicating the existence of 1O2 in addition to %OH. When VC was added to block all ROS, the absorbance value was 0.48. This experiment showed that at least two ROS (%OH, 1O2) were produced in the process of ultrasonic catalysis of ZnWO4.
of drug [38], more drugs were allowed to reach the surface of the acoustic catalyst and thus be adsorbed and degraded. 3.8. Reusability The sonocatalysts reusability after four sonocatalytic cycles were tested, where, 10.0 mg/L of MEL initial concentration, 20.0 mg of sonocatalyst, 0.278 W/cm2 of ultrasonic power intensity and irradiation time of 120 min were used as experimental conditions. After each cycle, the sonocatalyst was washed with distilled water and anhydrous ethanol alternately, then dried at 80 °C for successive reuse. The results were shown in Fig. 6c. It could be seen that the reused catalyst showed a little decrease in the degradation efficiency of the catalytic activity from run1 (76.1%) to run4 (53.7%). These observations indicated that the structure of ZnWO4 was relatively stable under the reaction conditions and might not be seriously affected by the reactants environment and ultrasound, hence its acoustic catalytic activity was relatively stable [39]. Considering the above results, it could be concluded that ZnWO4 could be safely used for four cycles.
3.10. Possible mechanism in the process of sonocatalytic degradation In this study, ultrasonic was used to replace light to excite ZnWO4. It is well known that sonocatalytic degradation undergoes a number of important processes. Firstly, under the irradiation of ultrasonic, tiny bubbles in the solution would be produced. After nucleation, they collapsed. This process was called ultrasonic cavitation effect [40](Step I in Fig. 8) [37]. Secondly, the cavitation effect would generate sonoluminescence and hot spot (local high temperature (5000 K) and high pressure (1000 atm) regions). Sonoluminescence could result in the formation of the light flash of average photon energy of 6 eV to excite ZnWO4 [40]. Thus, the e− on valence band of ZnWO4 could be transitted to conduction band and the h+ are generated on VB at the same time, forming e− – h+ pairs. The nano-microspheres structure of obtained ZnWO4 (pH = 6) possessed the largest BET surface area (27.068 m2/g) which could provide more active sites on its surface, thus, more e− and h+ could be produced. Meanwhile, in the extreme environment of high temperature and high pressure, the water vapor in the cavitation bubble was cracked into free radicals such as HO2% and % O, some of which would be combined to form H2O2. However, through H2O2, we could get more %OH and other ROS with strong oxidability (Step П in Fig. 8). The detailed producing of h+ and ROS were shown in the following:
3.9. Detection of reactive oxygen species (ROS) It has been proved in the literature that in the process of ultrasonic catalytic degradation, a large amount of ROS will be produced which could oxidize and decompose compounds into carbon dioxide and water [40,41]. In order to quantitatively study the species of ROS, three ROS scavengers were added: D-Man (%OH scavenger), L-His (1O2 and % OH scavengers) and VC (all ROS scavengers). Fig. 7 presented the UV–Vis absorption spectra of MEL at different conditions. It could be seen from Fig. 7A that under stirring, there was a slight decrease in absorption peak for MEL + Vc/D-Man/L-His solutions compared to origin MEL solution. As depicted in Fig. 7B, under the synergistic effect of ultrasound and ZnWO4, MEL was oxidized and decomposed by all
Fig. 7. UV–vis spectra of MEL solutions at the different conditions. ([MEL] = 10.0 mg/L, [ZnWO4] = 1.00 g/L, US/stirring time = 120 min, Ultrasonic power intensity (0.278 W/cm2) intensity) = 0.278 W/cm2.) 6
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Fig. 8. Proposed mechanism for sonocatalytic degradation of MEL in presence of ZnWO4.
Catalyst +))) → Catalyst(h+) + Catalyst(e−) h + H2O → HO% + H +
the highest.
+
3.12. Comparison of sonocatalytic with photocatalytic and sonophotocatalytic efficiencies of ZnWO4
e− + O2 →%O2− H2O + ))) → HO% + H+
Fig. 9b showed the photocatalytic/sonocatalytic/sonphotocatalytic degradation ratios of MEL under irradiation with ZnWO4 synthesized at pH of 6. As can be seen from Fig. 9b, the degradation ratio of MEL increased with reaction time for all the three experimental methods and the order was sonphotocatalytic activity > sonocatalytic activity ≫ photocatalytic activity. The sonocatalytic effect of ZnWO4 was significant but photocatalytic activity of ZnWO4 under simulated sunlight was much weaker (ranging from 5.0% to 9.0%). ZnWO4 in photocatalytic experiment mainly absorbed high energy ultraviolet light (254 nm) [44,45], which accounted for less than 5% of the output intensity of simulated sunlight. Less light intensity excited ZnWO4 and less photo-induced electrons and holes was obtained [46]. Therefore, the photocatalytic activity of ZnWO4 under simulated sunlight was weak. In the sonophotocatalytic experiment, the degradation ratio of MEL was a little higher than sonocatalytic experiment, but there was no obvious synergistic effect.
O2 + ))) →%O %O2− + H+ → HO2% HO% + %O → HO2% 2HO2% → O2 + H2O2 2H2O2 + %O2− → %2OH + 2OH− + O2 Thirdly, all kinds of ROS and h+ with strong oxidation ability degraded the surrounding MEL molecules, eventurally, to CO2, H2O and inorganic molecules, resulted in a decrease in the peak of UV–visible absorption of MEL (Step Ш in Fig. 8). 3.11. The contrastive degradation ratio of different compounds In this section, ZnWO4 were used to investigate its sonocatalytic degradation performance on several kinds of organic compounds (their names, structures and detection wavelength are shown in Table S1). These organic pollutants selected as degradation models were identified in varying concentrations in natural waters [42,43]. In the experiment, the dosage of ZnWO4 was 20.0 mg (1.0 g/L), the compounds concentration was 20 mg/L. The adsorption/degradation ratio of each compounds are shown in Fig. 9a. It could be seen from Fig. 9a that ZnWO4 could degrade these compounds under US irradiation to some extent. The order of percentage of adsorption degradation was: MB > CIP > LEV > AMA > MEL. The results showed that the cationic MB/CIP in aqueous solution with smaller molecular weight was more easily adsorbed by ZnWO4, the adsorption capacity of ZnWO4 for LEV was in the middle, and ZnWO4 has the weakest adsorption capacity for anionic AMA/MEL. Hence, simply from the point of view of adsorption, the catalyst ZnWO4 is more likely to adsorb positively charged compounds on the surface in the solution. Meanwhile, the order of degradation percentage was: MB > MEL > CIP > AMA > LEV. Due to the different structure of compounds, ROS produced in catalytic process have different destructive effects on different structures. As far as MEL was concerned, its benzothiazine ring was more susceptible to ROS’s attack and degradation. Therefore, the degradation ratio of MEL was
4. Conclusions In conclusion, ZnWO4 were synthesized by hydrothermal method and for the first time used as sonocatalyst. ZnWO4 nano/micro particles were obtained by changing five acidity conditions and characterized by XRD, SEM, EDS and nitrogen adsorption–desorption isotherm measurements. Interestingly, the sonocatalytic activity of ZnWO4 on degradation of MEL varied with the synthetic pH values, with ZnWO4 under pH = 6 exhibiting the best sonocatalytic performance. Under the condition of 10 mg/L MEL concentration, 20 mg catalyst dosage, 120 min irradiation time, 0.278 W/cm2 ultrasonic power intensity, the degradation ratio on MEL have reached 75.7%. At this synthesized pH value, nano-microspheres structure of ZnWO4 displayed the smallest particle size (40–60 nm), the largest BET surface area (27.068 m2/g) and more active site to generate ROS and h+, which were responsible for the excellent sonocatalytic performances. In addition, measurements of sonocatalytic effect indicated that the degradation efficiency of MEL increased with the increase of US power and time. Results of dynamics studies demonstrated that the sonocatalytic degradation conforms to the first-order reaction kinetic equation and the rate constant is 0.0136 min−1. The mechanism of the sonocatalytic process was 7
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Fig. 9. The contrastive degradation ratio of different organic dyes (a) and the contrastive degradation ratio of different catalytic way (b).
suggested with regard to sequence of events: nucleation, sonoluminescence, hot spot and generation of ROS/h+. Finally, sonphotocatalytic effect of ZnWO4 was slightly better than that of sonocatalytic effect and sonocatalytic effect was much higher than that of photocatalytic effect under simulated sunlight. This result suggested a new way for the harmless treatment of drugs using ZnWO4 as an sonocatalyst in aqueous media.
[13]
[14]
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Acknowledgements
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The authors greatly acknowledge the research project of education department of Liaoning province (LQN201715) for financial supports.
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Appendix A. Supplementary data [18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.104815.
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