Preparation of Ce4+-doped BaZrO3 by hydrothermal method and application in dual-frequent sonocatalytic degradation of norfloxacin in aqueous solution

Ultrasonics - Sonochemistry 42 (2018) 356–367

Contents lists available at ScienceDirect

Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Preparation of Ce4+-doped BaZrO3 by hydrothermal method and application in dual-frequent sonocatalytic degradation of norfloxacin in aqueous solution ⁎

Hongbo Zhanga, Jing Qiaoa, Guanshu Lib, Siyi Lia, Guowei Wanga, Jun Wanga, , Youtao Songb, a b

T

⁎

College of Chemistry, Liaoning University, Shenyang 110036, PR China College of Environment, Liaoning University, Shenyang 110036, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Dual-frequent ultrasonic irradiation Sonocatalyst (Ce4+-doped BaZrO3) Sonocatalytic degradation Norfloxacin (NOR) Reactive oxygen species (ROS)

In this paper, the dual-frequent sonocatalytic degradation of norfloxacin (NOR), an antibiotic, caused by Ce4+doped BaZrO3 is studied. The used Ce4+-doped BaZrO3 as a novel sonocatalyst with highly efficient and stable sonocatalytic activity is prepared via hydrothermal method. The prepared sample is characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectra (DRS) and Fourier transform infrared spectra (FT-IR) in order to investigate the structure, morphology and chemical composition. The dual-frequent sonocatalytic activity of prepared Ce4+-doped BaZrO3 powder is evaluated through sonocatalytic degradation of norfloxacin (NOR) as a model organic pollutant. Some influencing factors such as single/dual-frequent ultrasonic frequent, cerium and zirconium molar proportions, ultrasonic irradiation time and used times are studied in detail by using UV–vis spectra. The generated reactive oxygen species (ROS) during the dual-frequent sonocatalytic degradation process of norfloxacin (NOR) are confirmed by using two different trapping agents. The holes (h+) and hydroxyl radicals (%OH) are identified and the holes plays a major role during the oxidation process. Finally, the possible mechanism for the dual-frequent sonocatalytic degradation of norfloxacin (NOR) caused by Ce4+-doped BaZrO3 is proposed. The experimental results show that the Ce4+-doped BaZrO3 displays a good sonocatalytic activity under dual-frequent ultrasonic irradiation. Under optimal conditions, the most of norfloxacin (NOR) can be removed under dual-frequent ultrasonic irradiation for 150 min.

1. Introduction Nowadays, many pharmaceutical products could be detected in the effluent of pharmaceutical industry and river water [1]. The illegally discharged pharmaceutical products are also considered to be the emerging pollutants in environment [2]. Among these pharmaceutical products, the antibiotics are regarded as common micro-pollutants [3,4]. In general, antibiotics have widely been applied in the treatments of various infections for human body and animals due to the broadspectrum antibacterial activity [5]. The accidental or long-term overuse of antibiotics has caused several serious problems such as damage of human organism, decrease of the function of biological community diversity and increase of drug resistance and so on [6,7]. If these antibiotics in water and environment were not properly and timely disposed, they would not only destroyed environment and ecosystems system, but also threatened human health through drinking water [8]. In the past, some treatment techniques of antibiotics were used in

⁎

practice, such as coagulation [9], adsorption [10], air floatation [11] and biological treatment methods [12] and so on. However, there are some disadvantages and limitations for these traditional treatment methods, for example, high cost investment, low efficiency, uncertain therapeutic effects and incomplete decomposition [13]. None of these methods can effectively dispose the antibiotics, at the same time the use of these methods often leads to the second pollution. Considering the above negative factors, the recently proposed sonocatalytic degradation based on semiconductor catalyst, as a novel Advanced Oxidation Process, is regarded to be a feasible method for the treatment of wastewaters containing toxic organic pollutants, especially, non- or lowtransparent and high concentration effluents [14,15]. Compared with the traditional approaches, sonocatalytic degradation method has many advantages such as easy operation, strong penetrability and low investment [16]. The sonocatalytic degradation of organic pollutants roots in the ultrasonic cavitation effect in aqueous medium. Ultrasonic cavitation effect can produce hot spots (> 5000 K)

Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Wang), [email protected] (Y. Song).

https://doi.org/10.1016/j.ultsonch.2017.11.043 Received 25 August 2017; Received in revised form 27 November 2017; Accepted 28 November 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

and high pressure (> 1000 atm) cause sonoluminescence with wide wavelength of lights [17]. The high temperature can cause the thermal dissociation of water molecules to produce a small number of highly oxidative hydroxyl radicals (%OH) and hydrogen radicals (%H) [18]. The hydrogen radicals (%H) react with oxygen (O2) dissolved in aqueous solution, forming superoxygen radical anions (%O2−) [16]. These produced reactive oxygen species (ROS) can degrade organic pollutant molecules, but the degradation ratio is very low due to limited reactive oxygen species (ROS). Nevertheless, when some semiconductor oxides are used as catalysts, they can be effectively activated by appropriate wavelength of lights in sonoluminescence resulted from cavitation effect. That is, under ultrasonic irradiation, the electrons (e−) on valence band (VB) of semiconductor oxides can transit to conduction band (CB) and the holes (h+) are generated on valence band (VB) at the same time, forming electrons (e−)-holes (h+) pairs [19]. The generated holes (h+) can directly degrade organic pollutants [20]. The generated electrons (e−) react with oxygen (O2) dissolved in aqueous solution, producing the superoxygen radical anions (%O2−). Up till now, the used irradiation source in sonocatalytic degradation of organic pollutants generally is single-frequent ultrasound [16,17,21], which may generate weak cavitation effect and low excitation energy. Compared with single-frequent ultrasound, the dual-frequent ultrasound can intensify the cavitation effect [22]. When two different frequent ultrasound waves are used in the same medium, the superposition phenomenon can occur in the focal acoustic field [23]. Such superposition produces the large amplitude vibration waves in ultrasound wave spreading process, which promotes the increase of cavitation bubble number and kind [24]. Therefore, the dual-frequent ultrasonic irradiation can generate wider wavelength range and stronger high-energy lights. The use of dual-frequent ultrasonic is more beneficial to activate wide band semiconductor sonocatalysts. In actual applications, it is vitally important to choose an effective and stable semiconductor oxide as sonocatalyst for better carrying out the sonocatalytic degradation of organic pollutants under ultrasonic irradiation, especially, to use wide band-gap semiconductor. The utilization of some zirconate compounds in photocatalytic field has been reported for their excellent catalytic activity [25,26]. Barium zirconate (BaZrO3) possesses a wide band-gap of 5.05 eV, which have a valence band with strong positive potential and a conduction band with strong negative potential [27]. However, for some semiconductor sonocatalysts, the easy recombination of photogenerated electrons (e−) and photogenerated holes (h+) decreases the sonocatalytic degradation activity of semiconductor sonocatalysts. Some studies showed that doping of metal ions (for example: rare earth Ce4+ ion) can restrain the recombination of photogenerated electron (e−) – hole (h+) [28–30]. The rare earth metal ions with 4f electron configurations can act as highly efficient electron reservoirs to trap electrons in photocatalytic reaction process [31]. It also induces the red shift of the absorption edge and allows the absorption of visible light through the charge transfer or transition of d and f orbital electrons [32]. In addition, the rare earth metal ions as dopants may introduce some new energy levels between the valence band and conduction band of the semiconductor [33]. Among these doped rare earth metal ions, the valent state of cerium can convert between Ce3+ in Ce2O3 and Ce4+ in CeO2 due to its unique Ce3+/Ce4+ redox couple [34]. Moreover, the different electronic structures of Ce3+ with 4f15d0 and Ce4+ with 4f05d0 are the origin of distinct optical properties, which also leads to high charge mobility [35]. Therefore, a new sonocatalyst, Ce4+-doped BaZrO3, can be obtained, when the cerium as dopant is added into BaZrO3. This new sonocatalyst can perform the effective sonocatalytic degradation of antibiotics. In this study, dual-frequent sonocatalytic degradation of antibiotics caused by Ce4+-doped BaZrO3 was studied. The used Ce4+-doped BaZrO3 powders were prepared through hydrothermal method. The sample structure and composition were characterized by various techniques and the sonocatalytic activity of Ce4+-doped BaZrO3 was

Scheme 1. Molecular structure of norfloxacin (NOR).

studied through sonocatalytic degradation of norfloxacin (NOR) under dual-frequent ultrasonic irradiation. Meanwhile, some main influence factors on the sonocatalytic degradation activity of Ce4+-doped BaZrO3 were investigated. Furthermore, the generated reactive oxygen species (ROS) during the dual-frequent sonocatalytic degradation process of norfloxacin (NOR) was confirmed by using two different trapping agents and the holes was the major reactive species during the oxidation process. The mechanism of dual-frequent sonocatalytic degradation process of Ce4+-doped BaZrO3 was also proposed. 2. Experimental 2.1. Materials and reagents Barium chloride dihydrate (BaCl2·2H2O, analytical purity) and Zirconium oxychloride (ZrOCl2·8H2O, analytical purity) (Sinopharm Chemical Regent Co, Ltd, China) were used to prepare the pure BaZrO3. Cerium nitrate hexahydrate (Ce(NO3)3·6H2O) (99.5%, Sinopharm Chemical Regent Co, Ltd, China) as doping ions was used to prepare Ce4+-doped BaZrO3. Norfloxacin (98% purity, Sinopharm Chemical Regent Co, Ltd, China) (as shown in Scheme 1) was used to undergo the sonocatalytic degradation experiment under dual-frequent ultrasonic irradiation. All the reagents were of analytical purity grade, and were directly used without further purification. 2.2. Preparation of Ce4+-doped BaZrO3 as sonocatalysts The pure and Ce4+-doped BaZrO3 powders were prepared by using hydrothermal method. The synthetic reaction occurred as follows:

BaCl2·2H2 O+ ZrOCl2·8H2 O+ 4NaOH → BaZrO3 + 4NaCl + 12H2 O

BaCl2·2H2 O+ ZrOCl2·8H2 O+ Ce(NO3)3·6H2 O+ 4NaOH → BaCe x Zr1 −x O3 + 4NaCl + 18H2 O 4.42 g BaCl2·2H2O and equimolar mixture of ZrOCl2·8H2O and Ce (NO3)3·6H2O powders, whose molar proportions of Ce and Zr were adopted as 0.000:1.000, 0.016:0.984, 0.032:0.968, 0.048:0.952 and 0.064:0.936, respectively, were dissolved into 20 mL distilled water with magnetic stirring and heated until transparent. 15 mol/L NaOH solution was dropped to the above transparent solution with stirred vigorously and then precipitated hydroxides were formed. The mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 180 °C for 12 h. The products were washed with distilled water and ethanol several times, and dried at 80 °C overnight. Finally, the different molar proportions of Ce4+-doped BaZrO3 particles were obtained and stored for the sonocatalytic degradation. 2.3. Characterization of prepared Ce4+-doped BaZrO3 powders The prepared Ce4+-doped BaZrO3 particles as sonocatalysts were characterized by X-ray powder diffractometer (XRD, D-8, Bruker-axs, Germany, Ni filtered Cu Kα radiation in the range of 2θ from 20° to 80°) and scanning electron microscopy (SEM, JEOL JSM-5610LV, Hitachi Corporation, Japan) to determine the crystalline phase and surface 357

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

Scheme 2. Apparatus of dual-frequent ultrasonic irradiation.

morphology. Energy dispersive X-ray analysis (EDX, JEOL JSM5610LV, Hitachi Corporation, Japan) and X-ray photoelectron spectroscopy (XPS, Escalab 250XI, Thermo, USA) were used to determine the element types and composition content of prepared Ce4+-doped BaZrO3. UV–vis diffuse reflectance spectra (DRS, UV-3600, SHIMADZU, Japan) was used measure the optical diffuse absorption spectra of prepared pure BaZrO3 and different molar proportions of Ce4+-doped BaZrO3. FTIR spectroscopy (FT-IR, Nicolet 5700, Beckman Coulter, USA) was used to further confirm the chemical composition of prepared Ce4+-doped BaZrO3. UV–vis spectrometer (Cary 50, Varian Company, USA) was used to inspect the sonocatalytic degradation processes of norfloxacin (NOR) in aqueous solution. Controllable dual-frequent combined Ultrasonics apparatus (XH-2008DE, Beijing Xianghu Science and Technology Development Company, China) and Serial-Ultrasonics apparatus (KQ-300, Kunshan Company, China) were adopted to irradiation the norfloxacin (NOR) solution, operating at ultrasonic frequent of 40 KHz, 25 KHz and 40 KHz + 25 KHz, output power of 50 W through numerical control adjust. Apparatus of ultrasonic irradiation was shown in Scheme 2. 2.4. Measurements of sonocatalytic activity of prepared Ce4+-doped BaZrO3 Sonocatalytic degradation experiments of norfloxacin (NOR) was carried out in a 100 mL erlenmeyer flask placed in an ultrasonic irradiation apparatus (40 KHz + 25 KHz, 300 (length) × 180 mm (width) × 120 mm (height), KQ-300 (50 W, six transducers), Kunshan ultrasonic apparatus Company and XH-2008DE, Beijing Xianghu Science and Technology development limited company, China) under air atmosphere. In the common experiment, 1.00 g/L Ce4+-doped BaZrO3 powders and 10.00 mg/L norfloxacin (NOR) concentration in 100 mL total volume were adopted to perform the sonocatalytic degradation. Before ultrasonic irradiation, the above solution was stirred for 30 min in the dark to achieve adsorption–desorption equilibrium. And then, the degradation of norfloxacin (NOR) was performed under dual-frequent ultrasonic irradiation for 150 min. Every 30 min, a certain of sample was took out from irradiated solution and its UV–vis spectrum was measured. The dual-frequent sonocatalytic degradation ratios of norfloxacin (NOR) were determined from the absorbance change by using the following equation:

Fig. 1. XRD patterns of prepared BaZrO3 powder (a), standard card of pure BaZrO3 (JCPDS 06-0399) (b) and (0.064:0.936) Ce4+-doped BaZrO3 powder (c).

the sonocatalytic degradation of norfloxacin (NOR) in aqueous solution. In addition, the mechanism of dual-frequent sonocatalytic degradation of organic contaminant caused by prepared Ce4+-doped BaZrO3 was investigated in detail by using isopropanol and EDTA as trapping agents.

Degradation ratio(%) = [C0−Ct ]/Co × 100 where C0 is the initial concentration of norfloxacin (NOR) solution and At is the instant concentration after a certain of ultrasonic irradiation time (t). Synchronously, the some influencing factors such as Ce and Zr molar proportions and ultrasonic frequent on the sonocatalytic activity of prepared Ce4+-doped BaZrO3 powders were also examined through

3. Results and discussion 3.1. XRD patterns of prepared pure BaZrO3 and Ce4+-doped BaZrO3 powders Fig. 1 shown the X-ray diffraction (XRD) patterns of prepared pure 358

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

composition of BaZrO3. It demonstrates that the prepared sample should be BaZrO3. From Fig. 3(b–e), all element peaks of Ce4+-doped BaZrO3 can be found clearly. The percents of Ba, Zr, O and Ce are close to the composition of Ce4+-doped BaZrO3. These results are consistent with the analyses of XRD above as well.

BaZrO3 (a), standard card of BaZrO3 (JCPDS 06-0399) (b) and prepared (0.064:0.936) Ce4+-doped BaZrO3 powders (c). From Fig. 1(a) it can be seen that the main characteristic diffraction peaks of prepared pure BaZrO3 appear at 2θ = 30.04°, 42.96°, 53.30°, 62.22° and 70.98°, corresponding with (1 1 0), (1 1 1), (2 0 0), (2 1 1), (2 2 0) and (0 1 3) planes [36]. All peaks of prepared pure BaZrO3 can be indexed to the standard card of BaZrO3 (JCPDS 06-0399) in Fig. 1(b) [33]. From Fig. 1(c), no new diffraction peak was found in doped (0.064:0.936) Ce4+-doped BaZrO3 sample, which indicates that the doped Ce ions into the BaZrO3 do not cause any significant change in the crystal structure. However, it can be found that the intensity of diffraction peaks for (0.064:0.936) Ce4+-doped BaZrO3 decline after increasing a certain amount of Ce, which demonstrates some changes in the size of crystallites. In comparison to pure BaZrO3, the slight shift of diffraction peaks of (0.064:0.936) Ce4+-doped BaZrO3 occurred toward the higher angle [37]. The diffraction pattern also indicates that the samples are monophasic in nature. The sharp and high peaks of the samples indicate the high degree of crystallinity for the pure BaZrO3 and (0.064:0.936) Ce4+-doped BaZrO3 particles.

3.4. XPS spectra of prepared Ce4+-doped BaZrO3 powder In order to confirm the composition and analyze chemical bonds, XPS spectrum of the prepared (0.064:0.936) Ce4+-doped BaZrO3 was measured in the range of binding energy from 1200 eV to 0 eV. From Fig. 4(o), it can be found that three strong peaks appear at 779.11 eV, 181.64 eV and 530.65 eV, respectively, which should belong Ba (3d), Zr (3d) and O (1s) [36]. It indicates that the prepared sample contains Ba, Zr and O elements. In addition, a weak peak is also found at 899.32 eV, which belongs to Ce (3d) [38]. It indicates that Ce4+ ions have been doped into BaZrO3, replacing partial Zr4+ ions. From Fig. 4(a–d), the characteristic peaks of Ce, Ba, O and Zr can be clearly observed, respectively, which is consistent with the composition of Ce4+-doped BaZrO3. High resolution XPS spectrum (Fig. 4(a)) of Ce gives multiple peaks of Ce (3d), which is due to multiplet splitting effects, relating to the different oxidation states of Ce [39]. The peaks originated from Ce4+ appear at 882.3 eV and 888.9 eV from Ce (3d5/2) and 898.2 eV, 899.32 eV and 916.8 eV from Ce (3d3/2) [40], respectively. The signal of Ba (3d) in spectrum (Fig. 4(b)) is divided into two peaks, where Ba (3d5/2) peak with lower binding energy and Ba (3d3/2) peak with higher binding energy locate at 779.11 eV and 793.7 eV [38], respectively. It can be confirmed that, in Ce4+-doped BaZrO3, the binding energy of Ba (3d) is consistent with the divalent oxidation state of Ba [36]. In O (1s) spectrum (Fig. 4(c)), two peaks are situated at 529.12 eV and 530.65 eV, respectively. The lower binding energy is attributed to O(Ce/Zr) bonds and the higher binding energy belongs to Ba-O bonds. For Zr (3d3/2) and Zr (3d5/2) (Fig. 4(d)), their binding energy are located at 181.64 eV and 184.87 eV, respectively. The obtained data are consistent with the tetravalent oxidation state of Zr [36]. Based on XPS, the calculated atomic percents of Ba, Zr, O and Ce are 15.05%, 11.39%, 67.8% and 5.76%, respectively. They are close to the composition of (0.064:0.936) Ce4+-doped BaZrO3. The adsorbed water from atmosphere causes slightly high content of O. Due to the efficient charge transfer between adjacent components, all peaks of the sample emerge with slight shift, which demonstrates the formation of corresponding chemical bonds in prepared (0.064:0.936) Ce4+-doped BaZrO3.

3.2. SEM images of prepared pure BaZrO3 and Ce4+-doped BaZrO3 powders The scanning electron microscopic (SEM) images of prepared pure BaZrO3 (a) and (0.064:0.936) Ce4+-doped BaZrO3 (b) powders were used to investigate the shape, size and morphology in this section. In Fig. 2(a) a large number of approximately sphere particles with size range of 500–800 nm can be seen, which apparently are the prepared BaZrO3 particles. It also can be found that these approximate sphere particles have unsmooth surface. From Fig. 2(b) it can be found that there are a lot of sphere particles with diameter ranging from 1.0 μm to 1.2 μm, which should belong to the micron-sized (0.064:0.936) Ce4+doped BaZrO3. Compared with the BaZrO3 in Fig. 2(a), the surface color of (0.064:0.936) Ce4+-doped BaZrO3 particles is apparently deepened. The doping of Ce4+ ions into BaZrO3 lattice sites is the main reason for change of surface structure. 3.3. EDX spectra of prepared pure BaZrO3 and Ce4+-doped BaZrO3 powders In order to investigate the chemical composition, the EDX spectra of prepared pure BaZrO3 (a) and Ce4+-doped BaZrO3 (b-e) powders with different molar proportions (0.000:1.000, 0.016:0.984, 0.032:0.968, 0.048:0.952 and 0.064:0.936) are determined and the results are shown in Fig. 3. In Fig. 3(a), it can be seen that three kinds of elements exist in sample, which are Ba, Zr and O, respectively. The percents of the elements are Ba: 19.64%, Zr: 16.92% and O: 63.44%, respectively. The atomic ratio of Ba, Zr and O is very close to the standard 1:1:3

3.5. UV–vis diffuse reflectance spectra of prepared pure BaZrO3 and Ce4+doped BaZrO3 powders with different Ce and Zr molar proportions The UV–vis diffuse reflectance spectra of the prepared pure BaZrO3 and Ce4+-doped BaZrO3 powders with different Ce and Zr molar

Fig. 2. SEM images of prepared BaZrO3 (a) and (0.064:0.936) Ce4+-doped BaZrO3 (b) powders.

359

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

also estimated by using the Tauc relation as showkn in below [33]:

αhν = A(hν−Ebg)1/2 where α, h, ν, Ebg and A represent absorption coefficient, Planck’s constant (6.6260 × 10−34 J·s), photon energy, band gap and proportionality constant, respectively. For all samples, the energy band gaps are obtained from the intercepts of the linear region in the curve of a plot of (αhν)2 on the Y-axis verses photon energy (hν) on the X-axis as shown in Fig. 5. The energy band gap of the pure BaZrO3 is estimated to be 5.05 eV, which is quite close to the value reported in the literature [27], it indicates that the pure BaZrO3 is a wide band semiconductor photocatalyst. As Ce and Zr molar proportions increase, the energy gap decreases, which can be ascribed to increase of lattice defects. Lattice defects increase the intermediary energy levels in the band-gap region of disordered Ce4+-doped BaZrO3 particles. The band gaps of Ce4+doped BaZrO3 are 5.05 eV, 4.85 eV, 4.65 eV, 4.50 eV and 3.82 eV, respectively, for 0.000:1.000, 0.016:0.984, 0.032:0.968, 0.048:0.952 and 0.064:0.936 molar proportions. It proves that adding Ce is helpful for expanding the used lights range of BaZrO3. The probable reasons are as follows: (I) the doping of Ce ions could induce slight lattice distortion, which will create some structural defects [41]. (II) due to minor difference of the ionic radii between Ce4+ (0.87 Å) and Zr4+ (0.72 Å), Zr4+ ion can be replaced by Ce4+ within the crystal lattice, which will also cause the change of lattice parameters [36]. That is, because Zr4+ is slightly smaller than Ce4+, the lattice parameters display slight increase. They lead to the variations of the unit cell volume, micro-strain and dislocation density [36,42]. It is demonstrated in Fig. 5 that the energy band-gap of Ce4+-doped BaZrO3 apparently decreases compared with that of pure BaZrO3. 3.6. FT-IR spectroscopy of prepared pure BaZrO3 and Ce4+-doped BaZrO3 powders with different Ce and Zr molar proportions FT-IR spectroscopy analyses were carried out in the range of 4000–400 cm−1 to further confirm the binding characteristics and chemical composition of prepared samples. Fig. 6(a–e) gives the FTIR spectra of pure BaZrO3 powder and Ce4+-doped BaZrO3 samples with different molar proportions of Ce and Zr. For all the samples, an intense and broad absorption band around 3450 cm−1 and a weak absorption band around 1620 cm−1 can be seen and they are assigned to OeH stretching vibration and bending vibration. It indicates the existence of OeH [43]. The skeletal vibration spectra of Ce4+-doped BaZrO3 with different molar proportions give a broad absorption band around 520 cm−1, belonging to Zr-O bonds [44]. With the increase of doped Ce amount, the absorption peak of Zr-O was shifted from 520 cm-1 to 480 cm-1. The peak shift may be due to the increment of cation vacancy in lattice, which indicates that doped ions (Ce4+) occupy the sites those previously belonged to the zirconium ions (Zr4+) and create new bonds with the surrounding oxygen atoms. In theory, the ionic radius (0.87 Å) of Ce4+ is relatively close to that (0.72 Å) of Zr4+. Therefore, Zr4+ can be replaced by Ce4+. In addition, the presence of Ba-O bond gives rise to an absorption peak within a range of 1450–1400 cm−1 [45]. All these observed peaks can confirm the formation of Ce4+-doped BaZrO3 particles.

Fig. 3. EDX spectra of BaZrO3 (a), (0.016:0.984) (1.62%) Ce4+-doped BaZrO3 (b), (0.032:0.968) (3.30%) Ce4+-doped BaZrO3 (c), (0.048:0.952) (5.00%) Ce4+-doped BaZrO3 (d) and (0.064:0.936) (6.84%) Ce4+-doped BaZrO3 (e) powders.

proportions were measured at room temperature in the range of wavelength from 200 nm to 800 nm. Fig. 5 shows that the BaZrO3 has absorption at 250 nm in ultraviolet region. When different amounts of Ce are doped into BaZrO3, a new absorption edge appears approximately 350–500 nm. Also, it can be found that the absorption peak at 350–500 nm of Ce4+-doped BaZrO3 generally rises along with increasing molar proportion of Ce and Zr from 0.000:1.000 to 0.064:0.936. It indicates that the increases of molar proportions of Ce and Zr from 0.000:1.000 to 0.064:0.936 are helpful for expanding absorption range of BaZrO3. In Fig. 5, the energy band gaps (Ebg) of the as-prepared samples are

3.7. Comparison of sonocatalytic degradation ratios of norfloxacin (NOR) under dual-frequent and single-frequent ultrasonic irradiations The UV–vis spectra (in the wavelength range from 200 nm to 800 nm) of norfloxacin (NOR) solution (10.00 mg/L) under single-frequent (25 kHz and 40 kHz) and dual-frequent (25 kHz + 40 kHz) ultrasonic irradiations were given in Fig. 7(a). For UV–vis spectrum of original norfloxacin (NOR) solution, it can be observed that there is a strong peak at the wavelength of 272 nm, which belongs to the π-π∗ electron transition of double bond, conjugated double bonds or benzene ring. In addition, it can be seen that there are two relatively weak peaks 360

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

Fig. 4. XPS spectra of (0.064:0.936) Ce4+-doped BaZrO3 powder (a) and high resolution XPS spectra of Ce (3d) (b), Ba (3d) (c), O (1s) (d) and Zr (3d) (e).

degradation ratios of norfloxacin (NOR) are much higher than ones under single-frequent (25 kHz or 40 kHz) ultrasonic irradiation. It indicates that the prepared Ce4+-doped BaZrO3 under dual-frequent (40 + 25 kHz) ultrasonic irradiation displays much higher photocatalytic activity. In addition, for three courses three main peaks of norfloxacin (NOR) solutions all show synchronous decrease, which indicates that the norfloxacin (NOR) molecules can be completely destroyed once under ultrasonic irradiation, and that the intermediate products in sonocatalytic degradation process may not be generated. Therefore, dual-frequent sonocatalytic degradation can effectively and completely decompose norfloxacin (NOR). The influence of ultrasonic irradiation time on sonocatalytic degradation of norfloxacin (NOR) under dual-frequent and single-frequent ultrasonic irradiations was shown in Fig. 7(b). It can be seen that the

at the wavelength of 323 nm and 327 nm, respectively, which belong to the n-π∗ electron transitions of some heteroatoms with lone pair electrons in the double bond. Under single-frequent (25 kHz and 40 kHz) and dual-frequent (25 kHz + 40 kHz) ultrasonic irradiations, three main peaks of norfloxacin (NOR) in aqueous solutions all decrease obviously in the presence of Ce4+-doped BaZrO3. After 150 min, the corresponding sonocatalytic degradation ratios based on the absorption peak at wavelength of 272 nm are 38.29%, 48.38% and 69.81%, respectively, for 25 kHz, 40 kHz and 40 + 25 kHz. Based on the peak at wavelength of 323 nm, they are 37.54%, 61.03% and 69.92%, respectively, for 25 kHz, 40 kHz and 40 + 25 kHz. And based on the peak at wavelength of 327 nm, they are 37.01%, 61.33% and 70.01%, respectively, for 25 kHz, 40 kHz and 40 + 25 kHz. Apparently, under dualfrequent (40 + 25 kHz) ultrasonic irradiation, the sonocatalytic 361

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

Fig. 5. UV–vis diffuse reflectance spectra (DRS) of prepared Ce4+-doped BaZrO3 powders with different Ce and Zr molar proportions.

3.8. Influences of Ce and Zr molar proportions and ultrasonic irradiation time on sonocatalytic degradation of norfloxacin (NOR) and corresponding reaction kinetics

sonocatalytic degradation ratios of norfloxacin (NOR) all rise along with increasing ultrasonic irradiation time for three frequent ultrasonic irradiation. It can be clearly found that the sequence based on the sonocatalytic degradation ratios were 25 kHz < 40 kHz < 40 + 25 kHz at any ultrasonic irradiation moment. After 150 min ultrasonic irradiation, the degradation ratios of norfloxacin (NOR) reach 38.29%, 48.38% and 69.81%, respectively, for 25 kHz, 40 kHz and 40 + 25 kHz. It further illustrates that the sonocatalytic degradation of norfloxacin (NOR) is very effective in the presence of Ce4+-doped BaZrO3 under dual-frequent ultrasonic irradiation. The dual-frequent sonocatalytic activity of Ce4+-doped BaZrO3 is higher than the single-frequent sonocatalytic activity.

The influences of different Ce and Zr molar proportions (0.000:1.000, 0.016:0.984, 0.032:0.968, 0.048:0.952 and 0.064:0.936) in Ce4+-doped BaZrO3 powders on dual-frequent sonocatalytic degradation of norfloxacin (NOR) were shown in Fig. 8(a). It can be seen that, for all courses in the absence of any catalyst and presences of different molar proportions of Ce4+-doped BaZrO3 powders under dualfrequent ultrasonic irradiation, the degradation ratios of norfloxacin (NOR) rise with increasing ultrasonic irradiation time. However, for onefold dual-frequent ultrasonic irradiation without any catalyst, the 362

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

proportion of Ce and Zr is increased from 0.032:0.968 to 0.048:0.952, the sonocatalytic degradation ratio of norfloxacin (NOR) is obviously increased. It indicates that a suitable Ce doping amount (0.048:0.952) can give the highest sonocatalytic degradation ratio of norfloxacin (NOR). It is the reason that (0.048:0.952) Ce4+-doped BaZrO3 is selected as sonocatalyst in all experiments. However, the degradation ratio of norfloxacin (NOR) using ultrasonic irradiation alone is only 15.11%. It illustrated that onefold ultrasonic irradiation has a low degradation capability to norfloxacin (NOR). As shown in Fig. 8(b), it can be clearly seen that, in the absence and presence of the prepared Ce4+-doped BaZrO3 powders with different molar proportions, three main absorption peaks of norfloxacin (NOR) solutions showed synchronous decline under ultrasonic irradiation for 150 min. It indicates that any norfloxacin (NOR) molecules can be destroyed thoroughly by Ce4+-doped BaZrO3 once under dual-frequent ultrasonic irradiation. The degradation ratios of norfloxacin (NOR) based on the absorbance changes of three main peaks have been calculated in Table (in Fig. 8(b)). From Table (in Fig. 8(b)), it can be found that, in the presence of the prepared (0.048:0.952) Ce4+-doped BaZrO3 powders, the degradation ratios of norfloxacin (NOR) based on three main peaks are very close. However, the degradation ratios of norfloxacin (NOR) based on three main peaks are obviously different for other sonocatalysts. It indicates that, for (0.048:0.952) Ce4+-doped BaZrO3 powders, intermediate products may not be generated in degradation process. As shown in Fig. 8(c), for (0.048:0.952) Ce4+-doped BaZrO3 system, three main peaks of norfloxacin (NOR) gradually decrease with the extension of ultrasonic irradiation time. The corresponding degradation ratios based on the absorption peak at wavelength of 272 nm are 5.40%, 18.10%, 37.17%, 43.18%, 50.73% and 69.81%, respectively, for 0 min, 30 min, 60 min, 90 min, 120 min and 150 min. The degradation ratios of norfloxacin (NOR) based on the absorbance changes of three main peaks have been calculated in Table (in Fig. 8(c)). For (0.048:0.952) Ce4+-doped BaZrO3 system, it can be found that the degradation ratios of norfloxacin (NOR) based on three main peaks are approximate at the same time. It also indicates that the norfloxacin (NOR) molecules have been completely destroyed and intermediate products may not be generated. For the sake of further discussing the sonocatalytic degradation ratios of norfloxacin (NOR) caused by these sonocatalysts, the simulative first-order reaction kinetics was constructed. The simulative first-order model is very acceptant and intelligible, so the reaction rates can be compared directly by reaction rate constants (k). From Fig. 8(d) it can be seen that all −ln(Ct/ Co) = −kt data were calculated for first-order reaction. All calculated values of −ln(Ct/Co) are approximately linear relationships with the

Fig. 6. FT-IR spectra of BaZrO3 and Ce4+-doped BaZrO3 powders with different Ce and Zr molar proportions (0.000:1.000 (a), 0.016:0.984 (b), 0.032:0.968 (c), 0.048:0.952 (d) and 0.064:0.936 (e)).

degradation ratio of norfloxacin (NOR) increases only slightly. In the presence of Ce4+-doped BaZrO3, the degradation ratio of norfloxacin (NOR) increases obviously, which indicates that the norfloxacin (NOR) in aqueous solution can be effectively decomposed by Ce4+-doped BaZrO3 as sonocatalyst under dual-frequent ultrasonic irradiation. In addition, at the same time, the dual-frequent sonocatalytic degradation ratios of norfloxacin (NOR) rise with increasing Ce and Zr molar proportions. Under dual-frequent ultrasonic irradiation for 150 min, the sonocatalytic degradation ratios of norfloxacin (NOR) are 28.89%, 32.05%, 46.80%, 69.81% and 75.90%, respectively, for 0.000:1.000, 0.016:0.984, 0.032:0.968, 0.048:0.952 and 0.064:0.936 Ce4+-doped BaZrO3. Obviously, for pure BaZrO3, the sonocatalytic degradation ratio of norfloxacin (NOR) is very low, indicates the low sonocatalytic activity of pure BaZrO3. It can be found that the doping of Ce ions enhance the sonocatalytic activity of BaZrO3. When the molar

Fig. 7. UV–vis spectra of norfloxacin (NOR) solution (a) and comparison of sonocatalytic degradation ratios of norfloxacin (NOR) under dual-frequent and single-frequent ultrasonic irradiations (b) (1.00 g/L (0.048:0.952) Ce4+-doped BaZrO3 powder, 10.00 mg/L norfloxacin (NOR) solution, 25 °C temperature, 100 mL total volume, 150 min ultrasonic irradiation, 300 W output power and 40 KHz + 25 KHz frequent).

363

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

Fig. 8. The influence of Ce and Zr molar proportions (a) in Ce4+-doped BaZrO3 on dual-frequent sonocatalytic degradation of norfloxacin (NOR), UV–vis spectra of norfloxacin (NOR) in aqueous solution at 300 min irradiation time (b) under different ultrasonic irradiation time (c) and corresponding reaction kinetics (d). (1.00 g/L Ce4+-doped BaZrO3 powder, 10.00 mg/L norfloxacin (NOR), 25 °C temperature, 100 mL total volume, 150 min ultrasonic irradiation, 300 W output power and 40 KHz + 25 KHz frequency.)

ultrasonic irradiation time (t). The kinetic equations corresponding to US, US + (0.000:1.000) BaZrO3, US + (0.016:0.984) Ce4+-doped BaZrO3, US + (0.032:0.968) Ce4+-doped BaZrO3, US + (0.048:0.952) Ce4+-doped BaZrO3 and US + (0.064:0.936) Ce4+-doped BaZrO3 are −ln(Ct/ C0) = 0.0322x − 0.0483 (R2 = 0.9482), −ln(Ct/C0) = 0.0705x − 0.0793 (R2 = 0.9822), −ln(Ct/C0) = 0.0754x − 0.0604 (R2 = 0.9903), −ln(Ct/ C0) = 0.1252x − 0.08 (R2 = 0.9869), −ln(Ct/C0) = 0.2142x − 0.2228 (R2 = 0.9461) and −ln(Ct/C0) = 0.2489x − 0.2506 (R2 = 0.9344), respectively. The rate constants were 0.0322 min−1, 0.0705 min−1, 0.0754 min−1, 0.1252 min−1 and 0.2142 min−1, respectively, for US, US + (0.000:1.000) BaZrO3, US + (0.016:0.984) Ce4+-doped BaZrO3, US + (0.032:0.968) Ce4+-doped BaZrO3, US + (0.048:0.952) Ce4+-doped BaZrO3 and US + (0.064:0.936) Ce4+-doped BaZrO3. Thus, the order of dual-frequent sonocatalytic degradation of norfloxacin (NOR) for all courses can be judged as US < US + (0.000:1.000) BaZrO3 < US + (0.016:0.984) Ce4+-doped BaZrO3 < US + (0.032:0.968) Ce4+-doped BaZrO3 < US + (0.048:0.952) Ce4+-doped BaZrO3 < US + (0.064:0.9 36) Ce4+-doped BaZrO3. 3.9. Influence of used times of Ce4+-doped BaZrO3 on dual-frequent sonocatalytic degradation of norfloxacin (NOR)

Fig. 9. The influences of sonocatalyst used times on dual-frequent sonocatalytic degradation of norfloxacin in aqueous solution. (1.00 g/L (0.048:0.952) Ce4+-doped BaZrO3 powder, 10.00 mg/L norfloxacin solution, 25 °C temperature, 100 mL total volume, 150 min ultrasonic irradiation, 300 W output power and 40 KHz + 25 KHz frequency.)

The reusability of sonocatalyst is one important factor for estimating the stability and performance in real application. Thus, the influence of reused times on dual-frequent sonocatalytic activity of Ce4+-doped BaZrO3 powder was investigated through degradation of norfloxacin (NOR) in aqueous solutions. From Fig. 9, it can be seen that the dual-

frequent sonocatalytic degradation ratios of norfloxacin (NOR) just slightly decline along with the increase of used times in presence of (0.048:0.952) Ce4+-doped BaZrO3. The corresponding degradation ratios are 69.81%, 66.64%, 62.71% and 57.85%, respectively, for four 364

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

cycles in order. For fourth times used, the prepared (0.048:0.952) Ce4+doped BaZrO3 still displays a relatively high sonocatalytic activity under dual-frequent ultrasonic irradiation. The results demonstrate that the dual-frequent sonocatalytic activity of the prepared Ce4+-doped BaZrO3 is steady and the Ce4+-doped BaZrO3 as sonocatalyst can be reused many times. Therefore, the Ce4+-doped BaZrO3 powder as sonocatalyst combined with dual-frequent ultrasonic irradiation can be effectively applied to treat various organic pollutants.

3.10. Influence of trapping agents on dual-frequent sonocatalytic degradation of norfloxacin caused by Ce4+-doped BaZrO3 powder In order to understand the reaction mechanism of dual-frequent sonocatalytic degradation of norfloxacin (NOR) caused by Ce4+-doped BaZrO3, it is very crucial to identify the formation of hydroxyl radicals (%OH) and holes (h+). In recent years, many researchers have reported that ethylene diamine tetraacetic acid (EDTA) is hole (h+) trapping agent and isopropyl alcohol (iso-IPA) is hydroxyl radicals (%OH) trapping agent [45,46]. Therefore, in this part, EDTA and iso-IPA were chosen as trapping agents of holes (h+) and hydroxyl radicals (%OH), respectively to determine the formation of hydroxyl radicals (%OH) and holes (h+). The molar proportions of trapping agents and norfloxacin (NOR) were changed from 0:1 to 15:1 and a series of experiments were carried out. All systems were performed under the same experimental conductions. From Fig. 10 it can be found that the dual-frequent sonocatalytic degradation ratios of norfloxacin (NOR) apparently decrease along with increasing molar proportions of EDTA and norfloxacin (NOR). The degradation ratios are 69.81%, 58.47%, 48.26% and 23.90%, respectively, for 0:1, 1:1, 5:1 and 15:1 molar proportions. However, when the iso-IPA as trapping agent of hydroxyl radicals (% OH) was added, the dual-frequent sonocatalytic degradation ratios of norfloxacin (NOR) decrease slightly and are 69.81%, 68.40%, 67.41% and 60%, respectively, for 0:1, 1:1, 5:1 and 15:1 molar proportions. Therefore, it can be concluded that the dual-frequent sonocatalytic degradation of norfloxacin (NOR) were resulted from hole (h+) oxidation on valence band of Ce4+-doped BaZrO3. In addition, the hydroxyl radicals (%OH) oxidation plays a secondary role. Thus, it follows that the dual-frequent sonocatalytic degradation is through the combined action of holes (h+) and hydroxyl radicals (%OH), but mainly caused by the hole (h+) oxidation.

Fig. 11. Sonocatalystic degradation principle of organic pollutant in the presence of Ce4+-doped BaZrO3 powder under dual-frequent ultrasonic irradiation.

3.11. Possible mechanism and process on dual-frequent sonocatalytic degradation of norfloxacin (NOR) caused by Ce4+-doped BaZrO3 powder In order to better understand the reaction process of dual-frequent sonocatalytic degradation of organic pollutants caused by prepared Ce4+-doped BaZrO3 powder, the investigation on related mechanism is necessary. Based on the research results above mentioned, the possible mechanism of dual-frequent sonocatalytic degradation of organic pollutants was proposed as illustrated in Fig. 11. Generally, when liquid solution is irradiated by ultrasound, the miraculous cavitation effect will occur. The detailed process of cavitation effect involves the forming and expanding of cavitation bubbles and aggregating of acoustic energy. Finally, these cavitation bubbles fast collapse and burst [47]. In the bursting process of cavitation bubbles, the high temperatures, high pressures and sonoluminescence can be produced. At the same time, a lot of energy is released. The produced high temperature (> 5000 K) and high pressure (> 1000 atm) sufficiently break down some water molecules (H2O), producing hydroxyl radicals (%OH) and hydrogen radicals (%H) [18]. The hydrogen radicals (%H) react with oxygen (O2) dissolved in aqueous solution, forming the superoxygen radical anions (%O2−) [16]. The generated reactive oxygen species (ROS) can degrade surrounding organic pollutant molecules. Nevertheless, the degradation efficacy of organic pollutants is very low under onefold ultrasonic irradiation due to limited cavitation bubbles. The some semiconductor oxides (for example: TiO2, ZnO and ZrO2) can be used as sonocatalysts to enhance the degradation ratio of organic pollutants under ultrasonic irradiation. Being similar to photocatalytic reaction, this process is called as sonocatalytic reaction. The sonocatalytic degradation of organic pollutants is concerned with both ‘‘hot spot” and sonoluminescence caused by ultrasonic cavitation [15]. Sonoluminescence process can produce the lights with wide wavelength range. However, the wide band-gap semiconductor catalyst can be excited only by high-energy ultraviolet lights in sonoluminescence. For prepared BaZrO3 as a wide band-gap (Ebg = 5.05 eV) semiconductor catalyst, under ultrasonic irradiation the electrons (e−) on valence band (VB) can transfer to conduction band (CB) and the holes (h+) are produced on valence band (VB) at the same time, forming photogenerated electrons (e−)-holes (h+) pairs. Through a series of chemical reactions, these high active electrons (e−) and holes (h+) can generate various reactive oxygen species (ROS) to carry out the sonocatalytic degradation of organic pollutants [18]. The doping of Ce4+ ions into BaZrO3 can restrain the recombination of photogenerated electrons (e−) and holes (h+). In Ce4+-doped BaZrO3, the Ce4+ ion can get an electron (e−) (pathway 1) and then is reduced to Ce3+ ion (pathway 2). The generated Ce3+ ion as electron donors easily gives electron and is oxidized by oxygen (O2) dissolved in aqueous solution (pathway 3), forming Ce4+ ion. In sonocatalytic degradation of organic pollutants, the circular reaction of Ce3+/Ce4+ is continuously carried out. The electrons (e-) from Ce3+ ions react with

Fig. 10. The influence of different molar proportions of trapping agents and norfloxacin (NOR) on dual-frequent sonocatalytic degradation of norfloxacin (NOR) in aqueous solution. (1.00 g/L (0.048:0.952) Ce4+-doped BaZrO3 powder, 10.00 mg/L norfloxacin (NOR) solution, 25 °C temperature, 100 mL total volume, 150 min ultrasonic irradiation, 300 W output power and 40 KHz + 25 KHz frequency. t: trapping agent and n: norfloxacin.)

365

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

frequent ultrasonic irradiation. The prepared (0.048:0.952) Ce4+doped BaZrO3 powder displays a good sonocatalytic activity during degradation of norfloxacin (NOR) under dual-frequent ultrasonic irradiation. The dual-frequent sonocatalytic degradation ratio can reach 69.81% for 10 mg/L initial concentration of norfloxacin (NOR), 1.00 g/ L (0.048:0.952) Ce4+-doped BaZrO3 addition amount, 150 min dualfrequent ultrasonic irradiation and 300 W ultrasonic power. The experimental results indicate that, in the presence of Ce4+-doped BaZrO3, the sonocatalytic degradation ratio of norfloxacin (NOR) under dualfrequent ultrasonic irradiation is apparently better than that under single-frequent ultrasonic irradiation. The Ce4+ ion as dopant can slightly decrease the band gap of BaZrO3, which leads to a wide light response range of Ce4+-doped BaZrO3 powders. Also, the doped Ce4+ ion can efficiently restrain recombination of photogenerated electrons (e−) and holes (h+). The trapping agent experiments show that the dual-frequent sonocatalytic degradation is through a combined action of holes (h+) and hydroxyl radicals (%OH), but mainly caused by the holes (h+) oxidation.

the molecular oxygen (O2) dissolved in aqueous solution, producing the superoxygen radical anions (%O2−). The superoxygen radical anions (% O2−) become the hydroxyl radicals (%OH) through a series of chemical reactions [48,49]. The high active electrons (e−) from conduction band (CB) are continuously consumed through reacting with molecular oxygen (O2). Apparently, the presence of Ce4+ ions can promote the separation of photogenerated electrons (e−) and holes (h+). The generated holes (h+) can directly degrade organic pollutants, leading to volatile degradation by-products or CO2, H2O and mineral acids through entire mineralization. In general, the wide band-gap semiconductor catalysts have high catalytic activity in the degradation of organic pollutants. However, because of the wide band-gap (Ebg = 5.05 eV), BaZrO3 only can use the high energy ultraviolet light in sonoluminescence. Nevertheless, the high energy ultraviolet lights in sonoluminescence are only a small part. In order to effectively excite the wide band-gap BaZrO3 to carry out sonocatalytic degradation of organic pollutants, the sufficient high energy ultraviolet lights is necessary. The use of dual-frequent ultrasound can further intensify cavitation effect. When two different frequent ultrasound waves are used in the same medium, the superposition phenomenon can occur in the focal acoustic field [23]. Such superposition produces the large and wider amplitude vibration waves in ultrasound wave spreading process, which promotes the increase of cavitation bubble number and kind. Apparently, the increase of cavitation bubble number can enhance the intensity of sonoluminescence, and that the increase of cavitation bubble kind can broaden the wavelength range of sonoluminescence. Therefore, the use of dual-frequent ultrasonic irradiation can generate much wider wavelength range and much stronger high-energy lights. The possible process is thought as following: Dual-frequent ultrasonic irradiation (cavitation effect) → light (sonoluminescence) + heat (hot spot)

Acknowledgements The authors greatly acknowledge the National Science Foundation of China (21371084 and 31570154), Major science and technology project of water pollution control and management in China (2015ZX07202-012) and Key Laboratory Basic Research Foundation of Liaoning Provincial Education Department (L2015043) for financial support. The authors also thank our colleagues and other students for their participating in this work. References [1] S.B. Hu, L. Li, M.Y. Luo, Y.F. Yun, C.T. Chang, Aqueous norfloxacin sonocatalytic degradation with multilayer flower-like ZnO in the presence of peroxydisulfate, Ultrason. Sonochem. 38 (2017) 446–454. [2] S.X. Zuo, Y. Chen, W.J. Liu, C. Yao, X.Z. Li, Z.Y. Li, C.Y. Ni, X.H. Liu, A facile and novel construction of attapulgite/Cu2O/Cu/g-C3N4 with enhanced photocatalytic activity for antibiotic degradation, Ceram. Int. 43 (2017) 3331–3336. [3] A. Speltini, M. Sturini, F. Maraschi, A. Profumo, A. Albini, Microwave-assisted extraction and determination of enrofloxacin and danofloxacin phototransformation products in soil, Anal. Bioanal. Chem. 404 (2012) 1565–1569. [4] M. Sturini, A. Speltini, F. Maraschi, E. Rivagli, L. Pretali, L. Malavasi, A. Profumo, E. Fasani, A. Albini, Sunlight photodegradation of marbofloxacin and enrofloxacin adsorbed on clay minerals, J. Photo. Photobiol. A: Chem. 299 (2015) 103–109. [5] J.M. Greenhalgh, J.R. Edwards, A comparative study of the in vitro activity of meropenem and representatives of the major classes of broad-spectrum antibiotics, Clin. Microbiol. Infect. 3 (1997) 20–31. [6] F. Liu, J. Wu, G.G. Ying, Z. Luo, H. Feng, Changes in functional diversity of soil microbial community with addition of antibiotics sulfamethoxazole and chlortetracycline, Appl. Microbiol. Biotechnol. 95 (6) (2012) 1615–1623. [7] W.D. Kong, Y.G. Zhu, B.J. Fu, P. Marschner, J.Z. He, The veterinary antibiotic oxytetracycline and Cu influence functional diversity of the soil microbial community, Environ. Pollut. 143 (2006) 129–137. [8] K.R. Solomon, D.G. Hillis, L. Lissemore, P.K. Sibley, Risks of agricultural pharmaceuticals in surface water systems and soils, ACS. Symp. 1108 (2009) 191–204. [9] T. Saitoh, K. Shibata, K. Fujimori, Y. Ohtani, Rapid removal of tetracycline antibiotics from water by coagulation-flotation of sodium dodecyl sulfate and poly(allylamine hydrochloride) in the presence of Al(III) ions, Sep. Purif. Technol. 187 (2017) 76–83. [10] T.J. Al-Musawi, F. Brouers, M. Zarrabi, Kinetic modeling of antibiotic adsorption onto different nanomaterials using the Brouers-Sotolongo fractal equation, Environ. Sci. Pollut. Res. 4 (24) (2017) 4048–4057. [11] J. Chen, Y.S. Liu, J.N. Zhang, Y.Q. Yang, L.X. Hu, Y.Y. Yang, J.L. Zhao, F.R. Chen, G.G. Ying, Removal of antibiotics from piggery wastewater by biological aerated filter system: treatment efficiency and biodegradation kinetics, Bioresour. Technol. 238 (2017) 70–77. [12] R.B.P. Marcelino, M. Leão, R.M. Lago, C.C. Amorim, Multistage ozone and biological treatment system for real wastewater containing antibiotics, J. Environ. Manage. 195 (2017) 110–116. [13] E. Ghasemi, H. Ziyadi, A.M. Afshar, M. Sillanpää, Ron oxide nanofibers: a new magnetic catalyst for azo dyes degradation in aqueous solution, Chem. Eng. J. 264 (2015) 146–151. [14] G.S. Li, H.B. Zhang, C.S. Wei, Y.Y. Huang, X.K. Dou, Y.D. Wang, J. Wang, Y.T. Song, Preparation of (5.0%) Er3+:Y3Al5O12/Pt-(TiO2-Ta2O5) nanocatalysts and application in sonocatalytic decomposition of ametryn in aqueous solution, Ultrason. Sonochem. 34 (2017) 763–773. [15] J. Wang, Z. Jiang, Z.H. Zhang, Y.P. Xie, X.F. Wang, Z.Q. Xing, R. Xu, X.D. Zhang,

H 2O + heat (hot spot) →%OH + %H %H + O

2

→%O

2

−

+H

+

Ce4 +-doped BaZrO3 + light → [Ce4 +-doped BaZrO3] ∗

Ce4 +−doped BaZrO3 → h+/VB/Ce4 +−doped BaZrO3 + e−/CB/Ce4 + −doped BaZrO3 h+/VB/Ce4 +−doped BaZrO3 + e−/CB/Ce4 +−doped BaZrO3 → light + heat Organic pollutants(NOR) + h+/VB/BaZrO3 → D+ → →D++ → →→CO2 + H2 O+ NO−3 + F−

e−/CB/Ce4 +-doped BaZrO3 → Ce3 +-doped BaZrO3 Ce

3+

%O HO

2 2

H 2O

-doped BaZrO

−

+H

+

3

→HO

+O

2

→%O

2

−

+ Ce

4+

-doped BaZrO

3

2

+ H 2O →H 2O2 + %OH 2

+ heat (hot spot) →2%OH

Organic pollutants (NOR) + %OH →D+ →→D++ →→→CO + NO3− + F −

2

+ H 2O

4. Conclusions In this work, the Ce4+-doped BaZrO3 as a new sonocatalyst was prepared by using hydrothermal method. And then, the sonocatalytic activity of Ce4+-doped BaZrO3 was evaluated through the sonocatalytic degradation of norfloxacin (NOR) in aqueous solution under dual366

Ultrasonics - Sonochemistry 42 (2018) 356–367

H. Zhang et al.

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23] [24]

[25]

[26]

[27] [28]

[29]

[30]

[31]

[32]

[33] R.B. Urby, L.A.D. Torres, P. Salas, C.A. Chavez, O. Mez, Strong broad green UVexcited photoluminescence in rare earth (RE = Ce, Eu, Dy, Er, Yb) doped barium zirconate, Mater. Sci. Eng: B 176 (2011) 1388–1392. [34] N.Z. Yang, R.T. Guo, W.G. Pan, Q.L. Chen, Q.S. Wang, C.Z. Lu, S.X. Wang, The deactivation mechanism of Cl on Ce/TiO2 catalyst for selective catalytic reduction of NO with NH3, Appl. Surf. Sci. 378 (2016) 513–518. [35] S. Chong, G. Zhang, N. Zhang, Y. Liu, T. Huang, H. Chang, Diclofenac degradation in water by FeCeOx catalyzed H2O2: Influencing factors, mechanism and pathways, J. Hazard. Mater. 334 (2017) 150–159. [36] D. Sharma, R.K. Upadhyay, B. Satpati, V.R. Satsangi, R. Shrivastav, U.V. Waghmare, S. Dass, Electronic band-offsets across Cu2O/BaZrO3 heterojunction and its stable photo-electro-chemical response: first-principles theoretical analysis and experimental optimization, Renewable Energy 113 (2017) 503–511. [37] G. Kiruthigaa, C. Manoharan, M. Bououdina, S. Ramalingam, C. Raju, Structural, optical and photocatalytic properties of Ce-doped SnS nanoflakes, Solid State Sci. 44 (2015) 32–38. [38] S.K. Jaiswal, J. Hong, K.J. Yoon, J. Son, H. Lee, J. Lee, Optical absorption and XPS studies of (Ba1-xSrx)(Ce0.75Zr0.10Y0.15)O3-δ electrolytes for protonic ceramic fuel cells, Ceram. Int. 42 (2016) 10366–10372. [39] X.L. Yang, W.Q. Zhang, C.G. Xia, X.M. Xiong, X.Y. Mu, B. Hu, Low temperature ruthenium catalyst for ammonia synthesis supported on BaCeO3 nanocrystals, Catal. Commun. 11 (2010) 867–870. [40] A. Dutta, P.K. Mukhopadhyay, T.P. Sinha, D. Das, S. Shannigrahi, Structural and magnetic properties of double perovskite oxide Ba2CeSbO6, Solid State Sci. 58 (2016) 64–69. [41] H. Eskandarloo, A. Badiei, M.A. Behnajady, A. Tavakoli, G.M. Ziarani, Ultrasonicassisted synthesis of Ce doped cubic-hexagonal ZnTiO3 with highly efficient sonocatalytic activity, Ultrason. Sonochem. 29 (2016) 258–269. [42] Y. Chen, K.R. Liu, Fabrication of magnetically recyclable Ce/N co-doped TiO2/ NiFe2O4/diatomite ternary hybrid: Improved photocatalytic efficiency under visible light irradiation, J. Alloy Compd. 697 (2017) 161–173. [43] J.H. Xu, J. Xiang, H. Ding, T.Q. Yu, J.L. Li, Z.G. Li, Y.W. Yang, X.L. Shao, Synthesis and electrical properties of BaCeO3-based proton conductors by calcinations of metal-polyvinyl alcohol gel, J. Alloy Compd. 551 (2013) 333–337. [44] E. Finocchio, H.A. Alessandro, M. Videla, S. Specchia, Surface chemistry and reactivity of Pd/BaCeO3·2ZrO2 catalyst upon sulphur hydrothermal treatment for the total oxidation of methane, Appl. Catal., A 505 (2015) 183–192. [45] Y. Parganiha, J. Kaur, V. Dubey, R. Shrivastava, S.J. Dhoble, Synthesis and luminescence study of BaZrO3:Eu3+ phosphor, Superlattices Microstruct. 88 (2015) 262–270. [46] Y.Y. Huang, G.W. Wang, H.B. Zhang, G.S. Li, D.W. Fang, J. Wang, Y.T. Song, Hydrothermal-precipitation preparation of CdS@(Er3+:Y3Al5O12/ZrO2) coated composite and sonocatalytic degradation of caffeine, Ultrason. Sonochem. 37 (2017) 222–234. [47] C. Yang, Q.S. Li, L.M. Tang, K. Xin, A.L. Bai, Y.M. Yu, Synthesis, photocatalytic activity and photogenerated hydroxyl radicals of monodisperse colloidal ZnO nanospheres, Appl. Surf. Sci. 357 (2015) 1928–1938. [48] Y.Y. Huang, H.B. Zhang, C.S. Wei, G.S. Li, Q. Wu, J. Wang, Y.T. Song, Assisted sonocatalytic degradation of pethidine hydrochloride (dolantin) with some inorganic oxidants caused by CdS-coated ZrO2 composite, Sep. Purif. Technol. 172 (2017) 202–210. [49] H.B. Zhang, Y.Y. Huang, G.S. Li, G.W. Wang, D.W. Fang, Y.T. Song, J. Wang, Preparation of Er3+:Y3Al5O12/WO3-KNbO3 composite and application in treatment of methamphetamine under ultrasonic irradiation, Ultrason. Sonochem. 35 (2017) 478–488.

Sonocatalytic degradation of acid red B and rhodamine B catalyzed by nano-sized ZnO powder under ultrasonic irradiation, Ultrason. Sonochem. 15 (2008) 768–774. H.B. Zhang, C.S. Wei, Y.Y. Huang, G.S. Li, Q. Wu, J. Wang, Y.T. Song, Preparation of Er3+:Y3Al5O12/KNbO3 composite and application ininnocent treatment of ketamine by using sonocatalytic decomposition method, J. Hazard. Mater. 317 (2016) 667–676. Y.L. Pang, A.Z. Abdullah, Comparative study on the process behavior and reaction kinetics in sonocatalytic degradation of organic dyes by powder and nanotubes TiO2, Ultrason. Sonochem. 19 (2012) 642–651. R.A. Torres, F. Abdelmalek, E. Combet, C. Petrier, C. Pulgarin, A comparative study of ultrasonic cavitation and Fenton’s reagent for bisphenol A degradation in deionised and natural waters, J. Hazard. Mater. 146 (2007) 546–551. L.N. Yin, J.Q. Gao, J. Wang, B.X. Wang, R.Z. Jiang, K. Li, Y. Li, X.D. Zhang, Enhancement of sonocatalytic performance of TiO2 by coating Er3+:YAlO3 in azo dye degradation, Sep. Purif. Technol. 81 (2011) 94–100. J. Wang, Z. Jiang, L.Q. Zhang, P.L. Kang, Y.P. Xie, Y.H. Lv, R. Xu, X.D. Zhang, Sonocatalytic degradation of some dyestuffs and comparison of catalytic activities of nano-sized TiO2, nano-sized ZnO and composite TiO2/ZnO powders under ultrasonic irradiation, Ultrason. Sonochem. 16 (2009) 225–231. J. Wang, S.Y. Zhou, J. Wang, S.G. Li, J.Q. Gao, B.X. Wang, P. Fan, Improvement of sonocatalytic activity of TiO2 by using Yb, N and F-doped Er3+:Y3Al5O12 for degradation of organic dyes, Ultrason. Sonochem. 21 (2014) 84–92. S.F. Xiong, Z.L. Yin, Z.F. Yuan, W.B. Yan, W.Y. Yang, J.J. Liu, F. Zhang, Dual-frequency (20/40 kHz) ultrasonic assisted photocatalysis for degradation of methylene blue effluent: Synergistic effect and kinetic study, Ultrason. Sonochem. 19 (2012) 756–761. Z. Eren, N.H. Ince, Sonolytic and sonocatalytic degradation of azo dyes by low and high frequency ultrasound, J. Hazard. Mater. 177 (2010) 1019–1024. L. Zhao, J. Ma, X.D. Zhai, Enhanced mechanism of catalytic ozonation by ultrasound with orthogonal dual frequency for the degradation of nitrobenzene in aqueous solution, Ultrason. Sonochem. 17 (2010) 84–91. S.Z. Ajabshir, M.S. Niasari, Photo-catalytic degradation of erythrosine and eriochrome black T dyes using Nd2Zr2O7 nanostructures prepared by a modified Pechini approach, Sep. Purif. Technol. 179 (2017) 77–85. X. Chen, Y.T. Liu, X.N. Xia, L.L. Wang, Popcorn balls-like ZnFe2O4-ZrO2 microsphere for photocatalytic degradation of 2,4-dinitrophenol, Appl. Surf. Sci. 407 (2017) 470–478. D. Kim, S. Miyoshi, T. Tsuchiya, S. Yamaguchi, Percolation conductivity in BaZrO3BaFeO3 solid solutions, Solid State Ionics 262 (2014) 875–878. W.T. Zhang, J. Ma, Y. Gao, D.S Zhou, X.F. Chen, Improved electrochemical performance of Li2Fe1-xCexSiO4/C cathode material via cerium ions doping for lithiumion batteries, Mater. Chem. Phys. 198 (2017) 83–89. R. Watanabe, Y. Saito, C. Fukuhara, Enhancement of ethylbenzene dehydrogenation of perovskite-type BaZrO3 catalyst by a small amount of Fe substitution in the Bsite, J. Mol. Catal. A: Chem. 405 (2015) 57–64. T. Wang, W.C. Yang, T.T. Song, C.F. Li, L.Y. Zhang, H.Y. Wang, L.Y. Chai, Cu doped Fe3O4 magnetic adsorbent for arsenic: synthesis, property, and sorption application, RSC Adv. 5 (2015) 50011–50018. M.S. Quezada, G.R. Ortiz, V. Suárez, G.M. Mendoza, L.L. Rojas, E.N. Cerón, F. Tzompantzi, S. Robles, R. Gómez, A. Mantilla, Photodegradation of phenol using reconstructed Ce doped Zn/Al layered double hydroxides as photocatalysts, Catal. Today 271 (2016) 213–219. C. Belver, J. Bedia, M.A. Álvarez-Montero, J.J. Rodriguez, Solar photocatalytic purification of water with Ce-doped TiO2/clay heterostructures, Catal. Today 266 (2016) 36–45.

367