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Sonocatalytic degradation of diclofenac with FeCeOx particles in water
Ultrasonics Sonochemistry 34 (2017) 418–425
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Sonocatalytic degradation of diclofenac with FeCeOx particles in water Shan Chong, Guangming Zhang ⇑, Zhongheng Wei, Nan Zhang, Ting Huang, Yucan Liu School of Environment & Natural Resource, Renmin University of China, Beijing 100872, China
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Article history: Received 18 May 2016 Received in revised form 18 June 2016 Accepted 18 June 2016 Available online 20 June 2016 Keywords: Diclofenac FeCeOx Ultrasound Oxygen vacancy Singlet oxygen
a b s t r a c t This paper studies the sonocatalytic degradation of diclofenac in water using FeCeOx-catalyzed ultrasound. The effects of pre-adsorption and gas addition were investigated. Nitrogen adsorption/desorption, SEM, XRD, Raman and XPS analyses of FeCeOx before and after sonication were characterized. The proposed mechanism was based on the microstructure changes of FeCeOx and reactive-species-scavenging performances. The results show that FeCeOx has excellent performance in catalyzing an ultrasonic system in water, and 80% of diclofenac was removed in 30 min ([Diclofenac] = 20 mg/L, FeCeOx amount = 0.5 g/L, pH = 6, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K). The Fe, Ce, and O elements remained highly dispersed in the structure of FeCeOx, and the solid solution structure of FeCeOx remained stable after the reaction. Ce (III) was gradually oxidized to Ce (IV) and Fe (III) was gradually reduced to Fe (II) after the reaction, which indicates that Fe and Ce ions with different valences coexisted in dynamic equilibrium. The amount of oxygen vacancies in FeCeOx significantly decreased after the reaction, which indicates that oxygen vacancy participated in the ultrasonic process. Singlet oxygen 1O2 was the primary reactive species in the degradation process, and the hydroxyl radicals OH and superoxide radical anion O2 also participated in the reaction. FeCeOx had excellent chemical stability with negligible leaching ions in the ultrasonic process. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Many pharmaceuticals have been detected in surface water, ground water and even drinking water and are considered ‘‘emerging pollutants” because of their adverse effects to human and aquatic organisms [1,2]. Diclofenac is one of the most frequently detected emerging pollutants in the aquatic environment [3]. The compound is a widely used non-steroidal anti-inflammatory pharmaceutical to relieve analgesic, antiarthritic and antirheumatic pains [4]. Exposure to diclofenac in aquatic environments leads to liver and kidney function damage in fish [5], and adverse effects have been found in birds as a result of diclofenac accumulation in the tissues of organisms [6]. Some studies have shown that most pharmaceuticals are partially removed in conventional water treatment processes; in particular, the removal efficiencies for diclofenac in wastewater treatment plants are typically 21–40% [7]. The limited removal effects signify the necessity of an innovative and effective diclofenac treatment method in water. Advanced oxidation processes (AOPs) have received considerable attention for the removal of pharmaceuticals from water [8– ⇑ Corresponding author. E-mail addresses: [email protected] (S. Chong), [email protected] (G. Zhang), [email protected] (Z. Wei), [email protected] (N. Zhang), [email protected] (T. Huang), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.ultsonch.2016.06.023 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
11]. Ultrasonic irradiation has been investigated as an effective AOP for diclofenac [12]. The mechanisms on the sonolysis of diclofenac mainly focused on the acoustic cavitation effect and free radicals effect. Naddeo et al. studied 20 kHz ultrasound-induced degradation of diclofenac and found that diclofenac conversion was improved by enhanced cavitation effect and production of free radicals [13]. The effect of acoustic cavitation plays a dominant role in the ultrasonic process, which involves the formation, expansion and collapse of micro bubbles and generation of high temperatures (up to 5000 K) and high pressures (above 1000 atm) [14]. Water molecules are dissociated into highly reactive species, which play an important role in the ultrasonic process to degrade organic pollutants. Ultrasonic irradiation could generate free radicals which had strong oxidation ability. The free radicals generated by ultrasonic irradiation and ‘‘hot spot” effects could degrade organic pollutants effectively [15]. In addition, the unique features of ultrasonic irradiation are the enhancement of mass transfer and improved surface properties of solid particles by ultrasonic power. Then, the chemical consumption and waste sludge generation rates can be reduced [16]. However, ultrasound alone is highly energy intensive and insufficient to totally degrade pollutants in water. Therefore, catalysts have been introduced to enhance ultrasonic processes to improve the treatment efficiency and reduce energy consumption [17,18]. Wang et al. investigated the sonocatalytic activity of Fe-doped
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mixed crystal TiO2 catalyst on degradation of azo fuchsine and proposed sonoluminescence and ‘‘hot spot” to explain the sonocatalytic degradation process [19]. Wang et al. also investigated the catalytic activities of CeO2/TiO2, SnO2/TiO2 and ZrO2/TiO2 composites on the sonocatalytic degradation of organic dyes and found that restraining the recombination of electron hole pairs was the important mechanism of sonocatalytic degradation in the presence of semiconductor composites [20]. Recently, ceria-based materials have been reported as heterogeneous catalysts for the degradation of organic pollutants in water [21,22]. In particular, Fe-doped CeO2 particles can provide massive active sites and produce large oxygen vacancies, which facilitate the adsorption of pollutant molecules and activation of reactive species during the reaction process [23]. Zou et al. investigated the enhancement of photo activity of transition metal ion (1 mol% Fe, Cu, Mn, and Zn) doped CeO2 nanocatalysts on Rhodamine B degradation [24]. The results showed that the catalytic activity was closely related to the oxygen vacancies and the valence of the doped ions and Fe doped CeO2 could improved separation of photo generated electrons and holes. The feature of Fe-doped CeO2 particles has obvious advantages in the ultrasonic process; the cavitation and interfacial effects are enhanced [25,26]. There are three zones of reaction in the ultrasonic process: cavitation bubble, supercritical interface, and bulk solution in the reaction solution. Thus, such enhancements will benefit the sonocatalytic reaction. In our previous study, Fe-doped CeO2 (FeCeOx) was successfully synthesized by an ultrasonic impregnation method and exhibited an excellent catalytic ability in removing diclofenac in the heterogeneous Fenton process [27]. This work was performed to investigate the application of FeCeOx catalysis in an ultrasonic system. We examined the removal of diclofenac in water using an FeCeOxcatalyzed ultrasonic system and investigated the reaction mechanisms of this sonocatalytic process. N2 adsorption-desorption, Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectrometer (Raman) and X-ray photoelectron spectroscopy (XPS) were used to investigate the changes in morphologic and physical/chemical properties of FeCeOx before and after sonication. The effect of gas addition on the diclofenac degradation efficiency in the ultrasonic process was investigated. To analyze the contributions of hydroxyl radicals (OH), superoxide radical anion (O2 ) and singlet oxygen (1O2) to diclofenac degradation, these active species were scavenged using dimethyl sulfoxide (OH quencher), 1,4-benzoquinone (O2 quencher) and sodium azide (1O2 quencher). The stability of FeCeOx was detected by determining leaching metal ions in the ultrasonic process. 2. Experimental 2.1. Reagents and materials All chemicals in this study were of analytical grade and used without further purification. Ferrous chloride hexahydrate (FeCl24H2O) and sodium hydroxide (NaOH) were purchased from Xilong Chemical Co. Ltd, China. Cerous nitrate (Ce(NO3)36H2O) was purchased from Tianjin Guangfu Fine Chemical Institute, China. Diclofenac was purchased from Tokyo Chemical Industry Co. Ltd, Japan. Sodium azide was purchased from Tokyo USP Co. Ltd, Japan. Dimethyl sulfoxide and 1,4-benzoquinone were obtained from Xilong Chemical Co., Ltd, China. The solutions were prepared with deionized water, which was purified using a Millipore Milli Q UV Plus system. 2.2. Synthesis of FeCeOx CeO2 was prepared using a precipitation method [18]. Ce (NO3)36H2O (0.01 mol) was dissolved in 200 ml of deionized water
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with vigorous mechanical stirring. A stoichiometric quantity of NaOH was added drop-wise and maintained at a reaction temperature of 35 °C. After 4 h, the solution was centrifuged at 3000 rpm for 3 min to separate the products. The settled products were aged for 12 h at room temperature and subsequently dried at 100 °C for 3 h. The resulting CeO2 was dipped in a FeCl24H2O solution (mole ratio of Fe and Ce = 1:9) under ultrasonic irradiation for 20 min, and the mixture was subsequently filtered. The solid was calcined for 2 h at 400 °C in a muffle furnace to obtain the final FeCeOx catalyst. 2.3. Characterization of FeCeOx before and after sonication The specific surface area and porosity of the catalyst were measured based on nitrogen adsorption/desorption isotherms at 77 K on a BeiShiDe 3H-2000PS2 specific surface and pore size analysis instrument. The surface morphology of the samples was investigated using a Hitachi S 4700 scanning electron microscope (SEM) analyzer with a secondary electron detector at different scales and magnifications. The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/max-rc diffractometer with Cu radiation. Raman measurements were performed at room temperature using a Via+ Reflex Raman spectrometer with excitation light of 514 nm. The X-ray photoelectron spectra (XPS) were measured on an EScalab 250Xi spectrometer, which was equipped with an XR6 monochromated X-ray source. 2.4. Determination of diclofenac removal All diclofenac degradation experiments were conducted in 150-ml beakers. The solution was adjusted with diluted NaOH and H2SO4 to the desired initial pH and a final volume of 100 ml. The desired amount of the FeCeOx catalyst was immediately introduced into the reaction solution. The ultrasonic probe was inserted into the beaker with the tail end approximately 1.5 cm below the liquid level. At set intervals, 1.0 ml of supernatant from the sample solution was analyzed immediately after filtration with a 0.45-lm membrane filter. Control experiments were performed under identical conditions. The reaction temperature was controlled by a Yiheng DK-600A constant temperature instrument by circulating water. Diclofenac was determined with a Waters e2695 high-performance liquid chromatography instrument (HPLC), which was equipped with a C18 reversed phase column (4.6 mm 150 mm I.D.). Elution conditions: mobile phase was composed of a 70/30 v/v acetonitrile and acetic acid solution (0.2%); flow rate: 1 ml/min; injection volume: 10 lL; column temperature: 30 °C; kmax: 275 nm. The leached Fe and Ce ions were measured by graphite furnace atomic absorption spectrometric method with a Hitachi Z-2000 Atomic Absorption Spectrophotometer (AAS) using iron lamp and cerium lamp as light resource, respectively. 3. Results and discussion 3.1. Role of FeCeOx under sonication To evaluate the feasibility of using a FeCeOx-catalyzed ultrasonic process to remove diclofenac, the diclofenac removal performance of FeCeOx was compared in the absence and presence of ultrasonic irradiation. Fig. 1 shows that the diclofenac removal efficiencies with ultrasound alone and FeCeOx alone were 22% and 60%, respectively, in 40 min. The diclofenac removal with FeCeOx alone was mainly attributed to the adsorption ability of the material. In the FeCeOx/ultrasound system, an 80% diclofenac removal efficiency was achieved in 30 min, which indicates that FeCeOx
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Fig. 1. Diclofenac removal performance in the absence and presence of ultrasound by FeCeOx. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 5, FeCeOx amount = 0.5 g/L, ultrasonic density = 1.5 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K.
can catalyze the ultrasonic process to remove diclofenac in water. Compared with ultrasound alone and FeCeOx alone, the significantly accelerated removal rate and improved removal efficiency of diclofenac in the FeCeOx/ultrasound combined system show that the presence of the FeCeOx catalyst facilitates the removal process under ultrasonic irradiation. Several studies have investigated diclofenac removal in the ultrasonic process. Nie et al. found that 44%, 29%, 26% and 21% diclofenac degradation efficiencies were achieved in 60 min under argon-, oxygen-, air- and nitrogensaturated conditions, respectively ([Diclofenac] = 0.05 mmol/L, pH = 7, ultrasonic density = 160 W/cm3, ultrasonic frequency = 585 kHz, temperature = 277 K) [28]. Michael et al. observed an 80% diclofenac removal efficiency in 120 min in a sonocatalytic process with TiO2 as the catalyst ([Diclofenac] = 10 mg/L, [TiO2] = 500 mg/L, ultrasonic density = 8.4 W/cm2, ultrasonic frequency = 20 kHz, xenon lamp power = 1 kW, temperature = 298 K) [29]. Thus, the obtained 80% diclofenac removal efficiency in 40 min in this study indicates that FeCeOx has excellent performance in catalyzing an ultrasonic system for water treatment. 3.2. Characterization of FeCeOx before and after sonication To detect the changes in the morphology and structural characteristics of FeCeOx before and after sonication, nitrogen adsorption/ desorption, SEM, XRD, Raman and XPS analyses were applied to characterize the catalyst. 3.2.1. Nitrogen adsorption/desorption analysis To detect the changes in the specific surface area and microscopic pore structure of FeCeOx before and after sonication, the samples were measured with a nitrogen adsorption/desorption analysis instrument. The N2 adsorption-desorption isotherm curves of FeCeOx before and after sonication are shown in Fig. 2. Both curves belong to Type IV, which indicates the mesoporous structure of FeCeOx before and after sonication. Based on the BET method, the specific surface area, pore volume and pore diameter of FeCeOx before sonication were approximately 68.43 m2/g, 0.1566 ml/g and 9.37 nm, respectively, whereas those after sonication were 75.44 m2/g, 0.1769 ml/g and 9.38 nm, respectively. Hence, FeCeOx had a higher specific surface area and pore volume after sonication. Because the micro jets and short waves, which are induced by the cavitation effect, continuously shock the surface of
Fig. 2. Nitrogen adsorption-desorption isotherms of FeCeOx before and after sonication.
FeCeOx during the ultrasonic process, the pore volume increases with the increase in number of jet holes and the specific surface area increases in the form of corrosion. The pore structures of FeCeOx after sonication remained mesoporous, which demonstrates the stable structure of FeCeOx in the ultrasonic process, which is beneficial for practical applications. 3.2.2. SEM analysis To analyze whether the changes occurred on morphology and crystal structure of FeCeOx before and after sonication, the samples were measured with a SEM analyzer equipped with element mapping detector. The changes in morphology of FeCeOx before and after sonication are shown in Fig. 3(a), (b), (c) and (d). The morphology of planar structure with a smooth surface of FeCeOx was observed before sonication in Fig. 3(a). Interestingly, both smooth and rough surfaces of FeCeOx were observed after 30 min of sonication in Fig. 3(b), (c) and (d). The different morphologies were caused by shock waves and micro-jets that formed when the cavitation bubbles collapsed. The consequence of the shocks on the surface of FeCeOx was in the form of corrosion and subsequently appeared similar to a smooth pebble under continuous shocks. Thus, the degree of the micro-jet shock resulted in different morphologies of FeCeOx after sonication. The element distribution maps of Ce, O and Fe elements were also detected to illustrate whether the changes occurred on the crystal structure of FeCeOx before and after sonication. As shown in Fig. 3(e), (f), (g) before sonication and Fig. 3(h), (i), (j) after sonication, there are no obvious changes of element distribution before and after sonication, indicating that crystal structure of FeCeOx remained stable in the reaction. The Fe, Ce, and O elements highly dispersed in the structure of FeCeOx. The relatively sparse dispersion of Fe crystals was caused by the low mole ratio of Fe and Ce (1:9), in which the solid solution structure was formed as detected in our previous study [27]. The Fe, Ce, and O elements also remained highly dispersed in the structure of FeCeOx, which indicates the stable solid solution structure of FeCeOx. 3.2.3. XRD analysis To detect the crystal structure of FeCeOx before and after sonication, the samples were measured with a XRD diffractometer. Fig. 4 shows the X-ray diffraction patterns of CeO2 and FeCeOx before and after sonication. All of the peaks of the samples had the CeO2 phase (JCPDS 34-0394) with no detectable diffraction
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Fig. 3. SEM images of FeCeOx (a) morphology before sonication (1640 times); (b), (c), (d) morphology of different particles after sonication (800, 1100, 2000 times, respectively); (e), (f), (g) elements distribution before sonication; (h), (i), (j) elements distribution after sonication.
peak of iron oxide, and a similar phenomenon was found in Ce1 xFexO2 d solid-solution film [30]. Fig. 4 shows that the peaks of FeCeOx both before and after sonication slightly shifted towards higher diffraction angles and were slightly broader than those of CeO2 because a lattice constriction formed after Fe ions were doped
into CeO2 [30]. More oxygen vacancy defects also formed in this new structure because Fe ions are smaller than Ce ions. Clearly, there was no obvious change of diffraction peaks after FeCeOx was used, which indicates that the ultrasonic process hardly affects the crystal structure of FeCeOx.
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Fig. 4. XRD spectra of CeO2 and FeCeOx before and after sonication.
3.2.4. Raman analysis To detect the changes of oxygen vacancies of FeCeOx before and after sonication, the samples were measured with a Raman spectrometer. The Raman spectra of CeO2 and FeCeOx before and after sonication are shown in Fig. 5. As observed in Fig. 5(a), the strong band at 462 cm 1 belongs to the vibration mode of the F2g symmetry in a cubic fluorite CeO2 lattice [21]. For FeCeOx before and after sonication in the ultrasonic process, the peak positions slightly shifted towards a lower wave number because a solid solution with Fe ions formed in the CeO2 lattice [31]. To detect the state of oxygen vacancy in CeO2 and FeCeOx before and after sonication, a partial range scan was made from 500 cm 1 to 1400 cm 1. The peak at 1172 cm 1 was assigned to the secondorder phonon mode of the fluorite structure [26]. The peak at 593 cm 1 is associated with oxygen vacancies in the CeO2 lattice [32]. The amount of oxygen vacancies can be evaluated based on the value of I593/I1172, which was calculated using the ratio of Raman peak area at 593 cm 1 and that at 1172 cm 1. As shown in Fig. 5(b), the amount of oxygen vacancies increased after Fe was doped into CeO2. This increase is attributed to the lattice distortion of the CeO2 crystallite after Fe ions substituted Ce ions. Moreover, the amount of oxygen vacancies in FeCeOx significantly decreased after use. Other studies propose that oxygen vacancies play an important role when Ce oxides are used as the catalyst in a Fenton-like reaction [21,26]. The results in this study suggest that the oxygen vacancy participated in the ultrasonic process and Fenton-like reaction. 3.2.5. XPS analysis To analyze the composition and states of the elements in FeCeOx before and after sonication, the samples were determined by XPS analyzer. The XPS survey spectra of FeCeOx before and after sonication are shown in Fig. 6(a). The photoelectron peaks demonstrate the presence of Ce, O, and Fe elements on the surface of fresh and used composites. Moreover, high-resolution scans were obtained for the O 1s, Ce 3d, and Fe 2p regions. Fig. 6(b) shows the O 1s spectra of the fresh and used samples: both had two peaks at 529.6 and 530.9 eV, which correspond to the O2 and OH groups, respectively [33]. The O2 group is characteristic of lattice oxygen in FeCeOx [34], and the OH group was formed by the adsorbed oxygen species on the surface of FeCeOx. The content percentages of O2 and OH groups were 71.49% and 28.51% in the fresh sample, respectively. After the reaction, the values changed to 71.67% and 28.33%, respectively. There was no obvious change of O2 and OH groups of FeCeOx before and after sonication.
Fig. 5. Raman spectra of CeO2 and FeCeOx (a) before and (b) after sonication.
Fig. 6(c) shows the Ce 3d spectra of FeCeOx before and after sonication. The peaks at 882.6 and 901.2 eV are attributed to Ce 3d5/2 and Ce 3d3/2, respectively [30]. The double peaks at 882.5/901.0, 889.3/907.8 and 898.6/916.9 eV are ascribed to the final states of the Ce (IV) species, whereas the double peak at 884.3/902.5 eV is assigned to the final state of Ce (III) species [35]. The content percentages of Ce (III) and Ce (IV) species were 22.58% and 77.42%, in the fresh sample, respectively. After the reaction, the values changed to 17.80% and 82.20%, respectively. The decrease of Ce (III) species and increase of Ce (IV) species suggest that Ce (III) was gradually oxidized to Ce (IV) after the reaction. Fig. 6(d) shows the Fe 2p spectra of FeCeOx before and after sonication. The binding energy at 710.7 eV with satellites at 712.4 and 719.2 eV can be ascribed to Fe (II) species, and the binding energy at 724.1 eV with satellites at 732.6 and 716.0 eV can be ascribed to Fe (III) in iron oxides [36]. The content percentages of Fe (II) and Fe (III) were 17.16% and 82.84%, respectively. After the reaction, the values changed to 37.59% and 62.41%. The increase of Fe (II) species and decrease of Fe (III) species suggest that Fe (III) was gradually reduced to Fe (II) after the reaction. This analysis of the O 1 s, Ce 3d, and Fe 2p regions indicates that all of the Fe and Ce ions with different valences coexisted in dynamic equilibrium, which facilitated the electron transfer in the reaction. 3.3. Potential reaction mechanisms To investigate the contributions of reactive species to the FeCeOx-catalyzed ultrasonic process, several scavengers were
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Fig. 6. XPS spectra of FeCeOx before and after sonication: (a) survey scan; (b) O 1s; (c) Ce 3d; (d) Fe 2p energy regions.
introduced to inhibit the action of each reactive species. The scavengers dimethyl sulfoxide, benzoquinone and sodium azide were used to quench hydroxyl radicals (OH), superoxide radical anion (O2 ) and singlet oxygen (1O2), respectively, which might involve in the removal process [37,38]. As shown in Fig. 7, the diclofenac degradation efficiency decreased during the period of 1–15 min when dimethyl sulfoxide and benzoquinone were added to the solution. The change in trend demonstrates that the capture of OH and O2 can slightly inhibit the diclofenac degradation in 1–15 min. Thus, it is proposed that OH and O2 participated in the reaction, but they were not the primary contributing species because the degradation was not significantly inhibited by dimethyl sulfoxide and benzoquinone. Interestingly, the addition of sodium azide inhibited the diclofenac degradation efficiency by approximately 60%, which indicates that 1 O2 plays a key role in diclofenac degradation. This result differs from those of other ultrasonic reactions, where OH radicals are the major reactive species. Hamdaoui et al. found that OH radical was the primary reactive species for 4-CP ultrasonic degradation and the dominant mechanism was the reaction of 4-CP molecules with OH radicals at the gas-bubble-liquid interface [39]. Huang et al. also found that OH radicals played the key role in BPA degradation in an ultrasonic Fenton-like process that was catalyzed by Fe3O4 nanoparticles [40]. Elshafei used nano-sized Fe2O3 and CuO to degrade nitrobenzene in an ultrasonic-assisted heterogeneous Fenton reaction at neutral pH conditions, and sonication played a
Fig. 7. Effect of radical scavengers on the diclofenac degradation in the ultrasonic process. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 6, FeCeOx amount = 0.5 g/L, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K, [scavengers] = 5 mmol/L.
major role in enhancing the production of OH radicals in the presence of solid oxides [41]. Because OH radicals are the common reactive radicals in advanced oxidation processes, the study
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regarding the contribution of 1O2 was relatively small, particularly in the ultrasonic process. Thus, few studies examined the important role of 1O2 in ultrasonic reactions. For the first time, diclofenac in water was removed in a FeCeOx-catalyzed ultrasonic process, and it is meaningful that 1O2 was found to play a key role in this study. Interestingly, Wang et al. found that both 1O2 and OH were generated when nano-sized TiO2 powder was used as the catalyst under ultrasonic irradiation to degrade organic dyes [42]. Harada et al. also found that TiO2 nano particles in the micelles could generate 1O2 by sonication [43]. Because both CeO2 and TiO2 are semiconducts and contain rich O vacancies, FeCeOx may have similar characteristics with TiO2 and generate 1O2 under ultrasonic irradiation. According to the above analyses, the solid-solution structure of FeCeOx remained stable and all Fe and Ce ions with different valences coexisted in dynamic equilibrium after the reaction. However, the oxygen vacancies of FeCeOx significantly decreased after use, and the capture of 1O2 also obviously inhibited the reaction. Thus, it may be concluded that the 1O2 generation is significantly related to the oxygen vacancies in FeCeOx composites. Some studies also reported a strong relationship between the 1O2 generation and oxygen vacancies in other metal oxide composites. Ding et al. found that 1O2 was generated from the crystal decomposition of NaBiO3 in an acidic solution [44]. Zhang et al. also found that the dominant reactive species in bisphenol A degradation system with Bi(V)/Bi(III) composites was 1O2, which was produced when lattice oxygen was released from the composite [45]. Li et al. deduced that 1 O2 was produced in the vacancies of the Fe-Co PBA structure by the rapid relevant radical reaction process [37]. To further study the contribution of oxygen vacancies of FeCeOx in the removal process, gases of O2, N2 and air were continuously pumped into the solution in the ultrasonic process. As shown in Fig. 8, N2 addition had no effect on diclofenac removal. However, the addition of O2 and air inhibited the diclofenac removal efficiency by approximately 10% and 3%, respectively. Because the oxygen vacancy could be oxidized by O2 and subsequently against electron transfer in the reaction, the diclofenac removal efficiency decreased after the O2 and air addition. Moreover, the degree of inhibition depended on the O2 content in the gas, which can explain the different inhibition extents of O2 and air. To further verify the stability of the crystal structure of FeCeOx in the ultrasonic process, the leaching Fe and Ce ions in the solu-
Fig. 9. Metallic-ion leaching in the diclofenac degradation process. Experimental conditions: [Diclofenac] = 20 mg/L, FeCeOx amount = 0.5 g/L, pH = 6, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K.
tion during the reaction were analyzed. Fig. 9 shows that the concentrations of both Fe and Ce were less than 0.2 mg/L and the percentages of leaching Fe and Ce were 0.23% and 0.024%, respectively, which were much less than the amounts of Fe and Ce in FeCeOx. As characterized in Section 3.2, FeCeOx maintained a stable solid-solution structure after the reaction, where the structure Fe and Ce ions flowed in the lattice instead of dissolving in water. Therefore, FeCeOx exhibited an excellent chemical stability with negligible ion leaching in the ultrasonic process.
4. Conclusions FeCeOx has excellent performance in catalyzing an ultrasonic system in water, and 80% diclofenac removal efficiency was achieved within 30 min. The solid solution structure of FeCeOx remains stable, and Fe and Ce ions flow in the lattice instead of dissolving in water. The morphology of FeCeOx changes after use, and the degree of the micro-jet shock results in different morphologies. Ce (III) is gradually oxidized to Ce (IV), and Fe (III) is gradually reduced to Fe (II) after the reaction, which indicates that Fe and Ce ions with different valences coexist in dynamic equilibrium. The oxygen vacancy participates in the ultrasonic process, and singlet oxygen 1O2 is the main active species in the degradation process. Acknowledgements The authors thank for the supports from Fundamental Research Funds for Central Universities and Research Funds of Renmin University of China (14XLNQ02, 15XNLD04). References
Fig. 8. Effects of gas addition on the diclofenac degradation in the FeCeOx-catalyzed ultrasonic process. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 6, FeCeOx amount = 0.5 g/L, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K, continuous bubbling gas = 0.1 ml/min.
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Sonocatalytic degradation of diclofenac with FeCeOx particles in water Shan Chong, Guangming Zhang ⇑, Zhongheng Wei, Nan Zhang, Ting Huang, Yucan Liu School of Environment & Natural Resource, Renmin University of China, Beijing 100872, China
a r t i c l e
i n f o
Article history: Received 18 May 2016 Received in revised form 18 June 2016 Accepted 18 June 2016 Available online 20 June 2016 Keywords: Diclofenac FeCeOx Ultrasound Oxygen vacancy Singlet oxygen
a b s t r a c t This paper studies the sonocatalytic degradation of diclofenac in water using FeCeOx-catalyzed ultrasound. The effects of pre-adsorption and gas addition were investigated. Nitrogen adsorption/desorption, SEM, XRD, Raman and XPS analyses of FeCeOx before and after sonication were characterized. The proposed mechanism was based on the microstructure changes of FeCeOx and reactive-species-scavenging performances. The results show that FeCeOx has excellent performance in catalyzing an ultrasonic system in water, and 80% of diclofenac was removed in 30 min ([Diclofenac] = 20 mg/L, FeCeOx amount = 0.5 g/L, pH = 6, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K). The Fe, Ce, and O elements remained highly dispersed in the structure of FeCeOx, and the solid solution structure of FeCeOx remained stable after the reaction. Ce (III) was gradually oxidized to Ce (IV) and Fe (III) was gradually reduced to Fe (II) after the reaction, which indicates that Fe and Ce ions with different valences coexisted in dynamic equilibrium. The amount of oxygen vacancies in FeCeOx significantly decreased after the reaction, which indicates that oxygen vacancy participated in the ultrasonic process. Singlet oxygen 1O2 was the primary reactive species in the degradation process, and the hydroxyl radicals OH and superoxide radical anion O2 also participated in the reaction. FeCeOx had excellent chemical stability with negligible leaching ions in the ultrasonic process. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Many pharmaceuticals have been detected in surface water, ground water and even drinking water and are considered ‘‘emerging pollutants” because of their adverse effects to human and aquatic organisms [1,2]. Diclofenac is one of the most frequently detected emerging pollutants in the aquatic environment [3]. The compound is a widely used non-steroidal anti-inflammatory pharmaceutical to relieve analgesic, antiarthritic and antirheumatic pains [4]. Exposure to diclofenac in aquatic environments leads to liver and kidney function damage in fish [5], and adverse effects have been found in birds as a result of diclofenac accumulation in the tissues of organisms [6]. Some studies have shown that most pharmaceuticals are partially removed in conventional water treatment processes; in particular, the removal efficiencies for diclofenac in wastewater treatment plants are typically 21–40% [7]. The limited removal effects signify the necessity of an innovative and effective diclofenac treatment method in water. Advanced oxidation processes (AOPs) have received considerable attention for the removal of pharmaceuticals from water [8– ⇑ Corresponding author. E-mail addresses: [email protected] (S. Chong), [email protected] (G. Zhang), [email protected] (Z. Wei), [email protected] (N. Zhang), [email protected] (T. Huang), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.ultsonch.2016.06.023 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
11]. Ultrasonic irradiation has been investigated as an effective AOP for diclofenac [12]. The mechanisms on the sonolysis of diclofenac mainly focused on the acoustic cavitation effect and free radicals effect. Naddeo et al. studied 20 kHz ultrasound-induced degradation of diclofenac and found that diclofenac conversion was improved by enhanced cavitation effect and production of free radicals [13]. The effect of acoustic cavitation plays a dominant role in the ultrasonic process, which involves the formation, expansion and collapse of micro bubbles and generation of high temperatures (up to 5000 K) and high pressures (above 1000 atm) [14]. Water molecules are dissociated into highly reactive species, which play an important role in the ultrasonic process to degrade organic pollutants. Ultrasonic irradiation could generate free radicals which had strong oxidation ability. The free radicals generated by ultrasonic irradiation and ‘‘hot spot” effects could degrade organic pollutants effectively [15]. In addition, the unique features of ultrasonic irradiation are the enhancement of mass transfer and improved surface properties of solid particles by ultrasonic power. Then, the chemical consumption and waste sludge generation rates can be reduced [16]. However, ultrasound alone is highly energy intensive and insufficient to totally degrade pollutants in water. Therefore, catalysts have been introduced to enhance ultrasonic processes to improve the treatment efficiency and reduce energy consumption [17,18]. Wang et al. investigated the sonocatalytic activity of Fe-doped
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mixed crystal TiO2 catalyst on degradation of azo fuchsine and proposed sonoluminescence and ‘‘hot spot” to explain the sonocatalytic degradation process [19]. Wang et al. also investigated the catalytic activities of CeO2/TiO2, SnO2/TiO2 and ZrO2/TiO2 composites on the sonocatalytic degradation of organic dyes and found that restraining the recombination of electron hole pairs was the important mechanism of sonocatalytic degradation in the presence of semiconductor composites [20]. Recently, ceria-based materials have been reported as heterogeneous catalysts for the degradation of organic pollutants in water [21,22]. In particular, Fe-doped CeO2 particles can provide massive active sites and produce large oxygen vacancies, which facilitate the adsorption of pollutant molecules and activation of reactive species during the reaction process [23]. Zou et al. investigated the enhancement of photo activity of transition metal ion (1 mol% Fe, Cu, Mn, and Zn) doped CeO2 nanocatalysts on Rhodamine B degradation [24]. The results showed that the catalytic activity was closely related to the oxygen vacancies and the valence of the doped ions and Fe doped CeO2 could improved separation of photo generated electrons and holes. The feature of Fe-doped CeO2 particles has obvious advantages in the ultrasonic process; the cavitation and interfacial effects are enhanced [25,26]. There are three zones of reaction in the ultrasonic process: cavitation bubble, supercritical interface, and bulk solution in the reaction solution. Thus, such enhancements will benefit the sonocatalytic reaction. In our previous study, Fe-doped CeO2 (FeCeOx) was successfully synthesized by an ultrasonic impregnation method and exhibited an excellent catalytic ability in removing diclofenac in the heterogeneous Fenton process [27]. This work was performed to investigate the application of FeCeOx catalysis in an ultrasonic system. We examined the removal of diclofenac in water using an FeCeOxcatalyzed ultrasonic system and investigated the reaction mechanisms of this sonocatalytic process. N2 adsorption-desorption, Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectrometer (Raman) and X-ray photoelectron spectroscopy (XPS) were used to investigate the changes in morphologic and physical/chemical properties of FeCeOx before and after sonication. The effect of gas addition on the diclofenac degradation efficiency in the ultrasonic process was investigated. To analyze the contributions of hydroxyl radicals (OH), superoxide radical anion (O2 ) and singlet oxygen (1O2) to diclofenac degradation, these active species were scavenged using dimethyl sulfoxide (OH quencher), 1,4-benzoquinone (O2 quencher) and sodium azide (1O2 quencher). The stability of FeCeOx was detected by determining leaching metal ions in the ultrasonic process. 2. Experimental 2.1. Reagents and materials All chemicals in this study were of analytical grade and used without further purification. Ferrous chloride hexahydrate (FeCl24H2O) and sodium hydroxide (NaOH) were purchased from Xilong Chemical Co. Ltd, China. Cerous nitrate (Ce(NO3)36H2O) was purchased from Tianjin Guangfu Fine Chemical Institute, China. Diclofenac was purchased from Tokyo Chemical Industry Co. Ltd, Japan. Sodium azide was purchased from Tokyo USP Co. Ltd, Japan. Dimethyl sulfoxide and 1,4-benzoquinone were obtained from Xilong Chemical Co., Ltd, China. The solutions were prepared with deionized water, which was purified using a Millipore Milli Q UV Plus system. 2.2. Synthesis of FeCeOx CeO2 was prepared using a precipitation method [18]. Ce (NO3)36H2O (0.01 mol) was dissolved in 200 ml of deionized water
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with vigorous mechanical stirring. A stoichiometric quantity of NaOH was added drop-wise and maintained at a reaction temperature of 35 °C. After 4 h, the solution was centrifuged at 3000 rpm for 3 min to separate the products. The settled products were aged for 12 h at room temperature and subsequently dried at 100 °C for 3 h. The resulting CeO2 was dipped in a FeCl24H2O solution (mole ratio of Fe and Ce = 1:9) under ultrasonic irradiation for 20 min, and the mixture was subsequently filtered. The solid was calcined for 2 h at 400 °C in a muffle furnace to obtain the final FeCeOx catalyst. 2.3. Characterization of FeCeOx before and after sonication The specific surface area and porosity of the catalyst were measured based on nitrogen adsorption/desorption isotherms at 77 K on a BeiShiDe 3H-2000PS2 specific surface and pore size analysis instrument. The surface morphology of the samples was investigated using a Hitachi S 4700 scanning electron microscope (SEM) analyzer with a secondary electron detector at different scales and magnifications. The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/max-rc diffractometer with Cu radiation. Raman measurements were performed at room temperature using a Via+ Reflex Raman spectrometer with excitation light of 514 nm. The X-ray photoelectron spectra (XPS) were measured on an EScalab 250Xi spectrometer, which was equipped with an XR6 monochromated X-ray source. 2.4. Determination of diclofenac removal All diclofenac degradation experiments were conducted in 150-ml beakers. The solution was adjusted with diluted NaOH and H2SO4 to the desired initial pH and a final volume of 100 ml. The desired amount of the FeCeOx catalyst was immediately introduced into the reaction solution. The ultrasonic probe was inserted into the beaker with the tail end approximately 1.5 cm below the liquid level. At set intervals, 1.0 ml of supernatant from the sample solution was analyzed immediately after filtration with a 0.45-lm membrane filter. Control experiments were performed under identical conditions. The reaction temperature was controlled by a Yiheng DK-600A constant temperature instrument by circulating water. Diclofenac was determined with a Waters e2695 high-performance liquid chromatography instrument (HPLC), which was equipped with a C18 reversed phase column (4.6 mm 150 mm I.D.). Elution conditions: mobile phase was composed of a 70/30 v/v acetonitrile and acetic acid solution (0.2%); flow rate: 1 ml/min; injection volume: 10 lL; column temperature: 30 °C; kmax: 275 nm. The leached Fe and Ce ions were measured by graphite furnace atomic absorption spectrometric method with a Hitachi Z-2000 Atomic Absorption Spectrophotometer (AAS) using iron lamp and cerium lamp as light resource, respectively. 3. Results and discussion 3.1. Role of FeCeOx under sonication To evaluate the feasibility of using a FeCeOx-catalyzed ultrasonic process to remove diclofenac, the diclofenac removal performance of FeCeOx was compared in the absence and presence of ultrasonic irradiation. Fig. 1 shows that the diclofenac removal efficiencies with ultrasound alone and FeCeOx alone were 22% and 60%, respectively, in 40 min. The diclofenac removal with FeCeOx alone was mainly attributed to the adsorption ability of the material. In the FeCeOx/ultrasound system, an 80% diclofenac removal efficiency was achieved in 30 min, which indicates that FeCeOx
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Fig. 1. Diclofenac removal performance in the absence and presence of ultrasound by FeCeOx. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 5, FeCeOx amount = 0.5 g/L, ultrasonic density = 1.5 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K.
can catalyze the ultrasonic process to remove diclofenac in water. Compared with ultrasound alone and FeCeOx alone, the significantly accelerated removal rate and improved removal efficiency of diclofenac in the FeCeOx/ultrasound combined system show that the presence of the FeCeOx catalyst facilitates the removal process under ultrasonic irradiation. Several studies have investigated diclofenac removal in the ultrasonic process. Nie et al. found that 44%, 29%, 26% and 21% diclofenac degradation efficiencies were achieved in 60 min under argon-, oxygen-, air- and nitrogensaturated conditions, respectively ([Diclofenac] = 0.05 mmol/L, pH = 7, ultrasonic density = 160 W/cm3, ultrasonic frequency = 585 kHz, temperature = 277 K) [28]. Michael et al. observed an 80% diclofenac removal efficiency in 120 min in a sonocatalytic process with TiO2 as the catalyst ([Diclofenac] = 10 mg/L, [TiO2] = 500 mg/L, ultrasonic density = 8.4 W/cm2, ultrasonic frequency = 20 kHz, xenon lamp power = 1 kW, temperature = 298 K) [29]. Thus, the obtained 80% diclofenac removal efficiency in 40 min in this study indicates that FeCeOx has excellent performance in catalyzing an ultrasonic system for water treatment. 3.2. Characterization of FeCeOx before and after sonication To detect the changes in the morphology and structural characteristics of FeCeOx before and after sonication, nitrogen adsorption/ desorption, SEM, XRD, Raman and XPS analyses were applied to characterize the catalyst. 3.2.1. Nitrogen adsorption/desorption analysis To detect the changes in the specific surface area and microscopic pore structure of FeCeOx before and after sonication, the samples were measured with a nitrogen adsorption/desorption analysis instrument. The N2 adsorption-desorption isotherm curves of FeCeOx before and after sonication are shown in Fig. 2. Both curves belong to Type IV, which indicates the mesoporous structure of FeCeOx before and after sonication. Based on the BET method, the specific surface area, pore volume and pore diameter of FeCeOx before sonication were approximately 68.43 m2/g, 0.1566 ml/g and 9.37 nm, respectively, whereas those after sonication were 75.44 m2/g, 0.1769 ml/g and 9.38 nm, respectively. Hence, FeCeOx had a higher specific surface area and pore volume after sonication. Because the micro jets and short waves, which are induced by the cavitation effect, continuously shock the surface of
Fig. 2. Nitrogen adsorption-desorption isotherms of FeCeOx before and after sonication.
FeCeOx during the ultrasonic process, the pore volume increases with the increase in number of jet holes and the specific surface area increases in the form of corrosion. The pore structures of FeCeOx after sonication remained mesoporous, which demonstrates the stable structure of FeCeOx in the ultrasonic process, which is beneficial for practical applications. 3.2.2. SEM analysis To analyze whether the changes occurred on morphology and crystal structure of FeCeOx before and after sonication, the samples were measured with a SEM analyzer equipped with element mapping detector. The changes in morphology of FeCeOx before and after sonication are shown in Fig. 3(a), (b), (c) and (d). The morphology of planar structure with a smooth surface of FeCeOx was observed before sonication in Fig. 3(a). Interestingly, both smooth and rough surfaces of FeCeOx were observed after 30 min of sonication in Fig. 3(b), (c) and (d). The different morphologies were caused by shock waves and micro-jets that formed when the cavitation bubbles collapsed. The consequence of the shocks on the surface of FeCeOx was in the form of corrosion and subsequently appeared similar to a smooth pebble under continuous shocks. Thus, the degree of the micro-jet shock resulted in different morphologies of FeCeOx after sonication. The element distribution maps of Ce, O and Fe elements were also detected to illustrate whether the changes occurred on the crystal structure of FeCeOx before and after sonication. As shown in Fig. 3(e), (f), (g) before sonication and Fig. 3(h), (i), (j) after sonication, there are no obvious changes of element distribution before and after sonication, indicating that crystal structure of FeCeOx remained stable in the reaction. The Fe, Ce, and O elements highly dispersed in the structure of FeCeOx. The relatively sparse dispersion of Fe crystals was caused by the low mole ratio of Fe and Ce (1:9), in which the solid solution structure was formed as detected in our previous study [27]. The Fe, Ce, and O elements also remained highly dispersed in the structure of FeCeOx, which indicates the stable solid solution structure of FeCeOx. 3.2.3. XRD analysis To detect the crystal structure of FeCeOx before and after sonication, the samples were measured with a XRD diffractometer. Fig. 4 shows the X-ray diffraction patterns of CeO2 and FeCeOx before and after sonication. All of the peaks of the samples had the CeO2 phase (JCPDS 34-0394) with no detectable diffraction
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Fig. 3. SEM images of FeCeOx (a) morphology before sonication (1640 times); (b), (c), (d) morphology of different particles after sonication (800, 1100, 2000 times, respectively); (e), (f), (g) elements distribution before sonication; (h), (i), (j) elements distribution after sonication.
peak of iron oxide, and a similar phenomenon was found in Ce1 xFexO2 d solid-solution film [30]. Fig. 4 shows that the peaks of FeCeOx both before and after sonication slightly shifted towards higher diffraction angles and were slightly broader than those of CeO2 because a lattice constriction formed after Fe ions were doped
into CeO2 [30]. More oxygen vacancy defects also formed in this new structure because Fe ions are smaller than Ce ions. Clearly, there was no obvious change of diffraction peaks after FeCeOx was used, which indicates that the ultrasonic process hardly affects the crystal structure of FeCeOx.
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Fig. 4. XRD spectra of CeO2 and FeCeOx before and after sonication.
3.2.4. Raman analysis To detect the changes of oxygen vacancies of FeCeOx before and after sonication, the samples were measured with a Raman spectrometer. The Raman spectra of CeO2 and FeCeOx before and after sonication are shown in Fig. 5. As observed in Fig. 5(a), the strong band at 462 cm 1 belongs to the vibration mode of the F2g symmetry in a cubic fluorite CeO2 lattice [21]. For FeCeOx before and after sonication in the ultrasonic process, the peak positions slightly shifted towards a lower wave number because a solid solution with Fe ions formed in the CeO2 lattice [31]. To detect the state of oxygen vacancy in CeO2 and FeCeOx before and after sonication, a partial range scan was made from 500 cm 1 to 1400 cm 1. The peak at 1172 cm 1 was assigned to the secondorder phonon mode of the fluorite structure [26]. The peak at 593 cm 1 is associated with oxygen vacancies in the CeO2 lattice [32]. The amount of oxygen vacancies can be evaluated based on the value of I593/I1172, which was calculated using the ratio of Raman peak area at 593 cm 1 and that at 1172 cm 1. As shown in Fig. 5(b), the amount of oxygen vacancies increased after Fe was doped into CeO2. This increase is attributed to the lattice distortion of the CeO2 crystallite after Fe ions substituted Ce ions. Moreover, the amount of oxygen vacancies in FeCeOx significantly decreased after use. Other studies propose that oxygen vacancies play an important role when Ce oxides are used as the catalyst in a Fenton-like reaction [21,26]. The results in this study suggest that the oxygen vacancy participated in the ultrasonic process and Fenton-like reaction. 3.2.5. XPS analysis To analyze the composition and states of the elements in FeCeOx before and after sonication, the samples were determined by XPS analyzer. The XPS survey spectra of FeCeOx before and after sonication are shown in Fig. 6(a). The photoelectron peaks demonstrate the presence of Ce, O, and Fe elements on the surface of fresh and used composites. Moreover, high-resolution scans were obtained for the O 1s, Ce 3d, and Fe 2p regions. Fig. 6(b) shows the O 1s spectra of the fresh and used samples: both had two peaks at 529.6 and 530.9 eV, which correspond to the O2 and OH groups, respectively [33]. The O2 group is characteristic of lattice oxygen in FeCeOx [34], and the OH group was formed by the adsorbed oxygen species on the surface of FeCeOx. The content percentages of O2 and OH groups were 71.49% and 28.51% in the fresh sample, respectively. After the reaction, the values changed to 71.67% and 28.33%, respectively. There was no obvious change of O2 and OH groups of FeCeOx before and after sonication.
Fig. 5. Raman spectra of CeO2 and FeCeOx (a) before and (b) after sonication.
Fig. 6(c) shows the Ce 3d spectra of FeCeOx before and after sonication. The peaks at 882.6 and 901.2 eV are attributed to Ce 3d5/2 and Ce 3d3/2, respectively [30]. The double peaks at 882.5/901.0, 889.3/907.8 and 898.6/916.9 eV are ascribed to the final states of the Ce (IV) species, whereas the double peak at 884.3/902.5 eV is assigned to the final state of Ce (III) species [35]. The content percentages of Ce (III) and Ce (IV) species were 22.58% and 77.42%, in the fresh sample, respectively. After the reaction, the values changed to 17.80% and 82.20%, respectively. The decrease of Ce (III) species and increase of Ce (IV) species suggest that Ce (III) was gradually oxidized to Ce (IV) after the reaction. Fig. 6(d) shows the Fe 2p spectra of FeCeOx before and after sonication. The binding energy at 710.7 eV with satellites at 712.4 and 719.2 eV can be ascribed to Fe (II) species, and the binding energy at 724.1 eV with satellites at 732.6 and 716.0 eV can be ascribed to Fe (III) in iron oxides [36]. The content percentages of Fe (II) and Fe (III) were 17.16% and 82.84%, respectively. After the reaction, the values changed to 37.59% and 62.41%. The increase of Fe (II) species and decrease of Fe (III) species suggest that Fe (III) was gradually reduced to Fe (II) after the reaction. This analysis of the O 1 s, Ce 3d, and Fe 2p regions indicates that all of the Fe and Ce ions with different valences coexisted in dynamic equilibrium, which facilitated the electron transfer in the reaction. 3.3. Potential reaction mechanisms To investigate the contributions of reactive species to the FeCeOx-catalyzed ultrasonic process, several scavengers were
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Fig. 6. XPS spectra of FeCeOx before and after sonication: (a) survey scan; (b) O 1s; (c) Ce 3d; (d) Fe 2p energy regions.
introduced to inhibit the action of each reactive species. The scavengers dimethyl sulfoxide, benzoquinone and sodium azide were used to quench hydroxyl radicals (OH), superoxide radical anion (O2 ) and singlet oxygen (1O2), respectively, which might involve in the removal process [37,38]. As shown in Fig. 7, the diclofenac degradation efficiency decreased during the period of 1–15 min when dimethyl sulfoxide and benzoquinone were added to the solution. The change in trend demonstrates that the capture of OH and O2 can slightly inhibit the diclofenac degradation in 1–15 min. Thus, it is proposed that OH and O2 participated in the reaction, but they were not the primary contributing species because the degradation was not significantly inhibited by dimethyl sulfoxide and benzoquinone. Interestingly, the addition of sodium azide inhibited the diclofenac degradation efficiency by approximately 60%, which indicates that 1 O2 plays a key role in diclofenac degradation. This result differs from those of other ultrasonic reactions, where OH radicals are the major reactive species. Hamdaoui et al. found that OH radical was the primary reactive species for 4-CP ultrasonic degradation and the dominant mechanism was the reaction of 4-CP molecules with OH radicals at the gas-bubble-liquid interface [39]. Huang et al. also found that OH radicals played the key role in BPA degradation in an ultrasonic Fenton-like process that was catalyzed by Fe3O4 nanoparticles [40]. Elshafei used nano-sized Fe2O3 and CuO to degrade nitrobenzene in an ultrasonic-assisted heterogeneous Fenton reaction at neutral pH conditions, and sonication played a
Fig. 7. Effect of radical scavengers on the diclofenac degradation in the ultrasonic process. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 6, FeCeOx amount = 0.5 g/L, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K, [scavengers] = 5 mmol/L.
major role in enhancing the production of OH radicals in the presence of solid oxides [41]. Because OH radicals are the common reactive radicals in advanced oxidation processes, the study
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regarding the contribution of 1O2 was relatively small, particularly in the ultrasonic process. Thus, few studies examined the important role of 1O2 in ultrasonic reactions. For the first time, diclofenac in water was removed in a FeCeOx-catalyzed ultrasonic process, and it is meaningful that 1O2 was found to play a key role in this study. Interestingly, Wang et al. found that both 1O2 and OH were generated when nano-sized TiO2 powder was used as the catalyst under ultrasonic irradiation to degrade organic dyes [42]. Harada et al. also found that TiO2 nano particles in the micelles could generate 1O2 by sonication [43]. Because both CeO2 and TiO2 are semiconducts and contain rich O vacancies, FeCeOx may have similar characteristics with TiO2 and generate 1O2 under ultrasonic irradiation. According to the above analyses, the solid-solution structure of FeCeOx remained stable and all Fe and Ce ions with different valences coexisted in dynamic equilibrium after the reaction. However, the oxygen vacancies of FeCeOx significantly decreased after use, and the capture of 1O2 also obviously inhibited the reaction. Thus, it may be concluded that the 1O2 generation is significantly related to the oxygen vacancies in FeCeOx composites. Some studies also reported a strong relationship between the 1O2 generation and oxygen vacancies in other metal oxide composites. Ding et al. found that 1O2 was generated from the crystal decomposition of NaBiO3 in an acidic solution [44]. Zhang et al. also found that the dominant reactive species in bisphenol A degradation system with Bi(V)/Bi(III) composites was 1O2, which was produced when lattice oxygen was released from the composite [45]. Li et al. deduced that 1 O2 was produced in the vacancies of the Fe-Co PBA structure by the rapid relevant radical reaction process [37]. To further study the contribution of oxygen vacancies of FeCeOx in the removal process, gases of O2, N2 and air were continuously pumped into the solution in the ultrasonic process. As shown in Fig. 8, N2 addition had no effect on diclofenac removal. However, the addition of O2 and air inhibited the diclofenac removal efficiency by approximately 10% and 3%, respectively. Because the oxygen vacancy could be oxidized by O2 and subsequently against electron transfer in the reaction, the diclofenac removal efficiency decreased after the O2 and air addition. Moreover, the degree of inhibition depended on the O2 content in the gas, which can explain the different inhibition extents of O2 and air. To further verify the stability of the crystal structure of FeCeOx in the ultrasonic process, the leaching Fe and Ce ions in the solu-
Fig. 9. Metallic-ion leaching in the diclofenac degradation process. Experimental conditions: [Diclofenac] = 20 mg/L, FeCeOx amount = 0.5 g/L, pH = 6, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K.
tion during the reaction were analyzed. Fig. 9 shows that the concentrations of both Fe and Ce were less than 0.2 mg/L and the percentages of leaching Fe and Ce were 0.23% and 0.024%, respectively, which were much less than the amounts of Fe and Ce in FeCeOx. As characterized in Section 3.2, FeCeOx maintained a stable solid-solution structure after the reaction, where the structure Fe and Ce ions flowed in the lattice instead of dissolving in water. Therefore, FeCeOx exhibited an excellent chemical stability with negligible ion leaching in the ultrasonic process.
4. Conclusions FeCeOx has excellent performance in catalyzing an ultrasonic system in water, and 80% diclofenac removal efficiency was achieved within 30 min. The solid solution structure of FeCeOx remains stable, and Fe and Ce ions flow in the lattice instead of dissolving in water. The morphology of FeCeOx changes after use, and the degree of the micro-jet shock results in different morphologies. Ce (III) is gradually oxidized to Ce (IV), and Fe (III) is gradually reduced to Fe (II) after the reaction, which indicates that Fe and Ce ions with different valences coexist in dynamic equilibrium. The oxygen vacancy participates in the ultrasonic process, and singlet oxygen 1O2 is the main active species in the degradation process. Acknowledgements The authors thank for the supports from Fundamental Research Funds for Central Universities and Research Funds of Renmin University of China (14XLNQ02, 15XNLD04). References
Fig. 8. Effects of gas addition on the diclofenac degradation in the FeCeOx-catalyzed ultrasonic process. Experimental conditions: [Diclofenac] = 20 mg/L, pH = 6, FeCeOx amount = 0.5 g/L, ultrasonic density = 3.0 W/cm3, ultrasonic frequency = 20 kHz, temperature = 298 K, continuous bubbling gas = 0.1 ml/min.
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