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Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution
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Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution Yixiong Pang a,b,c, Lingjun Kong a,∗, Hengyi Lei b,∗, Diyun Chen a, Gutha Yuvaraja a,c a
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, PR China School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510006, PR China c School of Civil Engineering, Guangzhou University, Guangzhou 510006, PR China b
a r t i c l e
i n f o
Article history: Received 18 April 2018 Revised 26 July 2018 Accepted 3 August 2018 Available online xxx Keywords: Tetracycline hydrochloride ZnFe2 O4 Microwave-induced Photocatalytic
a b s t r a c t In this research, ZnFe2 O4 was employed as heterogeneous catalyst for the degradation of tetracycline hydrochloride (TCH) by a novel combined microwave-induced (MW-induced) and photocatalytic oxidation. The ZnFe2 O4 catalyst was synthesized using co-precipitation method, and characterized by XRD, TEM, BET and UV–Vis DRS. A microwave electrodeless discharge lamp (MEDL) was introduced to this process as a light source. 91.6% of TCH degradation was obtained in MW/MEDL/ZnFe2 O4 system in 4 min. ZnFe2 O4 catalyst had both microwave-catalytic and visible-light photocatalytic activities. The operation parameters, such as microwave output, catalyst dosage, initial pH and recycle runs, showed different influence on TCH degradation. The analysis of degradation mechanism indicated that h+ was the main active species • for TCH degradation and little O2 − active species generated in MW/MEDL/ZnFe2 O4 system. The decomposition intermediates of TCH were identified by LC-MS. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Tetracycline hydrochloride (TCH), which widely used for antibacteria in the world, can be easily discharged into the river, soil, and even drinking water. TCH in the environment is considered as a potential risk for human living because it increases the antibiotics resistant of some pathogenic bacteria. Meanwhile, TCH may persist in the environment owing to their non-biodegradation. Consequently, efficient removal of TCH from wastewater is a great challenge for the scientists. Microwave-induced catalytic oxidation (MICO) has been widely reported as one of the most efficient AOPs, owing to its simple reaction conditions and high reaction speed [1]. Several methods of MICO are conducted to treat organic pollutant in wastewater, such as MW-Fenton [2], MW-Fenton-like [3], MW-persulfate [4] and MW-MEDL-photocatalyst [5]. Among these methods, MWMEDL-photocatalyst process may be considered as a novel way in wastewater treatment, because it combines the advantages of microwave and photocatalysis [6]. This process uses MEDL (microwave electrodeless discharge lamp) as light source which pro-
∗
Corresponding authors. E-mail addresses: [email protected] (L. Kong), [email protected] (H. Lei).
vides both ultraviolet and visible light. This system showed excellent performance for degrading organic pollutants, such as alizarin green [7], 2,4-D herbicide [8] and rhodamine B [9]. However, the light irradiances of ultraviolet wavelengths for MEDL are smaller than long wavelengths and they played only a minor role in the photochemical process [10]. Most photocatalysts that applied in MW/MEDL system are only excited by UV irradiation, such as TiO2 and ZnO [11]. MW/MEDL/photocatalyst process still have a number of drawbacks, the biggest being the insufficient utilization of energy owing to the poor microwave and visible light absorption of catalysts. Therefore, the main concern is finding out an appropriate catalyst for MW/MEDL/catalyst system. Ferrite, expressed as MFe2 O4 (M = Zn, Co, Mn or Cu), have been an area of activated research during the last several years [12–14]. They are widely used in environmental treatments because of their easy magnetic collection, high catalytic activity and good chemical stability [15,16]. The microwave-induced catalytic activity of ferrites have gained more attention [17,18] as a cleaner technology for wastewater treatment. Ferrites such as MnFe2 O4 [17] or NiFe2 O4 [19] were successfully employed for the degradation of organic pollutant from wastewater via MICO. In MW/ferrite process, electron-hole pairs generate on the surface of ferrite, which can oxide organic pollutant without other peroxide reagents [20]. Meanwhile, some researchers also have found that ferrites have
https://doi.org/10.1016/j.jtice.2018.08.008 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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photocatalytic activity [21]. Particularly, ZnFe2 O4 has better visiblelight photocatalytic activity owing to its relatively smaller band gaps (1.9 eV). However, most researchers only focus on its catalytic activity in photocatalysis process. Recently, High degradation percentage of organic dye was observed in MW/ZnFe2 O4 system [22], suggesting that ZnFe2 O4 might also have excellent microwave catalytic activity as other ferrites. Thus, the potential of ZnFe2 O4 in wastewater treatment has not been fully explored. Based on these reports, it can be expected that ZnFe2 O4 may reveal better performance in MW/MEDL system than in single MW or photocatalytic system. To the best of our knowledge, there is no report focusing on the combination of microwave-induced catalytic and photocatalytic activities of ZnFe2 O4 . In present work, degradation of organic contaminant in MW/MEDL/ZnFe2 O4 system is reported for the first time. ZnFe2 O4 is synthesized using co-precipitation method and characterized by TEM, XRD, UV–vis DRS and BET. MEDL is introduced to this process as a light source for exciting photocatalytic activity of ZnFe2 O4 under microwave irradiation. In this study, catalytic performance of ZnFe2 O4 is evaluated using TCH as a model pollutant. The effects of several operation parameters are investigated, such as catalyst dosage, microwave output, initial pH and reuse times. The mechanism of TCH degradation in MW/MEDL/ZnFe2 O4 is also discussed.
In quenching experiments, tert–butanol (TBA, C4 H9 OH), pbenzoquinone (C6 H4 O2 ) and EDTANa2 were added to the suspension before TCH degradation, respectively. In recycle experiments, the used catalysts were separated, and then washed with distilled water and dried at 65 °C before reused. The residual solutions were measured by a UV-2550 UV–vis spectrophotometer at 358 nm [23]. TCH degradation efficiency and reaction constant were calculated by Eq. S1 and Eq. S2, respectively.
2. Materials and methods
3. Results and discussion
2.1. Chemicals
3.1. Characterization
Analytical-reagent grade tetracycline hydrochloride (TCH, C22 H25 ClN2 O8 ), tert–butanol (TBA, C4 H9 OH), p-benzoquinone (C6 H4 O2 ) and EDTANa2 were purchased from Sigma-Aldrich. Chromatographic grade methanol (CH3 OH) and formic acid (HCOOH) were purchased from Sigma-Aldrich. Analytical-reagent grade Zn(NO3 )2 •6H2 O, Fe(NO3 )3 •9H2 O, NaOH and HCl were purchased from Sinopharm Chemical Reagent Co., Ltd., China.
The XRD patterns of as-prepared ZnFe2 O4 are presented in Fig. 1a. The diffraction peaks observed at 18.2°, 29.9°, 35.3°, 42.8°, 56.6° and 62.2° represent the Bragg reflection from the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 4 0) and (5 3 1) planes, respectively. These peaks can be well indexed to ZnFe2 O4 (JCPDS 22–1012). The XRD pattern of the ZnFe2 O4 catalyst used 5 runs is nearly the same as that of fresh catalyst, confirming that ZnFe2 O4 catalysts are stable in microwave-induced photocatalytic reaction. The TEM image of synthesized ZnFe2 O4 is shown in Fig. 1b. As can be seen, ZnFe2 O4 nanoparticles exhibit a diameter of 30–50 nm without serious aggregation. The UV–vis absorption spectra (Fig. 1c) indicated that ZnFe2 O4 absorbs the light in the region ranging from 200 to 600 nm, which is similar to the previous results [24]. Therefore, the MEDL, with emission spectra from 200 to 600 nm (Fig. S1a) can activate the as-prepared catalyst. The N2 adsorption–desorption isotherm of ZnFe2 O4 nanoparticles is shown in Fig. 1d. According to the IUPAC classification, the as-prepared ZnFe2 O4 sample exhibited the type IV isotherm. This indicates that the synthesized ZnFe2 O4 has mesoporous structure. The BET surface area of synthesized ZnFe2 O4 is calculated to be 50.05 m2 /g. To verify the microwave absorbing ability of ZnFe2 O4 , the rising temperature of as-prepared ZnFe2 O4 under microwave irradiation was measured. As shown in Fig. 1e, the temperature of as-prepared sample exceeded 420 °C within 6 min in the present of microwave irradiation (500 W), which indicates that ZnFe2 O4 is an excellent microwave absorbing material. Therefore, ZnFe2 O4 may show satisfying performance in microwave-induced photocatalytic systems.
2.2. Synthesis of ZnFe2 O4 ZnFe2 O4 was synthesized using the co-precipitation method. 0.01 mol Zn(NO3 )2 •6H2 O and 0.02 mol Fe(NO3 )3 •9H2 O were dissolved in 200 mL distilled water. Then 3 mol/L NaOH was added dropwise to the solution till pH around 10 with continuous magnetic-stirring. The temperature was maintained at 85 °C in the whole process. The precipitate was filtered, washed with double distilled water, and dried at 110 °C overnight. Then this precursor sample was calcined at 400 °C in a muffle furnace for 2 h. 2.3. Degradation of TCH The experiments were carried out in a modified microwave oven. Typically, the initial concentration of TCH, ZnFe2 O4 dosage and microwave output were fixed at 50 mg/L, 1.5 g/L and 500 w in each experiment, respectively, and the value of initial pH is 6. The as-prepared ZnFe2 O4 was introduced into TCH solution, followed by an adjustment of the initial pH. The total mixed solution of 50 mL was poured into a 500 mL quartz glass beaker. Before MW irradiation, the suspension was magnetically stirred in the dark for 60 min to ensure adsorption-desorption equilibrium. The reaction vessel was then exposed to MW irradiation for microwave-induced photocatalytic degradation, following an introduction of MEDL. The detail of experimental setups was depicted in Supplementary Material. The emission spectral of MEDL and the diagrams of experimental setups is shown in Fig. S1. Each degradation experiment was performed in duplicate within accepting error range (± 5%), and all results were expressed as mean value with error bar.
2.4. Analysis The crystalline phase of as-prepared materials was characterized using XRD. The morphology and structure of as-prepared materials were determined by TEM. The BET surface area of the asprepared materials was carried out by the nitrogen adsorption– desorption method using a NOVA40 0 0e surface analyzer. The ultraviolet-visible diffuse reflectance spectrum of ZnFe2 O4 was recorded in a UV-2501 PC UV–Visible spectrometer. The pHzpc of ZnFe2 O4 was determined by mass titration and the pH value was measured by a pH meter. The spectral emission of MEDL was measured by fiber spectrometer (USB200). The intermediates of TCH degradation was measured by LC-MS on Agilent 1100MSD spectrometry equipped with a C18 column. The detail condition of LCMS experiment is shown in Supplementary Material.
3.2. Degradation of TCH in different systems As shown in Fig. 2a, negligible degradation was obtained in single MW system owing to the little microwave absorption of TCH. Single ZnFe2 O4 system (without microwave) showed only 2.3% of TCH degradation within 4 min, indicating that the influence of adsorption on TCH was minimal under the same conditions. However, degradation of TCH could reach 42.3% at 4 min when ZnFe2 O4 catalyst combined with microwave, confirming that ZnFe2 O4 has
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 1. The (a) XRD patterns and (b) TEM image for synthesized ZnFe2 O4 ; The UV–vis diffuse reflectance spectra (c) and N2 adsorption-desorption isotherm (d) for ZnFe2 O4 ; The rising temperature curve (e) of ZnFe2 O4 under microwave irradiation.
microwave catalytic activity. According to other previous reports, organic contaminant can be oxidized in MW/ferrite system owing to the “hotspot” (> 10 0 0 °C) [17] which generated on ferrite’s surface. With the high heat, electron may be excited and transferred from valance band to conduction band, leading to the generation of e− -h+ pairs [25] (Eq. 1). A novel increasing of TCH degradation efficiency was observed when a MEDL was introduced into MW/ZnFe2 O4 system and the
TCH degradation reaction well followed pseudo-first order model with high correlation coefficient (R2 = 0.992, Fig. 2b). As shown in Table S1, the reaction rate of TCH degradation was higher than most photocatalytic degradation of TCH, which may be due to the combination between microwave-induced and photocatalytic activity of ZnFe2 O4 (Eq. 2). To identify the contribution of individual photocatalytic reaction in TCH degradation, the experimental setup (Fig. S1b) was modified to establish MEDL/ZnFe2 O4 system. As
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 2. Degradation efficiency of TCH (a) and first order kinetic modeling (b) in different systems; (c) TCH degradation in MEDL/ZnFe2 O4 with and without UV cut-off filter.
presented in Fig. 2c, TCH degradation only showed a little decrease with UV cut-off filter, suggesting that visible-light played the main role in photocatalytic degradation of TCH. Similar to previous studies, ZnFe2 O4 also shown satisfactory visible-light photocatalytic activity in this work, owing to its relatively smaller band gaps [26]. TiO2 is a common catalyst used in microwave-assisted photocatalytic system [5,27]. However, in the emission spectral of MEDL (Fig. S1a), it should be noted that the intensity of peaks at visiblelight region are stronger than the peaks at UV region. Most energy generated from MEDL cannot be utilized by TiO2 . Compared to other reports on microwave-assisted photocatalytic system, visiblelight can be utilized by catalyst in this study, because as-prepared ZnFe2 O4 exhibit strong absorption in whole region of MEDL emission. MW
ZnFe2 O4 −−−→ ZnFe2 O4 (h+ + e− )
(1)
ZnFe2 O4 + hv → ZnFe2 O4 (h+ + e− )
(2)
induce
3.3. Effect of different operation parameters The effect of catalyst dosage on TCH degradation is shown in Fig. 3a. TCH degradation efficiency and reaction rate (Fig. 3b) increased continuously from 56.5% to 91.6% and 0.0037 s−1 to 0.0107 s−1 by 4 min respectively when ZnFe2 O4 dosage increased from 0.5 g/L to 1.5 g/L. However, a slight decrease of degradation efficiency (∼10%) was observed when the catalyst dosage ex-
ceeded 2.0 g/L. It suggested that TCH degradation showed positive dependence on a relatively lower catalyst dosage. A similar trend appeared in visible-light photocatalytic degradation of tetracycline (TC) [28]. For example, Yang et al. [29] found that TC degradation efficiency showed reduction when higher dosage of Ag3 PO4 /Ag/BiVO4 /RGO catalyst was added into the reaction. In this work, higher dosage of ZnFe2 O4 catalyst also revealed detrimental effect on the degradation of tetracycline. Increasing dosage at an appropriate range can benefit the degradation of organic, but an excess of catalyst dosage increases the turbidity of suspension. Decreasing light penetration would reduce the photocatalytic performance [30]. When ZnFe2 O4 dosage was excessive for the reaction, redundant catalyst decreased the solution transmittance to light, so less light could irradiate on the surface of ZnFe2 O4 , which resulted in the inhibition of TCH degradation [27]. From these results, it can be concluded that a ZnFe2 O4 catalyst at 1.5 g/L is optimal dosage for TCH degradation. The effect of MW power output was also investigated. As illustrated in Figs. 3c and 3d, TCH degradation enhanced with the increase of MW output. The degradation and reaction rate were 29.7% and 0.0015 s-1 by 4 min at 100 w, and kept increasing to 91.6% and 0.0107 s-1 at 500 W. Similar trend also could be found in TOC removal with different MW outputs (Fig. 3e). Previous study demonstrates that the light intensity of MEDL can be adjusted by variations in MW power level [31]. The light intensity increased with the increase of MW output. In present work, as more microwave and light energy was introduced into the solution, more
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 3. Effects of ZnFe2 O4 dosage (a) and MW output (c) on TCH degradation efficiency; First order kinetic modeling of TCH degradation with different ZnFe2 O4 dosage (b) and MW output (d); (e) Effect of MW output on TOC removal; (f) Effect of recycle run on TCH degradation efficiency.
h+ could be excited on the surface of ZnFe2 O4 catalyst. This can be the reason for the positive effect of high microwave output on TCH degradation. In recycle tests, TCH degradation efficiency kept above 80% in the fifth run (Fig. 3f), suggesting that MW/MEDL/ZnFe2 O4 would be a promising way for wastewater treatment. The TCH degradation efficiency and reaction rates in acid condition were marginally higher than that in alkaline conditions (Fig. 4a). The effect of initial pH can be explained by the electrostatic interaction between TCH and ZnFe2 O4 . As shown in
Fig. 4b, there are four different kinds of ionic species in aqueous solution within the pH range from 2 to 11 [32]. TCH is considered to exist as a zwitterion between pH 3.3 and 7.7 [33]. When the initial pH value is higher than 7.7, most TCH exists as either monovalent or divalent anionic species. In present work, the pHZPC of ZnFe2 O4 was measured as 6.2. TCH existed both positive and negative charge species when initial pH value was in the range of 5.0 to 7.0. Electrostatic absorption still occurred between TCH and ZnFe2 O4 particles which favor the con-
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 4. Effect of pH on TCH degradation in MW/MEDL/ZnFe2 O4 (a) and (b) tetracycline ionic species. Inset in (a) is the corresponding reaction rate at different pH value.
tact between TCH and h+ . As initial pH is increased to 8.0, the ZnFe2 O4 surface would charge negatively [34]. Most TCH and ZnFe2 O4 existed as fully negatively charged species, leading to the electrostatic repulsion that inhibited the contact between TCH and h+ , which may decrease the degradation of TCH. 3.4. Mechanism of TCH degradation As we all known, active species such as holes (h+ ), hydroxyl radicals (OH• ), and superoxide radicals (O2 −• ) are commonly involved in the photo-degradation process [35]. In this study, t-BuOH, EDTANa2 and p-benzoquinone (BZQ) were serviced as the scavengers of OH• , h+ and O2 −• [26], respectively. As shown in Fig. 5a, 18.8% reduction of TCH degradation was occurred with the addition of t-BuOH, a 79.2% and 10.4% reduction were observed with the addition of EDTANa2 and BZQ, respectively. It suggested that h+ and OH• generated in MW/MEDL/ZnFe2 O4 system and played a role in the microwave-induced photocatalytic degradation of TCH. The generation of h+ species was owing to the hotspot and light excitation of active sites on ZnFe2 O4 surface. The OH• generated from the reaction between h+ and H2 O. This result also indicated that O2 −• had little contribution to the degradation of TCH,
Fig. 5. Degradation of TCH with different scavengers in MW/MEDL/ZnFe2 O4 system (a) and (b) generation pathway of active species.
which is similar to the result of previous reports. The researchers found that microwave-induced [36] or photo-generated [37] electron could not transfer O2 into O2 -• when the conduction band (CB) of catalyst is more positive than the standard redox potential Eθ (O2 / O2 -• ) (−0.33 eV vs. NHE). In present work, the electrons (e- ) generated on the conduction band (CB) of ZnFe2 O4 cannot reduce O2 to yield O2 −• , because the CB edge potential (0.41 eV vs. NHE) [24] of ZnFe2 O4 is more positive than −0.33 eV (NHE). Hence, electron made a negligible contribution in TCH degradation. The reduction of TCH degradation after the addition of EDTANa2 was highest among these scavengers, indicating that h+ was the dominating active species for the degradation of TCH. The proposed generation pathway of active species is shown in Fig. 5b. 3.5. Possible degradation pathway The degradation products identified from the data of LC-MS are presented in Table S2 (ESI + mode) and Table S3 (ESI − mode). Possible degradation pathway (Fig. 6) was deduced from the above results. The mass spectra corresponding to m/z 445 is assigned to TC [38]. The identified products were similar to the results reported by Chen et al. [28] The mass spectra corresponding to m/z 419, m/z 389 and m/z 296 are identified as the products formed from
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 6. Possible degradation pathway of TCH.
detachment of functional groups of TC molecule. The amino group, hydroxyl group or methyl group separated from TC molecule owing to the attack of active species, such as OH• and h+ . Secondly, as the reaction continued, ring-opening products (m/z 364 and m/z 301) were found in TCH degradation. Carboxyl groups were given by the cleavage of double bones, and then detached from the ringopening products by radical oxidation (m/z 270, m/z 224 and m/z 226). Further degradation of these compounds led to the formation of ring-opening product (m/z 243). The formation of oxalic acid (m/z 90) may be attributed to the further degradation of benzene ring products [39].
4. Conclusion This paper reports efficient degradation of TCH using MW/MEDL/ZnFe2 O4 system. A satisfactory degradation (91.6%) of TCH was observed in 4 min. Comparing MW/ZnFe2 O4 process, TCH degradation was enhanced significantly after adding MEDL. A marginal loss of catalytic activity when ZnFe2 O4 was reused in the fifth run. The results of quenching tests indicated that h+ was the dominant active species during TCH degradation and O2 − • had little contribution to the degradation of TCH. In conclusion, The MW/MEDL/ZnFe2 O4 process proposed in this study is a novel
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and environmentally friendly way for the degradation of TCH. This work is also benefit for exploring a novel ferrite-based system in wastewater treatment. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant nos. 51508116, U1501231); The Project of Guangdong Provincial Key Laboratory of radioactive contamination control and resources (Grant No. 2017B030314182). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.08.008. References [1] Remya N. Lin J-G. Current status of microwave application in wastewater treatment—a review. Chem Eng J 2011;166(3):797–813. [2] Liu S-T, Huang J, Ye Y, Zhang A-B, et al. Microwave enhanced Fenton process for the removal of methylene blue from aqueous solution. Chem Eng J 2013;215-216:586–90. [3] Li S, Zhang G, Wang P, Zheng H, et al. Microwave-enhanced Mn-Fenton process for the removal of BPA in water. Chem Eng J 2016;294:371–9. [4] Patil NN, Shukla SR. Degradation of Reactive Yellow 145 dye by persulfate using microwave and conventional heating. J Water Process Eng 2015;7:314–27. [5] Yu Y, Zhang T, Zheng L, Yu J. Photocatalytic degradation of hydrogen sulfide using TiO2 film under microwave electrodeless discharge lamp irradiation. Chem Eng J 2013;225:9–15. [6] Horikoshi S, Serpone N. Coupled microwave/photoassisted methods for environmental remediation. Molecules 2014;19(11):18102–28. [7] Xiong Z, Xu A, Li H, Ruan X, et al. Highly efficient photodegradation of alizarin green in TiO2 suspensions using a microwave powered electrodeless discharged lamp. Ind Eng Chem Res 2013;52(1):362–9. [8] Horikoshi S, Tsuchida A, Shinomiya T, Serpone N. Microwave discharge electrodeless lamps (MDELs). Part IX. A novel MDEL photoreactor for the photolytic and chemical oxidation treatment of contaminated wastewaters. Photochem Photobiol Sci 2015;14(12):2187–94. [9] Chae J-S, Jung D-S, Bae Y-S, Park SH, et al. Photo-catalytic degradation of rhodamine B using microwave powered electrodeless discharge lamp. Korean J Chem Eng 2010;27(2):672–6. [10] Zhang X, Wang Y, Li G. Effect of operating parameters on microwave assisted photocatalytic degradation of azo dye X-3B with grain TiO2 catalyst. J Mol Catal A-Chem 2005;237(1-2):199–205. [11] Horikoshi S, Matsubara A, Takayama S, Sato M, et al. Characterization of microwave effects on metal-oxide materials: Zinc oxide and titanium dioxide. Appl Catal B: Environ 2009;91(1-2):362–7. [12] Hong Y, Li C, Zhang G, Meng Y, et al. Efficient and stable Nb2 O5 modified g-C3 N4 photocatalyst for removal of antibiotic pollutant. Chem Eng J 2016;299:74–84. [13] Pang Y, Lei H. Degradation of p-nitrophenol through microwave-assisted heterogeneous activation of peroxymonosulfate by manganese ferrite. Chem Eng J 2016;287:585–92. [14] Ibrahim I, Ali IO, Salama TM, Bahgat AA, et al. Synthesis of magnetically recyclable spinel ferrite (MFe2 O4 , M=Zn, Co, Mn) nanocrystals engineered by sol gel-hydrothermal technology: high catalytic performances for nitroarenes reduction. Appl Catal B: Environ 2016;181:389–402. [15] Guan YH, Ma J, Ren YM, Liu YL, et al. Efficient degradation of atrazine by magnetic porous copper ferrite catalyzed peroxymonosulfate oxidation via the formation of hydroxyl and sulfate radicals. Water Res 2013;47(14):5431–8. [16] Wu J, Pu W, Yang C, Zhang M, et al. Removal of benzotriazole by heterogeneous photoelectro-Fenton like process using ZnFe2 O4 nanoparticles as catalyst. J Environ Sci: China 2013;25(4):801–7. [17] Fang X, Xiao J, Yang S, He H, et al. Investigation on microwave absorbing properties of loaded MnFe2 O4 and degradation of Reactive Brilliant Red X-3B. Appl Catal B: Environ 2015;162:544–50.
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Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution Yixiong Pang a,b,c, Lingjun Kong a,∗, Hengyi Lei b,∗, Diyun Chen a, Gutha Yuvaraja a,c a
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, PR China School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510006, PR China c School of Civil Engineering, Guangzhou University, Guangzhou 510006, PR China b
a r t i c l e
i n f o
Article history: Received 18 April 2018 Revised 26 July 2018 Accepted 3 August 2018 Available online xxx Keywords: Tetracycline hydrochloride ZnFe2 O4 Microwave-induced Photocatalytic
a b s t r a c t In this research, ZnFe2 O4 was employed as heterogeneous catalyst for the degradation of tetracycline hydrochloride (TCH) by a novel combined microwave-induced (MW-induced) and photocatalytic oxidation. The ZnFe2 O4 catalyst was synthesized using co-precipitation method, and characterized by XRD, TEM, BET and UV–Vis DRS. A microwave electrodeless discharge lamp (MEDL) was introduced to this process as a light source. 91.6% of TCH degradation was obtained in MW/MEDL/ZnFe2 O4 system in 4 min. ZnFe2 O4 catalyst had both microwave-catalytic and visible-light photocatalytic activities. The operation parameters, such as microwave output, catalyst dosage, initial pH and recycle runs, showed different influence on TCH degradation. The analysis of degradation mechanism indicated that h+ was the main active species • for TCH degradation and little O2 − active species generated in MW/MEDL/ZnFe2 O4 system. The decomposition intermediates of TCH were identified by LC-MS. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Tetracycline hydrochloride (TCH), which widely used for antibacteria in the world, can be easily discharged into the river, soil, and even drinking water. TCH in the environment is considered as a potential risk for human living because it increases the antibiotics resistant of some pathogenic bacteria. Meanwhile, TCH may persist in the environment owing to their non-biodegradation. Consequently, efficient removal of TCH from wastewater is a great challenge for the scientists. Microwave-induced catalytic oxidation (MICO) has been widely reported as one of the most efficient AOPs, owing to its simple reaction conditions and high reaction speed [1]. Several methods of MICO are conducted to treat organic pollutant in wastewater, such as MW-Fenton [2], MW-Fenton-like [3], MW-persulfate [4] and MW-MEDL-photocatalyst [5]. Among these methods, MWMEDL-photocatalyst process may be considered as a novel way in wastewater treatment, because it combines the advantages of microwave and photocatalysis [6]. This process uses MEDL (microwave electrodeless discharge lamp) as light source which pro-
∗
Corresponding authors. E-mail addresses: [email protected] (L. Kong), [email protected] (H. Lei).
vides both ultraviolet and visible light. This system showed excellent performance for degrading organic pollutants, such as alizarin green [7], 2,4-D herbicide [8] and rhodamine B [9]. However, the light irradiances of ultraviolet wavelengths for MEDL are smaller than long wavelengths and they played only a minor role in the photochemical process [10]. Most photocatalysts that applied in MW/MEDL system are only excited by UV irradiation, such as TiO2 and ZnO [11]. MW/MEDL/photocatalyst process still have a number of drawbacks, the biggest being the insufficient utilization of energy owing to the poor microwave and visible light absorption of catalysts. Therefore, the main concern is finding out an appropriate catalyst for MW/MEDL/catalyst system. Ferrite, expressed as MFe2 O4 (M = Zn, Co, Mn or Cu), have been an area of activated research during the last several years [12–14]. They are widely used in environmental treatments because of their easy magnetic collection, high catalytic activity and good chemical stability [15,16]. The microwave-induced catalytic activity of ferrites have gained more attention [17,18] as a cleaner technology for wastewater treatment. Ferrites such as MnFe2 O4 [17] or NiFe2 O4 [19] were successfully employed for the degradation of organic pollutant from wastewater via MICO. In MW/ferrite process, electron-hole pairs generate on the surface of ferrite, which can oxide organic pollutant without other peroxide reagents [20]. Meanwhile, some researchers also have found that ferrites have
https://doi.org/10.1016/j.jtice.2018.08.008 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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photocatalytic activity [21]. Particularly, ZnFe2 O4 has better visiblelight photocatalytic activity owing to its relatively smaller band gaps (1.9 eV). However, most researchers only focus on its catalytic activity in photocatalysis process. Recently, High degradation percentage of organic dye was observed in MW/ZnFe2 O4 system [22], suggesting that ZnFe2 O4 might also have excellent microwave catalytic activity as other ferrites. Thus, the potential of ZnFe2 O4 in wastewater treatment has not been fully explored. Based on these reports, it can be expected that ZnFe2 O4 may reveal better performance in MW/MEDL system than in single MW or photocatalytic system. To the best of our knowledge, there is no report focusing on the combination of microwave-induced catalytic and photocatalytic activities of ZnFe2 O4 . In present work, degradation of organic contaminant in MW/MEDL/ZnFe2 O4 system is reported for the first time. ZnFe2 O4 is synthesized using co-precipitation method and characterized by TEM, XRD, UV–vis DRS and BET. MEDL is introduced to this process as a light source for exciting photocatalytic activity of ZnFe2 O4 under microwave irradiation. In this study, catalytic performance of ZnFe2 O4 is evaluated using TCH as a model pollutant. The effects of several operation parameters are investigated, such as catalyst dosage, microwave output, initial pH and reuse times. The mechanism of TCH degradation in MW/MEDL/ZnFe2 O4 is also discussed.
In quenching experiments, tert–butanol (TBA, C4 H9 OH), pbenzoquinone (C6 H4 O2 ) and EDTANa2 were added to the suspension before TCH degradation, respectively. In recycle experiments, the used catalysts were separated, and then washed with distilled water and dried at 65 °C before reused. The residual solutions were measured by a UV-2550 UV–vis spectrophotometer at 358 nm [23]. TCH degradation efficiency and reaction constant were calculated by Eq. S1 and Eq. S2, respectively.
2. Materials and methods
3. Results and discussion
2.1. Chemicals
3.1. Characterization
Analytical-reagent grade tetracycline hydrochloride (TCH, C22 H25 ClN2 O8 ), tert–butanol (TBA, C4 H9 OH), p-benzoquinone (C6 H4 O2 ) and EDTANa2 were purchased from Sigma-Aldrich. Chromatographic grade methanol (CH3 OH) and formic acid (HCOOH) were purchased from Sigma-Aldrich. Analytical-reagent grade Zn(NO3 )2 •6H2 O, Fe(NO3 )3 •9H2 O, NaOH and HCl were purchased from Sinopharm Chemical Reagent Co., Ltd., China.
The XRD patterns of as-prepared ZnFe2 O4 are presented in Fig. 1a. The diffraction peaks observed at 18.2°, 29.9°, 35.3°, 42.8°, 56.6° and 62.2° represent the Bragg reflection from the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 4 0) and (5 3 1) planes, respectively. These peaks can be well indexed to ZnFe2 O4 (JCPDS 22–1012). The XRD pattern of the ZnFe2 O4 catalyst used 5 runs is nearly the same as that of fresh catalyst, confirming that ZnFe2 O4 catalysts are stable in microwave-induced photocatalytic reaction. The TEM image of synthesized ZnFe2 O4 is shown in Fig. 1b. As can be seen, ZnFe2 O4 nanoparticles exhibit a diameter of 30–50 nm without serious aggregation. The UV–vis absorption spectra (Fig. 1c) indicated that ZnFe2 O4 absorbs the light in the region ranging from 200 to 600 nm, which is similar to the previous results [24]. Therefore, the MEDL, with emission spectra from 200 to 600 nm (Fig. S1a) can activate the as-prepared catalyst. The N2 adsorption–desorption isotherm of ZnFe2 O4 nanoparticles is shown in Fig. 1d. According to the IUPAC classification, the as-prepared ZnFe2 O4 sample exhibited the type IV isotherm. This indicates that the synthesized ZnFe2 O4 has mesoporous structure. The BET surface area of synthesized ZnFe2 O4 is calculated to be 50.05 m2 /g. To verify the microwave absorbing ability of ZnFe2 O4 , the rising temperature of as-prepared ZnFe2 O4 under microwave irradiation was measured. As shown in Fig. 1e, the temperature of as-prepared sample exceeded 420 °C within 6 min in the present of microwave irradiation (500 W), which indicates that ZnFe2 O4 is an excellent microwave absorbing material. Therefore, ZnFe2 O4 may show satisfying performance in microwave-induced photocatalytic systems.
2.2. Synthesis of ZnFe2 O4 ZnFe2 O4 was synthesized using the co-precipitation method. 0.01 mol Zn(NO3 )2 •6H2 O and 0.02 mol Fe(NO3 )3 •9H2 O were dissolved in 200 mL distilled water. Then 3 mol/L NaOH was added dropwise to the solution till pH around 10 with continuous magnetic-stirring. The temperature was maintained at 85 °C in the whole process. The precipitate was filtered, washed with double distilled water, and dried at 110 °C overnight. Then this precursor sample was calcined at 400 °C in a muffle furnace for 2 h. 2.3. Degradation of TCH The experiments were carried out in a modified microwave oven. Typically, the initial concentration of TCH, ZnFe2 O4 dosage and microwave output were fixed at 50 mg/L, 1.5 g/L and 500 w in each experiment, respectively, and the value of initial pH is 6. The as-prepared ZnFe2 O4 was introduced into TCH solution, followed by an adjustment of the initial pH. The total mixed solution of 50 mL was poured into a 500 mL quartz glass beaker. Before MW irradiation, the suspension was magnetically stirred in the dark for 60 min to ensure adsorption-desorption equilibrium. The reaction vessel was then exposed to MW irradiation for microwave-induced photocatalytic degradation, following an introduction of MEDL. The detail of experimental setups was depicted in Supplementary Material. The emission spectral of MEDL and the diagrams of experimental setups is shown in Fig. S1. Each degradation experiment was performed in duplicate within accepting error range (± 5%), and all results were expressed as mean value with error bar.
2.4. Analysis The crystalline phase of as-prepared materials was characterized using XRD. The morphology and structure of as-prepared materials were determined by TEM. The BET surface area of the asprepared materials was carried out by the nitrogen adsorption– desorption method using a NOVA40 0 0e surface analyzer. The ultraviolet-visible diffuse reflectance spectrum of ZnFe2 O4 was recorded in a UV-2501 PC UV–Visible spectrometer. The pHzpc of ZnFe2 O4 was determined by mass titration and the pH value was measured by a pH meter. The spectral emission of MEDL was measured by fiber spectrometer (USB200). The intermediates of TCH degradation was measured by LC-MS on Agilent 1100MSD spectrometry equipped with a C18 column. The detail condition of LCMS experiment is shown in Supplementary Material.
3.2. Degradation of TCH in different systems As shown in Fig. 2a, negligible degradation was obtained in single MW system owing to the little microwave absorption of TCH. Single ZnFe2 O4 system (without microwave) showed only 2.3% of TCH degradation within 4 min, indicating that the influence of adsorption on TCH was minimal under the same conditions. However, degradation of TCH could reach 42.3% at 4 min when ZnFe2 O4 catalyst combined with microwave, confirming that ZnFe2 O4 has
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 1. The (a) XRD patterns and (b) TEM image for synthesized ZnFe2 O4 ; The UV–vis diffuse reflectance spectra (c) and N2 adsorption-desorption isotherm (d) for ZnFe2 O4 ; The rising temperature curve (e) of ZnFe2 O4 under microwave irradiation.
microwave catalytic activity. According to other previous reports, organic contaminant can be oxidized in MW/ferrite system owing to the “hotspot” (> 10 0 0 °C) [17] which generated on ferrite’s surface. With the high heat, electron may be excited and transferred from valance band to conduction band, leading to the generation of e− -h+ pairs [25] (Eq. 1). A novel increasing of TCH degradation efficiency was observed when a MEDL was introduced into MW/ZnFe2 O4 system and the
TCH degradation reaction well followed pseudo-first order model with high correlation coefficient (R2 = 0.992, Fig. 2b). As shown in Table S1, the reaction rate of TCH degradation was higher than most photocatalytic degradation of TCH, which may be due to the combination between microwave-induced and photocatalytic activity of ZnFe2 O4 (Eq. 2). To identify the contribution of individual photocatalytic reaction in TCH degradation, the experimental setup (Fig. S1b) was modified to establish MEDL/ZnFe2 O4 system. As
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 2. Degradation efficiency of TCH (a) and first order kinetic modeling (b) in different systems; (c) TCH degradation in MEDL/ZnFe2 O4 with and without UV cut-off filter.
presented in Fig. 2c, TCH degradation only showed a little decrease with UV cut-off filter, suggesting that visible-light played the main role in photocatalytic degradation of TCH. Similar to previous studies, ZnFe2 O4 also shown satisfactory visible-light photocatalytic activity in this work, owing to its relatively smaller band gaps [26]. TiO2 is a common catalyst used in microwave-assisted photocatalytic system [5,27]. However, in the emission spectral of MEDL (Fig. S1a), it should be noted that the intensity of peaks at visiblelight region are stronger than the peaks at UV region. Most energy generated from MEDL cannot be utilized by TiO2 . Compared to other reports on microwave-assisted photocatalytic system, visiblelight can be utilized by catalyst in this study, because as-prepared ZnFe2 O4 exhibit strong absorption in whole region of MEDL emission. MW
ZnFe2 O4 −−−→ ZnFe2 O4 (h+ + e− )
(1)
ZnFe2 O4 + hv → ZnFe2 O4 (h+ + e− )
(2)
induce
3.3. Effect of different operation parameters The effect of catalyst dosage on TCH degradation is shown in Fig. 3a. TCH degradation efficiency and reaction rate (Fig. 3b) increased continuously from 56.5% to 91.6% and 0.0037 s−1 to 0.0107 s−1 by 4 min respectively when ZnFe2 O4 dosage increased from 0.5 g/L to 1.5 g/L. However, a slight decrease of degradation efficiency (∼10%) was observed when the catalyst dosage ex-
ceeded 2.0 g/L. It suggested that TCH degradation showed positive dependence on a relatively lower catalyst dosage. A similar trend appeared in visible-light photocatalytic degradation of tetracycline (TC) [28]. For example, Yang et al. [29] found that TC degradation efficiency showed reduction when higher dosage of Ag3 PO4 /Ag/BiVO4 /RGO catalyst was added into the reaction. In this work, higher dosage of ZnFe2 O4 catalyst also revealed detrimental effect on the degradation of tetracycline. Increasing dosage at an appropriate range can benefit the degradation of organic, but an excess of catalyst dosage increases the turbidity of suspension. Decreasing light penetration would reduce the photocatalytic performance [30]. When ZnFe2 O4 dosage was excessive for the reaction, redundant catalyst decreased the solution transmittance to light, so less light could irradiate on the surface of ZnFe2 O4 , which resulted in the inhibition of TCH degradation [27]. From these results, it can be concluded that a ZnFe2 O4 catalyst at 1.5 g/L is optimal dosage for TCH degradation. The effect of MW power output was also investigated. As illustrated in Figs. 3c and 3d, TCH degradation enhanced with the increase of MW output. The degradation and reaction rate were 29.7% and 0.0015 s-1 by 4 min at 100 w, and kept increasing to 91.6% and 0.0107 s-1 at 500 W. Similar trend also could be found in TOC removal with different MW outputs (Fig. 3e). Previous study demonstrates that the light intensity of MEDL can be adjusted by variations in MW power level [31]. The light intensity increased with the increase of MW output. In present work, as more microwave and light energy was introduced into the solution, more
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Fig. 3. Effects of ZnFe2 O4 dosage (a) and MW output (c) on TCH degradation efficiency; First order kinetic modeling of TCH degradation with different ZnFe2 O4 dosage (b) and MW output (d); (e) Effect of MW output on TOC removal; (f) Effect of recycle run on TCH degradation efficiency.
h+ could be excited on the surface of ZnFe2 O4 catalyst. This can be the reason for the positive effect of high microwave output on TCH degradation. In recycle tests, TCH degradation efficiency kept above 80% in the fifth run (Fig. 3f), suggesting that MW/MEDL/ZnFe2 O4 would be a promising way for wastewater treatment. The TCH degradation efficiency and reaction rates in acid condition were marginally higher than that in alkaline conditions (Fig. 4a). The effect of initial pH can be explained by the electrostatic interaction between TCH and ZnFe2 O4 . As shown in
Fig. 4b, there are four different kinds of ionic species in aqueous solution within the pH range from 2 to 11 [32]. TCH is considered to exist as a zwitterion between pH 3.3 and 7.7 [33]. When the initial pH value is higher than 7.7, most TCH exists as either monovalent or divalent anionic species. In present work, the pHZPC of ZnFe2 O4 was measured as 6.2. TCH existed both positive and negative charge species when initial pH value was in the range of 5.0 to 7.0. Electrostatic absorption still occurred between TCH and ZnFe2 O4 particles which favor the con-
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Fig. 4. Effect of pH on TCH degradation in MW/MEDL/ZnFe2 O4 (a) and (b) tetracycline ionic species. Inset in (a) is the corresponding reaction rate at different pH value.
tact between TCH and h+ . As initial pH is increased to 8.0, the ZnFe2 O4 surface would charge negatively [34]. Most TCH and ZnFe2 O4 existed as fully negatively charged species, leading to the electrostatic repulsion that inhibited the contact between TCH and h+ , which may decrease the degradation of TCH. 3.4. Mechanism of TCH degradation As we all known, active species such as holes (h+ ), hydroxyl radicals (OH• ), and superoxide radicals (O2 −• ) are commonly involved in the photo-degradation process [35]. In this study, t-BuOH, EDTANa2 and p-benzoquinone (BZQ) were serviced as the scavengers of OH• , h+ and O2 −• [26], respectively. As shown in Fig. 5a, 18.8% reduction of TCH degradation was occurred with the addition of t-BuOH, a 79.2% and 10.4% reduction were observed with the addition of EDTANa2 and BZQ, respectively. It suggested that h+ and OH• generated in MW/MEDL/ZnFe2 O4 system and played a role in the microwave-induced photocatalytic degradation of TCH. The generation of h+ species was owing to the hotspot and light excitation of active sites on ZnFe2 O4 surface. The OH• generated from the reaction between h+ and H2 O. This result also indicated that O2 −• had little contribution to the degradation of TCH,
Fig. 5. Degradation of TCH with different scavengers in MW/MEDL/ZnFe2 O4 system (a) and (b) generation pathway of active species.
which is similar to the result of previous reports. The researchers found that microwave-induced [36] or photo-generated [37] electron could not transfer O2 into O2 -• when the conduction band (CB) of catalyst is more positive than the standard redox potential Eθ (O2 / O2 -• ) (−0.33 eV vs. NHE). In present work, the electrons (e- ) generated on the conduction band (CB) of ZnFe2 O4 cannot reduce O2 to yield O2 −• , because the CB edge potential (0.41 eV vs. NHE) [24] of ZnFe2 O4 is more positive than −0.33 eV (NHE). Hence, electron made a negligible contribution in TCH degradation. The reduction of TCH degradation after the addition of EDTANa2 was highest among these scavengers, indicating that h+ was the dominating active species for the degradation of TCH. The proposed generation pathway of active species is shown in Fig. 5b. 3.5. Possible degradation pathway The degradation products identified from the data of LC-MS are presented in Table S2 (ESI + mode) and Table S3 (ESI − mode). Possible degradation pathway (Fig. 6) was deduced from the above results. The mass spectra corresponding to m/z 445 is assigned to TC [38]. The identified products were similar to the results reported by Chen et al. [28] The mass spectra corresponding to m/z 419, m/z 389 and m/z 296 are identified as the products formed from
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Fig. 6. Possible degradation pathway of TCH.
detachment of functional groups of TC molecule. The amino group, hydroxyl group or methyl group separated from TC molecule owing to the attack of active species, such as OH• and h+ . Secondly, as the reaction continued, ring-opening products (m/z 364 and m/z 301) were found in TCH degradation. Carboxyl groups were given by the cleavage of double bones, and then detached from the ringopening products by radical oxidation (m/z 270, m/z 224 and m/z 226). Further degradation of these compounds led to the formation of ring-opening product (m/z 243). The formation of oxalic acid (m/z 90) may be attributed to the further degradation of benzene ring products [39].
4. Conclusion This paper reports efficient degradation of TCH using MW/MEDL/ZnFe2 O4 system. A satisfactory degradation (91.6%) of TCH was observed in 4 min. Comparing MW/ZnFe2 O4 process, TCH degradation was enhanced significantly after adding MEDL. A marginal loss of catalytic activity when ZnFe2 O4 was reused in the fifth run. The results of quenching tests indicated that h+ was the dominant active species during TCH degradation and O2 − • had little contribution to the degradation of TCH. In conclusion, The MW/MEDL/ZnFe2 O4 process proposed in this study is a novel
Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008
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Please cite this article as: Y. Pang et al., Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.008