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Rare Met. DOI 10.1007/s12598-017-0940-7

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Preparation of Fe3O4@C@TiO2 and its application for oxytetracycline hydrochloride adsorption Qi-Li Hu, Lin-Shan Wang* Xiao-Min Hu

, Nan-Nan Yu, Ze-Fei Zhang, Xin Zheng,

Received: 5 August 2016 / Revised: 13 November 2016 / Accepted: 18 June 2017 Ó The Nonferrous Metals Society of China and Springer-Verlag GmbH Germany 2017

Abstract The magnetic Fe3O4@C@TiO2 microspheres with multilevel yolk–shell structure were successfully prepared by combining sol–gel and simple hydrothermal methods. The features of the as-obtained Fe3O4@C@TiO2 microspheres were investigated by Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), N2 adsorption– desorption measurements and transmission electron microscopy (TEM). Fe3O4@C@TiO2 was used as an adsorbent to explore its adsorption properties of oxytetracycline hydrochloride (OTC-HCl) by changing initial concentration and time. The results suggest that the maximum adsorption of Fe3O4@C@TiO2 is 87.3 mgg-1, and the time reaching the absorption equilibrium is 60 min. Langmuir model fits to data better than the Freundlich model, and the kinetic properties are well described by the pseudo-second-order model. In addition, the synthesized composites’ reusability without obvious deterioration in performance is demonstrated by three cycles. Keywords Fe3O4@C@TiO2; Adsorption; Oxytetracycline hydrochloride; Recycle

Q.-L. Hu, L.-S. Wang*, N.-N. Yu, Z.-F. Zhang, X. Zheng College of Science, Northeastern University, Shenyang 110819, China e-mail: [email protected] X.-M. Hu College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China

1 Introduction Nowadays, the safety of water gained extensive attention of every country [1]. Antibiotics, as a kind of antibacterial compounds, have aroused particular concern because of their excessive application to human beings, agriculture and planting [2, 3]. As their ineffective metabolism and low absorption of the treated species, a large number of antibiotics are let off through the excretory system [4, 5]. As for it, more and more scientists pay attention to the research of the abuse of antibiotics. Antibiotics are chemical substance produced by microorganism at low temperature; they are often used to cure the infection of human and animals. Besides, people often use antibiotics as growth promoters in animal husbandry and aquaculture industry [6–9]. According to the chemical structure, antibiotics are divided into several species: penicillin, aminoglycoside, tetracycline, chloramphenicol and forests amide. Almost all those species include circles; according to the classification, different structures mean different properties [10]. As the stability of antibiotics and the bacteriostatic action of itself, natural biodegradation process cannot remove from the environment effectively and it can cause accumulation from day to day [8]. At present, many countries have detected the existence of antibiotics in their rivers, lakes [11, 12], underground water and even drinking water. Lots of developed countries have regarded the antibiotics pollution as the most serious environmental problem, and researches in the removal of antibiotics are developing rapidly now. Oxytetracycline hydrochloride is a tetracycline-class broad-spectrum antibiotic that can cure the diseases of human; it is also one of the most effective medicines for livestock breeding and aquaculture. China has produced more than 10,000 ton oxytetracycline hydrochloride in 2003, which is about 65%

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of the world’s total. Meanwhile, China is a sales power of tetracycline drugs and the export of the drug has reached 13,400 ton in 2008 [13]. But the abuse of oxytetracycline hydrochloride and the accumulation in the water lead to a large number of resistance genes and threaten the health of human being [14, 15]. So, it is necessary to find a way to remove this type of antibiotics. At present, there are many kinds of antibiotic wastewater processing methods, mainly including solvent extraction, supercritical fluid extraction, microwave extraction technology, membrane separation technology as well as solid-phase extraction [16]. Compared with other methods, adsorption method is wildly used because of easy operation and low requirement for objective conditions. As an active photocatalyst, TiO2 is wildly used in the field of wastewater treatment and degradation of organic matter [17]. In the process of photocatalysis, almost all the free radicals generate and annihilate on the surface catalyst, which means that the reaction is occurred on the surface. So the research about the property of catalyst is important [18], and it is also essential to study the adsorption of TiO2. As the antibiotics difficulties of powder TiO2, Fe3O4 is used to solve this problem. The magnetic quality of this material can not only solve problem of recycle but also avoid secondary pollution. But the structure of Fe3O4 may affect the photocatalysis property of TiO2; with the purpose of eliminating the influence of Fe3O4 to TiO2, inert layer was added between Fe3O4 and TiO2 [19, 20]. But almost all those researches are about the photocatalytic performance, and the length of dark processing time is 30 min. Referring to their synthetic method, the adsorption property of this particle is discussed, and the result of this paper may give a better figure about the length of processing time. Herein, it was carefully designed and synthesized a Fe3O4@C@TiO2 yolk–shell-structured catalyst which dressed both the catalytic activity and recovery issues for adsorption. Fe3O4@C@TiO2 was synthesized by hydrothermal method, in accordance with Fe3O4 and Fe3O4@C. Oxytetracycline hydrochloride (OTC-HCl) was set as the model adsorbate for the first time; the concentration and time span were varied to get the maximum adsorption and adsorption equilibrium time. Meanwhile, fitting of kinetic and adsorption isotherm models was studied in this paper, which could be helpful to comprehend the process of adsorption. Lastly, the property of reutilization was also studied.

from Sinopharm Chemical Reagent Co., Ltd. (Shenyang, China). All chemicals were of analytical grade and used without further purification. In this study, only deionized water was used. 2.2 Characterization Images of the materials and their surface elements were obtained by scanning electron microscopy (SEM, Ultra Plus, Carl Zeiss AG, Germany) equipped with energydispersive spectroscopy at an accelerating voltage of 15.0 kV. Transmission electron microscopy (TEM) images were obtained using a Tecnai G220 (Fei, USA) microscope. The specific surface area, N2 adsorption–desorption isotherms and pore size distribution were determined using the Brunauer–Emmett–Teller (BET) theory with a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). The samples were degassed at 150 °C for 12 h before N2 adsorption–desorption analysis. Crystal structures were obtained by X-ray diffractometer (XRD, PAN analytical, Holland) in 2h range from 20° to 80° at a scanning rate of 6 (°)min-1, with graphite-monochromatized CuKa radiation (k = 0.15406 nm) and a nickel filter. These Fourier transform infrared spectra (FTIR, Bruker, Germany) were recorded on from 400 to 4000 cm-1 using KBr disks. 2.3 Preparation of Fe3O4 nanoparticles and Fe3O4@C nanoparticles The magnetic particles were synthesized using a solvothermal reaction [21]. FeCl36H2O (2.04 g), CH3COONa (NaAc) (3.6 g) and C10H16N2O8–2Na (EDTA-2Na) (0.102 g) were dissolved in ethylene glycol (60 ml). After sonication treatment for 5 min, the dark yellow colloidal solution was transferred to a Teflon-lined stainless-steel autoclave. It was heated at 200 °C for 10 h and then naturally cooled down to room temperature. After that, the black precipitate was washed with ethanol three times and then dried at 60 °C. The Fe3O4 nanoparticles (0.3 g) were added to 60 ml 0.25 molL-1 glucose solution and then transferred and sealed into a Teflon-lined stainless-steel autoclave (100 ml in capacity). The autoclave was heated at 180 °C for 6 h and then allowed to cool to room temperature. The obtained black products Fe3O4@C were washed with deionized water three times and dried at 60 °C for about 12 h in vacuum drying oven.

2 Experimental 2.4 Preparation of Fe3O4@C@TiO2 microspheres 2.1 Materials OTC-HCl was purchased from Yeyuan Biotech Company (Shanghai, China). All other chemicals were purchased

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The Fe3O4@C@TiO2 core–shell microspheres were synthesized in the mixed solvent of ethanol and acetonitrile at room temperature by hydrolyzing tetrabutyl titanate Rare Met.

Preparation of Fe3O4@C@TiO2 and its application for oxytetracycline hydrochloride adsorption

(TBOT) in the presence of ammonia [19]. Allocate solution consists of ethanol and acetonitrile (volume ratio of 3:1), and two pieces of 40 ml (A) and 10 ml (B) were taken. In total, 0.2 g Fe3O4@C nanoparticles were added to A and then ultrasonically washed for 10 min. After that, the system was transferred into three flasks and fitted with electric mixer. 0.3 ml of ammonia was added to the three flasks and continued stirring for 30 min. In addition, 1 ml of tetrabutyl titanate was added to B in the condition of stirring with glass rod and ice water and continued stirring for 2 h after dripping. The as-prepared power was washed repeatedly with water and ethanol several times and dried at 60 °C for 12 h. 2.5 Adsorption of OTC-HCl by Fe3O4@C@TiO2 microspheres Fifty milligrams of Fe3O4@C@TiO2 microspheres was mixed with 1 L OTC-HCl solution (0–50 mgL-1). The mixture was shaken in oscillator (Shanghai, China) with 150 rmin-1 for 0–120 min at 25 °C to investigate the influence of contact time and initial concentrations on OTC-HCl adsorption. The Fe3O4@C@TiO2 microspheres were isolated from the solution magnetically, and the amount of adsorbed OTC-HCl was measured by the concentration change in the solution using ultraviolet–visible (UV–Vis) adsorption spectrophotometry (TU-1900, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) at 353 nm. The adsorbed amount of OTC-HCl (qt, mgg-1) is calculated using Eq. (1): qt ¼ ðc0  ct ÞV=W

ð1Þ

where c0 (mgml-1) is the initial OTC-HCl concentration, ct (mgml-1) is OTC-HCl concentration in supernatant at time (t, min), V (L) is the volume of OTC-HCl solution used and W (g) is the weight of Fe3O4@C@TiO2 microspheres used.

3 Results and discussion 3.1 Characterization of catalysts Figure 1 depicts FTIR spectra of Fe3O4, Fe3O4@C and Fe3O4@C@TiO2 microspheres. FTIR bands at low wave numbers (^700 cm-1) are obtained from vibrations of Fe– O bonds of iron oxide. The presence of magnetic nanoparticles can be seen by two strong absorption bands at around 632 and 585 cm-1 [22, 23], and Fe–O bond of bulk magnetite band can be seen at 569 cm-1 [24, 25]. The peaks at 1613 and 1706 cm-1 in Fig. 1b are ascribed to C=C and C=O vibrations [26], respectively, indicating the carbonization of glucose during hydrothermal reaction.

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Fig. 1 FTIR spectra of Fe3O4, Fe3O4@C and Fe3O4@C@TiO2 microspheres

Figure 1c shows that the broad adsorption band of TiO2 below 1000 cm-1 could be attributed to Ti–O–Ti vibration [27], and the peak of 1580 cm-1 also shows the stretch of Ti–O. Nevertheless, the crystal form of TiO2 and the cladding state should affirm through the detection of XRD and TEM. SEM images of as-prepared Fe3O4, Fe3O4@C and Fe3O4@C@TiO2 are shown in Fig. 2. The microspheres of Fe3O4 (Fig. 2a) exhibit regular spherical shape, and the nanoparticles disperse well; the morphology in Fig. 2b seems similar to that in Fig. 2a, which shows that the cladding of carbon does not change the shape and dispersing properties. After a further modification, TiO2 particles are highly dispersed on the surface of Fe3O4@C spheres, and the content of TiO2 is about 15.30 wt%, meaning that the cladding of TiO2 is not very good; the shape of this microsphere is the same as the former one. Figure 3(3) also shows that the dispersity of microspheres of Fe3O4@C@TiO2 is worse than that of Fe3O4@C. So, it is aimed to obtain the well-dispersed nanoparticles by synthesizing the new samples, which were sonicated for a longer time. The crystal structure and composition of Fe3O4@C@TiO2 microspheres were identified by XRD. A series of characteristic peaks of Fe3O4 at 2h of around 30.1°, 35.5°, 43.2°, 53.4°, 57.2° and 62.8° are, respectively, related to the reflections of (220), (311), (400), (422), (511) and (440) planes of magnetite Fe3O4 in Fig. 3(1), well indexed to the typical cubic inverse spinel structure (JCPDS No. 19-629) [28]. As the result of the carbon layer is amorphous and the crystal phase of Fe3O4 microspheres does not change during hydrothermal reaction with glucose, there are no other peaks observed in Fig. 3(2) [26]. In Fig. 3(3), the anatase TiO2 is formed on Fe3O4@C microspheres after solvothermal treatment, and peaks at 25.3°, 37.8°, 48.2°, 54.2°, 55.3° and 62.7° (2h) are observed, which are assigned to (101), (004), (200), (105), (211) and (204) planes of TiO2 with anatase tetragonal structure (JCPDS No. 21-1272) [29].

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Fig. 2 SEM images of a Fe3O4 microspheres, b Fe3O4@C and c Fe3O4@C@TiO2 microspheres

Fig. 3 XRD patterns of (1) Fe3O4, (2) Fe3O4@C and (3) Fe3O4@C@TiO2 microspheres

The morphology and structure of as-prepared samples were investigated by TEM. As shown in Fig. 4a, Fe3O4 microspheres have spherical shape with a diameter of about 180 nm. In contract with TiO2 in Fig. 4d, the carboxyl groups in EDTA-2Na stabilizer provide Fe3O4 microspheres with outstanding dispersibility in the solvents [30]. Figure 4b presents TEM image of Fe3O4@C, and it can be seen that these microspheres show a core– shell structure in which the black Fe3O4 is encapsulated into thin bright white carbon shell with about 10 nm in thickness. As a result of carbonization of glucose, the shell is proved to possess plenty of hydrophilic groups by FTIR analysis (Fig. 1), and this core–shell can not only make the surface of Fe3O4@C magnetic microspheres turn to be negatively charged but also offer an inert layer between Fe3O4 and TiO2 species [31]. As shown in Fig. 4c, Fe3O4@C@TiO2 microspheres present a core– shell structure and the black Fe3O4@C particles are coated with gray TiO2 shell that is about 40 nm in thickness. When coated by TiO2, positive ammonium ions were absorbed via electrostatic interaction, then the negatively charged :TiO- species were drew, and thereby the TiO2 shell was facilitated through a hydrothermal process [32]. All in all, the dispersibility of Fe3O4 (Fig. 4a), Fe3O4@C (Fig. 4b) and Fe3O4@C@TiO2 (Fig. 4c) is better than that of TiO2 (Fig. 4d).

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Barrett–Joyner–Halenda (BJH) model was used to calculate the adsorption–desorption isotherms branch, which reveals that the uniform pore size of Fe3O4@C is 21 nm (Fig. 5a) and that of Fe3O4@C@TiO2 is 3.2 nm (Fig. 5b). This phenomenon reveals that the coat of TiO2 changes the pore size of the samples. The Brunauer–Emmett–Teller (BET) surface area of Fe3O4@C (Fig. 5a) is 9.02 m2g-1 and that of Fe3O4@C@TiO2 (Fig. 5b) is 16.00 m2g-1. The BET total pore volumes of Fe3O4@C (Fig. 5a) and Fe3O4@C@TiO2 (Fig. 5b) are calculated to be 0.00265 and 0.0112 cm3g-1, respectively. All these data indicate that after TiO2 was coated on Fe3O4@C, the pore size becomes smaller and the number of pole increases, and thus, the surface area and the total pore volume increase. 3.2 Adsorption properties of Fe3O4@C@TiO2 microspheres 3.2.1 Effect of time and concentration on adsorption Figure 6a shows the effect of time on adsorption. As shown in Fig. 6a, as time goes, the adsorption amounts of OTCHCl by Fe3O4@C and Fe3O4@C@TiO2 microspheres have a certain difference, and Fe3O4@C@TiO2 microspheres have larger adsorption capacity on OTC-HCl than Fe3O4@C microspheres. The maximum adsorption amounts of Fe3O4@C are 26.1 mgg-1, and the adsorption equilibrium time is 80 min; as for Fe3O4@C@TiO2, the adsorption amounts are 87.3 mgg-1 and the adsorption equilibrium is 60 min. And the adsorption effects of Fe3O4@C@TiO2 are better than 58.5 mgg-1 and 150 min of magnetic halloysite nanotubes (MHNTs) [33] and also better than 483.3 mgg-1 and 24 h of CuFe2O4 [34]. By comparing Fig. 6a, b, the adsorption equilibration time of Fe3O4@C@TiO2 (Fig. 6b) is shorter than that of Fe3O4@C (Fig. 6a) and the adsorption amounts are higher. On the basis of BET analytical, the pore size of Fe3O4@C is bigger than that of Fe3O4@C@TiO2, but the surface area and the total pore volume are lower; maybe this is the reason why the adsorption efficiency of Fe3O4@C is bad and the adsorption equilibration time is long. The conclusion can be gotten through the above experimental results

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Preparation of Fe3O4@C@TiO2 and its application for oxytetracycline hydrochloride adsorption

Fig. 4 TEM images of a Fe3O4, b Fe3O4@C, c Fe3O4@C@TiO2 and d TiO2 microspheres

Fig. 5 N2 adsorption–desorption isotherms of a Fe3O4@C microspheres and b Fe3O4@C @TiO2 microspheres (dV/dD, referred as pore volume distribution shown in inset)

that the adsorption amount is magnified with the increase in solution concentration. And the results also indicated that concentration has a significant impact on the performance of Fe3O4@C@TiO2. 3.2.2 Adsorption isotherms The Freundlich and Langmuir equations are commonly adopted to describe the adsorption isotherms. The Langmuir model is based on the assumptions of adsorption

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homogeneity, such as uniform energetic adsorption sites, monolayer surface coverage and no interactions between adsorbate molecules on adjacent sites [35, 36]. The Freundlich isotherm can be applied to non-ideal sorption onto heterogeneous surfaces involving multilayer adsorption [37]. In this study, the Langmuir and Freundlich adsorption equations were both used to correlate with the obtained isotherm data. The Langmuir model is represented by the expression [38]:

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Fig. 6 Effect of a time (t) and b concentration (ce) on adsorption of OTC-HCl

ce ce 1 ¼ þ qe qm qm b

ð2Þ

where b is the Langmuir equilibrium adsorption constant (mlmg-1), qe is the equilibrium adsorption capacity per unit weight of adsorbent (mgg-1) and qm is the theoretical maximum adsorption capacity per weight unit of adsorbent (mgg-1). The Freundlich model is given by the equation [39]: ln qe ¼ ln KF þ Ce =n

ð3Þ

where KF and n are Freundlich constants. Adsorbate–adsorbent interactions can be characterized by adsorption isotherms, which have a key function in optimizing the use of adsorbents. Thus, fitting equilibrium data with empirical Langmuir and Freundlich equations is crucial to the practical design and operation of adsorption systems [40]. Well-fitted Langmuir and Freundlich parameters for OTC-HCl adsorption on Fe3O4@C@TiO2 are listed in Table 1. While the adsorption behavior is well agreed upon with the two models, the Freundlich isotherm with related coefficients (R2) of greater than 0.99 is more suitable for describing the adsorption progress, compared to R2 of greater than 0.9873 of the Langmuir isotherm, but the n values according to the Freundlich isotherm are higher than 1.0, indicating that the adsorption process is hard [41]. So, Langmuir model is fitter than Freundlich. The maximum adsorption capacity (qm) is found to be 87.3 mgg-1. All these data indicate that the process of adsorption is monolayer adsorption and TiO2 is the main part of this process. Table 1 Isotherm parameters for adsorption for OTC-HCl qe,exp/(mgg-1)

Langmuir constant -1

87.3

123

Freundlich constant 2

qm/(mgg )

b

R

251.9

0.01

0.999

KF

1/n

R2

3.5

1.2

0.996

3.2.3 Adsorption kinetics The kinetics of adsorption is an indispensable index to evaluate the efficiency of adsorption. The pseudo-firstorder model is expressed as follows [42]: 1 K1 1 þ ¼ qt qm t qm

ð4Þ

where K1 is the rate constant of pseudo-first-order adsorption (min-1), qm is the maximum adsorption capacity per unit weight of adsorbent (mgg-1) and qt is the amount (mgg-1) of OTC-HCl adsorbed at time (t, min). The pseudo-second-order model is expressed as shown in Eq. (5) [43]: t 1 t ¼ þ qt K2 q2m qm

ð5Þ

where K2 is the rate constant of pseudo-second-order adsorption (gmg-1min-1). The pore diffusion model is expressed as shown in Eq. (6) [44]: qt ¼ ki t0:5 þ c

ð6Þ

where ki is the rate of pore diffusion adsorption (gmg-1min-1). The figures of the pseudo-first-order, pseudo-secondorder kinetic models and pore diffusion model are listed in Table 2. The correlation factors obtained for the pseudosecond-order model (R2 = 0.968) and pseudo-secondorder model (R2 = 0.999) are much higher than those obtained for the pore diffusion model (R2 = 0.9301) for the adsorption of OTC-HCl. These results show that the pseudo-second-order model fits well with the experimental data, and it could be used to explain the adsorption kinetics of OTC-HCl onto Fe3O4@C@TiO2 microspheres. The results also indicate that the adsorption is based on chemical kinetic [45], and the adsorption rate of Fe3O4@C@TiO2 is in direct proportion to square of driving force [32].

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Preparation of Fe3O4@C@TiO2 and its application for oxytetracycline hydrochloride adsorption Table 2 Pseudo-first-order, pseudo-second-order and pore diffusion constant about adsorption of OTC-HCl qe,exp/(mgg-1)

Pseudo-first -1

87.3

Pseudo-second -1

2

qm/(mgg )

K1/min

R

85.4

4.08

0.968

-1

qm/(mgg )

K2/min

88.6

0.00237

Compared with adsorptional materials like active carbon, Fe3O4@C@TiO2 can degrade OTC-HCl though chemical reaction. 3.2.4 Repetitive use of catalyst The reusable performance is very important for practical application of catalyst and adsorbent. To investigate the repeatability of catalyst activity, recycling experiments were carried out. The recyclable of OTC-HCl was investigated using 10 mg Fe3O4@C@TiO2 microspheres and 10 ml OTC-HCl solution with initial concentration of 50 mgL-1. As shown in Fig. 7, after three cycles, the composite Fe3O4@C@TiO2 shows a slight drop in catalysis efficiency from 87.3 to 80.5 mgg-1. This indicates that the activity of the as-prepared Fe3O4@C@TiO2 has good repeatability. The decline during this recycling experiment may be attributed to the decrease in bonding points between Fe3O4@C@TiO2 and OTC-HCl [46]. The results reported in this study demonstrate that the magnetic Fe3O4@C@TiO2 microspheres are excellent candidates for remediation processes that can reuse and recycle the OTC-HCl easily.

4 Conclusion TiO2-coated Fe3O4@C core–shell-structured microspheres were successfully prepared by combining sol–gel and simple hydrothermal processes. Taking advantages of magnetite and TiO2 aerogel, the prepared Fe3O4@C@TiO2 particles

Fig. 7 Adsorption of OTC-HCl with recycled Fe3O4@C@TiO2

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Pore diffusion -1

2

R

Ki/(mgg-1min-0.5)

c

R2

0.999

-0.0516

1.639

0.792

feature high adsorption capacity and fast magnetic separation at room temperature. The adsorption equilibrium could be reached within 60 min, and the maximum adsorption amount is 87.3 mgg-1. The adsorption kinetics data fit well into the pseudo-second-order model, while the adsorption equilibrium results are best described by the Langmuir model. The magnetic particles of Fe3O4@C@TiO2 have repeatability, and no obvious decline in adsorption efficiency is observed after three repeated cycles. So, 60 min is a better length of dark processing time. Acknowledgements This study was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2013ZX07202-010).

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