J Nanopart Res (2017) 19:56 DOI 10.1007/s11051-017-3764-3

RESEARCH PAPER

Carbon dots/BiOCl films with enhanced visible light photocatalytic performance Weitian Lin & Xiang Yu & Yinghua Shen & Hongbin Chen & Yi Zhu & Yuanming Zhang & Hui Meng

Received: 12 October 2016 / Accepted: 24 January 2017 # Springer Science+Business Media Dordrecht 2017

Abstract Novel carbon dots with a diameter of 6 nm modified BiOCl (CDs/BiOCl) photocatalyst on FTO was synthesized via a facile immobilization method at room temperature. The crystalline structures, morphologies, optical properties, and photocatalytic properties were studied. The results showed that the CDs/BiOCl films exhibited higher photocatalytic activity than pure BiOCl. The 4 wt% CDs/BiOCl film showed the best photocatalytic activity, which was about eight times than that of pure BiOCl and excellent recyclability even after four recycles. Compared with other film photocatalysts, the photocatalytic activity of 4 wt% CDs/BiOCl was also higher than that of many other photocatalysts. The enhanced activity was ascribed to the enhanced light adsorption and the improvement of charge separation. Holes and superoxide radicals ·O2− were revealed as the

Weitian Lin and Xiang Yu are contributed equally to this work. Electronic supplementary material The online version of this article (doi:10.1007/s11051-017-3764-3) contains supplementary material, which is available to authorized users. W. Lin : X. Yu : Y. Shen : H. Chen : Y. Zhu (*) : Y. Zhang Department of Chemistry, Jinan University, Guangzhou 510632, People’s Republic of China e-mail: [email protected] X. Yu Analytical & Testing Center, Jinan University, Guangzhou 510632, People’s Republic of China H. Meng Siyuan laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou 510632, People’s Republic of China

dominant reactive species. The photocatalytic mechanism was proposed based on the results. Keywords Carbon dots . BiOCl . Film . Visible light . Photocatalytic activity . Nanocomposites

Introduction Photocatalysis is of great significance in curing environmental and energy problems due to its promising applications in water purification (Chan et al. 2011), solar energy storage (Ran et al. 2014), and hydrogen generation from water (Maeda et al. 2006). As the widely investigated photocatalyst, bismuth oxychloxide (BiOCl) has drawn considerable research interest, which exhibits relatively good photocatalytic activity due to its unique layered structure and suitable band gaps (Cheng et al. 2014; Li et al. 2014; Shenawi-Khalil et al. 2012; Xiong et al. 2014). Nevertheless, the practical applications of such a material have been limited by high recombination of the photo-generated electron–hole pairs. And it is necessary to facilitate the separation of electron–hole pairs to further enhance its photocatalytic activity. Various strategies have been employed to improve the photocatalytic activity, such as crystal facet exposure (Jiang et al. 2012), morphology control (Xiong et al. 2011), noble metal doping (Lin et al. 2014) and so on. Among them, coupling other materials with BiOCl to form nanocomposites is an effective means. Lee et al. reported that novel BiOCl/Bi2O3 photocatalyst demonstrated notably high photocatalytic activity over a wide

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composition range in decomposing 2-propanol in gas phase and terephthalic acid in aqueous solution, whereas the individual BiOCl and Bi2O3 showed a negligible efficiency (Chai et al. 2009). Nan et al. synthesized 3D BiOI/BiOCl composite microspheres, which demonstrated enhanced photocatalytic activities in the bisphenol-A degradation under visible light irradiation. The superior photocatalytic activity of this composite catalyst is attributed to the suitable band gap energies and the low recombination rate of the photo-generated electron–hole pairs due to the presence of BiOI/BiOCl heterostructures (Xiao et al. 2012). Carbon quantum dots (CDs) are fascinating carbon materials which have attracted massive research interests, because they display abundant charming characteristics, such as stability, high aqueous solubility, low toxicity, uniform distribution, excellent charge transport properties, easy functionalization, and so on (Lim et al. 2015; Zheng et al. 2015). Very recently, many studies for synthesis of CDs/semiconductor composites and their enhanced visible light photocatalytic activity have been reported (Deng et al. 2016; Di et al. 2015a; Xia et al. 2016). For example, Yu and his co-workers synthesized carbon self-doped TiO2 sheets for enhanced visible light photocatalytic activity (Yu et al. 2011). Zhang et al. utilized Fe2O3/CDs nanocomposites to investigate the effective photocatalitic activity toward the degradation of gas phase benzene and methanol under visible light irradiation (Zhang et al. 2011). Tang and his partners prepared CDs/m-BiVO4 nanospheres with various exposed facets and observed enhanced photocatalytic activities in degradation of MB under the irradiation of visible light (Tang et al. 2013). In order to combine remarkable properties of CDs and BiOCl, CDs can be coupled with BiOCl to form nanocomposite with higher photocatalytic performance. However, there have been only few reports on the preparation and investigation of CDs/BiOCl nanocomposites to date. Li et al. synthesized a novel CDs modified BiOCl ultrathin nanosheet photocatalyst via a facile solvothermal method. The 5 wt% CDs/BiOCl materials displayed the best performance, which showed a broad spectrum of photocatalytic degradation activity (Di et al. 2015a). Deng et al. reported two-dimensional interlaced BiOCl/carbon quantum dot composites by a templatefree coprecipitation method at room temperature, which exhibited 100% 2-nitrophenol removal under visible light irradiation (Deng et al. 2016). Xia et al. prepared controlled synthesis of CDs/BiOX (X = Br, Cl) hybrid

nanosheets with tunable CDs loading contents. Three weight percent CDs/BiOBr nanosheets showed the highest photocatalytic activity for the degradation of RhB, CIP, and BPA under visible light irradiation (Xia et al. 2016). But all the literatures focused on the CDs/ BiOX (X = Cl, Br, I) nanocomposite powders, which limited the practical application because of the disadvantages such as separating and recycling photocatalyst powders. Immobilization of photocatalyst powders is the most effective approach to solve this problem (Mu et al. 2012). However, no CDs/BiOX (X = Cl, Br, I) films are reported, which are more beneficial to recycling and regeneration. Additionally, the photocatalytic mechanism of CDs/BiOX composites is not determined, more similar composites are necessary to further confirm the photocatalytic mechanism. Herein, we synthesized CDs/BiOCl films on FTO by a simple and facile immobilization method at room temperature. The structures, morphologies, optical properties, and photocatalytic properties were studied. The as-prepared CDs/BiOCl films showed high visible light photocatalytic activity toward degradation of RhB and excellent recyclability. Accordingly, a reasonable photocatalytic mechanism was also proposed.

Experimental Sample preparation All the reagents were of analytical grade and were used without further purification. CDs were synthesized according to the literature followed by a one-step electrochemical method (Ming et al. 2012). The CDs/BiOCl was synthesized as follows: 1.5 g BiCl3 was dissolved in 50 mL ethanol containing 0.25 mL HCl and treated with ultrasonication for 30 min to form a clean solution. After that, different amounts of CDs were added into the s o l u t i o n a n d th e m i x t u r e w a s t r e a t e d w i t h ultrasonication at room temperature. Prior to the experiment, FTO substrate was ultrasonically cleaned in acetone, ethanol, and distilled water. A certain amount of the suspension was sucked by a pipette, smeared homogeneously on FTO, and dried at 80 °C for 30 min. Then, the treated FTO was soaked in distilled water and dried in an oven at 60 °C (Scheme 1). The added contents of CDs in the CDs/BiOCl materials were 0.8, 2, 4, 5, and 7 wt%, respectively. For comparison, BiOCl without the

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Scheme 1 Schematic procedure for fabricating CDs/BiOCl film

modification of CDs was prepared under similar conditions.

were detected by trapping with isopropanol (IPA), triethanolamine (TEOA), and benzoquinone (BQ).

Characterization

Photoelectrochemical measurements

The X-ray diffraction (XRD) was performed on an Xray diffractometer (XD-2) with 0.154 nm Cu Kα radiation as light source. The morphology of the as-prepared samples was analyzed by field emission scanning electron microscopy (FE-SEM, Zeiss ULTRA 55) and transmission electron microscopy (TEM, JEOL 2010F). Compositions and chemical state of the samples were characterized by energy-dispersive spectroscopy (EDS, Bruker/Quanta 200) and X-ray photoelectron spectroscopy (XPS, ESCALab250). The photoelectrochemical experiment was performed on an electrochemical system (CS 310). The optical properties of the products were investigated with a UV-Vis diffuse reflectance spectrum (DRS, Hitachi UV-3010) by using BaSO4 as a reference. The photoluminescence (PL) spectra of the samples were recorded using a RF-5301PC fluorescence spectrophotometer.

The photoelectrochemical and electrochemical impedance spectroscopy (EIS) measurements were conducted by using an electrochemical analyzer (SP-150, France) with a standard three-electrode configuration. The Pt wire was used as counter electrode, the saturated Ag/ AgCl electrode as the reference electrode and the asprepared samples as the working electrode. A 350-W Xe lamp equipped with an ultraviolet cut-off filter (λ > 420 nm) was utilized as the visible light source. And a 0.1 M Na2SO4 aqueous solution was used as the electrolyte.

Photocatalytic activity tests

Figure 1 displayed the XRD patterns of the assynthesized pure BiOCl and CDs/BiOCl with different CDs contents. The diffraction peaks centering at 12.0°, 24.1°, 25.9°, 32.5°, 33.4°, 36.5°, 40.9°, and 49.7° can be attributed to the diffractions of (001), (002), (101), (110), (102), (003), (112) and (113) crystal facets of tetragonal phase BiOCl crystals (JCPDS no.06-0249). No extra characteristic peaks were detected, indicating that there were no impurities in the products. However, no diffraction peaks of CDs were observed in the CDs/ BiOCl sample, which may due to the low CDs content in the samples (Zhang et al. 2012). The peak intensity of the (001) plane is relatively stronger than those of other planes, which probably resulted from the preferential growth orientation of the nanosheets (Ye et al. 2011).

The photocatalytic activities of the samples were evaluated by monitoring the degradation of RhB under visible light irradiation of a 350-W Xe lamp with a cut-off filter (λ > 420 nm). All experiments were carried out at room temperature. In each experiment, FTO substrate covered with sample was soaked in a beaker with 100 mL of RhB aqueous solution with a concentration of 2.5 mg/L. Prior to irradiation, the system was magnetically stirred in dark for 30 min to ensure the establishment of adsorption-desorption equilibrium. At certain intervals, a quantitative solution was collected and centrifuged. The solution was examined by UV-vis spectrophotometer. The active species in the photocatalytic system

Results and discussion Structure, morphology, and chemical composition characterization

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Fig. 1 XRD patterns of pure BiOCl and CDs/BiOCl with different CDs contents

The morphology and microstructure of the asprepared 4 wt% CDs/BiOCl film was investigated by SEM and TEM. As shown in Fig. 2a, the as-prepared 4 wt% CDs/BiOCl had nanosheet-like morphology, and the size of the nanosheets was about 400–600 nm.

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However, the CDs could not be observed clearly from the SEM images. Figure 2b showed a low magnification TEM image of pure BiOCl. Compared with pure BiOCl, large quantity of nanodots with size of about 6 nm could be observed (Fig. 2c) on the surface of the nanosheets, indicating the successful decoration of BiOCl by CDs Fig. 2d, e showed the HRTEM images of BiOCl and CDs, respectively, which were collected from the redmarked areas in Fig. 2c. The lattice spacing of 0.24 and 0.32 nm corresponded to (003) planes of BiOCl and (002) planes of CDs, respectively. The EDS pattern (Fig. 2f) indicated that the 4 wt% CDs/BiOCl sample contained Bi, O, Cl, and C elements, evidencing that the sample was CDs/BiOCl composite. XPS was employed to study the components and surface properties of the 4 wt% CDs/BiOCl films. In the XPS spectrum of Bi 4f in Fig. 3a, two strong peaks at 159.2 and 164.6 eV were assigned to Bi 4f7/2 and Bi 4f5/2, respectively, corresponding to Bi3+ in the crystal structure (Peng et al. 2013). The Bi 4f peaks in the 4 wt% CDs/BiOCl sample displayed a slight shift toward higher binding energies compared with pure

Fig. 2 a FE-SEM images of the 4 wt% CDs/BiOCl. b TEM images of BiOCl. c TEM images of 4 wt% CDs/BiOCl. d, e HRTEM images of BiOCl and CDs. f EDS of 4 wt% CDs/BiOCl

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BiOCl, which might be attributed to the interaction between BiOCl and CDs. On the other hand, for the spectrum of Cl 2p, shown in Fig. 3b, the peaks with binding energies at 198.1 and 198.5 eV were ascribed to Cl 2p3/2 and Cl 2p1/2, respectively, which was a characteristic of Cl− in the materials (Cheng et al. 2013). The O1s peak at 530.6 eV (Fig. 3c) could be ascribed to the oxygen in the BiOCl. In Fig. 3d, the peak at 284.7 eV was attributed to C–C bond with sp2 orbital. The binding energies of 286.1 and 287.3 eV were assigned to the C–O and C = O, respectively, suggesting the existence of CDs in the CDs/BiOCl films (Yu et al. 2014). The results of XPS analysis indicated the coexistence of BiOCl and CDs in the CDs/BiOCl composite. Optical absorption properties The optical properties of pure BiOCl and CDs/BiOCl with different CDs contents were studied by UV-vis diffuse reflectance spectroscopy and the results were displayed in Fig. 4a. The pure BiOCl sample did not

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absorb in the visible light region, with the absorption edge of approximately 370 nm. Compared with the pure BiOCl, all of the CDs/BiOCl samples exhibited absorption in the visible light region due to the presence of CDs in the composites. Moreover, the absorption intensity of the CDs/BiOCl samples was improved with the increase of CDs quantity. This result suggested that CDs played an important role in utilizing sunlight and increasing the formation of electron–hole pairs during light irradiation (Zhang et al. 2008), thus resulting in higher photocatalytic reactivity. The band gap energy (Eg) of the as-prepared samples was calculated according to the following formula (Shang et al. 2009):αhv = A(hv ‐ Eg)n/2, where α, h, v, and Eg are the absorption coefficient, Plank constant, light frequency, and band gap energy, respectively, while A is a constant. Among them, n depends on the characteristics of the optical transition of a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). For BiOCl, the value of n was 4 for the indirect transition (Li et al. 2015). The Eg of pure BiOCl and 4 wt% CDs/

Fig. 3 XPS spectra of samples BiOCl and 4 wt% CDs/BiOCl. a Bi 4f. b Cl 2p. c O1s. d C 1 s

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BiOCl could be estimated from the plot of (αhv)1/2 versus (hv). The intercept of the tangent to the x axis would give a good approximation of the band gap energy for the samples. As was shown in Fig. 4b, the Eg of pure BiOCl and 4 wt% CDs/BiOCl were estimated to be 3.2 and 2.9 eV, respectively. Obviously, the estimated band gap of 4 wt% CDs/BiOCl films was smaller than that of the BiOCl sample, illustrating that the introduction of CDs in BiOCl could improve the optical absorption property of BiOCl.

Photocatalytic properties The photocatalytic activities of the as-prepared samples were evaluated by monitoring the degradation of RhB under visible light irradiation with the sample as photocatalyst. As was shown in Fig. 5a, the degrees of RhB photodegradation by CDs/BiOCl were all higher than that of pure BiOCl. The optimum content of CDs was located at 4 wt%; the best photocatalytic activity of CDs/BiOCl was obtained. The 4 wt% CDs/BiOCl could degrade 99% of RhB in 3 h under visible light irradiation. The degradation rate constants in decomposing RhB with photocatalytic samples were calculated to be 0.23, 0.32, 0.52, 1.76, 0.57, and 0.69 h−1 for pure BiOCl, 0.8, 2, 4, 5, and 7 wt% CDs/BiOCl, respectively. The results implied that k values for all CDs/BiOCl samples were higher than that for pure BiOCl. The constant of 4 wt% CDs/BiOCl was about eight times that of pure BiOCl. However, a further increase of CDs content caused a decrease in the photocatalytic activity of RhB degradation. Because too many CDs dispersed on the BiOCl might shield the BiOCl from absorbing visible light (Di et al. 2015c, 2016). This indicated that

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proper content of the CDs was beneficial for improving the photocatalytic performance. Figure 5b showed the time-dependent absorption spectra of RhB solution in the presence of 4 wt% CDs/BiOCl. The absorption peak at 553 nm decreased gradually as the irradiation time increased. The color of RhB (inset) solution changed from red to light yellow and then faded completely during the reaction. This indicated that the 4 wt% CDs/BiOCl exhibited excellent photocatalytic activity on the degradation of RhB. And TOC analysis showed that 95% of dye was mineralized (Fig. S1). The stability and reusability of the photocatalyst are vital to the practical applications. The results for the recycling performance of 4 wt% CDs/BiOCl were shown in Fig. 5c. After 4 cycles, there was no obvious decrease of photocatalytic efficiency over RhB under visible light irradiation. In addition, the present photocatalyst could be directly separated from the aqueous system because of its immobilization. The above results indicated that the photocatalyst had high stability and could act as effective photocatalyst. The photocatalytic activity of 4 wt% CDs/BiOCl was compared with other film photocatalysts in Table 1. The photocatalytic activity of 4 wt% CDs/BiOCl film in this work was higher than that of many other photocatalysts indicating its great potential for practical applications in the removal of dyes. Possible photocatalytic mechanism of CDs/BiOCl PL emission spectra have been widely used to study the efficiency of photocatalyst in the photo-generated charge carrier trapping, migration, and transfer. It is well known that a lower PL intensity may indicate a lower recombination rate of electron–hole pairs under

Fig. 4 a UV-vis diffuse reflectance of the as-prepared samples. b The band gap energies (Eg) of the pure BiOCl and 4 wt% CDs/BiOCl

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Fig. 5 a Comparison of photocatalytic activities of the asprepared samples on the degradation of RhB under visible light irradiation (λ > 420 nm). b Variation of the absorption spectra of

RhB aqueous solution with the presence of the 4 wt% CDs/BiOCl film. c Recycling tests of 4 wt% CDs/BiOCl film for the degradation of RhB under visible light irradiation (λ > 420 nm)

irradiation (Xu et al. 2013a). As was shown in Fig. 6a, the PL spectrum of 4 wt% CDs/BiOCl composite was similar to that of pure BiOCl catalyst, but the emission intensity decreased significantly, which implies that the recombination rate of photo-generated charge carriers was lower in 4 wt% CDs/BiOCl and thus improved the photocatalytic activities.

The photoresponses of BiOCl and 4 wt% CDs/BiOCl in on-off cycles under visible light irradiation were investigated and the results are shown in Fig. 6b. As could be seen, the 4 wt% CDs/BiOCl composite presented higher photocurrent intensity. However, the photocurrent of pure BiOCl was weaker, which indicated that the 4 wt% CDs/BiOCl composite could reduce the

Table 1 Photocatalytic activities of various photocatalysts on the degradation of RhB Photocatalysts

Dye concentration (mg/L)

Photocatalytic activity

Reference

Films TiO2

10

50% degraded within 5 h (UV light)

Wang et al. 2012

BiOCl

1.0

52.5% degraded within 8 h (visible light)

Liang et al. 2013

BiOBr

5.0

80% degraded within 8 h (visible light)

Cuellar et al. 2015

ZnO:I/TiO2

2.4

97% degraded within 6 h (visible light)

Wang et al. 2015 Zhao et al. 2007

Bi2WO6

5.0

53% degraded within 12 h (visible light)

Bi2O(OH)2SO4

1.0

92% degraded within 7 h (visible light)

Zhang et al. 2015

CDs/BiOCl

2.5

99% degraded within 3 h (visible light)

This work

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Fig. 6 a PL spectra of pure BiOCl and 4 wt% CDs/BiOCl. b Transient photocurrent response of pure BiOCl and 4 wt% CDs/ BiOCl. c Electrochemical impedance spectra of pure BiOCl and

recombination rate of photo-generated electron–hole pairs (Di et al. 2015b). Electrochemical impedance spectroscopy (EIS) was used to investigate the charge transfer resistance and the separation efficiency of photo-generated electron–hole pairs. As displayed in Fig. 6c, the diameter of the Nyquist circle of the 4 wt% CDs/BiOCl was smaller than that of BiOCl. This indicated that 4 wt% CDs/BiOCl had a lower resistance than that of pure BiOCl and enhanced the separation efficiency of photo-induced electrons and holes (Xu et al. 2013b). The trapping experiment of active species during the photocatalytic reaction of 4 wt% CDs/BiOCl was conducted, as shown in Fig. 6d. In a series of experimental studies, isopropanol (IPA) (Di et al. 2014), benzoquinone (BQ) (Chen et al. 2013), and triethanolamine (TEOA) (Bai et al. 2014) were adopted as the traps for·OH radicals,·O2− radicals, and h+, respectively.

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4 wt% CDs/BiOCl. d The effect of reactive species in the photogradation process of RhB over 4 wt% CDs/BiOCl

Fig. 7 Schematic illustration of the proposed mechanism for photodegradation process of CDs/BiOCl composites under visible light irradiation

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When isopropanol was added, the photocatalytic degradation of RhB was not affected obviously, which indicated that the·OH was not the main active species. However, the photocatalytic degradation of RhB decreased obviously with the addition of benzoquinone or triethanolamine. The above results indicated that ·O2− radicals and h+ played an important role during the photocatalytic process. Based on the above results, we proposed a possible mechanism for the photocatalytic degradation of RhB over CDs/BiOCl. BiOCl could not be excited by the visible light irradiation because the BiOCl was a wide band gap semiconductor. But the up-converted PL property of CDs suggested that CDs can be used as an efficient energy transfer materials to convert visible and near-infrared light to shorter wavelengths light (Fig. S2). resulting in exciting BiOCl. The CDs/BiOCl samples exhibited absorption in the visible light region due to the presence of CDs in the composites. This result suggested that CDs played an important role in utilizing sunlight and increasing the formation of electron–hole pairs during light irradiation. When the system was irradiated by visible light, CDs absorbed long wavelength light and emitted shorter wavelength light due to photoluminescence property (Xia et al. 2016), resulting in that BiOCl was excited to generate electron–hole pairs. These photo-generated electrons on the CB of BiOCl tended to transfer to CDs due to the conjugated system of CDs (Zheng et al. 2015), which acted as electron acceptors, thus resulting in effective inhibition of electron–hole recombination. Then, the electron on the CDs reacted with O2 to generate·O2− and the holes on the valence band of BiOCl degraded the pollutant effectively. The reaction mechanism of improved photoactivity of the CDs/BiOCl composites was shown in Fig. 7.

Conclusion In summary, CDs/BiOCl composite photocatalyst on FTO was successfully synthesized. The as-prepared CDs/BiOCl films especially 4 wt% CDs/BiOCl exhibited significantly enhanced photocatalytic activity compared with the pure BiOCl and demonstrated excellent recyclability under visible light irradiation. The enhancement of visible light photocatalytic reactivity should be attributed to the enhanced light adsorption and the improved separation of photoinduced carriers

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between BiOCl and CDs. The photocatalytic mechanism was revealed as that·O2− generated by the reaction between O2 and photo-generated electron which firstly transferred to CDs then to the surface of BiOCl, and photo-generated holes degraded the pollutant. In addition, immobilization of CDs/BiOCl on FTO made their recycle much easier and realizable. This research can provide a new insight into developing new immobilized composite photocatalysts with high photocatalytic activity and excellent recyclability. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest. Funding This work was supported by the National Major Science and Techology Program for Water Pollution Control and Treatment (No. 2013ZX07105-005).

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