porous channel biochar for improving photocatalytic activity for degradation of tetracycline

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In-suit preparation of CdSe quantum dots/porous channel biochar for improving photocatalytic activity for degradation of tetracycline Qiuyue Men, Tao Wang, Changchang Ma∗, Lili Yang∗, Yang Liu, Pengwei Huo, Yongsheng Yan∗ Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China

a r t i c l e

i n f o

Article history: Received 17 December 2018 Revised 13 March 2019 Accepted 31 March 2019 Available online xxx Keywords: CdSe QDs Biochar Tetracycline (TC) Visible light Porous channel Photocatalytic

a b s t r a c t In this paper, a simple in-suit method was used to load CdSe quantum dots (QDs) onto HTC (abbreviated as Hydrothermal biochar, HTC) to form CdSe/HTC composites. Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy were used to detect and analyze the surface valence and composition of CdSe/HTC composites. The photocatalytic degradation activity was judged by photodegrading tetracycline (TC). Compared with pure CdSe quantum dots, the best photocatalytic degradation efficiency of CdSe/HTC complex containing 15% HTC was 73%, which was attributed to the high carrier transport efficiency of HTC and the inhibition reorganization of photogenerated electron-hole pairs. More importantly, the hydrothermal biochar used in this paper was extracted from waste biomass and had the characteristics of economy, environmental protection and high utilization rate, and was in line with the basic viewpoint of sustainable use of resources. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Water was an important part of nature and the most active element in the environment. Antibiotic contamination in water is one of the major challenges people face [1]. Antibiotics were one of the most widely used drugs in clinical practice, aquaculture and so on, which were released into the environment through the sewage discharge system, causing some damages to the balance of the microbial community [2]. There were many kinds of antibiotics, among which tetracycline (TC) was widely used because of its low cost and stable drug effect. The negative impact can not be ignored, because the accumulation of food and food chain transmission would cause dental malformations, skin rash and neuropathic allergic reactions and side effect on human health [3]. Therefore, it was necessary to treat the tetracycline residue in the wastewater. At present, there were many treatment methods for handling TC residues problems [1,4,5]. The photocatalytic technology was favored for its high degradation efficiency and no pollution of products. However, conventional photocatalysts such as ZnO, TiO2 , and WO3 have wide band gaps and are only responsive to ultraviolet light, because commercial UV lamps are expensive and have high energy requirements for large-scale utilization, which is not suitable for practical environments and industrial applications

∗

Corresponding authors. E-mail address: [email protected] (Y. Yan).

[6–9]. As a new type of nanomaterial, due to the limitation of excited electrons and holes, the optical and electronic properties of semiconductor quantum dots are different from those of bulk semiconductors [10,11], resulting in surface effects and multiple exciton effects [12], which attracts more and more researchers’ attention. The use of quantum dot materials to treat organic contaminants in water is considered to be a green and effective method [13,14], such as quantum photocatalyst Fe3 O4 QDs [15], ZnS2 QDs [16,17] SnO2 QDs [18] and BiOI QDs [19] and the like. Since the first successful synthesis of CdSe QDs by the Bawendi team in 1993, a series of important advances have been made in the field of photocatalysis. As far as we know, the CdSe QDs is a direct suitable bandgap metal chalcogenide semiconductor material whose electrons in valence are easily excited to the conduction band in the visible light region of the solar spectrum, and has good photochemical properties. Moreover, the CdSe QDs is a high-efficiency photocatalyst that oxidizes organic pollutants into CO2 and H2 O under visible light irradiation without generating secondary pollutant. However, CdSe QDs also has some disadvantages, such as easy aggregation, photogenerated electron-hole pairs easy to recombine, and difficult to recover and separate as a nanomaterial in water. In the natural environment, the concentration of TC in water was low. This leads to problems such as low reaction performance, slow rate, and poor degradation efficiency [20]. This severely limited the practical application of CdSe QDs. We need to find an economical, practical and stable material to solve the above problems.

https://doi.org/10.1016/j.jtice.2019.03.019 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Q. Men, T. Wang and C. Ma et al., In-suit preparation of CdSe quantum dots/porous channel biochar for improving photocatalytic activity for degradation of tetracycline, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/ 10.1016/j.jtice.2019.03.019

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In recent years, biochar is generally considered to be a good support for nanomaterials. They can effectively improve the stability of the photocatalyst and control the morphological growth of the nanoparticles [21]. In addition, biomass carbon has a large number of surface functional groups, good electrical conductivity, strong adsorption capacity, easy to obtain and easy to recover expensive metal phase [22], alleviating the problem of separation and recovery of photocatalyst from aqueous solution [23,24]. It has become a widely used and low cost functional material, and attracted wide attention of researchers. In this study, porous biochar was used as a carrier for CdSe quantum dots. Since HTC in CdSe QDs/HTC composites not only solved the problem of easy aggregation of CdSe quantum dots, but also effectively transferred electrons to the surface of biochar. It improves the separation efficiency of photogenerated electron-hole pairs and provides more reactive sites. In addition, porous HTC can enrich TC dispersed in water to the surface, which increases the efficiency of photocatalytic degradation, and it also provides more possibilities for photocatalyst recycling. Therefore, the excellent performance of HTC indicates that there is great potential for removing tetracycline contaminants from water in practical applications.

by using of NaOH solution (1 M). Afterward, the solution was deoxygenated by bubbling nitrogen. Then the precursors were mixed with the NaHSe solution and transferred to a round flask and treated at 80 °C for 4 h. After the container cooled to room temperature, the final products were collected by centrifugated, washed with the DI water and ethanol several times, and dried in a vacuum. The CdSe QDs sample was prepared by the same procedure to obtain CdSe QDs/HTC composites (marked as CHC) with different mass ratios of 5%, 10%, 15%, and 20% (wt%) HTC, and labeled as CHC-1, CHC-2, CHC-3 and CHC-4, respectively. 2.4. Characterizations

Bamboo was obtained from Jiangsu University, Zhenjiang, China. The collected biomass was washed by DI water three times to scour off impurity such as dust and dried for days. Raw biomass was cut into pieces, ground and passed through a 60 mesh sieve and oven-dried at 100 °C for 24 h. For hydrothermal biochar preparation, 3.0 g biomass powder, 0.5 g citric acid monohydrate and 30 mL DI water were sufficiently mixed, then transferred into a 50 mL stainless steel autoclave. The autoclave was kept at 200 °C for 4 h and cooled to room temperature. The products were filtered and washed with DI water and ethanol for several times, dried at 60 °C in a vacuum oven, as biochar precursor. Afterward, 1.0 g biochar precursor and 30 mL NaOH (1 M) were mixed, then filtered, washed and dried at 60 °C, finally carbonized in a muffle furnace under a flow of N2 to 650 °C keep with 1 h. The product was washed with HCl (1 M), DI water, ethanol and dried, marked as HTC.

The morphology and particle size of HTC, CdSe and CdSe/HTC composites was observed by Field Emission Scanning Electron Microscope (SEM, JSM-7001F, JEOL, Japan). The composition and crystal state of the crystal were determined using an X-ray Diffractometer (XRD, D8 ADVANCE, BRUKER, Germany), used CuKα , with a scanning speed of 7° min−1 , and scanning angles ranging from 10° to 80° Further, the structure of the photocatalyst was qualitatively analyzed by means of Fourier transform infrared spectrometer (FT-IR, Nicolet Nexus, Nicolet, USA) using a KBr beam splitter with a wavelength in the range of 40 0–40 0 0 cm−1 . The UV–vis absorption spectrum of the photocatalyst was measured on an ultraviolet-visible spectrophotometer (UV-Vis, Cary 8454, Agilent, USA). The Raman experiment was carried out using a Laser Raman Spectrometer (Raman, DXR, ThermoFisher, USA). Data such as nitrogen adsorption-desorption curves, specific surface area and pore size distribution were measured using a specific surface and porosity analyzer (TriStar II 3020, Micromeritics, USA). Among them, electrochemical impedance (EIS) transient photocurrent response and Mott-Schottky test were performed in electrochemical workstation (CHI660D, Chenhua, China) with CHC-3 as working electrode, platinum plate and Ag/AgCl (saturated KCl) as for the reference electrode, a Na2 SO4 solution having a concentration of 0.5 M was used as the electrolyte solution. The X-ray photoelectron spectroscopy (XPS) test was carried out on an AXIS Ultrabld spectrometer manufactured by Kratos, UK, and the X-ray source was Al’s Kα ray. During the experiment, the chamber vacuum was 10−7 , with the carbon source (C1s, E = 284.6 eV) from the instrument itself as the charge calibration standard. Photoluminescence spectra of photocatalysts obtained by high-grade steady-state fluorescence (PL, QuantaMaster TM 40, Photon Technology International, Inc.) can be used to evaluate the lifetime of photogenerated electron-hole pairs. In order to further detect superoxide radicals, singlet oxygen and other active species during the reaction, electron spin resonance (ESR, Bruker A300, Bruker, Germany) spectroscopy was introduced for testing and analysis. The degree of mineralization of tetracycline (TC) by CHC-3 was measured by total organic carbon/total nitrogen analyzer (TOC, MultiN/C 2100, Jena, Germany), the high-temperature catalytic oxidation temperature was 800 °C, and the injection volume was 500 uL. Finally, the high performance liquid chromatography-mass spectrometry (HPLC-MS) system, a 2.1 × 150 mm Zorbax ODS chromatography column, detecting the structure of the intermediates under 30 °C.

2.3. The preparation CdSe/HTC composite

2.5. Photocatalytic activity evaluation

1.1 mmol Se powder and 3 mmol NaBH4 were added to 15 mL deionized water under nitrogen atmosphere to form a black mixture. The mixture was continuously stirred under nitrogen atmosphere to produce a clear NaHSe solution. Then 0.13 g CdCl2 ·2·5H2 O, different quality HTC (0.0 068 g, 0 0137 g, 0.0204 g or 0.0272 g), and 170 μL methyl acrylic acid were separately dissolved in 30 mL DI water with magnetic stirring. Then adjusted the pH = 7

The photocatalytic activity of CHC composite was studied by analyzing the decomposition of tetracycline (TC) under visible light irradiation. The visible light source was formed by a 250 W Xe lamp and a 420 nm filter. Typically, 50.0 mg of the CHC photocatalyst was stirred and dispersed in a 100 ml TC of 20 mg/mL solution in a quartz degradation reaction bottle. Adsorption-desorption equilibrium has been reached by vigorous agitation for 30 min in

2. Experimental section 2.1. Materials Citric acid monohydrate (C6 H8 O7 . H2 O), sodium hydroxide (NaOH), methyl acrylic acid (C4 H6 O2 ), selenium (Se), cadmium chloridehemi (pentahydrate) (CdCl2 ·2·5H2 O), C10 H14 N2 Na2 O8 (EDTA-2Na), C3 H8 O (IPA), C6 H4 O2 (BQ), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, C6 H11 NO) were all supported by Aladdin Chemistry Co, Ltd. Hydrochloric acid (HCl), ethanol (C2 H5 OH), Sodium chloride (NaCl), Sodium sulfate (Na2 SO4 ), Sodium bicarbonate (NaHCO3 ), Sodium nitrate (NaNO3 ) were purchased from Sinopharm Chemical Reagent Co, Ltd. Distilled water (DI water) was used throughout the experimental procedures. 2.2. The preparation of biochar by hydrothermal carbonization

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published data (JCPDS No. 19–0191). The lattice constant was de˚ b = 6.077 A, ˚ and c = 6.077 A. ˚ It can be connoted by a = 6.077 A, firmed that the prepared catalyst CdSe was a cubic crystal structure. Compared with the XRD pattern of CHC-3, it can be seen that there is a slight difference, which proves that the introduction of HTC does not change the cubic structure of CdSe nanoparticles. Due to the lower content of HTC and weaker diffraction peak intensity, faint HTC characteristic diffraction peaks are detected. The Scherrer formula is used to approximate the size of the CdSe nanoparticles:

D=

Fig. 1. XRD patterns of HTC, CdSe QDs and CHC-3 (scanning speed of 7° min−1 ).

the dark. 4 mL sample solution was collected from the quartz reactor at intervals of 10 min. UV–vis spectrophotometer was used to detect the absorbance value at 357 nm for numerical analysis. 3. Results and discussion In order to understand the microstructure and reaction mechanism of CdSe/HTC composites, it was further illustrated by SEM, elemental diagram, XRD and so on. Among them, CHC-3 has the excellent photocatalytic activity, and the above characterization was based on a CHC-3 photocatalyst. 3.1. Structural and composition characterization of the CdSe QDs, HTC,and CHC-3 Fig. 1 shows the XRD patterns of the CdSe QDs, CHC-3, and HTC. The XRD pattern of pure CdSe clearly shows three distinct diffraction peaks around at 2θ of 25.47° , 42.12° , and 49.80° , which belong to the (111), (220) and (311) planes reflections, respectively. Fig. 1 reveals the XRD pattern of prepared CdSe well matches with

kλ β cos θ

(1)

k was the Scherrer constant, which was 0.89 [25], λ was the X-ray ˚ D was the average thickness wavelength, which was λ=1.5406 A, of the grain perpendicular to the crystal plane, and β was the half width of the diffraction peak of the measured sample, and θ was the diffraction angle. It was approximate that the grain size of CdSe QDs is between 8 nm and 9 nm.

3.2. Morphology and elemental composition studies Fig. 2 shows the microsurface structures of the prepared HTC and CHC-3. As shown in Fig. 2a and b, the pure HTC exhibites pores of different morphology and size. The abundant pore structure of biochar is more conducive to the full contact with antibiotics and improving photocatalytic efficiency. As can be clearly seen in Fig. 2c and d, many nanoparticle clusters are formed due to the deposition of CdSe nanoparticles. These clusters of nanoparticles grow relatively evenly on the surface of the HTC, making it coarse. which would produce more active sites for photocatalytic reactions [26]. Furthermore, the elemental distributions of the CHC-3 are also confirmed by Energy Dispersive Spectrometry (EDS) and elemental mapping images shown in Fig. 2(e–i), It can be observed from the Fig. 2(g–i) with a different color, the elements Se, Cd and C were uniformly distributed, indicating that CdSe QDs closely connected with HTC. Combined with previous XRD data analysis, CdSe nanoparticles are successful in loading onto the HTC surface by evaporation induction.

Fig. 2. SEM images of HTC (a, b) and CHC-3 (c, d) with different resolutions. The EDS (e) spectrum and elemental mapping (g-i) analysis of CHC-3.

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the relative desorption curves of CHC-3 and HTC at a pressure of 0.5–1.0 indicate the presence of mesopores and macropores [27]. The pore size distribution of CHC-3 was mainly distributed between 2 nm and 10 nm, mainly dominated by mesopores. Generally, the porous structure was more favorable for adsorbing TC, facilitating diffusion of the product, and promoting the progress of the photocatalytic reaction [28,29].

3.4. Raman spectroscopy of CdSe QDs, HTC and CHC-3

Fig. 3. The N2 adsorption-desorption isotherms of CdSe QDs, HTC, CHC-3.

3.3. BET and pore size distribution measurements The nitrogen adsorption-desorption isotherms curves and pore size distributions of CdSe, HTC, CHC-3 are shown in Fig. 3. It can be observed from Fig. 3. All the isotherms of the catalyst have the characteristics of a type IV isotherm curve. CdSe has a distinct H3 hysteresis loop at high relative pressures, which is related to the slit holes formed by the accumulation of particles. Similarly,

To determine more about the microscopic structure of CHC-3, Raman scattering spectra are measured. Fig. 4 shows the Raman spectra of HTC, CdSe QDs, CHC-3, respectively. For the HTC Raman spectra (Fig. 4a), there are two typical characteristic peaks at 1381 cm−1 and 1582 cm−1 , which are assigned to D band and G band of HTC [30]. The D band is correlated with the in-plane longitudinal phonon vibration and the G band is attributed to the in-plane vibration of the conjugated sp2 carbon atom in the twodimensional hexagonal lattice [31]. Fig. 4b shows the spectrum of CdSe QDs. The peak located at 204 cm−1 is owing to the longitudinal optical (LO) mode of CdSe, which is slightly offset from the LO phonon frequency of the CdSe bulk material due to the phonon limited field effect [32]. And the weak peak at 409.6 cm−1 is attributed to the double frequency optical phonon (2LO) mode of CdSe [33]. Interestingly, as shown in Fig. 4c, with the introduction of CdSe in HTC, we can see that the characteristic D bands and G bands of HTC are significantly weakened, which may be due to the scattering of CdSe QDs on the surface of HTC blocking the signal of Raman [34].

Fig. 4. Raman spectra of HTC (a), CdSe QDs(b), and CHC-3 (c).

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at 1635 cm−1 ,1325 cm−1 , 1250 cm−1 , 808 cm−1 are assigned to C=O stretching vibration [38], C–OH stretching vibration [39], C–O stretching vibration [40], -CH2 groups [41], respectively. The characteristic peaks of HTC and CdSe QDs in CHC-3 composites are obviously weakened, but some of their characteristic absorption peaks can still be found, which proves the successful synthesis of CdSe/HTC nanocomposites.

3.6. Surface valence analysis of CHC-3 by XPS

Fig. 5. FT-IR spectra of CHC-3, CdSe QDs, and HTC. (using a KBr beam splitter with a wavelength in the range of 40 0–40 0 0 cm−1 .).

Further to be analyzed surface chemical composition and valence states of CHC-3 by XPS. The full survey spectra show that the composite contained Cd, Se, C, and O elements (Fig. 6a). As shown in Fig. 6b, two characteristic peaks appear at 404.5 eV and 411.3 eV, which are assigned to Cd 3d5/2 and Cd 3d3/2 in CdSe [42,43]. The peaks observed at about 53.2 eV and 54.1 eV are the contributions of Se 3d5/2 and Se 3d3/2 (Fig. 6c). While the main peak appearing at 284.6 eV is ascribed to C–C band, the peak at 285.6 eV and 286.9 eV are correlated with C–O band, respectively (Fig. 6d) [43,44].

3.5. FT-IR spectral analysis of CdSe QDs, HTC and CHC-3

3.7. Optical properties UV-Vis DRS analysis

The possible interactions among CHC-3 are further confirmed by FT-IR spectra. As shown in Fig. 5, The FT-IR spectrum of pure HTC, CdSe and CHC-3. The pure HTC exhibits characteristic functional groups, the peak at 3300 cm−1 correlated with stretching vibration of -OH [35], and 3075 cm−1 attributed to the aromatic structure of HTC [36]. The peak at 1440 cm−1 originates from carboxylic anion stretching of HTC [37]. Beyond that, the peak

The absorption ranges of CdSe QDs, HTC and CHC-3 are further determined by UV-visible diffuse reflectance spectroscopy, and the optical properties of the photocatalysts are detected, as shown in Fig. 7a. When CdSe QDs are loaded onto HTC, the absorption range of light is further improved, which indicates that CHC-3 has higher light utilization [45]. The band gap energy of CdSe QDs is evaluated using a converted Kubelka-Munk formula using a graph of

Fig. 6. XPS survey spectra of CHC-3 (a), Cd 3d (b), Se 3d (c) and C 1 s (d).

Please cite this article as: Q. Men, T. Wang and C. Ma et al., In-suit preparation of CdSe quantum dots/porous channel biochar for improving photocatalytic activity for degradation of tetracycline, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/ 10.1016/j.jtice.2019.03.019

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Fig. 7. UV-Vis DRS of HTC, CdSe QDs, and CHC-3 (a); the (α hυ )1/2 versus (hυ ) plots to obtain the band gap of CdSe QDs(b).

Fig. 8. Mott–Schottky plots of CdSe QDs. (electrolyte solution: 0.5 mol/L Na2 SO4, platinum plate and Ag/AgCl as for the reference electrode).

light absorption energy: 1

(α hν ) 2 = K(hν −Eg ) hν =

(2)

1240

(3)

λ

Where h is the Planck constant, ν the frequency of light, Eg the 1

band gap energy value, α was the absorbance, (α hν ) 2 the ordinate, and hν is plotted on the abscissa, and the linear fit curve is shown in Fig. 7b. The band gaps of CdSe QDs is about 1.6 Ev [46,47], and its absorption wavelength is about 768 nm. Compared with the absorption wavelength of CdSe (780 nm) [48], the absorption edge of CdSe QDs exhibits a certain degree of blue shift. This indicates that the crystal size of the latter is larger than that of the former, which may be attributed to the effect of quantum confinement effect [49-51]. 3.9. Mott–Schottky analysis The position of the conduction band (CB) and the valence band (VB) in the photocatalytic reaction is also critical. In order to obtain the position of the conduction band edge, a Mott–Schottky diagram is used. The linear region has a positive slope means that CdSe QDs photocatalyst is an n-type semiconductor. It can be seen

Fig. 9. PL spectra of CdSe QDs and CHC-3.

from Fig. 8 that the flat band potential of CdSe obtained at different frequencies (from 1.0 to 10.0 KHz) is approximate −0.7 V. Since the position of the flat band potential of the n-type semiconductor is approximately equal to the position of the conduction band low, the conduction band position of the CdSe can be considered to be −0.5 V (vs. NHE) [52]. In combination with the calculation results of the transformed Kubelka-Munk formula, we can evaluate the valence band potential of CdSe QDs and use the formula:

EVB = ECB + Eg

(4)

Where Eg is the band gap of 1.6 eV, ECB was band potential. The EVB with valence band potential was about 1.1 V [53,54]. 3.10. PL analysis Studies have shown that photoluminescence emission is mainly caused by photo-induced electron-hole pair recombination. Lower PL intensity indicates that the photo-excited electron-hole pairs separated efficiency is higher [55]. Fig. 9 shows the steady-state fluorescence spectra of pure CdSe QDs and CHC-3, with all samples are excited at a wavelength of 337 nm [56]. It is worth noting that the PL intensity of CHC-3 is significantly lower than that of pure CdSe QDs, and the lifetime of photocarriers gets relatively prolonged. It proves that the recombination of photocarriers of CHC-3 composites is effectively inhibited under visible light irradiation.

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Fig. 10. (a) Photocurrent transient responses and (b) EIS Nyquist plots of CHC-3 and CdSe QDs. (electrolyte solution: 0.5 mol/L Na2 SO4, platinum plate and Ag/AgCl as for the reference electrode).

Fig. 11. (a) The CdSe QDs and different proportions of CHC degradation TC experiment and (b) The cyclic runs of CHC-3 degradation TC under visible light irradiation. (250 W Xe-lamp, Temp: 25 ± 1 °C, CTC = 20 mg/L, Photocatalyst dose 0.5 g/L and reaction time of 80 min).

3.11. Photoelectrochemical properties analysis The separation of photogenerated charge carriers and holes during photocatalytic degradation is of vital importance, and the degree of photocurrent response can indirectly prove the separation efficiency of photogenerated charge carriers. To further investigate the photochemical properties of composites, the repeatable response transient photocurrents of the CHC-3 and CdSe QDs electrodes under intermittently visible light irradiation are shown in Fig. 10a. The photocurrent density of CHC-3 is significantly higher than that of pure CdSe QDs, which may be attributed to the introduction of HTC. This indicates that the charge transfer and separation efficiency of CHC-3 composites is higher, and the recombination of photogenerated electron-hole pairs is effectively suppressed [57]. In addition, EIS is also an effective test method for investigating catalysts as photoelectrode surface properties and interface electron transfer (Fig. 10b). The impedance arc radius of obtained CHC-3 is smaller than that of pure CdSe QDs. It shows that introducing HTC reduces the interface electronic impedance. It may be that excellent conductivity of HTC contributes to improving the interface electron transfer [58]. In summary, we can know that the combination of HTC and CdSe QDs not only contributes to the separation of photogenerated electron-hole pairs but also enhances the ability of interface electron transfer to enhance photocatalytic activity.

3.12. Measurement of photocatalytic degradation activity and photocatalytic degradation stability of CHC-3 The degree of photocatalytic degradation of TC is the most effective verification of photocatalytic performance. Fig. 11a shows the degradation of TC under visible light conditions. As the content of HTC increases from 0 to 15%, the photocatalytic effect is gradually enhanced. However, when the content of HTC reaches 20%, the photocatalytic effect is significantly weakened, which may be due to the excessive HTC content, which leads to the covering of CdSe QDs photocatalyst, further inhibiting photocatalytic activity [6]. It is well known that the stability and recyclability of photocatalysts are also critical to practical applications. The four-cycle experiment is shown in Fig. 11b. After 4 cycles of visible light irradiation, the photocatalytic effect of CHC-3 did not change significantly. It still has excellent photocatalytic activity, indicating that the CHC photocatalyst has excellent stability. 3.13. Evaluation of the effect of CHC-3 on the degree of TC mineralization In order to further understand the decomposition of TC, the Total Organic Carbon (TOC) content test was carried out to evaluate the mineralization degree of CHC-3 to TC (Fig. 12). The mineralization rate of TC is 10.72% when it is irradiated by visible light

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Fig. 12. The TOC removal rate of CHC-3 under visible light irradiation. (250 W Xelamp, Temp: 25 ± 1 °C, CTC = 20 mg/L, Photocatalyst dose 0.5 g/L).

for 90 min. When the illumination time is extended to 150 min, the rate increases significantly to 35.86%, which is due to a large number of active radicals in the environment after the reaction is carried out for a period of time, further promoting the degree of mineralization of TC. 3.14. Study of active species and reaction mechanism in the photocatalytic degradation process It is generally believed that superoxide radicals (·O2 − ), hydroxyl radicals (·OH), and holes (h+ ) were the main active species.

Fig. 13 shows an active substance capture experiment of CHC-3 degradation of TC in the presence of different scavengers under visible light irradiation. Fig. 13a shows an active species capture experiment of CHC-3 degradation of TC in the presence of different scavengers under visible light illumination. The scavengers BQ, EDTA-2Na and IPA are added to the photocatalytic reaction system to capture ·O2 − , h+ and ·OH reactive species, respectively. It can be clearly observed that when no capture agent is added for photocatalytic degradation experiments, the photocatalytic efficiency is 73.1%. When IPA, EDTA-2Na and BQ capture agent was added, the removal rates of CHC-3 to TC are 68.4%, 30.5% and 15.6%, respectively. From the above experimental results, we can draw the conclusion that the role of ·OH is scant, and ·O2 − , h+ play a leading role. In addition, electron spin resonance (ESR) detection using DMPO further confirms the presence of ·O2 − active species. As shown in Fig. 13b, no ESR signature signals of ·O2 − are detected under dark conditions. Conversely, a strong DMPO-·O2 − the signature signals are detected, which means that the photocatalyst produce ·O2 − active species under illumination conditions [59]. By capturing the experimental results of the experiment, it can be known that h+ and ·O2 − are the main active substances in the photodegradation process. For the aerobic degradation process, 1O2 is an important active substance. Typically, holes in the system can oxidize superoxide radicals to 1O2 [60]. In order to detect the presence of singlet oxygen (1O2 ) active species in the reaction system, 1O2 is detected using 4-Oxo-TEMP. In the dark conditions, the ESR characteristic signals of 1O2 are not found. On the contrary, we can clearly observe the TEMPONE-·1O2 characteristic signal with a 1:1:1 intensity in Fig. 13c. It can be determined that singlet oxygen does exist and has a higher strength [61]. So it can be determined

Fig. 13. (a) Effects of different active material capturing agents on photocatalytic degradation TC of CHC-3 under visible light irradiation. After photocatalytic degradation of TC, DMPO and TEMP spin capture ESR spectrum, (b) CHC-3/CH3 OH/DMPO, (c) CHC-3/H2 O/TEMP, 250 W Xe-lamp, Temp: 25 ± 1 °C, CTC = 20 mg/L, Photocatalyst dose 0.5 g/L and reaction time of 80 min).

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that ·O2 − , h+ and 1O2 were the main active substances in photocatalytic degradation. 3.15. Influence of experimental variables on photocatalytic degradation processes 3.15.1. Effect of different water substrates on photocatalytic degradation of tC In order to evaluate the application effect of CHC-3 photocatalytic degradation TC in water in the environment, four different water substrates are tested (Table S1). As a benchmark, which includes deionized water. Fig. S1 shows the degradation effect of four different water substrates on TC within 80 min. The photocatalyst in DI (73.1%) shows the highest degradation efficiency at 80 min, followed by those in WS (40.9%), LW (33.9%), and TW (16.2%). Compared with deionized water (DW), the degradation rates of TC in WS, LW and TW get significantly lowered. It is important to note that although the WS, LW and TW of TC photocatalytic degradation efficiency is low, their removal efficiency on TC (WS: 88.6%, LW: 86.4%, TW: 87.0%) far high photocatalytic degradation efficiency, which may be due in dark reaction stage where the TW has a strong oxidizing hypochlorous acid ions to interact with the coexisting ions in the water so as to achieve the purpose of the decomposition of TC. For LW and WS, the components are relatively complex, and there are a variety of coexisting ions and microorganisms, which may change the intermediate products of TC decomposition and form some substances that are conducive to be degraded by the catalyst. Therefore, the photocatalytic efficiency of WS, LW and TW is low, while the removal efficiency of TC is high. In addition, there are significant differences in the removal efficiency of different water sources, which may be the result of a combination of many factors, including solution pH, anionic species and salt concentration, etc, and will be further discussed in the following sections. 3.15.2. Effect of initial solution pH on photocatalytic degradation of TC In the process of heterogeneous catalytic oxidation, the initial solution pH value is considered to be a key factor to affect the system efficiency, because it has a considerable impact on the hydrolysis of pollutants, surface properties, and catalyst interface points [62]. In order to investigate the influence of pH on the photocatalytic reaction, the initial concentration of TC was set as 20 mg/L, and 100 mL was taken. NaOH of 1 mol/L and HCl of 1 mol/L were added to adjust the pH value of the solution, respectively. The dosage of each group of catalysts was 0.05 g. The influence of pH on photocatalytic degradation of TC by CHC-3 is shown in Fig. S2(a-b) and Table S2. Under the environment of strong acid and strong base, the activity of photocatalyst was inhibited, and the degradation efficiency of TC was at a relatively low level. When pH =1 and pH =7, TC degradation rate is 65.2% and 70.9%, respectively. When pH =11, TC photocatalytic degradation efficiency is 68.2%. The catalyst activity was higher under weak acid environment. When pH = 5.0, TC removal rate was the highest, reaching 88.4%. In general, when pH value is more than 5, then the degradation efficiency decreases, which may be associated with the isoelectric point of CdSe [16], when pH value is around 5, CdSe may achieve equipotential and surface charge is neutral, bettering photogenerated electronic-holes on the surface of the catalyst to capture, reducing recombination rate of them, and effectively improving the photocatalytic degradation rate. When the pH value is less than 3, TC exists in the form of cationic. If pH value is between 4.0 and 7.0, TC is basically in the form of zwitterion. As for pH value is between 8.0 and 10.0, TC is in the form of anion. While the pH value is larger than 10.0, the form of existence is net negatively charged ion [63]. Therefore,

9

when pH=1, the presence of TC in the form of a cation will compete with oxygen for electrons, hindering the formation of superoxide radicals, and reducing the degradation efficiency. When pH value is 11, a large amount of OH− ions exist in the strong alkali solution, and OH- ions can act as a trapping agent for holes, which affects the generation of singlet oxygen. Because OH− and h+ can form partial hydroxyl groups, the degradation efficiency is not significantly reduced. Therefore, when pH value is 5, the interaction with the catalyst is enhanced and the degradation efficiency is the highest. 3.15.3. Effect of TC initial concentration on photocatalytic degradation The experiment selected fixed CHC-3 catalyst dosage of 0.5 g/L, 100 ml of different initial concentration of TC solution (10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L) photocatalysis time of 80 min, and investigated different photocatalytic degradation efficiency at initial concentration. The experimental results are shown in Fig. S3 and Table S3. The essence of photocatalytic reaction is a kind of free radical reaction. The photocatalyst firstly adsorbs the target pollutant to the surface of the catalyst and then the photocatalytic reaction occurs. As the concentration of target pollutant TC increases, the degradation rate decreases within the same treatment time range, that is, the concentration increases, and a longer reaction time is required to achieve the same removal rate. The reason may be that under the condition of constant light intensity and catalyst dose, the number of photogenic electron-holes generated in the same time is the same. The smaller the concentration of target pollutant TC is, the larger the relative value of its removal by oxidation is. Secondly, when the initial concentration is too high, the adsorption amount of target pollutants on the catalyst surface is too high, and the activity center of photocatalyst is easily blocked by pollutants, causing the catalyst to be deactivated, and the photocatalytic reaction efficiency is poor. Therefore, the degradation efficiency (70.9% and 57.2%, respectively) at the initial concentration of 30 mg/L and 40 mg/L is not as good as that at the low concentration of 10 mg/L and 20 mg/L(82.6% and 73.1%, respectively). Based on comprehensive multi-angle considerations, it is determined that the target pollutant is 10 mg/L as the optimal initial concentration. 3.15.4. Effect of photocatalyst dosage on photocatalytic degradation of TC The appropriate amount of catalyst is an important factor in photocatalytic oxidation. In order to investigate the influence of CHC-3 dosage on photocatalytic reaction, 100 mL solution was used for catalytic reaction, and the initial concentration of TC was 20 mg /L. The dosage of CHC-3 catalyst was 0.03 g, 0.05 g, 0.07 g and 0.09 g, respectively. Samples were taken every 20 min to determine the absorbance value. It can be seen from Fig. S4 and Table S4, with the increase of CHC-3 dosage, the degradation rate of TC pollutants increases. When the dosage is 0.07 g, the maximum degradation rate is 82.6%. The amount of catalyst continues to increase, and the reaction speed shows a decreasing trend. It can be seen that, within a certain range, increasing the amount of catalyst can improve the photocatalytic degradation rate, which is because TC pollutants have more opportunities to contact with the catalyst that generates more active species, and promotes the degradation of TC pollutants. However, too much catalyst can easily cause turbidity of the solution, reduce the transmittance, affect the scattering of light, cause light path blockage [64], reduce the utilization rate of the reaction system to the light source, and reduce the catalytic efficiency to some extent. When the dosage of the catalyst is too small, the probability of contact rate of the TC contaminant with the catalyst is reduced, and the effective photon cannot be utilized to the fullest extent, and the reaction rate is slow, and the

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degradation efficiency is low. This indicates that the amount of catalyst has a certain range of application, both in terms of degradation efficiency and economic cost. A reasonable dosage of catalyst can achieve the best catalytic effect. 3.15.5. Effect of salt concentration on photocatalytic degradation of TC The sources of salt-containing wastewater are wide and the water quantity is increasing year by year. It is very important to remove organic pollutants from salt-containing wastewater. Therefore, it is necessary to systematically evaluate the effect of salt concentration on the degradation of TC pollutants. In order to explore the influence of salt concentration on the removal of TC pollutants in water, a TC solution with an unadjusted initial pH concentration of 20 mg/L was used for catalytic reaction, and NaCl concentrations of 0.10 g/L, 0.30 g/L, 0.60 g/L, 0.80 g/L and 1.60 g/L were added to the TC solution. The sample is taken every 20 min and the absorbance value is determined. As shown in Fig. S5(a–b) and Table S5, the effect of 0.10 g/L NaCl on the degradation efficiency is negligible, and the degradation effect increases with the increase of salt concentration. When the salt concentration reaches 0.60 g/L, it reaches the peak value and continues to increase the salt concentration. The degradation efficiency is significantly reduced. Such phenomenon indicates that when the salt concentration reaches the appropriate value, the number of anions and cations in the system increases, which to some extent promotes the charge transfer rate in the process of organic matter oxidation, The electrons transferred combine with the active sites on the surface of the catalyst to improve the photocatalytic degradation efficiency. The degradation rate of the high concentration salt is lower, which may be that the ions cover the surface of the photocatalyst and occupy part of the active sites, resulting in a decrease in catalytic activity. 3.15.6. Effect of co-existing ions on photocatalytic degradation of TC In the natural environment, the water matrix contains a wide variety of ions, and the photocatalytic degradation process is affected by anions, which are not negligible because they could scavenge free radicals or produce active substances. In this part of the study, TC was the target pollutant, the initial concentration of fixed TC was 20 mg/L, the dosage of CHC-3 was 0.50 g/L, the anion concentration was 0.60 g/L and the photocatalytic time was 80 min. The effects of various inorganic anions (NO3 − , Cl− , HCO3 − , SO4 2− ) on the photocatalytic degradation of TC were investigated. As shown in Fig. S6 and Table S6, it was found that the degradation efficiency was significantly enhanced when Cl− was added to the system, which was consistent with the effect of salt concentration on photocatalytic degradation. When Cl− and HCO3 − coexist, the degradation efficiency decreases. On the one hand, HCO3 − can be dissociated into H+ and CO3 2- in aqueous solution, and can also be hydrolyzed to form OH− and H2 CO3 . The degree of hydrolysis is higher than the degree of dissociation, so the HCO3 − solution is alkaline. The effect of combined pH on the degradation of TC suggests that higher pH has an adverse effect on the degradation of TC. On the other hand, many literatures have also reported that HCO3 − is also a hole-trapping agent, which can transform the highly active hole into other weak free radicals (Eq. (5)). The combination of various factors lead to the strongest inhibition of TC degradation by HCO3 − . With the addition of NO3 − , the degradation efficiency is also reduced, which may be caused by the reduction reaction of NO3 − adsorption on the catalyst surface (Eq. (6)). When SO4 2− is added to the system, the degradation efficiency is improved but the effect is not obvious. It may be that SO4 2− reacts with h+ to produce ·SO4 2− (Eq. (7)), and ·SO4 2− reacts part of HCO3 − (Eq. (8)) [65], producing H+ to decrease the pH environment of the solution, resulting in improved degradation. The above results showed that Cl− and SO4 2− could promote the degradation

of TC, while HCO3 − and NO3 − inhibited the degradation of TC. − + HCO− 3 + hVB → •CO3 + H2 O

(5)

− − NO− 3 + H2 O + e → NO2 + 2OH

(6)

2− SO24− + h+ VB /OH → •SO4

(7)

2− − + •SO24− + HCO− 3 → SO4 + H + •CO3

(8)

3.16. Preliminary analysis of possible pathways for tetracycline degradation In order to analyze the intermediate products produced by the above photocatalytic reactions and to infer the possible TC degradation pathway, HPLC-MS tests were conducted on the products at 0 min, 40 min and 80 min, respectively. According to the Figure S7a mass spectrogram analysis, the peak at m/z = 445, which is a typical TC characteristic peak. It can be seen in Figure S7b that the characteristic peak of TC is significantly weakened, indicating that some TC has been decomposed. At this point, there are several new spectrum peaks, labeled as products A, B, C, D, E, G, and I. From the mass spectrum analysis, it can be concluded that there is a peak at m/z = 430 (Fig. S7b) which is an A product formed by the loss of N-methyl group due to the low bond energy of the N–C bond in TC [62]. The formation of product B may be the result of hydroxyl radical easily attacks the enol structure to undergo 1, 3-dipolar cycloaddition reaction [66]. The product C is formed by oxidation of the amino group. With the prolongation of the photocatalytic reaction time, the N-methyl group of product A is oxidized to the amino group to form product D, and further oxidized to the carbonyl group to form the product G. The formation of product I may be attributed to the fact that singlet oxygen attacks the break of the C=C double bond on the aromatic ring to produce ketone and carboxyl groups [67]. The continuous photocatalytic reaction, as shown in Figure S7c, can be seen that the reaction changes greatly at 40–80 min, and the peak intensity of the TC spectrum decreases to very weak level, indicating that the concentration of TC is already very low at this time, corresponding to the test results of photocatalytic activity. At the same time, some new spectrum peaks appeared, labeled as products F, H, J. combined with the previous mass spectrometry results, inferred that may be caused by further oxidative decomposition of product C, while other impurities are not inferred, which may be due to the low concentration of these substances, hard for mass spectrometry analyzer to identify [67]. In addition, combined with the results of TOC analysis, a large number of organic carbon compounds are indeed converted into inorganic carbon through photocatalytic oxidation. And the intermediates will eventually be mineralized into small molecules, such as CO2 , H2 O, NH4 + , etc. In summary, the possible degradation pathway of TC is shown in Fig. 14. The products of the photocatalytic reaction are mainly obtained by strong oxidation and the shedding of some groups (such as methyl, amino, hydroxyl, etc.) from the TC structure. Based on the above analysis results, the photocatalytic mechanism diagram of CHC-3 can be as shown in Fig. 15. The semiconductor CdSe QDs are photoexcited to form photogenerated charge carrier, and then electron-hole pairs are formed in the valence band and the conduction band according to the reaction [68], as described in Eqs. (1)–(9). According to the above equation, a reactive oxide is produced by a chemical reaction. Photo-generated electrons are excited from the valence band (VB) to the conduction band (CB). Because HTC has excellent electron conductivity, part of e− on CB of CdSe QDs could easily migrate to the surface of HTC, and HTC as a receptor for electrons effectively separates photogenerated electrons and holes to accelerate the conversion of

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11

Fig. 14. Possible degradation pathways of TC by CHC-3. (Photocatalyst dose 50 g/L, TC of 20 mg/L and reaction time of 80 min, a 2.1 × 150 mm Zorbax ODS chromatography column).

electrons [69]. Electrons (e− CB ) in a conductive bond are reduced with dissolved oxygen to produce ·O2 − active species (Eq. (10)) that can effectively degrade TC contaminants [70]. And a portion of the holes react with ·O2 − to produce a singlet oxygen active (Eq. (11)). The reactive species of the system increased further increase the degradation rate of TC. At the same time, the holes generated by photoexcited electrons further oxidize and decompose the macromolecular organic pollutants. Therefore, the synergistic action of superoxide radicals, holes and singlet oxygen can more effectively remove TC pollutants in water (Eq. (12)). − CdSe QDs + hν → h+ VB +eCB

(9)

eCB − +O2 → •O2 −

(10)

h+ + • O− 2 → 1O2

(11)

•O− 2 /h + /1O2 + tetracycline → CO2 + H2 O + moleculars

(12)

4. Conclusions In summary, CHC photocatalysts were synthesized by simple and effective in-suit methods, and the degradation of organic pollutants TC by CHC was significantly better than that of pure CdSe QDs. Due to the significant stability of the CHC composite, the degradation activity remained good after 4 cycles of the experiment. More importantly, the introduction of HTC is beneficial to alleviate the phenomenon that CdSe QDs are easy to aggregate and enrich the low concentration of TC in water to increase the reaction rate. In addition, excellent electron conductive performance of HTC effectively decreases photoelectron-hole pair recombination,

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Fig. 15. Schematic diagram of electron transfer and reaction mechanism during photocatalytic degradation.

which facilitates the further conversion of electrons and holes, which is critical for photocatalytic reactions. Therefore, CHC composites with stable and economical environmental protection have high potential value in practical applications.

Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. U1510126, 21676127, 21676115), Natural Science Foundation of Jiangsu Province (BK20180884), China Postdoctoral Science Foundation (No. 2017M621641).

Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.03.019.

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Please cite this article as: Q. Men, T. Wang and C. Ma et al., In-suit preparation of CdSe quantum dots/porous channel biochar for improving photocatalytic activity for degradation of tetracycline, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/ 10.1016/j.jtice.2019.03.019