Highly Efficient Degradation toward Tylosin in the Aqueous Solution

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Highly efficient degradation toward tylosin in the aqueous solution by carbon spheres/g-C3N4 composites under simulated sunlight irradiation Xuetao Guo, Hao Dong, Tianjiao Xia, Tiecheng Wang, Hanzhong Jia, and Lingyan Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01967 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Sustainable Chemistry & Engineering

Highly efficient degradation toward tylosin in the aqueous solution by carbon spheres/g-C3N4 composites under simulated sunlight irradiation Xuetao Guo†,§, Hao Dong†,‡, Tianjiao Xia*,†,§, Tiecheng Wang†,§, Hanzhong Jia†,§, Lingyan Zhu*,†,§ †

College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi, 712100, China ‡

School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, China

§

Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi, 712100, PR China

*Corresponding authors. E-mail addresses: [email protected] (T. Xia); [email protected](L. Zhu).

ACS Paragon Plus Environment

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ABSTRACT Carbon spheres (CS) were synthesized from monosaccharides (glucose), disaccharides (β-cyclodextrin) and polysaccharides (sucrose) using surfactants, and then by hydrothermal treatment with the g-C3N4 to form a composites. The photodegradation efficiency of tylosin（TYL）by modified carbon spheres/g-C3N4 composite materials was investigated. The results illustrated the efficiency of photodegradation of TYL by modified carbon spheres/g-C3N4 composites is higher than pure carbon spheres and g-C3N4. Furthermore, the surface characteristic and microstructure of the as-prepared composite materials were analyzed by different means. The results demonstrated that a small amount of carbon spheres embedded in g-C3N4 can form the heterojunction with an intimate interface, which boosts the absorption scope of visible light, enlarges surface areas and facilitates the segregation of electron-hole pairs of g-C3N4. Free radical quenching experiments showed that the main active groups of TYL photodegradation were superoxide radical (•O2−) and hydroxyl radical (•OH). Keywords: carbon spheres, g-C3N4, tylosin, photocatalytic

INTRODUCTION Antibiotics have been extensively application in medical and animal husbandry, and human beings are increasingly worried about the latent risks to human and ecotope 1. Due to the constant production and consumption, antibiotics are released directly and indirectly into the environment, mainly through excrement, sewage irrigation and sludge compost 2, 3. Nowadays, antibiotics and their metabolites are continually detected in the soils, surface water, underground water and even drinking water

4, 5

. Low levels of antibiotics are easily found in aquatic environments, and raising concerns

about the toxicological effects and negative impacts on natural ecosystems

5-7

. In addition, most

conventional water purification and wastewater effluent treatment are useless or less effective for these organic contaminants 8. Accordingly, the elimination of antibiotics has always attracted much attention in pollutants management. Various strategies including sorption, biological treatment, advanced oxidation and photocatalytic reduction have been used in antibiotics control, among which, most of those processing technic have shortcomings, such as energy-extensive consumption and inefficiencies at low pollutant concentration to treat these persistent antibiotics


9-11

. However,

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photocatalysis technology can achieve high efficient degradation low concentration of antibiotics under solar and visible light12, 13. Among all the known photocatalysts, graphitic carbon nitride (g-C3N4) has emerged as accelerated metal-free visible-light photocatalyst for water splitting and pollutant degradation on account of

nontoxicity,

steady

and

large

surface

area

14-16

.

Unfortunately,

the

photocatalytic performance of g-C3N4 was relatively low on account of the fast recombination of the photogenerated electron-hole pairs, which largely limit its practical applications

15, 17, 18

. Hence,

many researchers have now started to ameliorate the activity of g-C3N4 through various embellish strategies

19

. Constructing of semiconductor heterojunction is an effective and common way to

overcome the drawback

20, 21

. Nowdays, a good deal of work have been made to establish binary or

ternary heteroions, such as the complexus of g-C3N4 with Cu-Cu2O, NiMoO4, Sulfur and Mn+/CeO2-TiO2 22-25. However, the metal or harmful elements doped in these complexes are likely to be released into the water during photocatalytic degradation

26

. Meanwhile, the non-toxic and

harmless properties of the carbon material have attracted widespread attention 27. In addition, it has been attested the import of carbon materials impacts the competence of the photocatalyst

28

. For

example, unified with carbon materials for instance RGO and CNT it has been demonstrated a higher performance for the remove of pollutants 29. Among various types of carbon-based materials, carbon spheres have the potency applications on account of the distinct chemical and physical properties, including their size, microstructure and crystallinity 30. It is learned that incorporation of g-C3N4 with carbon spheres could heighten the electroconductivity and photo-electrochemical performance of g-C3N4 for hydrogen generation

30-32

. However, synthetic material and size uniformity of the carbon

spheres impact photocatalytic activity is still unknown. In this work, carbon spheres were prepared by thermal polycondensation of different precursor (glucose, β-cyclodextrin and sucrose). In addition, the differences between the synthesized carbon spheres were investigated using monosaccharides, disaccharides and polysaccharides as precursors. Furthermore, β-cyclodextrin is a type of cyclic oligosaccharide compound produced by starch under the action of enzymes and has been widely used as a carbon precursor

28, 33

. Hexadecyl

trimethyl ammonium bromide (CTAB) is comprehensive used in shape-command synthesis of nanostructures as an effective surfactant and stabilizer 34. Therefore, the size uniformity of the carbon spheres could be controlled by surfactants

35

. Tylosin (TYL), the extensive used antibiotics was



designated to evaluate the photo-degradation activity

36, 37

. To evaluate the photocatalytic activity,

photodegradation experiments were performed on the as-synthesized different types of CS and modified carbon spheres/g-C3N4 composites. Furthermore, the crystal texture, structure, morphology of T-CS/g-C3N4 were assessed by TEM, SEM, FTIR, XPS, XRD and UV–vis. By means of these results, the photocatalytic mechanism over as-synthesized photocatalyst was proposed.

MATERIALS AND METHODS Materials.

Glucose, β-cyclodextrin crystalline, sucrose, melamine and CTAB were provided by

Sinopharm Chemical Reagent Company. Benzoquinone (BQ), ammonium oxalate (AO) and isopropanol (IPA) supplied from Guangzhou Chemical Reagent Corporation. In addition, TYL (purity >95 %) afford from Sigma (StLouis, MO). Preparation of g-C3N4, CS and CS /g-C3N4 composites. polymerization with melamine as a precursor 38. Typically,

The g-C3N4 is synthesized by heat 15 g melamine powder was transferred

to the semi-covered crucible and then placed into the tube furnace. The temperature was elevated to 500 °C at a speed of 10 °C min-1 for 2 hours and then continued to warm to 550 °C for 2 hours 38. Subsequently, the yellow substance was obtained and milled into stive for later employ. The manufactured carbon spheres with uniform size is based on the previous study of the hydrothermal carbonization of glucose 30. In brief, 5 g of β-cyclodextrin crystalline was placed in the distilled water, the temperature was raised and mixed until it was completely dissolved to form a clear solution. Then shifted the solution to teflon autoclave and held at 200 °C for 12 hours. The brown product was gathered and washed three times with ultrapure water ethanol, then desiccation in a vacuum oven and named as CSC. Glucose and sucrose are also used in a similar way to synthesize carbon spheres, and the synthesized samples are named CSG and CSS respectively. For the preparation of uneven size of the carbon spheres, the surfactant is used to adjust the size of the synthetic carbon spheres

35

. Typically, 5 g β-cyclodextrin crystalline and 0.5 g CATB were

simultaneously added to 50 ml of distilled water, the rest of the synthesis steps are the same and the obtained product was called T-CSC. In addition, glucose and sucrose were synthesized in the same way and were denoted as T-CSG and T-CSS. T-CS/g-C3N4 photocatalyst is prepared with a pre-prepared carbon spheres and g-C3N4 as a precursor to hydrothermal treatment in scheme 1 35. In brief, 0.1 g of T-CS was mixed with 0.9 g of


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pure g-C3N4 and placed in 50 ml of deionized water. After 2 h of ultrasonic processing, the mixed solution was agitated at room temperature for 3 h to achieve uniform dispersion. The mixed solution was then moved to teflon reaction still and maintained at 150 °C for 12 h. The sample was centrifuged and dried in an oven. According to the type of synthetic carbon spheres, three samples were named as T-CSC/g-C3N4, T-CSS/g-C3N4 and T-CSG/g-C3N4. Scheme 1 shows the possible synthesis process of the compounds by different modified carbon spheres.

Scheme 1 Schematic diagram of preparation process of T-CS/g-C3N4 composites

Photocatalytic activity measurements and TYL analysis.

The photocatalytic efficiency of TYL was

appraised under a self-made reactor，the visible light was afforded by a 300 W Xe lamp outfited with a 420 nm light filter. The rate of light is surveyed to be 78.35 mW/cm2. In the degradation reactions, 0.1 g powder sample was added into the 200 mL TYL (5 mg/L) solution, and thenlucifuge agitated for 12 h to determine the sorption capacity of the photocatalyst. At a fixed point in time, about 1.5 mL solutions were gathered and then shifted to the chromatographic bottle for detection. The concentration of TYL was quantified by HPLC with a C18 column and UV detector. The mobile phases (1 mL min-1) were a mixture of acetonitrile (35%) and an aqueous solution (65%) adjusted to pH = 2.0 with 0.01 mol L-1 KH2PO4.



RESULTS AND DISCUSSION Evolution of structure and morphology.

Fig. 1. SEM images of CSG (a, b), CSC (e, f), CSS(i, j), T-CSG (c, d), T-CSC (g, h) and T-CSS(k, l).

The difference carbon spheres synthesized by different kinds of precursors by SEM was shown in Fig. 1 (a, b, e, f and i, j ), which showed that CS without surfactant modification has a uniform spherical morphology. Clearly, both sample CSG and CSC showed relative dispersibility (Fig. 1 b and f), while CSS showed significant agglomeration (Fig. 1 j). Also, we calculated the sizes of treated spheres based on the SEM images. The average diameters of the CSG, CSC and CSS particles are about 520, 890 and 870 nm, respectively. However, with the influence of surfactants, the diameter of the carbon spheres becomes larger and the size is not uniform. The SEM image of T-CSG and T-CSC (Fig. 1c and g) all shows a spherical shape with a diameter from 550 nm to 10,500 nm. Furthermore, the average diameters of the T-CSG, T-CSC and T-CSS particles are about 2,550, 3,380 and 1,550 nm, respectively. In contrast to T-CSG and T-CSC, the spherical structure of T-CSS is not very smooth (Fig. 1k and l), and it does not significantly change the agglomeration under surfactant modification. The above observation results show that the size of the carbon sphere expands with the addition of surfactant, and the uneven spherical structure gradually forms.


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Fig. 2. SEM and TEM images of T-CSG/g-C3N4 (a, b and c), T-CSC/g-C3N4 (d, e and f) and T-CSS/g-C3N4 (g, h and i).

After hydrothermal treatment, the SEM and TEM images of T-CSG, T-CSC and T-CSS complexus with g-C3N4 are exhibited in Fig. 2. Pure g-C3N4 layered structure with irregular particles can be found from the SEM images39. Moreover, the presence of the carbon spheres was demonstrated in T-CS/g-C3N4 composites, which showed good interlinkage between T-CS and g-C3N4. Moreover, from the TEM images (Fig. 2c and f), the conventional spherical shape was found in both T-CSG/g-C3N4 and T-CSC/g-C3N4. But for T-CSC/g-C3N4 (Fig. 2i), the ellipsoid shape shown can be attributed to the more severe agglomeration, which is in line with SEM results (Fig. 2d and e).



Fig. 3. (a) N2 adsorption–desorption isotherms for g-C3N4(inset) and T-CS/g-C3N4 composites;(b) pore size distribution curves for g-C3N4(inset) and T-CS/g-C3N4 composites.

For the sake of further analyze the introduction of TCS impacted the composites material, synthesized T-CS/g-C3N4 samples are analyzed by BET（Fig. 3）, which was pertain to type IV adsorption isotherms with antrorse prolonged embranchment sharp. According to the N2 adsorption/desorption isotherms, g-C3N4 has a specific surface area (SSA) of 7.012 m2/g. After the introduction of TCS, all the SSA of T-CS/g-C3N4 composites has been significantly improved, which were 11.314, 11.439 and 17.092 m2/g for T-CSG/g-C3N4, T-CSC/g-C3N4 and T-CSS/g-C3N4, respectively. In addition, the isotherms of all T-CS/g-C3N4 samples showed high absorption with higher pressure (P/P0) range (from 0.9 to 1.0), manifested formation of large mesopores and macropores24. Fig. 3 b show bore diameter is mostly of 50-600 nm, which is very large by contrast with g-C3N4. Furthermore, the average bore diameter of g-C3N4 and composites with T-CSG, T-CSC and T-CSS were 17.15, 223.21, 250.63 and 199.87 nm, respectively. For T-CS/g-C3N4 composites, the larger pore size may ascribe to the gap in the middle of carbon sphere and g-C3N4, which is in accord with SEM and TEM image.

Fig. 4. XRD patterns of six kinds of carbon spheres (a) and T-CS/g-C3N4 composites(b).

In order to analyze the crystalline structure of different types of carbon spheres and their composite with g-C3N4, we performed XRD analysis of all the composites as shown in Fig. 4. XRD of disparate carbon spheres were exhibited in Fig. 4a which attested the existed of amorphous carbon 40, 41

. For all carbon spheres, a peak at ∼ 24◦ was related to the (002) diffraction peak of carbon 27.

After CTAB modification, as-synthesized carbon spheres showed a higher degree of carbonation 42. In addition, all of the T-CS/g-C3N4 composites have the same XRD device as g-C3N4, indicating that


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T-CS/g-C3N4 samples retains original g-C3N4 crystal structure.The sharp diffraction maximum at ∼27.3°demonstrated graphitic materials as the (002) peak. And a minor angle diffraction maximum at 13.2° was equal to in-plane structure repeating units (100) 29. Particular, the intensity of (002) peak of the T-CS/g-C3N4 composites is receded in contrast to g-C3N4. This is owing to the composite of T-CS and g-C3N4 reduces the bending vibration of the planar structure and further indicates the successful synthesis of T-CS/g-C3N4 composites.

Fig. 5. FTIR spectra of six kinds of carbon spheres (a) and T-CS/g-C3N4 composites(b).

The chemical structure was further analyzed by FTIR as shown in Fig. 5. For all the different types of carbon spheres have shown a similar structure, indicating that the surfactant did not change the structure of the original carbon spheres. As shown in Fig. 4a, all of the carbon spheres spectrums reveal absorption bands at about 1102 to 1251 cm-1 which was accord with C-O stretching vibrations respectively

40

. In addition, the sharp band at 1618 and 1400 cm-1 can be attributed to skeletal

vibration of aromatic C=C, but absorption bands at 1702 cm-1 which correspond to C=O 43. Besides , the broadband of 3431 cm-1 is divided among stretching vibrations of O-H (hydroxyl or carboxyl) 43. However, it can be surveied peak strength of O-H decreases with surfactant modification. The oxygen surface functional groups can enhance the ability of the carbon spheres to adsorb polar molecules, the sorption of TYL may be reduced under the modification of the surfactant

44

. In

addition, all of the T-CS/g-C3N4 composites reveal a typical molecular structure of g-C3N4. Due to the composites manifested semblable absorptions scope of 1200–1700 cm-1 which put down to conjugated CN rings stretching vibrations and breathing mode of s-triazine at 806 cm-1 45. Nevertheless, a clear distinction in the scope of 3000–3500 cm-1 was observed, which the absorption band at 3432 cm-1 was put down to O-H and the band at 3191 cm-1 was accord with the N-H 45. The peak at 3430 cm-1 of the T-CS/g-C3N4 is higher than g-C3N4, mainly due to T-CS successful



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introduction.

Fig. 6.

XPS spectra of T-CS/g-C3N4 composites: (a) survey, (b)C 1s, (c) N 1s and (d) O 1s.

Chemical construction element status of the composite materials were investigated by XPS, and the result is demonstrated in Fig. 6. All of the as-synthesized samples showed that the three peaks were approximately at 285, 399 and 532 eV, which are attributable to C 1s, N 1s, and O 1s signals, consistent with composites composition

45

. However, compared with g-C3N4, C/N ratio for all

T-CS/g-C3N4 composites increased and the content of O element increased, which further attested carbon spheres is successfully combined with the g-C3N4. C 1s with high-resolution spectra for composites can be split into three portions incorporated sp2 C-C bonds of graphitic carbon (284.8 eV) 38, 46

, C–H, C–N (285.4 eV) and sp2-hybridized atomic carbon in N–C=N (288.2 eV)

12

. N 1s

spectrum (Fig. 6c) can be split into two peaks at 398.7 and 400.2 eV, which can be due to sp2 nitrogen (C=N–C) involved in triazine rings and N–(C)3 47. Moreover, O 1s spectrum can be split into two peaks with binding energies of 532.13 and 533.4 eV which due to C=O and adsorbed water 48. C-C and C=O peak intensities of T-CS/g-C3N4 composites increased significantly, which was attributed to the successful intervention of carbon spheres. This can further confirms the existence of the heterojunction structure, which may be beneficial to the transfer of electronics and then


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enhancing the photocatalytic activity.

Fig. 7.

PL spectra of g-C3N4, carbon spheres and T-CS/g-C3N4 composites

Fig. 7 showd PL spectra of g-C3N4, carbon spheres and T-CS/g-C3N4 composites with the excitation wavelength of 341 nm. Emission peak of carbon spheres and g-C3N4 were 438 nm, which can be put down to radiative recombination process of self-trapped excitations 49. Obviously, the PL emission of T-CS/g-C3N4 composites distinct decreased, illustrating the valid charge separation and slower photogenerated electron-hole recombination rate in contrast to carbon spheres and g-C3N4 50. It attested intense reciprocity in the middle of carbon spheres and pure g-C3N4 were occured and the interface structure between them is in favour of the interfacial charge transfer

50

. Noteworthy,

T-CSC/g-C3N4 composites displayed weakest PL curve, which was on account of valid interfacial charge transfer between T-CSC and g-C3N4. This efficient electron transfer process of T-CSC/g-C3N4 will not only benefit the electron-hole separation but also promote the increase of the photocatalytic efficiency 49.

Optical absorption performance and photocatalytic activity.



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Fig. 8. (a) UV–vis spectra of carbon spheres and T-CS/g-C3N4 composites; (b) Band gap of T-CS/g-C3N4 composites.

All the samples exhibit strong light absorption with UV-vis scope from 240 to 800 nm in Fig.8, which shows g-C3N4 had representative semiconductor absorption with the absorption edge at 450 nm

51

. T-CS/g-C3N4 composites enhanced optical absorption under visible range than g-C3N4,

indicating that the involvement of carbon spheres has a obvious influence on photoabsorption of g-C3N4. The results manifested carbon spheres enlarge absorption ranges of g-C3N4 in visible region, which is helpful to improve the photocatalytic activity. In addition, the band gap values of composite materials were calculated by plots of (αhv)1/2 versus photo energy

43

. The calculated band gaps of

g-C3N4, T-CSG, T-CSC and T-CSS composites with g-C3N4 were 2.54, 2.40, 2.37 and 2.29 eV. The results manifested doping with carbon spheres is beneficial to reducing the band gaps of g-C3N4. The decreasing of band gap was in favour of collect visible light and make for TYL remove52.


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Fig. 9. Time-course variation of C/C0 and ln(C0/C) of TYL solution under visible light illumination over T-CSG/g-C3N4 (a,b)，T-CSC/g-C3N4 (c,d) and T-CSS/g-C3N4 (e,f).

Different types of carbon spheres and T-CS/g-C3N4 composites were tested for photodegradation of TYL under simulated sunlight irradiation (Fig. 9). Nearly 1% of TYL was degraded within 120 minutes lack of catalyst, indicating TYL self-photolysis is limited. The unmodified carbon spheres has the strong sorption ability for TYL, in which the CSG and the CSC can adsorb about 50% TYL in the dark condition, and the CSS can adsorb about 20%. The sorption of TYL on the modified carbon spheres was significantly reduced, which may be due to reduction of oxygen functional groups on the modified carbon spheres relative to unmodified carbon spheres, and the result corresponds to the FTIR pattern

40

. As shown in Fig. 9, it can be found that the modified carbon

spheres have obvious improvement on the degradation of TYL, and the CSG and CSC have



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improved the photodegradation efficiency of about 1 times for the unmodified carbon spheres. This result may be attributed to the higher crystallinity and relative good dispersibility of the modified carbon spheres. In addition, by contrast with g-C3N4, T-CS/g-C3N4 composites put up notably higher photocatalytic activity. T-CSG/g-C3N4 and T-CSC/g-C3N4 are efficient for photodegradation of TYL, and nearly 98% of TYL is degraded after 45 min of irradiation. It is worth mentioning that for T-CSS/g-C3N4, its degradation of TYL is weaker than T-CSG/g-C3N4 and T-CSC/g-C3N4, which may be dedicated to more severe aggregation of carbon spheres formed by sucrose. The photodegradation dynamics can be well matched by pseudo first-order dynamics (ln(C0/C)=kt), thus the first-order rate constant (k) is employed to compare efficiency of all composites

45

. The k of

T-CSG/g-C3N4 , T-CSC/g-C3N4 and T-CSS/g-C3N4 is 1.84, 2.01, 1.43 times than g-C3N4, respectively. It indicates that heterostructured with carbon spheres materials can improve the photocatalytic degradation efficiency.

Fig. 10. The TOC removal efficiency using T-CSG/g-C3N4, T-CSC/g-C3N4 and T-CSS/g-C3N4.

The extent of TOC eliminate was exhibited in Fig. 10, which was distinct low at the first 30 min for all of the three composites, while T-CSS/g-C3N4 put up lower TOC removal rate owing to its low photodegradation efficiency. In the HPLC chromatographic peaks, TYL was transformed into different outcomes. At the same time, an evident transition arised at 30-60 min due to these outcomes, then they were entirely transformed into H2O and CO2.


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Fig. 11. Cycling runs for TYL photo-degradation of by T-CSG/g-C3N4(a) , T-CSC/g-C3N4(b) and T-CSS/g-C3N4(c).

Besides, the stability and reusability of composites were significant importance for practical application. Therefore, we tested four successive cycles for the photocatalytic degradation of TYL by T-CSG/g-C3N4 , T-CSC/g-C3N4 and T-CSS/g-C3N4. Fig. 11 shows that the as-prepared composite material has good recyclability, and there is no significant deactivation of the catalyst after 4 cycles in TYL degradation. The cycling experiments determined the stability and reusability of the samples.

Fig. 12. Different radical scavenging species existed by T-CSG (a) , T-CSC (b) and T-CSS (c) with g-C3N4 composites; (d) pH influence on TYL photodegradation.

For the sake of clarify TYL photodegradation mechanism by synthesized catalyst, AO, IPA and BQ were used as scavengers for the major active species hole (h+), •OH and •O2-, respectively. As indicated in Fig. 12, when AO was introduced, T-CS/g-C3N4 composites had no significant effect on TYL degradation, stated h+ is not primary liveness species12. At the same time, there is remarkable



restrain TYL degradation with existenceof

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IPA and BQ, declared •OH and •O2- worked in TYL

removal. In addition, according to the capture experiment we can see that adding BQ has more inhibitory effect than IPA. Therefore, the primary liveness species are •O2-, which plays a significantly important role53. In addition, pH influence were surveyed for redouble probe reaction mechanisms. Effect of pH on TYL photodegradation showed the same pattern for all the T-CS/g-C3N4 composites were shown in Fig. 12d. All the T-CS/g-C3N4 composites have high photodegradation efficiency (pH=3) for TYL under acidic conditions and normal photocatalytic performance at pH 7 to 11. This phenomenon may be attributed to the physical and chemical properties of TYL. At pH inferior to 7.1, primary species of TYL turned into ionized state with the decrease of pH

54

. TYL+ has a stronger ability to absorb light than TYL, which will promote TYL

photodegradation under visible light 55. Therefore, when the presence of TYL in aqueous solution is not charged, the efficiency of photodegradation returns to its original state. So, the photodegradation mechanism by composites were attested and consequences were demonstrated in Fig. 13. Under simulated sun irradiation, the electrons excited by g-C3N4 could quickly pass on to carbon spheres, thus restrained reunion electron-hole pairs. After CTAB modification, carbon spheres showed a higher carbonation. Furthermore, carbon spheres with a higher degree of carbonation can transfer electrons faster, thereby further improving the photodegradation efficiency. The process of charge transfer and degradation may be as follows: T-CS/g-C3N4 + hv →e− + h+

(1)

H2O + h+→ H+ + •OH

(2)

O2 +e− → •O2−

(3)

TYL+•O2−/ •OH→ Degradation products


(4)

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Fig. 13. Proposed photo-degradation mechanism of TYL by T-CS/g-C3N4 composites.

CONCLUSIONS In this study, carbon spheres are prepared by simple hydrothermal processes through different precursors and surfactants. After CTAB modification, as-synthesized carbon spheres showed a higher degree of carbonation. In addition, the T-CS and g-C3N4 form an intimate contact heterojunction, which obviously promoted photodegradation of TYL. T-CSG/g-C3N4 and T-CSC/g-C3N4 are efficient for photodegradation of TYL, and nearly 98% of TYL is degraded after 45 min of irradiation. T-CSC/g-C3N4 has a weaker degradation effect than the other catalysts, which may be due to more severe aggregation of carbon spheres formed by sucrose. The unique structure of the carbon spheres can accelerate absorption of visible light and restrain electron-hole pairs recombination, thereby achieving enhancement of photodegradation efficiency on TYL. Moreover, this synthetic strategy may provide a development idea for antibiotics remediation.

Conflict of Interests The authors declare no competing financial interest.

Acknowledgements



The work was supported by the China National Science Fund Program (No. 41503095) and China Postdoctoral Science Foundation (Grant 2018M631203).

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Preparation process of T-CS/g-C3N4 composites by different precursor (glucose, β-cyclodextrin and sucrose)


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Highly Efficient Degradation toward Tylosin in the Aqueous Solution

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