g-C3N4 nanosheet efficient for photocatalytic degradation of ciprofloxacin

Journal Pre-proof Facile synthesis of Z-scheme composite of TiO2 nanorod/g-C3N4 nanosheet efficient for photocatalytic degradation of ciprofloxacin Kang Hu, Ruiqi Li, Chenlu Ye, Anqi Wang, Weiqi Wei, Di Hu, Rongliang Qiu, Kai Yan PII:

S0959-6526(20)30102-5

DOI:

https://doi.org/10.1016/j.jclepro.2020.120055

Reference:

JCLP 120055

To appear in:

Journal of Cleaner Production

Received Date: 24 July 2019 Revised Date:

15 November 2019

Accepted Date: 7 January 2020

Please cite this article as: Hu K, Li R, Ye C, Wang A, Wei W, Hu D, Qiu R, Yan K, Facile synthesis of Z-scheme composite of TiO2 nanorod/g-C3N4 nanosheet efficient for photocatalytic degradation of ciprofloxacin, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Kang Hu: Methodology, Investigation, Writing- Original draft preparation; Ruiqi Li: Validation and partially perform some tests; Chenlu Ye: Formal analysis and stability test; Anqi Wang: Visualization and discussion; Weiqi Wei: Software and analysis; Di Hu: Resources and Facility; Rongliang Qiu: Writing - Review & Editing. Kai

Yan: Conceptualization,

acquisition;

Supervision,

Project

administration,

Funding

Wordcount: 6271 words Facile synthesis of Z-scheme composite of TiO2 nanorod/g-C3N4 nanosheet efficient for photocatalytic degradation of ciprofloxacin Kang Hua, Ruiqi Lia, Chenlu Yea, Anqi Wanga, Weiqi Weib, Di Hua, Rongliang Qiu a, Kai Yana,* a

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510006, P. R. China b

Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources,

Nanjing Forestry University, Nanjing 210037, P. R. China *Corresponding author E-mail: [email protected]

Abstract: Developing efficient and robust photocatalyst is crucial for antibiotic degradation in water treatment. In this study, the Z-scheme nanocomposite of 1D/2D TiO2 nanorod/g-C3N4 nanosheet was successfully fabricated for the efficient photocatalytic degradation of ciprofloxacin (CIP). 93.4% degradation of CIP was achieved in 60 min under the conditions of the nanocomposite containing 30 wt% g-C3N4, 15 µmol L-1 CIP and pH of 6.3. The effect of catalysts, CIP concentrations and pH on degradation were systemically studied. It was found that the photodegradation process fitted the pseudo-first-order kinetic model well when CIP concentration was higher than 10 µmol L-1. Under simulated sunlight irradiation, the nanocomposite exhibited 2.3 and 7.5-times CIP photodegradation rate than those of commercial TiO2 powder and g-C3N4 nanosheet, respectively. Besides, degradation kinetics and mechanisms were further investigated. Scavenging experiments and 1

electron spin resonance (ESR) technique confirmed that h+ and ·OH played a major role in the CIP degradation process. Three CIP degradation pathways were subsequently proposed. This work may provide an effective strategy to remove various antibiotics in water treatment. Keywords: photocatalytic degradation; ciprofloxacin; TiO2 nanorod/g-C3N4 nanosheet; kinetics; mechanisms

1. Introduction Fluoroquinolones are the third largest group of antibiotics which make up almost 17% of global pharmaceutical market (Bojer at al., 2017). Ciprofloxacin (CIP) is a second-generation quinolone antibiotic, which has been applied to human therapy and is one of the most widely used antibiotics all over the world owing to broad-spectrum antimicrobial activity resist various diseases. However, CIP could not be metabolically decomposed completely. More than 75% CIP is excreted from the living body in unmetabolized form then enters municipal wastewater eventually (Githinji et al., 2011). Besides, CIP is refractory to biodegradation in traditional wastewater treatment plants (WWTPs) and are continuously released into the environment (Wang et al., 2019; Wang et al., 2020). The residuals (249-405 ng L-1) have been detected in the waters worldwide (Rashid et al., 2019). CIP may lead to the bacterial drug resistance and be bio-toxic to certain probiotics even at very low concentration which will bring a serious threat to human and ecological environment. Thus, it is a challenge to develop effective technologies to remove CIP from water (Xu et al., 2

2019). Photocatalysis is one of the promising and effective technologies to eliminate the antibiotic residues in water (Zheng et al., 2018; Ye et al., 2019). Among the available photocatalyst materials, titanium dioxide (TiO2) has attracted widespread attentions owing to the non-toxicity, low-cost, high stability and great photocatalytic performance (Tang et al., 2019). However, TiO2 exhibits less response to visible light and easy recombination of photogenerated electron-hole pairs due to its wide band gap (ca. 3.2 eV) (Diao et al., 2017). It is prevalent to couple TiO2 with the other visible light-driven catalysts to form heterostructures (Ng et al., 2019; Djellabi et al., 2019). The reasonable synthesis of composite catalysts with suitable band is generally considered as a promising methodology to hinder recombination of electron-hole pairs (Deng et al., 2018a). Recently, g-C3N4 has attracted extensive attention due to its narrow band gap, high chemical and thermal stability and good performance of organic pollutant degradation (Hong et al., 2016). The TiO2/g-C3N4 composite has been verified to show the broad visible-light response range, and superior photogenerated electrons and holes separation. It has been shown a considerable applicative prospect for organic pollutant photocatalytic degradation. During the last a few years, several methods have been utilized to synthesize g-C3N4/TiO2 composites for the photodegradation of Rhodamine B (Chen et al., 2016), propylene (Li et al., 2017), norfloxacin (Liu et al., 2019a), phenol (Sheng et al., 2019), etc. Recently, Li et al. (2018) developed a 3D g-C3N4/TiO2/kaolinite composite showed a 92% degradation of CIP in 240 min. They demonstrated that the novel “sandwich” 3

structure could enlarge surface area, enhance visible-light absorption ability and improve separation and transfer efficiency of photogenerated charges. Guo et al. (2018) synthesized a novel TiO2@g-C3N4 hollow core@shell composite for CIP with removal efficiency of 74% in 120 min. The results illustrated that the TiO2@g-C3N4 photocatalyst provided the short diffusion distance, sufficient channels and the much freer path for photogenerated charge carriers. Albeit, the previous works have made great progress, there is still large upgrade space of TiO2/g-C3N4 to improve the performance. It is a challenge to design suitable Z-scheme composite to enhance CIP removal. Fabrication of different morphologies is another strategy to optimize the catalytic behavior of materials (Zhao et al., 2019; Hu et al., 2019). It is well known that the geometric shape has great effects on photocatalytic properties of TiO2 (Wu et al., 2012). Our previous work has also shown that TiO2 microsphere composes of TiO2 nanorods exhibited much enhanced photocatalytic performance due to the unique 1 D structure (Yan et al., 2014; Yan and Wu, 2015). The TiO2 nanorod has high aspect ratio, advantageous electron survivability and well-defined unidirectional transport channel for electrical carriers (Wang et al., 2014). Therefore, it can impede recombination of photogenerated electron-hole pairs, and enhance separation and transportation efficiencies of photogenerated charges (Feng et al., 2018). To the best of our knowledge, there are few reports studying systematically the Z-scheme composite of TiO2 nanorod and g-C3N4 nanosheet for CIP photodegradation. In this study, we reported an efficient Z-scheme composite of 1D/2D TiO2 4

nanorod/g-C3N4 nanosheet to degrade CIP under simulated sunlight irradiation. A variety of synthetic parameters regarding the physicochemical property, morphology and structure were analyzed by the combination of XRD, FTIR, XPS, UV-vis DRS, SEM, TEM and HR-TEM. Besides, the effect of catalysts, CIP concentrations and pH on degradation were systemically studied to investigate reaction kinetics and mechanisms. Scavenging experiments and electron spin resonance (ESR) technique were then conducted to identify primarily reactive species. Based on the identified intermediates by LC-MS, the degradation pathways were proposed.

2. Experimental section 2.1 Materials synthesis and characterization All chemicals and reagents are commercially available and utilized directly without any purification, the details are provided in Supplementary Information (SI). For synthesis of TiO2 nanorod, 2 g TiO2 powder was dissolved to 80 mL 10 mol L-1 NaOH solution, stirring at 22±1oC for 2 h to obtain a homogeneous solution. Then, added in the Teflon-lined stainless autoclave kept at 180℃ for 72 h. The obtained white sample was washed using ultrapure water and 0.1 mol L-1 HCl for several times, and finally dried at 80℃ for 10 h. To acquire TiO2 nanorod, the obtained sample was calcined at 450oC for 4 h with the rising rate at 8 oC min-1. For synthesis of g-C3N4 nanosheets, 3 g melamine was added into an alumina crucible with a cover. Then, it was kept at 650oC for 2 h with the rising rate at 2 oC min-1. Then, the obtained yellow bulk sample was grinded to acquire g-C3N4 nanosheets for further use. For synthesis 5

of the composites, g-C3N4 nanosheet 100 mg was dissolved into 100 mL ethanol. The mixed solution treated by ultrasonic for 2 h, then some amount of TiO2 nanorod was added with stirring at 800 rpm for 2 h. Finally dried at 80℃ for 10 h and grinded the as-obtained powder to get TiO2 nanorod/g-C3N4 nanosheet. By adjusting the dosage of TiO2 nanorod, a series of heterojunction composites were obtained. The samples were marked as TiO2 nanorod-CN X (X=10 wt%, 20 wt%, 30 wt%, 50 wt%, 70 wt%) based on the mass ratio of g-C3N4. The as-prepared samples were analyzed by the combination of XRD, FTIR, XPS, UV-vis DRS, SEM, TEM and HR-TEM. The details are shown in SI. 2.2 CIP photocatalytic degradation experiments The performance of TiO2 nanorod-CN for CIP degradation was investigated under simulated sunlight irradiation. The light source was a 500 W Xenon lamp (CEL-LAB500 E4, Beijing China Education Au-light Co., Ltd.) without any filter. 10 mg of the TiO2 nanorod-CN was added into 50 mL CIP solution and was treated by ultrasound for 2 min. Afterwards it was stirred for 60 min to reach adsorption equilibrium in dark. In tests, 1.5 mL samples were withdrawn every 10/20 min for analysis after filtered through 0.45 µm filters. The CIP concentration was measured using a Shimadzu LC-20A HPLC system with a SIL-20A liquid autosampler. 0.2% formic acid and methanol were used as mobile phase at the ratio of 70:30 with 1 mL min-1 flow rate. 10 µL sample was injected every analysis and the column temperature was 40oC. The detection wavelength was at 272 nm.

6

3

Results and discussion

3.1 Materials characterization The crystal structure of the as-prepared materials was firstly measured by XRD (Fig. S1a). Two clear diffraction peaks are found, 13.2° for the periodic tri-s-triazine units and 27.4° for the aromatic systems in g-C3N4 nanosheets (Deng et al.,2018a). The diffraction peaks of commercial TiO2 powder and TiO2 nanorod can be clearly identified with peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.0° and 62.7°, which belongs to the anatase TiO2 (JCPDS No. 99-0008) (Tang et al., 2019). Although the characteristic peaks of g-C3N4 nanosheet are not apparent when the mass ratios less than 20 wt%. The XRD pattern of the prepared TiO2 nanorod-CN containing more than 20 wt% g-C3N4 nanosheet exhibit the characteristic peaks of TiO2 nanorod and g-C3N4 nanosheet. To study the surface chemical compositions and groups, FTIR was used as shown in Fig. S1b, the strong broad band at ∼3435 cm−1 represents the adsorbed water and hydroxyl groups on surface (Zhang et al., 2018). It verifies the existence of hydroxyl groups on the composite surface. In the case of g-C3N4 nanosheets, several adsorption peaks in 1200-1640 cm-1 correspond to the aromatic CN heterocycles (Sheng et al., 2019; Zou et al., 2018) and the peak at ∼807 cm-1 represents the tri-s-triazine units (Deng et al., 2018b). For pure TiO2 nanorod, the strong absorption peaks in 500-800 cm−1 are attributed to the Ti–O and Ti–O–Ti bands (Martins et al., 2018). The prepared TiO2 nanorod-CN 30 wt% owns the typical characteristic peaks of both g-C3N4 and TiO2 nanorod. The further analysis was conducted to compare with the feature curve of the g-C3N4, the vibration peak of s-triazine units shows a 7

slight shift from 807 to 809 cm−1 in TiO2 nanorod-CN 30 wt%, which indicates the chemical combination existed on the interface between g-C3N4 and TiO2 nanorod. The general morphology and microstructure of the fabricated TiO2 nanorod, g-C3N4 nanosheet and TiO2 nanorod-CN were further analyzed by SEM and TEM. SEM/TEM images of pure g-C3N4 nanosheet show layered structures consisted of parallel nanosheets with wrinkle (Fig. S2a and S2b). For the pure TiO2 nanorod, nanorod morphology with the width of 30-60 nm and the length of 90-190 nm were measured (Fig. S3). Several big nanorods with 200 nm in width and 1600 nm in length were also found due to the combination of small ones (Fig. S4a and S4b). Fig. 1a and 1b show that dense TiO2 nanorods are loaded on g-C3N4 nanosheet to create 1D/2D heterojunction. HR-TEM of the composite (Fig. 1c) clearly depicts the nanorod structure of TiO2 with a 0.352 nm lattice spacing corresponding to the (101) plane and (002) plane of g-C3N4 with a 0.33 nm lattice spacing. SAED image (Inset in Fig. 1c) also indicates the combination for anatase phase of TiO2 nanorod on g-C3N4 nanosheet, which is well in accordance with the XRD results. The high-angle annular dark-field scanning TEM image (Fig. S5) and element mapping images (Fig. 1d-g) confirm that C, N, Ti and O atoms are relatively homogeneous distributions. XPS was further used to study the chemical state of the as-prepared samples. It shows that the composite is primarily composed of C, N, Ti, and O (Fig. S6). C 1s at 284.8 eV in the high-resolution spectra is attributed to the C–C bonds and 288.3 eV represents the sp3 N–C=N bonds in g-C3N4 (Fig. 2a) (Zhu et al., 2018). It can be shown that C–C in TiO2 nanorod-CN 30 wt% has obviously larger percentage than in 8

g-C3N4 nanosheet. Three individual peaks at 398.6 eV, 399.5 eV and 400.2 eV were observed in N 1s spectra which are attributed to C=N–C, N–(C)3 and C–N–H, respectively (Fig. 2b) (Shao et al., 2018). TiO2 nanorod-CN has more C–N–H bonds. In Fig. 2c, two peaks at 458.3 eV and 464.0 eV corresponds to Ti 2p3/2 and Ti 2p1/2 that indicate the presence of Ti4+ in the composite (Xu et al., 2018). Fig. 2d shows that O 1s at 529.6 eV and 531.2 eV assign to lattice oxygen bound (Ti–O) and surface hydroxyl groups (O–H), respectively (Wu et al., 2018). UV-vis DRS was further used to analyze the optical properties of as-prepared samples. TiO2 and g-C3N4 nanosheet revealed a basal absorption band at ∼390 nm and ∼475 nm, respectively (Fig. S7a) (Liu et al., 2019a). TiO2 nanorod-CN composites possess the absorption features of both TiO2 nanorod and g-C3N4. Comparing to TiO2, the composite makes a red shift towards a longer wavelength in 450-500 nm which indicates that the composites can response to visible light. As expected, the composites with different amounts of g-C3N4 show remarkable additional absorption in the region of ≤450 nm. Herein, Kubelka-Munk function and the equation (αhv) = A (hv − Eg)n were used to calculate the band gap energies (Zhao et al., 2018). The n=2 was used to calculate the direct-gap. According to the calculation, the band gaps of g-C3N4, commercial TiO2 powder and TiO2 nanorod-CN 30 wt% are ∼2.64, ∼3.27 and ∼2.95 eV, respectively (Fig. S7b). Comparing to commercial TiO2 powder, TiO2 nanorod shows a weak red shift and band gap is ∼3.26. Moreover, the photocurrent response of composites has been tested to analyze the effect of g-C3N4 amount on the separation efficiency of photoinduced carriers (Lu et 9

al., 2019). As shown in Fig. S8, stable photocurrents are generated with the lamp turned on and extremely decreased with the lamp turned off. The TiO2 nanorod -CN 30 wt% exhibits stronger photocurrent intensity than the other composites over 6 cycles. 3.2 Effect of catalysts As shown in Fig. 3a, CIP was self-decomposed around 20% in the absence of photocatalyst in 60 min under simulated sunlight irradiation. The g-C3N4 absorbed less 10% CIP under dark condition and removed 30% CIP in 60 min under simulated sunlight irradiation. Commercial TiO2 powder performed better that 15% and 70% CIP were absorbed and decomposed respectively. An obvious improvement in the removal of CIP was achieved by TiO2 nanorod-CN 30 wt%, 91% of CIP can be decomposed over the same timeline. Moreover, Fig. 3b illustrates that as the content ratio of g-C3N4 increased from 10 to 20 wt% in the composite, the pseudo-first-order rate constants (kobs) decreased from 0.0286 to 0.0223 min−1. The max kobs 0.0366 was arrived by TiO2 nanorod-CN 30 wt%. It was ∼2.3 and ∼7.5 times faster than the commercial TiO2 powder and g-C3N4 nanosheet. 3.3 Effect of initial CIP concentrations As shown in Fig. 4a, degradation efficiencies of different initial CIP concentrations were obtained in the range of 5 to 25 µmol L-1. An increase of the degradation efficiency under simulated sunlight irradiation is observed when the concentration increased to 15 µmol L-1. It is probably due to more CIP molecules in the solution are adsorbed by TiO2 nanorod-CN 30 wt% composite on active catalytic 10

sites. However, when the concentration is over 15 µmol L-1, the degradation efficiency starts to decrease. One possible reason is that active catalytic sites provided by 10 mg TiO2 nanorod-CN 30 wt% composite are not enough for a higher concentration. CIP molecules covered all the active catalytic sites and prevented dissolved O2 from being adsorbed on the photocatalyst (Saikia et al., 2015). As the electron acceptor, less the O2 adsorbed, less the degradation efficiency and rate constant achieved. Moreover, high concentration of CIP can adsorb partial light energy and reduce photons to activate the photocatalytic composite. Fig. 4b presents that with CIP concentration increases from 5 to 25 µmol L-1, kobs of CIP degradation increases from 0.0219 to 0.0381 min−1 and then decrease to 0.259 min−1. It is noteworthy that only when the concentration is higher than 10 µmol L-1, it follows the pseudo-first-order kinetic model well. 3.4 Effect of pH In this study, 0.1 M HCl or 0.1 M KOH was used to adjust pH to analyze the effect of different pH value on the CIP degradation efficiency. It was found that the lowest degradation efficiency of 64.8 % was obtained under the pH of 11. While the degradation efficiency increased with the pH decreased (Fig. 5). The maximum degradation efficiency (93.4%) reached at pH 6.3. The possible reason resulting in this difference is that CIP adsorption performance is depended on the catalyst surface charge. The isoelectric point of TiO2 nanorod-CN 30 wt% was measured to be ca. 6.3 (Fig. S9). Fig. S10 describes existence forms of CIP in different pH which depends on the pKa at 6.16 and 8.23 (Wang et al., 2018). When pH<6.3, positive charge 11

electrostatic repulsion occurred between CIP and TiO2 nanorod-CN 30 wt% impedes the photocatalytic degradation. It is considered that there is electrostatic attraction between CIP and negatively charged TiO2 nanorod-CN 30 wt% at pH>6.3 leading to enhance photocatalysis. But, the electrostatic repulsion between CIP and TiO2 nanorod-CN 30 wt% was aggravated with further increase pH resulting in CIP deprotonation, which cause CIP degradation efficiency to be lower. As shown in Table S1, the TiO2 and g-C3N4-based photocatalyst reported in updated literature are summarized. It shows that the TiO2 nanorod-CN composite is an effective photocatalytic catalyst with visible light response and great degradation efficiency to degrade CIP. 3.5 Roles of reactive species Scavenging experiment was conducted to identify different reactive species in the CIP degradation. Isopropanol, EDTA-2Na, furfuryl alcohol, TEMPOL and dimethyl sulfoxide were chosen as ·OH (Lai et al., 2019), h+ (Zhang et al., 2019), 1O2 (Cheng et al., 2019), ·O2- (Xia et al., 2019) and e- (Shi et al., 2014) scavenger. As shown in Fig. 6, the CIP degradation efficiency decreases to 24.3%, 49.3%, 71.2% and 79.7 % in the presence of EDTA-2Na, isopropanol, dimethyl sulfoxide and TEMPOL, respectively. In contrast, the degradation is almost no difference with the addition of furfuryl alcohol. Therefore, it is indicated that h+ and ·OH played a major role for CIP degradation. The e- and ·O2- played a moderate role in the photocatalytic process. It has been considered that e- can capture dissolved O2 to generate ·O2- to take part in the photocatalytic degradation process. Moreover, H2O2 generally exists in 12

reactions of superoxide radicals which is also another significate intermediate species in the photocatalytic process. The DPD method was applied to detect H2O2 which generated during the photodegradation of CIP (Bader et al., 1988; Schick et al., 1997). As shown in Fig. S11, g-C3N4 nanosheet, commercial TiO2 powder and TiO2 nanorod-CN 30 wt% can produce H2O2. Compared to g-C3N4 nanosheet and commercial TiO2 powder, TiO2 nanorod-CN 30 wt% has obvious stronger absorption peaks when DPD is added. This result verifies the presence of ·O2- radicals again and existence of another inevitable source for ·OH species. To further validate the presence of these radicals in the system, the electron spin resonance (ESR) technique was introduced. The tests were conducted in dark and simulated sunlight irradiation using Xenon light in 1 and 10 min. As shown in Fig. 7, there is no characteristic peak under the dark condition. In contrast, after irradiated with the Xenon light, the strong peaks for ·OH and ·O2- arose. Furthermore, the peak intensity enhanced with the lighting time. Therefore, it is confirmed the existence of ·OH and ·O2- in CIP photodegradation process. 3.6 Photocatalytic degradation mechanism The potentials of valence and conduction bands in TiO2 nanorod and g-C3N4 were calculated in SI, and the results showed in Table S2. Generally, h+ and e- are generated at the ECB and EVB, respectively. For the conventional TiO2/g-C3N4 heterojunction, h+ formed in TiO2 transfers to g-C3N4, and e- generated in g-C3N4 goes to TiO2 (Fig. S12). It has been reported that the standard redox potentials of ·OH/OHand ·OH/H2O are +1.99 eV and +2.38 eV, respectively, which are higher than valence 13

band potential of g-C3N4 (+1.54 eV) (Liu et al., 2019b). Therefore, the h+ in g-C3N4 is not able to oxidize OH- and H2O to form ·OH in conventional heterojunction. This study showed that h+ and ·OH were primarily responsible for the degradation of CIP. The ·OH was most likely formed via OH− and H2O oxidizing by h+ in EVB of TiO2 nanorod (+2.94 eV). Therefore, according to the above analysis, TiO2 nanorod-CN 30 wt% composite is direct contact Z-scheme structure for CIP degradation. In view of the above, a reasonable photocatalytic degradation mechanism of CIP on TiO2 nanorod-CN 30 wt% composite is illustrated in Fig. 8. The reactions took place in the system are described as follows: TiO2 nanorod/g-C3N4 + hv → TiO2 nanorod (h+, e−)/g-C3N4 (h+, e−)

(1)

TiO2 nanorod (h+, e−)/g-C3N4 (h+, e−) →TiO2 nanorod (h+) + g-C3N4 (e−)

(2)

g-C3N4 (e−) + O2 → ·O2−

(3)

·O2− + 2H+ → H2O2

(4)

H2O2 + g-C3N4 (e−) → ·OH + OH-

(5)

H2O + TiO2 nanorod (h+) → ·OH+ H+

(6)

OH-+ TiO2 nanorod (h+) → ·OH

(7)

h+/·OH/·O2− + CIP → Intermediates → CO2 + H2O + F-+ NO3-

(8)

The photocatalytic degradation of CIP is a complex process in which many intermediates generated. In this study, the main intermediates were identified based on an MS technique (SI) and theoretical analysis to understand the possible degradation pathways. These intermediates are summarized in Table S3. It can be found that at least six intermediates were generated during the CIP photodegradation. Fig. 9 shows 14

the proposed photocatalytic degradation pathways of CIP on TiO2 nanorod-CN 30 wt% under simulated sunlight irradiation. Three pathways were included, cleavage of the piperazine ring, decarboxylation of quinolone and hydroxylation. Specifically, intermediates A (m/z 362), C1 and C2 (m/z 334), E (m/z 291) and F (m/z 263) were derived from cleavage of the piperazine ring. The intermediate D (m/z 304) was arisen from the decarboxylation of quinolone. The intermediate B (m/z 348) was generated from the hydroxylation of CIP. For convenience, the sites of CIP atoms were labeled in Fig. S13. In pathway I, h+ prefers to attack N 11 and N 14 sites which leads to cleavage of the piperazine ring to generate E (Wang et al., 2018). Comparing with intermediate E, intermediate F decreased 28 on molecular weight. As reported by Deng et al. (2017), decarboxylation is quite common in MS. Therefore, intermediate F is proposed to be directly generated by decarboxylation of E. In pathway II, the molecular weight of intermediate B is 16 more than CIP which indicates maybe an oxygen atom is added or a hydrogen atom is substituted by a hydroxyl group. Here, intermediate B is proposed to derive from hydroxyl group substitution since hydroxylation is very common for CIP photodegradation (Deng et al., 2017). The further analysis shows that N 11 is a tertiary amine which has a strong electron absorption ability. Hence, the α-carbon (C 12) in piperazine ring is unstable and is preferred to be attacked by ·OH radical for hydroxyl substitution reaction in photocatalytic system (Zheng et al., 2018). In pathway III, ·O2- radical prefers to attack the piperazine side chain to form oxidative intermediates. Intermediate A is first generated during the ring opening oxidative process. Afterwards, intermediates C1 15

and C2 are derived from the loss of -CO group during the piperazinyl rings broken. Finally, intermediate C1 is transformed into E when “CH2CH2NH” is lost and further produce F (Chen et al., 2018).

4. Conclusions A Z-scheme composite of 1D/2D TiO2 nanorod-CN was successfully prepared via a facile process in this study. It was confirmed that dense TiO2 nanorods were loaded on g-C3N4 nanosheet to create 1D/2D heterojunction. The as-prepared TiO2 nanorod- CN is an effective photocatalytic catalyst with visible light response and great degradation efficiency to degrade CIP. The TiO2 nanorod-CN composite containing 30 wt% g-C3N4 had the best performance for 15 µmol L-1 CIP under pH of 6.3 with a degradation of 93.4% in 60 min. Furthermore, the photodegradation process fitted the pseudo-first-order kinetic model well when CIP concentration was higher than 10 µmol L-1. Moreover, scavenging experiments and ESR technique verified that h+ and ·OH played a major role in the CIP degradation process. According to main intermediates identified based on LC-MS, three rational photocatalytic degradation pathways were proposed. This work indicates that the Z-scheme composite of TiO2 nanorod-CN may be expected to be a promising candidate for various antibiotics degradation.

Acknowledgements This work was supported by National Ten Thousand Talent Plan, National Key R&D 16

Program of China (2018YFD0800700), National Natural Science Foundation of China (21776324), Key Research & Development Program of Guangdong Province (2019B110209003), the Fundamental Research Funds for the Central Universities (19lgzd25), and Hundred Talent Plan (201602) from Sun Yat-sen University.

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Figures Fig. 1. (a) SEM, (b) TEM, (c) HR-TEM (Inset was SAED), and (d-g) Element mapping images of TiO2 nanorod-CN. Fig. 2. High-resolution spectra of (a) C 1s, (b) N 1s, (c) Ti 2p, and (d) O 1s. Fig. 3. (a) CIP removal with different catalysts under simulated sunlight irradiation, 0.2 g L-1 catalyst, 20 µmol L-1 CIP; (b) Photocatalytic degradation kinetics. Fig. 4. (a) Comparison of CIP initial concentrations under simulated sunlight irradiation, 0.2 g L-1 TiO2 nanorod-CN 30 wt%; (b)Effect of CIP initial concentrations on the kinetics. Fig. 5. (a) Comparison of pH under simulated sunlight irradiation under the conditions of initial concentration 0.2 g L-1 catalyst, 15 µmol L-1 CIP; (b)Effect of pH 25

on photocatalytic degradation kinetics. Fig. 6. Effects of radical scavengers on the degradation of CIP (10 mg TiO2 nanorod-CN 30 wt% composite, 20 µmol L-1 CIP; 1 mM isopropanol, dimethyl sulfoxide, EDTA-2Na, furfuryl alcohol and TEMPOL). Fig. 7. ESR spectra of (a) ·OH and (b) ·O2- in dark and under simulated sunlight irradiation. Fig. 8. Photocatalytic mechanism scheme in TiO2 nanorod-CN Z-scheme heterojunction. Fig. 9. Possible degradation pathways of CIP on TiO2 nanorod-CN composites.

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Highlights: (1) Z-scheme nanocomposite of 1D/2D TiO2 nanorod/g-C3N4 nanosheet was fabricated. (2) The nanocomposite was efficient for photocatalytic degradation of ciprofloxacin. (3) h+ and ·OH played a major role in the CIP degradation process. (4) The photocatalytic degradation mechanisms for ciprofloxacin were discussed.

Conflict of Interest No conflict of interest exits in the submission and publication of this manuscript, and this manuscript is approved by all authors for submission. I declare on behalf of my coauthors that the work described is original research that has not been published elsewhere previously.