Ag3PO4@natural hematite heterojunction photocatalyst under simulated solar light

Journal of Hazardous Materials 371 (2019) 566–575

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Degradation of antibiotics in multi-component systems with novel ternary AgBr/Ag3PO4@natural hematite heterojunction photocatalyst under simulated solar light

T

Liwei Chena, Shengjiong Yangb, Yang Huangc, Baogang Zhangd, Fuxing Kanga, Dahu Dinga, , Tianming Caia ⁎

a

College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China College of Materials Science and Engineering, Shenzhen university, Shenzhen 518060, China d School of Water Resources and Environment, MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, China University of Geosciences (Beijing), Beijing 100083, China b c

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Photo-generated hole Superoxide radical Frontier electron density Teteracycline Sulfadiazine

Abatement of antibiotics from aquatic systems is of great importance but remains a challenge. Herein, we prepared ternary AgBr/Ag3PO4@natural hematite (AgBr/Ag3PO4@NH) heterojunction composite via a simple route for the photocatalytic degradation of antibiotic pollutants. By adjusting the dose of Ag species, four products with different Ag content (denoted as Ag0.5BrPFe, Ag1BrPFe, Ag1.5BrPFe, and Ag2BrPFe) were developed. Among them, Ag1.5BrPFe exhibited the best photocatalytic activity. Four antibiotics (i.e. ciprofloxacin (CIP), norfloxacin (NOR), sulfadiazine (SDZ), and tetracycline (TTC)) could be degraded with synthesized Ag1.5BrPFe in multi-component systems. Water matrix indexes including solution pH, coexisting anions, humic acids exhibited distinct effects on the degradation process. The results revealed that the degradation process was accelerated at acidic conditions while depressed at basic conditions. Superoxide radical and hole were detected by in situ electron spin resonance technique and played the dominant roles. The degradation pathway TTC was tentatively established followed with the identification of the degradation intermediates and computational analysis. This work would shed light on the photocatalytic degradation mechanism of organic pollutants by the AgBr/Ag3PO4@NH composite.

⁎ Corresponding author at: College of Resources and Environmental Sciences, Nanjing Agricultural University, Weigang No. 1, Xuanwu District, Nanjing 210095, China. E-mail address: [email protected] (D. Ding).

https://doi.org/10.1016/j.jhazmat.2019.03.038 Received 28 September 2018; Received in revised form 29 December 2018; Accepted 7 March 2019 Available online 08 March 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

2. Materials and methods

Antibiotics play a vital role in treating bacterial infections, promote growth and improve feed efficiency in animals [1]. However, its adverse effects on the natural ecosystem attract increasing attention in recent years. The selection pressure applied by the antibiotics has stimulated the evolution and spread of resistance genes (ARGs), regardless of their origins. Bacteria could then acquire the antibiotic resistance genes through horizontal gene transfer process and defeat antibiotics [2]. Consequently, the elimination of residual antibiotics from environment is of great importance. As a typical refractory organics, antibiotics can hardly been degraded by conventional biological processes. Alternatively, chemical oxidation processes (e.g. electro-Fenton and photo-Fenton processes [3], ozonation [4], chlorination [5], and so on) exhibit great potential for the decomposition of antibiotics. Among them, photocatalytic degradation of antibiotics attracts increasing attention due to its high efficiency and low energy consumption. Semiconductor oxides such as TiO2, ZnO, SnO2, WO3 have been frequently developed for the photocatalytic degradation of organic pollutants [6–8]. Specifically, hematite (α-Fe2O3) is the most stable iron oxide under ambient conditions and abundantly occurs in the earth [8]. More importantly, α-Fe2O3 is an n-type semiconductor and can absorb visible light since it has a narrow bandgap (ranging from 1.9 to 2.2 eV) [9]. This is particularly attractive because visible light accounts for around 46% of solar light energy [7]. However, the single-component semiconductor photocatalyst usually suffers from low light harvesting, high recombination rate of electron-hole pairs, and poor reusability [10]. Specifically, α-Fe2O3 is significantly limited by a high rate of photo-generated charge carriers due to its low carrier mobility (< 1 cm2 V−1 s−1), short hole-diffusion length (˜2-4 nm), short excited state lifetime (˜10 ps), and low absorption coefficient [11]. Our group found that only with assistance of H2O2 that the synthesized α-Fe2O3 could perform efficient photocatalytic property [12–14]. To this end, the combination of two semiconductors with narrow and wide bandgaps might be a promising option. Several α-Fe2O3 based heterostructures including TiO2/Fe2O3 and SnO2/Fe2O3 have been demonstrated to be more active than bare Fe2O3 [15–17]. Meanwhile, as another semiconductor photocatalyst, Ag3PO4 can harness the visible light (wavelength < 530 nm, bandgap = 2.45 eV [18]) for the decomposition of organic contaminants as well as water splitting and is even more active than TiO2, WO3 and N-TiO2 [19]. However, it suffers from the severe self-photocorrosion due to the excessive electrons accumulated on the conductive band. In addition, Ag3PO4is slightly soluble in aqueous solution (Ksp = 1.4 × 10−16, 0.02 g L−1at 25 °C), making it more unstable during the photocatalysis process. To this end, TiO2/Ag3PO4 [20], Co3O4/Ag3PO4 [21], MoO3/ Ag3PO4 [22], and WO3/Ag3PO4 [23] are fabricated. By doping an insoluble AgX (X = Cl, Br, I) nanoshell around Ag3PO4particle, the abovementioned shortages can also be greatly addressed. More importantly, AgX and Ag3PO4 possess matching band potentials, which benefits the fast transfer and separation of the photo-induced carriers [24]. Consequently, the aim of this work is to prepare a novel ternary AgBr/Ag3PO4@natural hematite (AgBr/Ag3PO4@NH) heterojunction composite. Based on the previous findings, we suppose that by loading AgBr/Ag3PO4 on hematite will simultaneously improve the photocatalytic activity and its stability. Four typical antibiotics (ciprofloxacin (CIP), norfloxacin (NOR), sulfadiazine (SDZ), and tetracycline (TTC)) are degraded by the prepared AgBr/Ag3PO4@NH in multi-component systems. The Frontier Electron Densities (FEDs) of the selected antibiotics are calculated by Gaussian 09 program to propose the photocatalytic degradation mechanisms.

2.1. Chemicals and reagents Natural hematite powder (main phase: quartz and α-Fe2O3) passed through 400 mesh was obtained from an iron ore plant located in Hebei province, China. The descriptions of the chemicals were given in Supporting information Text S1. 2.2. Preparation and characterizations The heterojunction catalysts were synthesized by in situ deposition of AgBr/Ag3PO4 onto natural hematite powder. Typically, 8 g AgNO3 was completely dissolved in 20 mL DW to make a 2.4 M AgNO3 solution. 5 g hematite powder was then added into the above solution and thoroughly mixed for 1 h. After that, 10 mL Na3PO4 solution (1.0 M) was dropwisely added into the mixture. The resulted mixture was agitated for 3 h. Finally, 10 mL KBr solution (1.5 M) was introduced dropwisely and agitated for another 3 h to make product AgBr/ Ag3PO4@NH. The final product was centrifugally separated and washed with DW for 5 times and completely dried in an oven at 60 °C. In this catalyst, the molar ratio of [Ag]:[Hematite] was 1.5:1 and thereby designated as Ag1.5BrPFe. Similarly, Ag0.5BrPFe, Ag1BrPFe, and Ag2BrPFe were also fabricated by adjusting the dosage of AgNO3 during the preparation process. Details of the characterizations were provided in the Supporting Information Text S2. 2.3. Photocatalytic degradation experiments The stock solutions of CIP (100 mg/L), NOR (20 mg/L), SDZ (100 mg/L), and TTC (100 mg/L) were separately prepared. To conduct the photocatalytic degradation experiments, a mixture of antibiotic stock solutions was freshly diluted with DW to make a solution containing 1 mg L−1 CIP, NOR, SDZ, and TTC. The inherent pH value of the mixture was 5.6 and adjusted with 0.1 M HCl or NaOH to a desired value. The photocatalytic degradation experiments were conducted in a 100 mL customized vessel surrounded with a circulating water jacket. To eliminate the effect of reaction temperature on the photocatalytic degradation process, tap water was circulated throughout the experiment. A 300 W Xe lamp equipped with an AM 1.5 G filter was used to supply the simulated solar light and placed above the vessel (10 cm away with the liquid surface). Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to establish an adsorptiondesorption equilibrium. The duration was determined based on the preliminary experiments (Fig. S1). Then, the solution was exposed to simulated solar light with continuous stirring. At regular time intervals, an aliquot of 1.5 mL was collected and filtered with 0.22 μm membrane. Noting that, the direct photolysis of the antibiotics within the tested period (30 min) was negligible. For the radical scavenging study, isopropanol (IPA), ethylenediamine teraacetic acid disodium salt (EDTA-2Na), and p-benzoquinone (BQ) were used as the chemical scavengers of hydroxyl radical, h+, and superoxide radical, respectively [25]. 2.4. In situ determination of radicals Electron spin resonance (ESR) signals of radical spin-trapped by DMPO were detected in situ to confirm the active radicals. For the determination of %OH and O2%−, 2 mg solid catalyst was dispersed into 10 mL DW to get a homogeneous mixture. Then, 0.2 mL mixture was

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mixed with 0.2 mL DMPO solution (50 mM). ESR signals of DMPO-OH and DMPO-O2 adducts were recorded on Bruker model ESR JES-FA200 spectrometer at ambient temperature. For the detection of photo-generated hole, the solid samples were dispersed with DW and directly measured on Bruker model ESR JES-FA200 spectrometer at 77 K. The microwave frequency was 9.853 GHz. The measurement at a center field was of 3510 G. The time constant was 1.250 ms and the sweep time was 19.456 s.

Table 1 Properties of prepared photocatalysts.

2.5. Theoretical calculations

Catalyst

Ag/Fe ratioa

SSABETb (m2/g)

Pore volume (cm3/g)

Direct band gap (eV)

Indirect band gap (eV)

Hematite Ag0.5BrPFe Ag1BrPFe Ag1.5BrPFe Ag2BrPFe

0.0 1.2 1.6 2.0 3.0

3.74 7.62 7.79 9.17 7.40

0.013 0.048 0.051 0.046 0.055

1.81 1.74 1.76 1.64 1.77

1.99 1.95 1.94 1.89 1.94

a

obtained through the XPS analysis. specific surface area calculated from multi-point Brunauer-Emmett-Teller (BET) method. b

The molecular orbital calculations and frontier electron densities (FEDs) were derived by a hybrid density functional B3LYP method with the 6-31 + G* basis set (B3LYP/ 6-31 +G*) in the Gaussian 09 program. The FEDs of the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and point charge were calculated to predict the reaction sites for ·OH, h+, and O2%− attacks, respectively [26–28].

impurity peaks are observed, revealing the high phase purity of the prepared AgBr/Ag3PO4@NH hybrid composites. All nitrogen adsorption/desorption curves (Fig. S3) follow the H3 hysteresis nature. The SSABET of bare hematite is 3.74 m2 g−1, which is quite low when comparing with other hematite nanostructures [12,29]. This is attributed to the bulky particle size of natural hematite sample. By combining with AgBr/Ag3PO4 nanoparticles, the surface area is enlarged. As given in Table 1, the AgBr/Ag3PO4@NH hybrids possess larger SSABET than bare hematite particles. It is supposed that the enlarged interaction area will be beneficial with respect to improving photocatalytic performance. XPS results clearly confirm the introduction of silver in the modified hematite-based hybrids (Fig. 2). Br 3d and P 2p XPS spectra are absent in natural hematite sample, whilst occur in the AgBr/Ag3PO4@hematite samples, indicating the successful formation of AgBr and Ag3PO4, respectively. Moreover, the Ag content increases along with the ratio of [Ag]:[Hematite] (Table 1). The high-resolution Ag 3d and Fe 2p spectra are given in Supporting Information Fig. S4, revealing the occurrence of Ag+ and Fe2O3. As shown in Fig. 3A, natural hematite can well absorb UV light and visible light with wavelength < 600 nm. The optical absorption is probably attributed to both direct transition and indirect transition [30]. The former one, corresponding to the ligand-metal transfer from O2− 2p → Fe3+ 3d causes the absorption in UV region, whilst the latter one results the absorption in visible light region through the metalmetal transfer (2Fe3+ → Fe2+ + Fe4+) [31]. Through the Tauc analysis (Fig. 3 B and C), the direct and indirect transition band gaps of bare hematite are estimated to be 1.81 and 1.99 eV respectively, which are in well accordance with the previous studies [30,32]. Moreover, after loading of AgBr/Ag3PO4, a red shift in the absorption edge is observed.

2.6. Analytical methods Concentration of dye MB was measured with a UV–vis spectrophotometer at the detection wavelength of 664 nm. Concentrations of antibiotics were detected with a high performance liquid chromatograph (HPLC, Fig. S2). More details of the HPLC analysis and the identification of degradation products were provided in Supporting Information Text S3. 3. Results and discussion 3.1. Characterization results As given in Fig. 1, all the diffraction peaks can be indexed to rhombohedral iron oxide (α-Fe2O3, JCPDS PDF# 79-0007) and hexagonal quartz (SiO2, JCPDS PDF# 85-0796) in natural hematite. Specifically, the peaks at 2θ degrees of 20.9° and 26.6° can be indexed to (100) and (011) crystal plans of hexagonal quartz, whilst those at 33.2°, 35.7° and 54.1° can be indexed to (104), (110) and (116) crystal planes of rhombohedral iron oxide, respectively. After deposition of AgBr/ Ag3PO4, the characteristic diffraction peaks of Ag3PO4 (JCPDS PDF# 84-0510) and AgBr (JCPDS PDF# 79-0149) are clearly identified. The strong and sharp diffraction peaks show that the obtained products are well crystallized. In addition, all the peaks can be matched well and no

Fig. 1. X-ray diffraction (XRD) patterns of natural hematite and AgBr/Ag3PO4@NH hybrid samples. 568

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Fig. 2. XPS spectra of natural hematite and prepared AgBr/Ag3PO4@NH hybrids.

The indirect band gap is correspondingly reduced to 1.95, 1.94, 1.89, and 1.94 eV, respectively. Undoubtedly, the decreased band gap is beneficial for the photocatalytic activity under irradiation of solar light. Morphological observations show that the bare hematite particles are irregularly schistose-like (Fig. 4). The particle size ranges from several hundred nanometers to micro meters. The nanoscale AgBr/ Ag3PO4 particles are uniformly distributed on the surface of schistoselike hematite. The particle size of AgBr/Ag3PO4 is quite homogeneous and around 50–200 nm. The similar structure has also been reported in previous publications [33,34].

decolorization rate is greatly accelerated, revealing the excellent solar light response of AgBr/Ag3PO4 [24]. The content of silver species in the ternary hybrids plays a role in the photocatalytic activity. Specifically, Ag1.5BrPFe photocatalyst performs the highest photocatalytic decolorization rate among others. This result clearly suggests that an optimal content of silver species is occurred in the hybrid composites [35,36]. The decolorization performance of MB with prepared Ag1.5BrPFe in successive four cycles is investigated to evaluate its stability. As illustrated in Fig. S5, the decolorization rate of Ag1.5BrPFe is still as high as 84% after four cycles. On the other hand, a significant decrease of the decolorization efficiency occurs for the AgBr/Ag3PO4 composite, indicating that AgBr/Ag3PO4 is not stable for successive use. Notably, the Ag1.5BrPFe prepared in this study is much more stable than the reported AgBr/Ag3PO4@TiO2 [34]. To further confirm the superiority of the prepared AgBr/Ag3PO4@ NH photocatalysts, the previously synthesized visible-light responsive photocatalysts are taken into comparison and the results are summarized in Table S2. It is clear that the AgBr/Ag3PO4@NH photocatalysts exhibit better performance for the degradation of dyes and antibiotics.

3.2. Photocatalytic activity evaluation The photocatalytic activity of AgBr/Ag3PO4@NH composites with different Ag content is evaluated by decolorization of MB. As illustrated in Fig. 5, bulky natural hematite powder possesses a poor photocatalytic activity under simulated solar light. No more than 40% of MB is decolorized by hematite within 40 min, much worse than commercial and synthesized nanoscale α-Fe2O3 products [13,30]. This is probably due to (i) the bulky size of hematite particles results into an extremely small surface area; and (ii) the occurrence of SiO2 (as revealed in the XRD pattern Fig. 1) reduces the content of solar light responsive αFe2O3. After the deposition of AgBr/Ag3PO4 nanoparticles, the

3.3. Photocatalytic degradation of antibiotics As depicted in Fig. S6, degradation efficiency increases with the

Fig. 3. UV–vis DRS patterns of natural hematite and AgBr/Ag3PO4@NH photocatalysts (A) and the corresponding direct (B) and indirect (C) Tauc plot analysis of optical band gap. 569

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Fig. 4. Scanning electron microscopic (SEM) and transition electron microscopic (TEM) images of natural hematite (A and C) and Ag1.5BrPFe (B and D).

dosage of catalyst. However, there is no remarkable change when the dosage is higher than 0.17 g L−1. This probably means an optimum dosage is preferable for the photocatalytic degradation of antibiotics. More importantly, the degradation kinetics of the four antibiotics roughly follows: SDZ > CIP ≈ NOR > TTC. The distinct degradation rate is believed to be related with the molecular structure of the different antibiotics. For example, the electron densities of these antibiotics are different, which might lead to some of them much susceptible to the radicals’ attack [37]. Therefore, different compounds possess distinct reaction rate constants with radical species [38,39]. It is reported that the second-order rate constants of CIP, NOR, SDZ, and TTC with ·OH are (4.1 ± 0.3) × 109 [40], (6.18 ± 0.18) × 109 [41], (3.7 ± 0.5) × 109 [42], and (6.3 ± 0.1) × 109 [43] M−1 s-1, respectively. There is no significant difference among these values. Therefore, the different kinetic rates might be driven by other reasons. Solution pH plays a critical role in the speciation of organic

Table 2 Photocatalytic degradation rate constants of antibiotics by Ag1.5BrPFe at different solution pH values. (Experimental conditions: [CIP] = [NOR] = [SDZ] = [TTC] = 1 mg L−1, [Catalyst] = 0.17 g L−1). Antibiotics

CIP NOR SDZ TTC

kapp (min−1) 3

5

7

9

0.16 0.19 0.34 0.10

0.14 0.15 0.33 0.07

0.12 0.12 0.23 0.06

0.03 0.04 0.14 0.03

compounds as well as in the surface electrical properties of solid catalysts [44]. As shown in Table 2, the acidic condition is favorable for the photocatalytic degradation process, whilst basic condition is inhibitory.

Fig. 5. Decolorization of MB with the prepared photocatalysts under simulated solar light and the corresponding pseudo-first-order kinetic rate constants. (Experimental conditions: [MB] = 5 mg L−1, [Photocatalyst] = 0.25 g L−1, pH unadjusted). 570

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The results can be explained by taking into consideration the properties of both catalyst and antibiotics at different pH conditions. As listed in Supporting Information Table S1, the pKa values of the four antibiotics are 6.09 and 8.74 (CIP), 6.34 and 8.75 (NOR), 1.57 and 6.5 (SDZ), 3.3, 7.7 and 9.7 (TTC), respectively. Therefore, at pH of 3, CIP, NOR, and TTC will be protonated to their cationic forms (Fig. S7), whereas the surface of Ag1.5BrPFe is negatively charged (see the zeta potential results in Table S3). Consequently, the adsorption of positively charged antibiotics on the surface of negatively charged Ag1.5BrPFe will be greatly facilitated. As well known, the heterogeneous catalytic processes are predominantly dependent on the interfacial reactions occurred on the surface of catalysts. Therefore, the facilitated interaction means a better degradation performance of organic compounds. However, the SDZ is not protonated at pH of 3. Therefore, there is no significant change in the degradation process for SDZ. At pH of 5, TTC is transferred to its neutral form. The attraction force caused by the electric is diminished. As a result, the degradation process is clearly inhibited. On the other hand, though the degrees of protonation of CIP and NOR are weakened when the pH increases to 5, the surface of Ag1.5BrPFe is much more negatively charged. Therefore, the degradation process is not significantly retarded. At pH of 7, CIP and NOR will be transferred to their neutral form. The degradation processes are slightly inhibited. Meanwhile, the SDZ will be dissociated to its anionic form. The repulsive force caused by the negatively charged Ag1.5BrPFe will significantly suppress the degradation process. The TTC is still at its neutral form, and the degradation process is not significantly altered. All these explanations well reflect the results shown in Table 2. When the solution pH value further increased to 9, CIP, NOR, and SDZ will be dissociated to their anionic forms [45]. Meanwhile, the surface of catalyst Ag1.5BrPFe is still negatively charged. Therefore, the repulsive force will inhibit the interaction between antibiotics and

catalyst, the degradation processes are greatly retarded. Nonetheless, the degradation kinetic is slightly decreased for TTC because its existence form is not changed. The versatile natural organic matters (NOM) in water may induce adverse effects on the photocatalytic degradation processes through the radiation attenuation, competition for active sites and surface deactivation of the catalyst [46–48]. In this case, the addition of humic acid (HA), a representative NOM clearly retards the photocatalytic degradation process (Fig. S8). In addition, the inhibition becomes more significant with increasing the dose of HA. However, the removal efficiencies of CIP, NOR and SDZ can still reach 90% even when there is 4 mg/L HA in the water matrix. This probably implies that the Ag1.5BrPFe is effective for the elimination of antibiotics from the NOM containing water systems. Inorganic ions naturally occurring in wastewater also can influence the photocatalytic degradation process of organic pollutants, depending on their nature, and concentration [49]. In this case, chloride ion exhibits a negative effect on the degradation of antibiotics (Fig. S9). This is not strange because: (i) the chloride ion can scavenge the photogenerated hole [50]; (ii) high concentration of chloride may be adsorbed on to the surface of catalyst and partially block the active sites [51,52]; (iii) ·OH has the potential to oxidize Cl− to less reactive chlorine or hypochlorous [53]. Unlike chloride ion, sulfate ion plays a stimulative role to the photocatalytic process (Fig. S10). This is probably due to the formation of reactive species such as SO4%− (Eq. (1)), which is known to be the primary oxidizing intermediate for the destruction of organic molecules in sulfate radical based oxidation processes [52–54].

h+ + SO24

SO•4

(1)

Compared with the parent antibiotics, the elimination of total organic carbon (TOC) is relatively tardive (69.5% reduction within

Fig. 6. ESR signals of DMPO-OH (A and C) and DMPO-O2 (B and D) during the photocatalytic degradation process by bare hematite and Ag1.5BrPFe. 571

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300 min, Fig. S11). This is expectable because some of the degradation intermediates are more stable toward the photocatalytic process [53,55–57]. Consequently, the complete mineralization of the organic compounds is usually rather time-consuming. To study the stability of the catalyst, the degradation of antibiotics in three successive cycles is also investigated. The results depicted in Fig. S12 confirm that the stability of the catalyst is quite satisfactory with the degradation of the antibiotics not significantly decreased, agreeing well with the MB decolorization result (Fig. S5).

confirm the higher separation rate, the electrochemical impedance spectroscopy (EIS) results are provided in Fig. 7B. Only one semicircle is observed on the EIS plane, suggesting that the photocatalytic reaction involves only the surface charge-transfer process [12]. The much smaller radius shown in Ag1.5BrPFe electrode indicates a lower solidstate interface layer resistance, leading to a higher transfer rate of the electron-hole pairs [60].

3.4. Reactive species in the photocatalytic process

In this study, the frontier electron densities (FEDs) of four antibiotics are calculated to predict the reaction sites for electron extraction and radicals attack. According to the Frontier Orbital Theory, an electron can be easily extracted at positions with higher values of 2FED2HOMO, while the addition of %OH usually occurs at a position with a higher FED2HOMO+FED2LUMOvalue. Meanwhile, the position with a more positive point charge is more easily attacked by O2%− [61,62]. Fig. S14 shows the optimized structure and atomic numbering of the four antibiotics. As shown in Supporting Information Table S4, the photo-generated hole most likely attacks CIP at N (4) position according to the 2FED2HOMO values. Similarly, An et al. also reported that the N (4) was likely the main initial position for direct hole oxidation [27]. Moreover, the %OH will prefer attack CIP at N (4), C (15), and C (9) positions based on the FED2HOMO+FED2LUMO values. This is consistent with the previous observations that quinolone ring is firstly attacked by %OH [27]. C (21) exhibits more positive point charges than others, revealing that C (21) position is preferentially attacked by O2%− through a nucleophilic reaction. Similarly, the N (5) position in NOR possesses the highest 2FED2HOMO value (Table S5), suggesting that the photo-generated hole firstly attacks this position. C (14) and C (12) positions are preferentially attacked by %OH due to the high FED2HOMO+FED2LUMO values. C (19) position would be firstly attacked by O2%− because of the highest point charge. Noting that, previous studies extensively detected the corresponding degradation products due to the decomposition and partial elimination reactions occurred on the piperazinyl and quinolone moieties of CIP or NOR [27,44,63], indicating the reliability of the prediction. For SDZ, the highest 2FED2HOMO is calculated at C (14) position (Table S6), revealing the holes attack this site firstly. The highest FED2HOMO+FED2LUMO is found at C (11) position and the highest point charge is found at S (22) position, respectively. Previous studies extensively reported the occurrence of hydroxylated SDZ during the AOPs

3.5. Identification of reaction sites

As well known, the photocatalyst can be excited to generate hole and electron pairs under irradiation of light. The photo-generated hole can directly or indirectly (react with H2O to form %OH) oxidize organic compounds, whist the photo-generated electron can be transferred to the acceptor molecules adsorbed on the surface such as oxygen to form O2%−. To confirm the formations of the reactive species in the photocatalytic reactions, the ESR signals of the spin-trapped radicals are recorded and given in Fig. 6. The four characteristic peaks of DMPO-OH (with hyperfine splitting constants of αN = 14.4 G and αH = 15.1 G, g = 2.0065) and six characteristic peaks of DMPO-O2 (with hyperfine splitting constants of αN = 8.9 G, αH = 4.6 G, and αH = 9.3 G, g = 2.0068) are clearly identified, indicating the involvement of %OH and O2%− in the photocatalytic process [58]. Besides, the ESR signal intensities are much stronger in Ag1.5BrPFe than in bare hematite system (Fig. 6 C and D), implying the effectiveness of Ag1.5BrPFe photocatalyst. These results are consistent with the degradation performance. To further distinguish the roles of different reactive species during the photocatalytic degradation process, the chemical scavengers are introduced into the heterogeneous systems respectively. Interestingly, the kapp values of the four antibiotics are nearly unchanged after the addition of IPA (Table 3), indicating the minor role of %OH in the photocatalytic degradation process. This observation seems to be contradictory with the detected EPR signals of DMPO-OH given in Fig. 10A. It should be noticed that there is no antibiotics introduced during the EPR determination. In that case, the hole will preferably react with H2O to form %OH. However, in the antibiotics degradation process, the reactions might be different. It is hypothesized that the holes directly rather than indirectly oxidize antibiotics during the degradation process. This speculation can be further verified by the following EDTA2Na scavenging assay, which shows that EDTA-2Na induces significant inhibition on the photocatalytic degradation of four antibiotics. This clearly indicates that the photo-generated hole mainly reacts with antibiotics rather than H2O during the photocatalytic degradation process. To confirm the formation of hole, the EPR signals (g = 2.0038) are also recorded and given in Supporting Information Fig. S13. Moreover, BQ generates the most remarkable inhibitions to the degradation processes of CIP (90.1%), NOR (100%), and SDZ (95.8%), indicating the crucial role of O2%− in the degradation processes. Based on the above discussion, it is suggested that Ag1.5BrPFe catalyst is more efficient than bare hematite for the photocatalytic degradation of antibiotics. Table 1 clearly illustrates that the band gap of Ag1.5BrPFe composite is lower than bare hematite, suggesting the better utilization of solar light. In addition, the recombination rate of the photo-generated electron and hole is also of great importance. As given in Fig. 7A, the photocurrent response sharply increases once the light irradiation is activated. Moreover, the Ag1.5BrPFe catalyst exhibits a higher photocurrent response than bare hematite. The photocurrent is steady and reproducible during several intermittent on-off irradiation cycles. The higher photocurrent response probably means higher separation rate of photo-generated electron and hole [59]. This is attributed to that the photo-generated electron is excited from the valence band (VB) to conduction band (CB) and then transfers to Ag species, inhibiting the direct recombination of electron and hole. To further

Table 3 Kinetic rate constants of CIP (A), NOR (B), SDZ (C), and TTC (D) degradations with the radical scavengers. (Experimental conditions: [CIP] = [NOR] = [SDZ] = [TTC] = 1 mg L−1, [Catalyst] = 0.17 g L−1, pH = 7, [IPA] = [EDTA-2Na] = [BQ] = 6 mM). Antibiotics

Quencher

CIP

Control IPA EDTA-2Na BQ Control IPA EDTA-2Na BQ Control IPA EDTA-2Na BQ Control IPA EDTA-2Na BQ

NOR

SDZ

TTC

572

RS quenched

kapp (min−1)

Inhibition rate (%)

– OH h+ O2%− – % OH h+ O2%− – % OH h+ O2%− – % OH h+ O2%−

0.171 0.153 0.019 0.017 0.170 0.165 0.025 0.000 0.236 0.232 0.067 0.010 0.067 0.071 0.012 0.016

0 10.5 88.9 90.1 0 2.9 85.3 100 0 1.7 71.6 95.8 0 0 82.1 76.1

%

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Fig. 7. Transient photocurrent response (A) and electrochemical impedance spectroscopy curves (B) of bare hematite and Ag1.5BrPFe ternary photocatalyst.

[64,65]. However, they did not indicate clearly the reaction site. Through the computational calculations conducted in this study, we propose that the C (14) and C (11) positions are the probable sites for the hydroxylation reaction. Cleavage of SeN bond is expected due to the attack of O2%− on the S (22) position. The corresponding product had been successfully identified previously [65]. For TTC, the C (9) position exhibits the highest 2FED2HOMO value (Table S7), revealing the preferable attack by photo-generated hole. Besides, C (14) and C (3) positions might also be attacked by hole because the 2FED2HOMO values are relatively high. The highest FED2HOMO+FED2LUMO is found at C (16) position. However, as evidenced by the radical scavenging experiment, the contribution of %OH is negligible. The highest point charge is found at C (20) position, suggesting it is readily attacked by O2%−.

attacked by free radical through electron-withdrawing substituent. Our prediction also clearly indicates that this double-bond is more likely to be attacked by hole. Consequently, the product A is believed to be produced through the initial 1,3-dipolar cycloaddition towards the C (9) = C (14) bond and a rearrangement with the hydroxyl at the C (14) position (structure is given in Supporting Information Table S8). The structure is then validated by the fragment ion at m/z 444 (-17 Da), corresponding to the loss of NH3 (17 Da). Product B with protonated form at m/z of 475 (30 Da more than TTC) is also detected in the MS spectra. Ji et al. [67] and Zhu et al. [68] all detected this protonated product, while different molecular structures were proposed according to their characteristic fragment ions. Notably, both of them indicated that the hydroxylation reaction occurred at C (9) position. As revealed by the computational prediction, C (3) position might also be attacked by hole. Therefore, based on the reaction site prediction and fragment ions (m/z 249) alignment, the possible structure of product B is tentatively proposed in Table S8. Product C with protonated form at m/z of 459 (14 Da more than TTC) is also reported in other AOPs [67]. Based on the theoretical calculations, it is suggested that the C (3) position is attacked by hole and the subsequent oxidation reaction occurs. Its structure can be validated by its fragment ion at m/z 442 (-17 Da), corresponding to the loss of CH3 (17 Da).

3.6. Degradation pathways of TTC As shown in Supporting Information Fig. S15, five main intermediates as well as TTC are detected in the HPLC-MS spectra. The product A with protonated form at m/z of 461 (16 Da more than TTC) can be easily assigned to be the hydroxyl TTC because the mass difference (16 Da) is exactly equal to the OH addition. Wang et al. [66] claimed that the C (9) = C (14) bond was more susceptible to be

Fig. 8. Proposed degradation pathways of TTC during the photocatalytic degradation processes by Ag1.5BrPFe under simulated solar light irradiation. 573

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Products D and E with protonated forms at m/z of 431 (14 Da less than TTC) and 417 (28 Da less than TTC) are detected through MS analysis. Further MS/MS analyses under product ion scan mode (Table S8) indicate that they are probable 4-demethyltetracycline and 4-dedimethyltetracycline, respectively. Though the structures are beyond the computational prediction, they have been well documented in previous studies [67]. Noting that, as evidenced by the computational calculations, the C (20) position would be preferentially attacked by O2%−. However, no corresponding products are detected by HPLC-MS. A probable reason is that the intermediate is very unstable and can not accumulate during the degradation process. Overall, the predictions match well with the proposed structures of the intermediates. Based on the above discussion, the photocatalytic degradation pathways of TTC by Ag1.5BrPFe under simulated solar light are established in Fig. 8.

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4. Conclusion In summary, AgBr/Ag3PO4@NH photocatalysts with different compositions were fabricated via simple routine. The deposition of silver species greatly reduced the band gap and therefore enhanced the photocatalytic activity of natural hematite. And hematite increased the stability of AgBr/Ag3PO4 in turn. Four ubiquitous antibiotics (CIP, NOR, SDZ, TTC) could be simultaneously degradation by prepared Ag1.5BrPFe under simulated solar light illumination. Among them, SDZ exhibited the highest degradation kinetic rate constant (0.23 min−1 at neutral condition). Acidic condition was favorable for the photocatalytic degradation process, whilst basic condition was inhibitory. EPR and radical scavenging experiments revealed that photo-generated hole and O2%− dominated the degradation process, whilst %OH played a negligible role. Finally, based on the HPLC-MS/MS and frontier electron density calculations, the degradation pathways of TTC were tentatively proposed. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 51608274), National Science and Technology Major Project (No. 2017ZX07204001-06), and the Research Foundation of Jiangsu Environmental Protection Department (No. 2017002). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.03.038. References [1] A.K. Sarmah, M.T. Meyer, A.B.A. Boxall, A global perspective on the use, sales, exposure pathways, occurrence, fate and effects of veterinary antibiotics (VAs) in the environment, Chemosphere 65 (2006) 725–759. [2] H.K. Allen, J. Donato, H.H. Wang, K.A. Cloud-Hansen, J. Davies, J. Handelsman, Call of the wild: antibiotic resistance genes in natural environments, Nat. Rev. Microbiol. 8 (2010) 251. [3] X. Liu, Y. Zhou, J. Zhang, L. Luo, Y. Yang, H. Huang, H. Peng, L. Tang, Y. Mu, Insight into electro-Fenton and photo-Fenton for the degradation of antibiotics: mechanism study and research gaps, Chem. Eng. J. 347 (2018) 379–397. [4] N.F.F. Moreira, J.M. Sousa, G. Macedo, A.R. Ribeiro, L. Barreiros, M. Pedrosa, J.L. Faria, M.F.R. Pereira, S. Castro-Silva, M.A. Segundo, C.M. Manaia, O.C. Nunes, A.M.T. Silva, Photocatalytic ozonation of urban wastewater and surface water using immobilized TiO2 with LEDs: micropollutants, antibiotic resistance genes and estrogenic activity, Water Res. 94 (2016) 10–22. [5] Q. Guo, Z. Du, B. Shao, Simulation and experimental study on the mechanism of the chlorination of azithromycin, J. Hazard. Mater. 359 (2018) 31–39. [6] K.M. Lee, C.W. Lai, K.S. Ngai, J.C. Juan, Recent developments of zinc oxide based photocatalyst in water treatment technology: a review, Water Res. 88 (2016) 428–448. [7] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997–3027. [8] M. Mishra, D.-M. Chun, α-Fe2O3 as a photocatalytic material: a review, Appl. Catal. A Gen. 498 (2015) 126–141.

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