Fast mineralization of acetaminophen by highly dispersed Ag-g-C3N4 hybrid assisted photocatalytic ozonation

Separation and Purification Technology 216 (2019) 1–8

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Fast mineralization of acetaminophen by highly dispersed Ag-g-C3N4 hybrid assisted photocatalytic ozonation ⁎

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Yu Ling, Gaozu Liao , Peng Xu, Laisheng Li

Guangdong Engineering Research Center for Drinking Water Safety, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ag-g-C3N4 Photocatalytic ozonation Acetaminophen Synergistic effect

Highly dispersed Ag-g-C3N4 hybrid was synthesized successfully by a simple calcination method in present study. The samples were characterized by TEM, XRD, XPS, BET and UV–vis DRS. The results indicate that Ag element are existent as metallic silver and highly dispersed in the matrix of g-C3N4 nanosheet. It was used as active catalyst for photocatalytic ozonation of acetaminophen (ACE) (solar light/Ag-g-C3N4/O3). The results showed that the apparent rate constant of mineralize ACE by solar light/4% Ag-g-C3N4/O3 during 120 min is almost 2 times as large as that with solar light/g-C3N4/O3. The mechanism of photocatalytic ozonation with 4% Ag-gC3N4 was further confirmed. In this system, Ag acts as not only a good photo-generated electron acceptor for photocatalysis but also a beneficial decomposition center for ozone. Subsequently, holes and hydroxyl radicals both make contribution to the mineralization of ACE in solar light/4% Ag-g-C3N4/O3 process. A superior synergistic effect was observed in mineralization of ACE according to the synergy index of 5.3. Furthermore, Effects of various pH values and silver contents on the ACE mineralization were evaluated. The pathway of ACE degradation in photocatalytic ozonation system with 4% Ag-g-C3N4 was also proposed.

1. Introduction Recently, a mass of pharmaceutical and personal care products (PPCPs) were discarded into the environment, which caused growing global concern. These PPCPs and their metabolites possessed potential hazard to human health and ecosystem even at very low concentration levels [1–3]. As an effective painkiller, acetaminophen (ACE) was used for the relief of mild to moderate pain associated with headache, backache, arthritis and postoperative pain all over the world. It has been detected at concentrations of µg L−1 in water environment [4,5]. Due to its physicochemical properties, ACE is very hard to degrade by conventional activated sludge method. Some new technologies were applied as the feasible approaches to treat ACE, such as constructed wetland [6], emulsion liquid membrane [7], advanced oxidation processes (AOPs) [8]. Among them, AOPs are regarded as the most effective, including photocatalysis [9,10], electro-Fenton process [11,12], ultrasonic [13], UV/H2O2 and O3 [14]. However, there still exist some defects for these processes. Fast and complete mineralization was hard to achieve by pristine AOPs. Consequently, coupled AOPs technologies turned out to be available methods for efficiency improvement [15–17]. Recently, photocatalytic ozonation with g-C3N4 was considered as an efficient process for removing of refractory organics [18,19]. Under ⁎

visible light irradiation, the more negative conduction band potential of g-C3N4 facilitated the electrons transfer and ozone trapping reaction. Subsequently, yield of hydroxyl radicals and organics degradation efficiency were enhanced. But for pristine g-C3N4, the lack of active sites may limit the decomposition of O3. In our previous study, Ag component were incorporated in TiO2 or g-C3N4, the introduction of Ag nanoparticles not only promote the separation of photo-generated carriers, but also accelerate O3 decomposition as the active sites [20,21]. However, the Ag particle size in the Ag/g-C3N4 composite was ranged in dozens of nanometers. Indeed, the size effect of metal nanoparticles is of great importance for the catalytic performance. It was reported that the reduction of dimension for metal particles could enhance the photoexcited electrons to the active sites further [22,23]. Especially when downsizing the metal nanocluster to single atoms can maximize the atom utilization efficiency and boost catalytic performance [24–26]. For instance, highly dispersed metal catalysts anchored on gC3N4 shown efficient performance for many catalytic reactions [27–30]. Herein, we described a simple one-step strategy to prepare highly dispersed Ag-g-C3N4 hybrid. It was involved to photocatalytic ozonation of ACE. Enhanced catalytic activity is hoped to be got by highly dispersed Ag atoms in photocatalytic ozonation of ACE under simulated solar light irradiation.

Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Liao), [email protected] (L. Li).

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https://doi.org/10.1016/j.seppur.2019.01.057 Received 20 November 2018; Received in revised form 10 January 2019; Accepted 22 January 2019 Available online 23 January 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. TEM (a) and EDX elemental mapping images of 4% Ag-g-C3N4: carbon, (b), nitrogen (c) and silver (d).

added to 1 L ACE solution with a concentration of 10 mg L−1. Ozone was generated from ozone generator (ANSEROS, COM-AD-01-OEM) and the flow rate was maintained at 1 L min−1. The yield of ozone was 50 mg h−1. It was continuously bubbled into the solution through a porous glass plate which was immobilized at the bottom of reactor. At the beginning of reaction, the suspension was firstly bubbled with O2 for 40 min to establish adsorption/desorption equilibrium. Samples were withdrawn at specific time intervals and filtered using syringe filters (pore size: 0.45 µm). The filtrate was collected and analyzed further. As a quencher, Na2S2O3 solution was added into the filtrate to avoid the continuous oxidation.

2. Material and methods 2.1. Reagents Acetaminophen (C14H22N2O3, ≥98.0%) was obtained from Shanghai Macklin Co. Ltd. Dicyandiamide (C2H4N8, > 99%) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Other reagents were purchased from Tianjin Kermel Chemical Reagents Ltd. and used without purification. Deionized water was used to prepare all aqueous solutions. 2.2. Synthesis of Ag-g-C3N4

2.4. Characterization and analysis procedures In a typical synthesis, 0.4 g dicyandiamide was firstly dissolved in 50 mL deionized water. A certain amount of silver nitrate was added in the solution under magnetic stirring (the mass ratio between silver nitrate and dicyandiamide was 0, 1%, 3%, 4%, 6% and 8%, respectively). After the resultant solution was stirred at 35 °C for 1 h, the mixture was heated at 80 °C and keeps stirring until the water was removed. Finally, the obtained solid was calcined at 550 °C for 2 h under N2 atmosphere. After cooling to room temperature, a series of Ag-g-C3N4 was successfully obtained and denoted as x% Ag-g-C3N4 (x = 0, 1, 3, 4, 6 and 8 represent the mass ratio of silver nitrate/dicyandiamide).

The structure and EDX elemental mapping images of the samples were studied by transmission electron microscopic (TEM, JEM-2100HR, JEOL, Japan). The crystalline phase was determined by X-ray diffraction (XRD, BRUKER D8 ADVANCE) using Cu Ka radiation. X-ray photoelectron spectra (XPS) was examined by a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI, America). The specific surface area of the samples was calculated by Brunauer-Emmett-Teller analysis (BET, ASAP 2020, Micromeritics, America) using nitrogen adsorption-desorption isotherms at −196 °C. The optical absorption properties of the samples were measured using a UV–Vis spectrophotometer (U-3010, HITACHI, Japan) with BaSO4 reference. ACE concentration was measured by high performance liquid chromatography (Shimadzu, LC10A HPLC) equipped with a UV detector (SPD-10AV) at 248 nm and a Diamonsil 5U C18 column (5 μm, 250 mm × 4.6 mm, Dikma technologies). The mobile phase was composed of 70% ultrapure water and 30% methanol (1.0 mL min−1 flow rate). The intermediates in ACE degradation were detected by liquid chromatography/mass spectrometry (LC-MS, Agilent 1260-6460, America) equipped with an electrospray ionization source, in positive ionization mode. The total organic carbon (TOC) was analyzed by a

2.3. Photocatalytic ozonation experiment Photocatalytic ozonation experiments were conducted in a 1 L glass tubular photoreactor (h = 400 mm, Φ in = 85 mm), equipped with a high pressure xenon long-arc lamp (GXH500W, Beijing NBET Technology Co., Ltd) as the simulated solar light irradiation source. The lamp was placed inside a quartz water-cooling thimble to maintain the temperature at 25 °C. The illumination intensity of Xenon lamp was 48 mW cm−2 (Measured by Radiometers of model FZ-A, Photoelectric Instrument Factory Beijing Normal University). 0.25 g catalyst was 2

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spectrum (Fig. 4b), three peaks at 284.9, 288.3 and 293.8 eV, which are identified as the adventitious carbon (CeC), the sp2-bonded carbon (NeC]N) and π-π*, respectively. The N 1s spectrum (Fig. 4c) could be divided into three peaks centered at 398.5, 399.8 and 400.7 eV, which are attributed to the sp2-hybridized aromatic nitrogen (C]NeC), the tertiary nitrogen (Ne(C)3), the free amino groups (CeNeH), respectively. The classical Ag3d peaks (Fig. 4d) at 368.6 eV (Ag 3d3/2) and 374.6 eV (Ag 3d5/2) were accompanied by a spin energy separation of 6.0 eV, indicated the predominance existence of metallic silver [30,31]. The optical properties of the two samples were studied by UV–vis DRS. As depicted in Fig. 5, the absorption edge of 4% Ag-g-C3N4 shift about 25 nm compared to g-C3N4 while the main absorption edge of 4% Ag-g-C3N4 was measured to be 490 nm. Furthermore, 4% Ag-g-C3N4 owned stronger optical absorption intensity than g-C3N4 in virtue of the surface plasmon resonance of Ag particles.

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3.2. Photocatalytic ozonation of ACE

Shimadu TOC 5000 analyzer.

The photocatalytic ozonation efficiency of 4% Ag-g-C3N4 is evaluated through the mineralization of ACE solution under simulated solar light irradiation. As shown in Fig. 6A, the TOC removal ratio of ACE is only 1.9% during 120 min for simulated solar light irradiation. This value was increased to 6.8% in solar light/4% Ag-g-C3N4. For ozonation alone and catalytic ozonation process with 4% Ag-g-C3N4, 23.1% and 28.8% of ACE is mineralized. When photocatalysis and ozonation are coupled, 53.8% of ACE was mineralized in solar light/g-C3N4/O3. The TOC removal ratio is enhanced dramatically after Ag incorporated in gC3N4. 83.1% of ACE is mineralized in solar light/4% Ag-g-C3N4/O3. The highly dispersed Ag element on g-C3N4 not only accelerated the transfer of photogenerated electrons, but also improved the utilization efficiency of ozone, which strengthened the synergistic effect between photocatalysis and ozonation. The mineralization of ACE accords with pseudo-first order kinetics by linear transforms ln(TOC0/TOCt) = kt, where TOC0 is the initial TOC of ACE, TOCt is the TOC concentration of ACE at time t, and k is apparent rate constant. ACE mineralization apparent rate constant (k) in different processes were calculated. As shown in Fig. 6B, the apparent rate constant for solar light and O3 along were 0.0002 min−1 and 0.0025 min−1, respectively. The k value for solar light/4% Ag-g-C3N4/ O3 reached 0.0143 min−1 which was 2 times as much as that of solar light/g-C3N4/O3 process. According to Eq. (1), the synergy factor (η) of solar light/4% Ag-g-C3N4/O3 system in mineralization of ACE was calculated as 5.3 which exhibited an excellent synergistic effect in the coupling system. The result further confirmed the positive promotion of catalytic activity by highly dispersed Ag element.

3. Results and discussion 3.1. Catalysts characterization Fig. 1 shows the TEM image of Ag-g-C3N4 sample. It can be seen the hybrid possess two-dimensional nanosheets with chiffon-like wrinkles. There is no obvious Ag accumulated on the g-C3N4, which is proved further by the greater magnification image displayed in Fig. 1(b). It could be inferred that Ag are highly dispersed in the matrix of g-C3N4 nanosheet. The elemental mapping of Ag-g-C3N4 (Fig. 1c–e) indicates that the signals of C, N and Ag are uniform distribution across the nanosheet structure; especially Ag elements are atom-dispersed on the gC3N4. Fig. 2 shows nitrogen adsorption-desorption isotherms of 4% Agg-C3N4 and g-C3N4. The isotherms of the two samples are in accord with type IV, which means the presence of mesopores. Correspondingly, the BET specific surface area of 4% Ag-g-C3N4 was 6.3 m2 g−1, which is a little less than that of pristine g-C3N4 (10.9 m2 g−1). Fig. 3 presents the XRD patterns of X% Ag-g-C3N4 and g-C3N4. The strong diffraction peak at 27.5° is attribute to the interlayer stacking of aromatic systems which marked as (0 0 2) plane, and the relatively weak peak at 13.1° labeled as (1 0 0) plane matches the in-plain structural packing motif of tristriazine units. However, the diffraction peaks of Ag are not seen because the low amount of Ag was well dispersed on the surface of g-C3N4. To determine the valence states of each element in 4% Ag-g-C3N4, XPS measurement was carried out and the result is shown in Fig. 4. The survey spectrum (Fig. 4a) indicate that the hybrid photocatalyst mainly contain C, N and Ag element. In the C 1s 002

η (Synergy factor) =

In this study, the effects of initial pH on ACE mineralization have been considered. As shown in Fig. 7, the TOC removal ratio of ACE is increased gradually as pH value varied from acid station to neutral condition. In photocatalytic ozonation process, the increase of pH value is beneficial for the transformation of O3 to hydroxyl radicals (%OH), which degrade organic compounds more effectively than O3. Mena [32] reported that dissolved ozone concentration in photocatalytic ozonation process was detected from 6.1 × 10−6 M to 1 × 10−6 M when pH value increase from 3 to 7, indicating more consumption of O3 at neutral condition owing to its efficient decomposition. However, continue increasing pH has little effect on the reaction. We can see the TOC removal ratio of ACE keep stable if the pH increased to 11 for solar light/4% Ag-g-C3N4/O3 process. This is because the isoelectric point of Ag-g-C3N4 and pKa of acetaminophen was reported as < 3 and = 9.5, respectively [10,33]. It means both Ag-g-C3N4 and ACE were negatively

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nanosheet for 4% Ag-g-C3N4. It is very beneficial for the separation of photo-generated charges and decomposition of O3. However, excess Ag components play a negative role in ACE degradation. The ACE mineralization efficiency was reduced to 71.6% and 68.0% when the amount of Ag was raised to 6% and 8%. Excessive Ag amount involved accumulation on the g-C3N4 nanosheet. As can be seen in the TEM image of 8% Ag-g-C3N4, Ag nanoparticles with dimension of few nanometers were formed. This would reduce the amount of active sites and promote the recombination of photogenerated charges again on the catalyst surface [34].

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The stability of 4% Ag-g-C3N4 for ACE degradation was also studied. The samples were further reused for four cycles in solar light/4% Ag-gC3N4/O3 system. As shown in Fig. 9, the mineralization efficiency of ACE slightly decreased from 83.1% to 79.9% during 2 h, which exhibited that 4% Ag-g-C3N4 owns excellent catalytic ability and stability in photocatalytic ozonation coupling system.

Wavelength (nm) Fig. 5. The UV–Vis DRS spectra of 4% Ag-g-C3N4 and g-C3N4.

charged at pH = 11. In this condition, electrostatic repulsion between Ag-g-C3N4 surface and ACE play an opposite part in the reaction.

3.6. Mechanism discussion of photocatalytic ozonation with 4% Ag-g-C3N4 3.4. Effect of Ag amount In order to explore the degradation mechanism of 4% Ag-g-C3N4 over this coupling system, dissolved ozone concentration was studied during 120 min among different oxidation processes. As shown in Fig. 10A, the dissolved ozone concentration increased dramatically during the initial 30 min, and then dissolution/consumption equilibrium was established. For O3 alone process, the dissolved ozone concentration was higher than that of solar light/O3, which was related to ozone decomposition under simulated solar light irradiation. Addition

The amount of Ag might significantly influence the catalytic performance of Ag-g-C3N4. As shown in Fig. 8(A), all Ag-g-C3N4 composites exhibited much higher catalytic activity than that of pure g-C3N4. The catalytic activity of Ag-g-C3N4 hybrid increased with raising Ag content. The 4% Ag-g-C3N4 sample revealed the most efficient capability for photocatalytic ozonation of ACE. According to the results of TEM characterization, Ag element is highly dispersed in the matrix of g-C3N4 4

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Fig. 8. (A) Photocatalytic ozonation of ACE over the Ag-g-C3N4 with different Ag ratio; (B) TEM images of 8% Ag-g-C3N4. Ozone dose:50 mg h−1; flow rate of oxygen: 1.0 L min−1; catalyst dose: 0.25 g L−1; initial concentration of ACE solution: 10 mg L−1; volume of ACE solution: 1.0 L; T = 25 °C.

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Time (min) Fig. 6. (A) TOC removal of ACE by different processes; (B) The apparent rate constants of ACE for different processes. Ozone dose:50 mg h−1; flow rate of oxygen: 1.0 L min−1; catalyst dose: 0.25 g L−1; initial concentration of ACE solution: 10 mg L−1; volume of ACE solution: 1.0 L; T = 25 °C.

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Fig. 9. Photocatalytic ozonation of ACE by 4% Ag-g-C3N4 for four cycles. Ozone dose:50 mg h−1; flow rate of oxygen: 1.0 L min−1; catalyst dose: 0.25 g L−1; initial concentration of ACE solution: 10 mg L−1; volume of ACE solution: 1.0 L; T = 25 °C.

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Time (min) Fig. 7. Photocatalytic ozonation of ACE by 4% Ag-g-C3N4 at different pH condition. Ozone dose:50 mg h−1; flow rate of oxygen: 1.0 L min−1; catalyst dose: 0.25 g L−1; initial concentration of ACE solution: 10 mg L−1; volume of ACE solution: 1.0 L; T = 25 °C.

depicted in Fig. 10B, 98.1% of ACE was degraded during 20 min in solar light/4% Ag-g-C3N4/O3 without any scavenger. 54.8% and 43.7% of ACE were degraded in same condition when TBA and TEOA were added, respectively. Obviously the degradation of ACE was inhibited by adding TBA and TEOA, which means both %OH and holes contributed to the degradation of ACE in photocatalytic ozonation coupling system. The schematic diagram of possible mechanism for 4% Ag-g-C3N4 hybrid in photocatalytic ozonation was proposed and depicted in Fig. 11. Under simulated solar light irradiation, both photo-generated electrons and holes were produced when 4% Ag-g-C3N4 was excited (gC3N4 + hv → e− + h+). The highly dispersed Ag element on g-C3N4 acts as electron captors to separate photo-generated electrons. As an

of g-C3N4 and Ag-g-C3N4 reduce the equilibrium concentration greatly because of the photo-generated electron trapping by O3. Moreover, the concentration of dissolved ozone in solar light/4% Ag-g-C3N4/O3 was much lower than that of solar light/g-C3N4/O3 process. It was ascribed to the highly dispersed Ag on g-C3N4 accelerated ozone decomposition. For further understanding the mechanism of the ACE degradation over 4% Ag-g-C3N4, the oxidative substances were measured through trapping by tert butyl alchol (TBA, 5 mmol L−1) and triethanolamine (TEOA, 5 mmol L−1) as %OH and holes scavenger respectively. As 5

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3.7. Analysis of the intermediates for ACE degradation

Fig. 11. Proposed mechanism of photocatalytic ozonation by 4% Ag-g-C3N4 for degrading ACE.

According to the earlier literatures, ACE degradation pathways in ozonation and photocatalysis started mainly through hydroxylation because of the formation of abundant %OH as the most dominant reactant. One is attack on the para position of phenolic function, leading to 1,4-hydroquinone [10,36]; Another attack occurs on the ortho or meta position of phenolic function, yielding di-hydroxylated or tri-hydroxylated intermediate [36,37]. The pathway for photocatalytic ozonation of ACE was detected by LC/MS technique. As shown in Fig. 12, the peak at acquisition time of 3.24 min is attributed to ACE, which

efficient active site for ozone decomposition [20,21,35], these electrons were transported to Ag and react with O3 (e− + O3 → %O3−). The %O3− happened a series of reactions and produced abundant %OH (%O3− + H+ → HO3%, HO3% → O2 + %OH). Meanwhile, the consumption of photo-generated electrons lead to more holes occur on g-C3N4, which could also oxidize ACE. Finally, the formed %OH and holes both contribute to the degradation of ACE.

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[4]

[5]

[6]

[7]

[8]

[9] [10] [11] [12]

[13]

Fig. 14. Proposed degradation pathway for photocatalytic ozonation of ACE.

[14]

could be elucidated from the mass spectra (m/z = 152, Fig. 13A). We can see the concentration of ACE is reduced rapidly in the first 20 min and disappears at 30 min. The intermediate at acquisition time of 2.05 min is captured and identified as C6H6NO3 (m/z = 140, Fig. 13B). Its concentration increased in the beginning of reaction, reaching its maximum concentration after 10 min, and then decreased until it almost disappeared after 120 min. This intermediate generated from the cleavage of the bond between nitrogen and carbon from carbonyl group for tri-hydroxylated ACE [37]. The pathway 1 presented in Fig. 14 could explained its transformation. In addition, the pathway 2 may also exist in ACE degradation process [38], but the product was not observed during LC/MS analysis. On the basis of these results, the pathway for photocatalytic ozonation of ACE using 4% Ag-g-C3N4 was proposed in Fig. 14, and followed by subsequent reactions to form water and carbon dioxide.

[15]

[16] [17]

[18] [19]

[20]

[21]

[22]

4. Conclusions [23]

In summary, 4% Ag-g-C3N4 was synthesized successfully by a simple calcination method. Under simulated solar light irradiation, the catalytic ability of 4% Ag-g-C3N4 was further improved by involving Ag nanoparticles as the separators of photon-generated carriers. As the decomposition centre of ozone, the Ag of 4% Ag-g-C3N4 provided more electrons to ozone decomposition and produced more %OH than pure gC3N4. Finally, both %OH and holes contributed to the photocatalytic ozonation of ACE, and strengthened catalytic ability was realized. This study supplied an excellent catalyst to photocatalytic ozonation coupling system under simulated solar light irradiation.

[24]

[25] [26]

[27]

Acknowledgements [28]

This work was supported by the National Nature Science Foundation of China (No. 21207042).

[29]

References [30] [1] Q. Bu, B. Wang, J. Huang, S. Deng, G. Yu, Pharmaceuticals and personal care products in the aquatic environment in China: a review, J. Hazard. Mater. 262 (2013) 189–211. [2] T. Lin, S. Yu, W. Chen, Occurrence, removal and risk assessment of pharmaceutical and personal care products (PPCPs) in an advanced drinking water treatment plant (ADWTP) around Taihu Lake in China, Chemosphere 152 (2016) 1–9. [3] D.A. Algren, Review of n-acetylcysteine for the treatment of acetaminophen

[31]

[32]

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(paracetamol) toxicity in pediatrics, in: Second Meeting of the Subcommittee of the Expert Committee on the Selection and Use of Essential Medicines, 2018, pp. 1–18. N.K. Stamatis, I.K. Konstantinou, Occurrence and removal of emerging pharmaceutical, personal care compounds and caffeine tracer in municipal sewage treatment plant in Western Greece, J. Environ. Sci. Health B 48 (2013) 800–813. Q. Sun, M. Lv, A. Hu, X. Yang, C.P. Yu, Seasonal variation in the occurrence and removal of pharmaceuticals and personal care products in a wastewater treatment plant in Xiamen, China, J. Hazard. Mater. 277 (2014) 69–75. V.H.N. Phong, T. Koottatep, S.K. Chapagain, A. Panuvatvanich, C. Polprasert, K.H. Ahn, Removal of acetaminophen from wastewater by constructed wetlands with scirpus validus, Environ. Eng. Res. 21 (2016) 164–170. A.L. Ahmad, N.D. Zaulkiflee, A. Kusumastuti, M.M.H. Shah Buddin, Removal of acetaminophen from aqueous solution by emulsion liquid membrane: emulsion stability study, Ind. Eng. Chem. Res. DOI: 10.1021/acs.iecr.8b03562. M. Klavarioti, D. Mantzavinos, D. Kassinos, Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes, Environ. Int. 35 (2009) 402–417. X. Zhang, F. Wu, X. Wu, P. Chen, N. Deng, Photodegradation of acetaminophen in TiO(2) suspended solution, J. Hazard. Mater. 157 (2008) 300–307. L.M. Yang, L.E. Yu, M.B. Ray, Degradation of paracetamol in aqueous solutions by TiO2 photocatalysis, Water Res. 42 (2008) 3480–3488. C.C. Su, C.A. Cada, M.L.P. Dalida, M.C. Lu, Effect of UV light on acetaminophen degradation in the electro-Fenton process, Sep. Purif. Technol. 120 (2013) 43–51. M.D.G. Luna, M.L. Veciana, C. Su, M. Lu, Acetaminophen degradation by electroFenton and photoelectro-Fenton using a double cathode electrochemical cell, J. Hazard. Mater. 217–218 (2012) 200–207. E. Villaroel, J. Silva-Agredo, C. Petrier, G. Taborda, R.A. Torres-Palma, Ultrasonic degradation of acetaminophen in water: Effect of sonochemical parameters and water matrix, Ultrason. Sonochem. 21 (2014) 1763–1769. R. Andreozzi, V. Caprio, R. Marotta, D. Vogna, Paracetamol oxidation from aqueous solutions by means of ozonation and H2O2/UV system, Water Res. 37 (2003) 993–1004. T.E. Agustina, H.M. Ang, V.K. Vareek, A review of synergistic effect of photocatalysis and ozonation on wastewater treatment, J. Photochem. Photobiol. C: Photochem. 6 (2005) 264–273. J.D. Xiao, Y.B. Xie, H.B. Cao, Organic pollutants removal in wastewater by heterogeneous photocatalytic ozonation, Chemosphere 121 (2015) 1–17. A. Aguinaco, F.J. Beltrán, J.F. García-Araya, A. Oropesa, Photocatalytic ozonation to remove the pharmaceutical diclofenac from water: Influence of variables, Chem. Eng. J. 189–190 (2012) 275–282. G.Z. Liao, D.Y. Zhu, L.S. Li, B.Y. Lan, Enhanced photocatalytic ozonation of organics by g-C3N4 under visible light irradiation, J. Hazard. Mater. 280 (2014) 531–535. J. Yin, G.Z. Liao, D.Y. Zhu, P. Lu, L.S. Li, Photocatalytic ozonation of oxalic acid by g-C3N4/graphene composites under simulated solar irradiation, J. Photochem. Photobiol. A: Chem. 315 (2016) 138–144. Y. Ling, Z.G. Liao, H.Y. Xie, J. Yin, Y.J. Huang, H.W. Feng, S.L. Li, Coupling photocatalysis with ozonation for enhanced degradation of Atenolol by Ag-TiO2 microtube, J. Photoch. Photobiol. A 329 (2016) 280–286. Y. Ling, Z.G. Liao, H.W. Feng, Y. Liu, S.L. Li, Excellent performance of ordered Ag-gC3N4/SBA-15 for photocatalytic ozonation of oxalic acid under simulated solar light irradiation, J. Photochem. Photobiol. A 349 (2017) 108–114. C. Gao, S.M. Chen, Y. Wang, J.W. Wang, X.S. Zheng, J.F. Zhu, L. Song, W.K. Zhang, Y.J. Xiong, Heterogeneous single-atom catalyst for visible-light-driven high-turnover CO2 reduction: the role of electron transfer, Adv. Mater. 30 (2018) 1704624. S.W. Cao, H. Li, T. Tong, H.C. Chen, A.C. Yu, J.G. Yu, H.M. Chen, Single-atom engineering of directional charge transfer channels and active sites for photocatalytic hydrogen evolution, Adv. Funct. Mater. 28 (2018) 1802169. Z. Chen, Q. Zhang, W.X. Chen, J.C. Dong, H.R. Yao, X.B. Zhang, X.J. Tong, D.S. Wang, Q. Peng, C. Chen, W. He, Y.D. Li, Single-site Au catalyst for silane oxidation with water, Adv. Mater. 30 (2017) 1704720. L. Liu, H. Su, F. Tang, X. Zhao, Q. Liu, Confined organometallic Au1Nx single-site as an efficient bifunctional oxygen electrocatalyst, Nano Energy 46 (2018) 110. Z. Chen, S. Pronkin, T.P. Fellinger, K. Kailasam, G. Vilé, D. Albani, F. Krumeich, R. Leary, J. Barnard, J.M. Thomas, J. Pérez-Ramírez, M. Antonietti, D. Dontsova, Merging single-atom-dispersed silver and carbon nitride to a joint electronic system via copolymerization with silver tricyanomethanide, ACS Nano 10 (2016) 3166–3175. Z. Chen, S. Mitchell, E. Vorobyeva, R.K. Leary, R. Hauert, T. Furnival, Q.M. Ramasse, J.M. Thomas, P.A. Midgley, D. Dontsova, M. Antonietti, S. Pogodin, N. López, J. Pérez-Ramírez, Stabilization of single metal atoms on graphitic carbon nitride, Adv. Funct. Mater. 27 (2017) 1605785. G. Vilé, D. Albani, M. Nachtegaal, Z. Chen, D. Dontsova, M. Antonietti, N. López, J. Pérez-Ramírez, A stable single-site palladium catalyst for hydrogenations, Angew. Chem. Int. Ed. 54 (2015) 11265–11269. X. Li, W. Bi, L. Zhang, S. Tao, W. Chu, Q. Zhang, Y. Luo, C. Wu, Y. Xie, Single-atom Pt as Co-catalyst for enhanced photocatalytic H2 evolution, Adv. Mater. 28 (2016) 2427–2431. Y.B. Wang, X. Zhao, D. Cao, Y. Wang, Y.F. Zhu, Peroxymonosulfate enhanced visible light photocatalytic degradation bisphenol A by single-atom dispersed Ag mesoporous g-C3N4 hybrid, Appl. Catal. B 211 (2017) 79–88. K.H. Leong, B.L. Gan, S. Ibrahim, P. Saravanan, Synthesis of surface plasmon resonance (SPR) triggered Ag/TiO2 photocatalyst for degradation of endocrine disturbing compounds, Appl. Surf. Sci. 319 (2014) 128–135. E. Mena, A. Rey, B. Acedo, F.J. Beltrán, S. Malato, On ozone-photocatalysis synergism in black-light induced reactions: Oxidizing species production in photocatalytic ozonation versus heterogeneous photocatalysis, Chem. Eng. J. 204–206

Separation and Purification Technology 216 (2019) 1–8

Y. Ling et al.

Chem. Res. 30 (1991) 217–221. [36] D. Vogna, R. Marotta, A. Napolitano, M. D'Ischia, Advanced oxidation chemistry of paracetamol. UV/H2O2-induced hydroxylation/degradation pathways and (15)Naided inventory of nitrogenous breakdown products, J. Org. Chem. 67 (2002) 6143–6151. [37] A.G. Trovo, R.F.P. Nogueira, A. Aguera, A.R. Fernandez-Alba, S. Malato, Paracetamol degradation intermediates and toxicity during photo-Fenton treatment using different iron species, Water Res. 46 (2012) 5374–5380. [38] N.H.E. Najjar, A. Touffet, M. Deborde, R. Journel, N.K.V. Leitner, Kinetics of paracetamol oxidation by ozone and hydroxyl radicals, formation of transformation products and toxicity, Sep. Purif. Technol. 136 (2014) 137–143.

(2012) 131–140. [33] Z.P. Chen, S. Pronkin, T.P. Fellinger, K. Kailasam, G. Vilé, D. Albani, F. Krumeich, R. Leary, J. Barnard, J.M. Thomas, J.P. Ramírez, M. Antonietti, D. Dontsova, Merging single atom dispersed silver and Carbon Nitride to a joint electronic system via co-polymerization with silver tricyanomethanide, ACS Nano 10 (2016) 3166–3175. [34] S. Sakthivel, M.V. Shankar, M. Palanichamy, B. Arabindoo, D.W. Bahnemann, V. Murugesan, Enhancement of photocatalytic activity by metal deposition: characterisation and photonic efficiency of Pt, Au and Pd deposited on TiO2 catalyst, Water Res. 38 (2004) 3001–3008. [35] S. Imamura, M. Ikebata, Decomposition of ozone on a silver catalyst, Ind. Eng.

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