Accepted Manuscript Role of humic substances in the photodegradation of naproxen under simulated sunlight Yong Chen, Lu Liu, Jing Su, Jianfeng Liang, Bo Wu, Jiaolan Zuo, Yuegang Zuo PII:

S0045-6535(17)31333-4

DOI:

10.1016/j.chemosphere.2017.08.110

Reference:

CHEM 19808

To appear in:

ECSN

Received Date: 5 June 2017 Revised Date:

3 August 2017

Accepted Date: 20 August 2017

Please cite this article as: Chen, Y., Liu, L., Su, J., Liang, J., Wu, B., Zuo, J., Zuo, Y., Role of humic substances in the photodegradation of naproxen under simulated sunlight, Chemosphere (2017), doi: 10.1016/j.chemosphere.2017.08.110. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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1 2 Role of humic substances in the photodegradation of naproxen under simulated

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sunlight

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Yong Chena,*, Lu Liua, Jing Sua, Jianfeng Lianga, Bo Wua, Jiaolan Zuoa, Yuegang Zuob

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a

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Technology, Wuhan, 430074, China

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b

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North Dartmouth, MA 02747, United States

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School of Environmental Science and Engineering, Huazhong University of Science and

Department of Chemistry & Biochemistry, University of Massachusetts Dartmouth,

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Revised and Submitted to Chemosphere

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(August 3, 2017)

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*

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E-mail address: [email protected] (Y Chen)

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Corresponding author. Tel.: +86-27-87792406; fax: +86-27-87792101.

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ABSTRACT

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Humic substances (HS) including humic acid (HA) and fulvic acid (FA) are ubiquitous in

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the natural waters. Although numerous studies documented their role in photodegradation

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of organic pollutants, the competitive effects of photosensitization and light-screening of

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HS on the photodegradation of pollutants are not yet clear. In this work, the role of HS in

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the photodegradation of the pharmaceutical naproxen (NP) was studied under simulated

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sunlight. The direct photodegradation quantum yield of NP in deionized water was 2.1 ×

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10−2, and the apparent quantum yields for photosensitized degradation of NP in the

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presence of FA and HA were 2.3 × 10−4 and 2.6 × 10−5, respectively. Both direct and

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photosensitized photodegradation decreased with increasing pH, consistent with the trend

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of singlet oxygen (1O2) reaction rate constants of NP. HA inhibited the photodegradation

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of naproxen thoroughly. In contrast, FA accelerated the photodegradation of NP at lower

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substrate concentration and light intensity, and vice versa. Direct photodegradation of NP

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declined sharply with spectral radiation attenuation of UV region, when HS-mediated

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photosensitization predominantly accounted for the photodegradation. The direct

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photodegradation was ascribed to decomposition of excited triplet state of naproxen

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(3NP∗) and self-sensitization effect involving 1O2. The FA-mediated photodegradation

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was mainly attributed to 1O2 oxidation in aerated solution. These findings are important

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for assessing the competitive effects of humic substances on the photodegradation of

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pollutants under various conditions in natural waters.

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Keywords: Emerging contaminants; Humic acid; Fulvic acid; Photosensitization;

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Light-screening

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1. Introduction The naturally occurring humic substances are ubiquitous in surface waters (both fresh

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and marine waters), groundwaters, and soil porewaters where they play an essential role

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in numerous environmentally important processes (Zuo et al., 1997; Schmitt-Kopplin et

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al., 1998). Humic substances are yellowish-brown in color and have light absorption

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overlapped with the spectrum of terrestrial sunlight. It is well known that HS can undergo

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photochemical reactions to form hydroxyl radical (•OH) (Vaughan et al., 1998), singlet

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oxygen (1O2) (Haag and Hoigné, 1986), superoxide radical anion and its conjugate acid

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(O2•−/HO2•) (Goldstone and Voelker, 2000), H2O2 (Draper and Crosby, 1983), hydrated

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electron (eaq−) and the excited triplet state of humic substances (3HS∗) (Zepp et al., 1987).

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The formed •OH is readily quenched by HS itself (Hoigné et al., 1988), and eaq− is

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reactive to some electrophilic halogenated compounds (Burns et al., 1997). O2•−/HO2•

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and H2O2 act indirectly as important precursors of reactive transients, but their oxidation

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capabilities are weak and they hardly induce appreciable transformation of organic

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contaminants (Richard et al., 2005). Both 1O2 and 3HS∗ play an important role in the

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indirect photodegradation of pollutants in natural waters (Vione et al., 2014).

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The photosensitized degradation of pollutants in the presence of HS has already been

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reported (e.g. Boreen et al., 2003; Richard et al., 2005; Chen et al., 2009; Yan et al. 2014;

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Li et al., 2016). HS-mediated indirect photodegradation is considered a major attenuation

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pathway for pollutants that show little sunlight absorption (Oliveira et al., 2016; Silva et 4

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al., 2016a, 2016b). Mostly, HS act as inner filter and inhibit the direct photodegradation

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of organic pollutants (Vione et al., 2010). However, as mentioned above, HS are also

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capable to produce the reactive species such as 1O2 and 3HS∗, which accelerate the

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photodegradation. Thus, HS could play dual roles in the degradation of pollutants that are

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susceptible to direct photolysis. The total effect depends on the competition between

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light-screening and photosensitization. Due to different reaction conditions, there are

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many studies reporting contrasting roles of HS in photodegradation even for the same

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compounds (e.g. Han et al., 2009; Leech et al. 2009; Chowdhury, et al., 2010; Atkinson et

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al., 2011; Caupos et al.; 2011; Bao et al., 2014). Therefore, it is essential to evaluate the

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dual roles of HS in the photodegradation to rationalize the paradoxical conclusions.

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Different sources of HS may result in different photoreactivity (Batista, et al. 2016;

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Porras et al., 2016). Interestingly, we have found that the incident light intensity can also

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affect the dual roles of HS in the photodegradation of organic pollutant (Chen et al.,

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2013). However, the effect of other reaction parameters, including substrate concentration,

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solution pH, and spectral radiation attenuation is still unknown.

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Naproxen is a synthetic non-steroidal anti-inflammatory drug (NSAID) and frequently

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prescribed as an analgesic, antiarthritic, and antirheumatic. It has been widely detected in

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the effluents of wastewater treatment plants (WWTPs), surface waters, and even drinking

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water with concentrations ranging from ng L−1 to µg L−1 (Verenitch et al., 2006; Benotti

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et al., 2009; Lindholm-Lehto et al., 2016; Gumbi, et al., 2017). Naproxen is photolabile 5

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and associated with a high incidence of both photoallergic and phototoxic reactions due

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to the production of reaction oxygen species and more toxic photoproducts (DellaGreca

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et al., 2004; Isidori et al., 2005; Bracchitta et al., 2013). Several studies on the

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photodegradation of naproxen have been published (Bosca et al., 2001; Arany et al., 2013;

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Durán-Álvarez et al., 2015; Avetta et al., 2016; Vulava et al., 2016). Although the role of

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singlet oxygen is discussed, the photodegradation mechanism especially in HS solutions

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is not fully understood.

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The objective of this study is, therefore, to (i) determine quantum yields of direct and

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indirect photodegradation of naproxen, (ii) investigate the effect of naproxen

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concentration, incident light intensity, pH, and spectral radiation attenuation on the dual

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role of HS in the photodegradation of naproxen, (iii) probe into direct and indirect

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photodegradation mechanism of naproxen by quenching experiments.

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2. Materials and methods

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2.1. Chemicals

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Naproxen (99%), pyridine (PYR, 99%) and sorbic acid (99%) were purchased from

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Sigma. p-nitroanisole (PNA, > 98%) was supplied by TCI. Humic acids (C, 35.06%; H,

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3.77%; O, 60.11%; N, 1.06%) were obtained from Sigma. Fulvic acids (C, 51.04%; H,

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5.22%; O, 39.13%; N, 4.62%) were extracted from weathered coal by acetone-sulfuric

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acid method and supplied by Henan ChangSheng Corporation. 2-propanol (99%) and 6

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NaN3 (99%) were supplied by Wuhan chemicals corporation. All chemicals used were of at least analytical-reagent grade.

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2.2. Photodegradation experiments

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The photodegradation experiments were performed in a capped cylindrical Pyrex

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vessel (40 mm i.d., containing 50 mL of solution) with tubing in the cap to allow for

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bubbling solutions. The light source used was a 150-W Xenon Short Arc Lamp (Zolix

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Corporation, Beijing) and a cooling fan of the photochemical instrument controlled the

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temperature at 22 ± 1 °C. The light of wavelengths lower than 300 nm was filtered out

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with Pyrex glass to simulate sunlight. For the spectral radiation attenuation experiments,

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320, 340, 360, 380, and 400 nm filters were used to cut off the light lower than the

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corresponding wavelengths. Light intensity was controlled by the input of constant

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electric current and monitored using the p-nitroanisole/pyridine (PNA/PYR) actinometer

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(Dulin and Mill, 1982). Deoxygenation was achieved by bubbling nitrogen gas into the

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solutions all the time. In general, 5.0 µM NP was irradiated in the presence of FA and HA

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(10 mg L−1) at pH 7.0. Inhibition experiments were carried out with addition of 10 mM

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2-propanol, NaN3, and sorbic acid in the reaction solutions. All photodegradation

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experiments were conducted in phosphate (10 mM) buffered solutions. Aliquots of

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samples (∼300 µL) were withdrawn at various intervals and substrate decay was

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measured by HPLC. Three sequential replicates were included in all photodegradation

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experiments.

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2.3. Determination of quantum yields of photodegradation The quantum yield for direct photodegradation of NP was measured at pH 7.0 under

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simulated sunlight. The value was calculated according to Eq. (1) (Dulin and Mill, 1982).

φ
=


∑λ ελ  φ  ∑λ ελ


(1)

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where s and a represent the substrate and actinometer, respectively. k is the rate constant

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for direct photodegradation (s−1), Lλ values are lamp irradiance at a specific wavelength,

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obtained from the manufacturer, ελ values are the molar absorptivity of the substrate or

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actinometer (M−1 cm−1) (Fig. 1 and A1), φ is the quantum yield of photodegradation. The

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quantum

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wavelength-independent and calculated according to Eq. (2) (Leifer 1988).

yield

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p-nitroanisole/pyridine

actinometer

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(PNA/PYR)

The apparent quantum yields (φi) of indirect photodegradation of NP in FA and HA

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solutions were determined using a filter to cut off the light with wavelength lower than

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360 nm to exclude direct photodegradation of NP. The determination of apparent

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quantum yield has been described elsewhere as Eq. (3) (Chen et al., 2009).

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φ =

  × ∑ (1 − 10

!" #$ )

(3)

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where r is the photosensitized degradation rate in the HS solution, Ir is the incident light

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intensity expressed in Einstein L−1 s−1 which is obtained by actinometry, Fλ is the spectral

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distribution of the light emitted by the lamp, ελ is the absorption coefficient (L mg−1 cm−1)

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of HS at specific wavelength, b is the path length (cm), and c is the concentration of HS 8

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(mg L−1).

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2.4. Determination of 1O2 reaction rate constant Bimolecular reaction rate constants between 1O2 and NP at different pH were

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determined using Rose Bengal method (Latch et al., 2003). The detailed procedures were

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shown in Supplementary information.

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2.5. Analytical procedures

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UV-vis spectra were recorded on a Hitachi U3100 spectrophotometer. Carbon,

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hydrogen, nitrogen, and oxygen contents of HA and FA were determined using a Vario

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Micro cube Elemental Analyzer (Germany). The concentrations of NP and PNA were

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determined by Shimadzu Essential LC-15C HPLC system with Agilent HC-C18 column

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(5 µm, 250 mm × 4.6 mm). The mobile phase of NP was a mixture of methanol and pH

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2.5 phosphate buffer solutions (68: 32, v/v). For PNA, the mobile phase was a mixture of

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acetonitrile and H2O (50: 50, v/v). The flow rate was 1 mL/min. The detection

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wavelength for NP and PNA was 232 and 300 nm, respectively.

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3. Results and discussion

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3.1. Quantum yields of photodegradation of NP

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As shown in Fig. A2, the photodegradation of NP followed pseudo-first-order kinetics

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and the rate constant of direct photodegradation was 4.6 × 10−3 min−1 in aqueous solution.

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The quantum yield of PNA was 5.7 × 10−3. According to Eq. (1), the quantum yield of 9

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direct photodegradation of NP was calculated to be 2.1 × 10−2, which was similar to the

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reported values (Packer et al., 2003; Marotta et al., 2013). The pseudo-first-order rate

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constants of NP photodegradation were 8.4 × 10−3 and 5.9 × 10−4 min−1 in FA and HA

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solutions. The incident light intensity determined by the PNA/PYR actinometer was 8.7 ×

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10−8 Einstein L−1 s−1. Combined with the UV-vis absorption of the solutions (Fig. A3), the

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apparent quantum yields of indirect photodegradation of NP in FA and HA solutions were

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calculated to be 2.3 × 10−4 and 2.6 × 10−5, respectively. The values were similar to the

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ones of amine drugs in the same system (Chen et al., 2009). The quantum yield of

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indirect photodegradation in FA solution was approximately one order of magnitude

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higher than that of HA, indicating that FA exhibits stronger photoreactivity compared to

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HA.

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3.2. Effect of initial concentration of NP

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As shown in Fig. 1, NP has obvious light absorption at wavelength above 300 nm,

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overlapped with the spectrum of simulated sunlight. There was a characteristic absorption

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peak at 330 nm with molar extinction coefficient 2258.6 M−1 cm−1. Generally, the light

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absorption was of medium intensity at λ > 300 nm. Fig. 2 illustrates the direct

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photodegradation of NP and the effect of HA and FA at different substrate concentration.

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The pseudo-first-order rate constant of direct photodegradation increased from 0.010 to

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0.027 min−1 when the concentration range increased from 2.0 to 20.0 µM. Similar

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phenomena were found in previous study (Arany et al., 2013). Fig. 2 shows that the addition of HA markedly inhibited the degradation of NP within

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the concentration range of 2.0 to 20 µM, and the inhibition increased with increasing

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concentration of HA (Fig. A4). Likewise, the presence of FA inhibited the

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photodegradation at high concentration of NP, e.g., 10.0 and 20.0 µM. The inhibition

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effect of HS was in agreement with the literature (Packer et al., 2003; Aydin 2015; Avetta

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et al., 2016; Koumaki et al., 2015; Vulava et al., 2016). Interestingly, when the

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concentration of NP decreased to less than 10.0 µM, the presence of FA accelerated the

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photodegradation noticeably. Fig. A4 further shows that the enhancement effect increased

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with increasing concentration of FA at low NP concentration. According to the

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Stark-Einstein law, the photodegradation rate of NP in absence (rdλ) and presence (riλ) of

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HS was given by Eq. (3) and (4) (Leifer, 1988),

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rdλ = 2.303φdλIλFλελcl

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riλ = IλFλ(1 − 10(−(α c′ + ε c)l))(φdλελc + φiλαλc′)/(αλc′ + ελc)

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λ

λ

(3) (4)

where φ is the photodegradation quantum yield of NP, Iλ is the intensity of incident light,

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αλ and ελ are the absorption coefficient (L mg−1 cm−1) of HS and molar extinction

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coefficient (M−1 cm−1) of NP at wavelength λ, c and c′ are the concentration of NP and

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HS, respectively. The absorption of HS was far higher than that of NP, so the Eq. (4) can

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be simplified to

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riλ = IλFλ(1 − 10(−α c′l)){(φdλελc/αλc′) + φiλ} λ

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(5)

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Thus, the ratio of riλ to rdλ was obtained by Eq. (3) and (5) as follows

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riλ/rdλ = (1 − 10(−α c′l)){1/(2.303αλc′l) + φiλ/(2.303φdλελcl)}

(6)

λ

Eq. (6) indicates that when the concentration of HS is constant, the ratio is inversely

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proportional to the concentration of NP. Therefore, increase of NP concentration

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decreases the ratio of riλ to rdλ. When the ratio is less than one, inhibition will occur in the

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presence of HS. The direct photodegradation of NP declines to nearly zero at the

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wavelength 340 nm due to the negligible light absorption, so competition between

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light-screening and photosensitization only occurs within the wavelength of 300 to 340

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nm. Fig. 3 illustrates the fitting of Eq. (6) according to the determined quantum yields

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and spectra of NP and HS. As shown in Fig. 3a, the ratios of riλ/rdλ are less than one for

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HA in the concentration range of 2.0 to 20 µM due to the low apparent quantum yield (φi),

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which interprets the inhibition effect of HA on the photodegradation. Similar effect was

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also found for the presence of FA at higher concentrations, e.g., 10.0 and 20.0 µM (Fig.

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3b). In contrast, at lower concentration of 2.0 µM, the ratio of riλ/rdλ is higher than one

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within the wavelength of 300 to 340 nm. Although partial values of ratio are lower than

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one for the concentration of 5.0 µM, especially in the absorption peak (330 nm) of NP,

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the higher values in the other wavelengths offset it and thus enhancement was observed.

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The fitting of Eq. (6) was in good agreement with experimental results. Therefore,

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photoscreening vs. photosensitization was related to NP concentration and the

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photoreactivity of HS. The real concentration of NP is far lower in natural waters, so it is

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expected that FA acts mainly as photosensitizer for the photodegradation under sunlight.

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The photodegradation rate of pollutants is readily controlled by light intensity. As

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shown in Fig. 4, the direct and indirect photodegradation of NP increased with increasing

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light intensity. The addition of HA inhibited the photodegradation of NP thoroughly. In

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contrast, FA enhanced the photodegradation of NP under lower irradiation. An interesting

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phenomenon was that an inhibition effect was found when the light intensity increased to

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1.0 × 10−7 Einstein L−1 s−1. The direct photodegradation of NP in deionized water became

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faster compared to that in the presence of FA. This was consistent with the report that the

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increase of light intensity led to the role change of HA from enhancement to inhibition

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during the other pollutant photodegradation (Chen et al., 2013). This phenomenon is a

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little puzzling because irradiation intensity with the same light source appears to show no

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effect on the ratio of riλ/rdλ according to Eq. (6). It is likely due to the decrease of ratio of

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φi/φd with increasing light intensity since the quantum yields are considered only

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relatively constant, especially for apparent quantum yield (φi). According to Eq. (6), when

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the decrease of φi/φd value becomes noticeable enough, it probably results in the decrease

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of ratio of riλ/rdλ to less than one finally. Accordingly, light intensity was also important

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factor affecting the dual roles of HS for photodegradation of pollutant.

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3.4. Effect of spectral radiation attenuation A series of filters were used to simulate the spectral radiation attenuation caused by

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chromophoric dissolved organic matter. The UV region radiation of simulated sunlight

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was gradually cut off from 320 to 400 nm by filters. The light absorption of NP at 340 nm

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is very weak with absorbance of 0.0016 and it has little absorption at 360 nm (Fig. 1).

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Accordingly, the direct photodegradation rate of NP alone decreased sharply with the use

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of 340 nm filter (cut-off below 340 nm) and nearly declined to zero at 360 nm (Fig. 5).

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The photodegradation in the presence of FA and HA also decreased but not as fast as the

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degradation in deionized water. The photodegradation was predominantly attributed to

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the photosensitization effect under irradiation of light above 340 nm in the presence of

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FA and HA. In natural waters, light attenuation exponentially decreases with increasing

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wavelength due to the absorption by chromophoric dissolved organic matter, which has

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great implications for the penetration of UV and visible radiation of sunlight in water

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columns (Markager et al., 2000; Marchisio et al., 2015). The spectral radiation

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attenuation renders photosensitization become a more important pathway for degradation

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of pollutants in underwater.

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3.5. Effect of pH

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As shown in Fig. 6, the photodegradation of NP in the absence and presence of HS

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decreased with increasing pH. FA and HA exhibited photosensitization and 14

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photo-screening effect in the pH range of 3.0 to 9.0, respectively. The photodegradation

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of NP all decreased in deionized water and HS solutions. It has already been reported that

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low pH facilitates the decomposition of NP (Sokol et al., 2017). The decrease was

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especially pronounced at neutral and alkaline conditions in the presence of FA. The

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UV-vis spectra of NP show negligible variation at different pH (Fig. A5), so the effect of

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pH was not related to the change of spectra. The pKa of NP was reported to be 4.2 (Tülp

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et al., 2009), and thus NP exists as protonated and depronated forms at pH 3.0 and 5.0

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while it is almost fully deprotonated at pH 7.0 and 9.0. Accordingly, the deprotonation of

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NP is unfavorable to the direct and indirect photodegradation. As discussed in the

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mechanism section, singlet oxygen (1O2) plays important role in the photodegradation of

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NP. The bimolecular rate constants for the interaction between naproxen and the 1O2 at

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pH 3.0, 5.0, 7.0, 9.0 were determined to be (1.4 ± 0.1) × 107, (9.0 ± 0.4) × 106, (6.6 ± 0.3)

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× 106, (5.4 ± 0.2) × 106 M−1 s−1, respectively (Fig. A6). The 1O2 reaction rate constants of

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NP also decreased with increasing pH, indicating that pH effect in aerated solution was

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probably attributed to the role of singlet oxygen.

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3.6. Quenching experiments for photodegradation of NP

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A series of quenching experiments were conducted to investigate the direct and indirect

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photodegradation mechanism of NP. To exclude the effect of direct photodegradation, the

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light below 360 nm was cut off for the examination of FA-mediated indirect 15

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photodegradation. As shown in Table 1, deoxygenation facilitated both direct and indirect

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photodegradation of NP. Compared to photodegradation in air-saturated solution, the

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reaction rate increased by 6.3- and 20.6-fold in the deionized water and FA solution,

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respectively. It is well known that oxygen is the quencher of excited triplet states of

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organic compounds (Halladja et al., 2007). Accordingly, the direct and indirect

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photodegradation was related to the excited triplet state of NP (3NP∗) and FA (3FA∗) in the

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photosystem. Sorbic acid is also a quencher of excited triplet state (Grebel et al., 2011),

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and it was added in the reaction solutions for further insight into the photoreaction

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mechanism. As shown in Table 1, for direct photodegradation, the addition of sorbic acid

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almost inhibited the photodegradation of NP in the N2-saturated solution, whereas the

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presence of •OH quencher 2-propanol showed little effect. Accordingly, the degradation

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was ascribed to the decomposition of 3NP∗ in the N2-saturated solution. Compared to the

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deoxygenation condition, the addition of both 1O2 quencher NaN3 and sorbic acid

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markedly inhibited the photodegradation of NP in the air-saturated solution. Likewise, the

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addition of 2-propanol showed little effect. Therefore, the direct photodegradation in the

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air-saturated solution was related to

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photodegradation of NP produced 1O2 and O2•−, of which 1O2 participated the direct

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photodegradation process to induce the self-sensitization degradation (Moore et al., 1988;

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Bosca et al., 2001; Musa et al., 2008; Bracchitta et al., 2013; Marotta et al., 2013; Avetta

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et al., 2016). Similar photodegradation process was also observed for the degradation of

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1

O2 and

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NP∗. It was reported that the

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other pollutants in the previous work (Chen et al., 2009 and 2012). In the FA solution, indirect photodegradation was the sole pathway for the degradation

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of NP since the light below 360 nm was cut off (Fig. 5). As shown in Table 1, the addition

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of 2-propanol exhibited little effect on the photodegradation of NP in the air-saturated

300

solution, indicating that •OH had negligible effect in this system. In contrast, the addition

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of NaN3 almost inhibited the photodegradation. Thus, the indirect photodegradation was

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predominantly ascribed to 1O2 produced in the FA solution. In N2-saturated solution, the

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role of 1O2 was excluded since no dissolved oxygen existed in the system. Likewise, the

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addition of 2-propanol showed little effect on the photodegradation, suggesting •OH

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played negligible role in the photodegradation of NP. In both air- and N2-saturated

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solutions, the addition of sorbic acid did not showed appreciable effect on the

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photodegradation of NP in the presence of FA. The ineffective quenching for 3FA∗ was

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possibly attributed to the more rapid reaction between 3FA∗ with dissolved oxygen or NP

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in the solution. Therefore, direct photodegradation of NP was attributed to 3NP∗ and 1O2,

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and FA-mediated indirect photodegradation were related to 1O2 in aerated solution, while

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3

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solutions, respectively.

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NP∗ and 3FA∗ were responsible for direct and indirect photodegradation in deoxygenated

313 314 315

4. Conclusions The photodegradation rate of NP depended on initial NP concentration, pH, incident 17

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light intensity, and spectral radiation attenuation. The increase of initial concentration,

317

incident light intensity facilitated the direct photodegradation in deionized water. The

318

direct photodegradation of NP decreased sharply with the spectral radiation attenuation in

319

the UV region, whereas photosensitization continued to act as significant role in the

320

degradation. Both direct and HS-mediated indirect photodegradation rates decreased with

321

increasing pH. The bimolecular rate constants for the interaction between NP and the 1O2

322

at pH 3.0, 5.0, 7.0, 9.0 were (1.4 ± 0.1) × 107, (9.0 ± 0.4) × 106, (6.6 ± 0.3) × 106, (5.4 ±

323

0.2) × 106 M−1 s−1, respectively. Due to light-screening effect, HA inhibited the

324

degradation of NP thoroughly at different substrate concentrations and light intensity. In

325

contrast, the inhibition or enhancement depended on the reaction parameters in the

326

presence of FA. When the direct photodegradation was rapid enough, FA acted as

327

inhibitor in the photosystem. In contrast, when the reaction parameter was unfavorable

328

for the direct photodegradation, such as decrease of substrate concentration and incident

329

light intensity, the presence of FA accelerated the photodegradation. The spectral

330

radiation attenuation decreased the photodegradation of NP sharply, whereas HS

331

continued to act as significant role in the degradation. In aerated solutions, the direct

332

photodegradation of NP was attributed to

333

photosensitized degradation was ascribed to 1O2.

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334 335

Acknowledgments 18

NP∗ and

3

1

O2, and the FA-mediated

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This work was supported by the National Natural Science Foundation of China

337

(21377043 and 21677054).

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338 339

Appendix A. Supplementary data

340

Supplementary data associated with this article can be found in the online version.

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341 References

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Captions for Table and Figures

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Fig. 1. UV-vis absorption of NP and the relative intensity of the 150-W Xenon Short Arc

512

Lamp.

513

Fig. 2. Effect of initial concentration of NP on the photodegradation in the absence (a),

514

presence of FA (b) and HA (c) (10.0 mg/L) at pH 7.0.

515

Fig. 3. Rate ratios of photodegradation in presence to absence of HA (a) and FA (b) at

516

different NP concentrations. The light of wavelength lower than 300 nm was cut off by

517

Pyrex glass.

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Fig. 4. Effect of light intensity on the photodegradation of NP (5.0 µM) in the absence (a),

519

presence of FA (b) and HA (c) (10.0 mg/L) at pH 7.0.

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Fig. 5. Effect of spectral radiation attenuation on the photodegradation of NP (5.0 µM) in

521

the absence (a), presence of HA (b) and FA (c) (10.0 mg/L) at pH 7.0. The light is cut off

522

below the corresponding wavelength.

523

Fig. 6. Effect of pH on the photodegradation of NP (5.0 µM) in absence (a), presence of

524

HA (b) and FA (c) (10.0 mg/L).

525

Table 1 The pseudo-first-order rate constants kobs (h−1) for photodegradation of NP under

526

different conditions in deionized water and FA solution, respectively.

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Table 1 The pseudo-first-order rate constants kobs (h−1) for photodegradation of NP

7

under different conditions in deionized water and FA solution, respectively.

9.31 ± 0.19

N2 + 2-propanol

3.64 ± 0.17

9.20 ± 0.22

N2 + sorbic acid

0.103 ± 0.004

9.24 ± 0.30

air

0.57 ± 0.05

0.45 ± 0.02

air + NaN3

0.23 ± 0.01

0.020 ± 0.001 0.43 ± 0.01

0.24 ± 0.01

0.32 ± 0.01

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air + sorbic acid

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FA solution

N2

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Deionized water

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3.5

1.0 0.5 0.0 200

OH

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ε/ 10 ( M cm )

O

2.5

300

400

500

40

30

Relative intensity

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3.0

2.0

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20 600

Wavelength (nm)

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Fig. 1. UV-vis absorption of NP and the relative intensity of the 150-W Xenon Short

19

Arc Lamp.

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24 0.030

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b

0.005 2

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c

8 10 12 14 16 18 20 22

Concentration (µΜ)

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Fig. 2. Effect of initial concentration of NP on the photodegradation in the absence (a),

27

presence of FA (b) and HA (c) (10.0 mg/L) at pH 7.0.

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(a)

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2.0 µ M 5.0 µM 10.0 µM 20.0 µM

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ri λ/rλd

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0.0 250 260 270 280 290 300 310 320 330 340

Wavelength (nm)

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(b)

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2.0 µM 5.0 µM 10.0 µM 20.0 µM

1.5 1.0 0.5

Wavelength (nm)

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35

Fig. 3. Rate ratios of photodegradation in presence to absence of HA (a) and FA (b) at

36

different NP concentrations. The light of wavelength lower than 300 nm was cut off

37

by Pyrex glass.

38

4

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0.013 0.012 0.011 0.010 0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002

a

b

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k ob s (min )

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c

7.00E-008 8.00E-008 9.00E-008 1.00E-007 -1

-1

Intensity (Einstein L s )

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Fig. 4. Effect of light intensity on the photodegradation of NP (5.0 µM) in the absence

47

(a), presence of FA (b) and HA (c) (10.0 mg/L) at pH 7.0.

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340

360

380

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Light absorption (nm)

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Fig. 5. Effect of spectral radiation attenuation on the photodegradation of NP (5.0 µM)

54

in the absence (a), presence of HA (b) and FA (c) (10.0 mg/L) at pH 7.0. The light is

55

cut off below the corresponding wavelength.

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0.025 0.020 0.015

a

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3

4

5

6

pH

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Fig. 6. Effect of pH on the photodegradation of NP (5.0 µM) in absence (a), presence

61

of HA (b) and FA (c) (10.0 mg/L).

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► HA inhibited the photodegradation of naproxen thoroughly. ► FA acted as dual roles in the photodegradation of naproxen (NP). ► 3NP∗ and 1O2 account for direct photodegradation of naproxen.

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► FA-mediated photodegradation was attributed to 1O2 in aerated solution.