Environ Sci Pollut Res (2014) 21:7797–7804 DOI 10.1007/s11356-014-2721-2

RESEARCH ARTICLE

Photodegradation of naproxen in water under simulated solar radiation: mechanism, kinetics, and toxicity variation Dujuan Ma & Guoguang Liu & Wenying Lv & Kun Yao & Xiangdan Zhang & Huahua Xiao

Received: 17 October 2013 / Accepted: 28 February 2014 / Published online: 19 March 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The main objective of this study was to investigate the degradation mechanism, the reaction kinetics, and the evolution of toxicity of naproxen in waters under simulated solar radiation. These criteria were investigated by conducting quenching experiments with reactive oxygen species (ROS), oxygen concentration experiments, and toxicity evaluations with Vibrio fischeri bacteria. The results indicated that the degradation of naproxen proceeds via pseudo first-order kinetics in all cases and that photodegradation included degradation by direct photolysis and by self-sensitization via ROS; the contribution rates of self-sensitized photodegradation were 1.4 %, 65.8 %, and 31.7 % via ·OH, 1O2 and O2•−, respectively. Furthermore, the oxygen concentration experiments indicated that dissolved oxygen inhibited the direct photodegradation of naproxen, and the higher the oxygen content, the more pronounced the inhibitory effect. The toxicity evaluation illustrated that some of the intermediate products formed were more toxic than naproxen. Keywords Naproxen . Simulated solar radiation . Mechanisms . Toxicity . Kinetics

Introduction In recent years, the literature has increasingly reported that pharmaceuticals and personal care products (PPCPs) have been frequently detected as emerging pollutants in potable Responsible editor: Philippe Garrigues D. Ma : G. Liu (*) : W. Lv : K. Yao : X. Zhang : H. Xiao School of Environmental Science and Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Panyu District, Guangzhou 510006, People’s Republic of China e-mail: [email protected]

water, surface water, groundwater (Andreozzi et al. 2003; Boyd et al. 2003; Vieno et al. 2007), and soil (Xu et al. 2009). In many cases, the concentrations of pharmaceuticals are reduced during the wastewater treatment process through microbial degradation or adsorption into activated sludge. However, some concentration remains, and these residuals are discharged into various aquatic environments in the form of the original drugs or metabolic intermediates (Cunningham et al. 2006; Kümmerer 2009). Accordingly, the importance of emerging environmental pollution by PPCPs and their metabolites in aquatic environments has raised increasing concern. Naproxen (NP) is a synthetic non-steroidal antiinflammatory drug (NSAID) and is mostly used in its sodium salt form in medical care as an analgesic, antiarthritic, and antirheumatic. Presently, owing to its minor side effects and excellent tolerance, NP becomes one of the best-selling OTCs in the world (DeArmond et al. 1995; Ekman 2012). The chemical characteristics of NP are described in Table 1. NP enters the aquatic environment by direct disposal in households or in medical care facilities and via excretion of human and animal wastes; it has been found in STP effluents in concentrations ranging from 0.1 ng·L−1 to 7.69 μg·L−1 (Verenitch et al. 2006) and in surface waters at concentration levels as high as 250 ng·L−1 (Bendz et al. 2005). Furthermore, NP has also been detected in drinking water (Hernando et al. 2006, Benotti et al. 2009). Its presence at low or high concentrations could bring about harmful toxicological consequences. For instance, people who ingest trace amounts of NP for a long time may have a higher risk of having a heart attack or a stroke than people who are not exposed to this medication (Ali et al. 2011; Manrique-Moreno et al. 2010). Because STPs are not sufficiently effective to eliminate pharmaceutical compounds such as NP from their effluent, new techniques are being tested to diminish NP concentrations in treated wastewater (Boyd et al. 2005; Hasan et al.

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Table 1 Chemical characteristics of NP Drug

CAS

Naproxen

22204-53-1

a

Chemical structure

MW

LogKow

pKa

water solubility (mg/L at 25 °C)

230.26

3.18a

4.15a

15.9a

Data from (Trenholm et al. 2006)

2013, Kim et al. 2009, Méndez-Arriaga et al. 2008). Photochemical degradation may be the most important loss mechanism for pharmaceuticals in surface water. Thus, knowledge of this process is essential to understanding the persistence of PPCPs (Chen et al. 2012; García-Galán et al. 2012; Ge et al. 2009; Zhang et al. 2011) and predicting their environmental fate and the risk of long-term exposure. The phototransformation of NP in aqueous solutions under UV radiation has been recently investigated (Boscá et al. 1990, Kim et al. 2009, Marotta et al. 2013, Méndez-Arriaga et al. 2008, Molinari et al. 2006). The ecotoxicity of NP and its photoproducts was assayed on aquatic organisms, including bacteria, microcrustaceans, and algae (DellaGreca et al. 2003, Isidori et al. 2005, Méndez-Arriaga et al. 2008). The reported results indicated that NP could be degraded effectively under UV radiation, and two suitable kinetic models (in the absence and in the presence of oxygen) were developed to simulate the system’s behavior. Furthermore, some photoproducts were more ecotoxic than NP, both under acute and chronic conditions, whereas genotoxic and mutagenic effects were not found. It was reported that certain organic pollutants absorbed photons and transferred energy as electrons to other chemicals with the formation of reactive oxygen species (ROS), which subsequently oxidized and degraded the pollutants. This process was designated as the self-sensitized photodegradation of pollutants (Chen et al. 2009; Cogan and Haas 2008; Ge et al. 2010; Zhan et al. 2006, 2011). Although some degradation processes of NP under UV radiation have been developed, the reaction schemes and the kinetics of its photodegradation under simulated solar radiation have not been described, a study aims to determine whether NP underwent selfsensitization photodegradation or whether the generated ROS initiated the degradation of NP under simulated solar radiation, which is of practical significance for understanding the environmental fate and transformation of naproxen in an aquatic environment and assessing its environmental effect. In this study, NP was selected as a model compound to investigate its photodegradation mechanism and kinetics in aqueous solution under simulated solar irradiation. For this purpose, a quenching experiment, a dissolved oxygen concentration experiment, and a kinetic model for NP degradation were used to test whether NP

underwent self-sensitization photodegradation. The toxicity evolution of the photodegradation process was investigated.

Materials and methods Materials Naproxen (99 %) was purchased from Sigma-Aldrich Company. HPLC-grade methanol and acetonitrile were obtained from Shanghai ANPEL Scientific Instrument Co., Ltd. (Shanghai, China). Sodium azide (99 %) and p-quinone (99 %) were obtained from Adamas Reagent Co., Ltd. (Switzerland). Acetic acid (99.5 %), isopropanol (99.7 %), and formic acid (88 %) were all obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (Sichuan, China). All of the reagents used were of analytical grade without further purification. Ultra-pure water (resistance>18.2 MΩ) produced by a water purification system (TKA, Germany) was used for preparing all aqueous solutions. Vibrio fischeri was obtained from the Institute of Soil Science, Chinese Academy of Sciences (Nanjing, China). Preparation of NP solution An NP stock solution of precisely (1 g·L−1) was prepared in methanol and stored in a brown glass volumetric flask at 4 °C in the refrigerator. Then an adjustable pipette was used to add 5 mL NP stock solution dropwise into the volumetric flask with approximately 250 mL of ultra-pure water under ultrasonic vibration conditions. A 10 mg·L−1 reaction solution of NP was prepared by diluting this flask to a volume of 500 mL. The pH value was then adjusted to 4 using sulfuric acid and sodium hydroxide. Photodegradation experiments An SGY-IIB.Y1 rotary photochemical reactor (Nanjing STO Co. Ltd., China) (Fig. 1) and a solar-simulated light source (350 W xenon lamp with 290 nm filter, λ>290 nm) were used to perform the photodegradation experiments in a chamber. A 20 mL reaction solution was placed into a 25 mL quartz tube.

Environ Sci Pollut Res (2014) 21:7797–7804

Inlet of cool water

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Outlet of cool water

Pseudo first-order kinetics is generally used to express the photodegradation of a micropollutant (Liu et al. 2009). The rate constant k was calculated from the first-order equation:

Circulating water well

Quartz tube

Kinetics of NP degradation

Light source

Light filter

Fig. 1 Rotary photochemical reactor

By adopting a double-walled quartz cooling jacket and an additional miniature air-cooling system, reaction solution temperatures were kept constant at about 25 °C during irradiation. Irradiance was measured with a full-spectrum type optical power meter (CEL-NP2000, Beijing China Education Aulight Co., Ltd.), by which the total power was checked to ensure irradiance consistency during each step of the experiment. Irradiances were measured at 99.6–104.2 mW·cm−2 for the full-spectrum. Throughout all experiments, an aliquot was taken out every 10 min from the rotary photochemical reactor and immediately analyzed using a reversed-phase high-performance liquid chromatography (HPLC) system, which consisted of two LC-20AT Binary HPLC pumps and a diode array detector. Dark controls were performed simultaneously under the same conditions. Each of the experiments was carried out at least in triplicate, and the results are reported as the mean± 95 % confidence interval when available.

dC ¼ −kC dt

ð1Þ

Where C is the concentration of NP, k is the rate constant, and t is the reaction time. By integrating Eq. (1), the following equation was obtained: ln

Ct ¼ −kt C0

ð2Þ

Where Ct is the NP concentration at time t and C0 is the initial concentration of NP. Furthermore, when the concentration of NP is reduced to 50 % of its initial concentration, the half-life (t1/2) can be calculated from the rate constant as shown in the following equation: t 1=2 ¼

ln2 k

ð3Þ

Acute toxicity test To indicate the toxicity variation during the photodegradation of NP, a 15-min Microtox bioassay using V. fischeri was carried out according to the Chinese national standard test method GB/T 15441-1995 with the following modification: A LuminMax-C luminometer was used to quantify the luminescence intensities. The luminescence inhibition rate (I%) was calculated as follows (I=luminescence).   I sample I% ¼ 1−  100% ð4Þ I blank Results and discussion

Analytical methods The concentrations of the NP solutions were determined using an HPLC system, which consisted of two LC-20AT Binary HPLC pumps and a diode array detector (Shimadzu, Japan). The analytical column (ZORBAX Eclipse XDB-C18 column, 2.1 mm×150 mm, particle size 5 μm) and guard column (ZORBAX Eclipse XDB-C18, 2.1×12.5 mm, particle size 5 μm) were both purchased from Agilent, USA. The column temperature was 40 °C, and the injection volume was 4 μL. An isocratic elution was used with a mobile phase of 50:50 (v:v) HPLC-grade acetonitrile and ultra-pure water (containing 0.3 % acetic acid, pH 3.0) with a flow rate of 0.2 mL min−1. The detection wavelength was set at 254 nm.

Effect of initial NP concentration on the photodegradation of NP The degradation percentage η (%) of NP is calculated by the Eq. (5): η ¼ ðC 0 −C Þ=C 0  100%

ð5Þ

The effect of different initial concentrations of NP on the degradation of NP in the presence of simulated solar radiation or in the dark control was investigated (Fig. 2). It was shown

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According to the literature (Boreen et al. 2008), we can estimate the contribution of ·OH, 1O2, and O2•− to the overall photodegradation of NP as follows:

1.0

A B C D E F G H I J

[NP]/[NP]0

0.8

0.6

0.4

k •OH k−k isopropanol ≈ k k

ð6Þ

k 1O2 k isopropanol −k NaN3 ≈ k k

ð7Þ

R•OH ¼

R1O2 ¼

0.2

0.0 0

10

20

30

40

50

60

Time(min)

Fig. 2 Photodegradation of five different initial NP concentrations under dark and light conditions. Light: a [NP]=10 mg·L−1; b [NP]=20 mg·L−1; c [NP]=30 mg·L−1; d [NP]=40 mg·L−1; (e) [NP]=50 mg·L−1. Dark: f [NP]=10 mg·L−1; (g) [NP]=20 mg·L−1; (h) [NP]=30 mg·L−1; (i) [NP]= 40 mg·L−1; (j) [NP]=50 mg·L−1

that NP could hardly be decomposed in the absence of simulated solar radiation. In contrast, more than 90 % of NP was degraded in 50 min under simulated solar radiation (350 W xenon lamp), which suggested that the degeneration of NP was the result of photon absorption. Experiments conducted to examine the plots of [NP]/[NP]0 versus time for initial NP concentrations of 10, 20, 30, 40, and 50 mg·L−1, resulted in degradation rates of 96.5, 95.8, 94.2, 93.1, and 91.7 %, respectively. The degradation rate constant of NP increased with decreasing initial concentrations of NP, which could be explained by considering the competition for absorption of the limited quantity of available photons by the NP. As the initial concentration of NP increased, the number of available photons did not change, leading to a decrease of the number of photons available per molecule of NP. Therefore, the degradation rate of NP decreased.

RO2 •−

k O2 •− k−k p quinone ≈ k k

ð8Þ

Where R•OH, R1O2 , and RO•−2 are the contribution rates of self-sensitized photodegradation via ·OH, 1O2, and O2•−, respectively; k•OH, k 1O2 , and k O •− are the rate constants of self2

sensitized photodegradation via ·OH, 1O2, and O2•−, respectively; kisopropanol, kNaN3, and kp-quinone are the rate constants for the addition of isopropanol, sodium azide, and p-quinone in water. k is the rate constant of photodegradation of NP alone. It was calculated that R•OH is 1.4 %, R1O2 is 65.8 %, and Ro2 •− is 31.7 %. One may infer that the following processes occur during NP photodegradation: (1) NP absorbs photons and forms an excited NP (singlet (1NP) and triplet (3NP)), followed by direct photodegradation; (2) excited NP transfers energy to dissolved oxygen in water with the formation of reactive oxygen species, which subsequently cause the self-sensitized photooxidation of NP. The photodegradation rate constant (k), the half-life period (t1/2), and R2 are listed in Table 2. Time(min) 0

10

20

30

40

50

60

0.5

Mechanisms and kinetic models for the photodegradation of NP

0.0 -0.5 -1.0

ln[NPX]/[NPX0]

Quenching experiments were used to test whether NP underwent self-sensitization via reactive oxygen species (ROS). Figure 3 indicated that the addition of isopropanol (·OH quencher) (Buxton et al. 1988), sodium azide (1O2 and·OH quencher) (Miolo et al. 2002) and p-quinone (O2•− quencher) (Chen et al. 2008) each produced an inhibition effect on the NP degradation. Isopropanol had little effect on the degradation of NP. In contrast, sodium azide and p-quinone showed significant inhibition effects. These experimental results indicated that the photodegradation of NP might be included a self-sensitization photooxidation process via ·OH, 1 O2, and O2•− under simulated sunlight.

-1.5 -2.0 -2.5

NP Isoprpanol NaN3

-3.0

p-quinone

-3.5 -4.0

Fig. 3 Effects of isopropanol (100 mmol·L−1), NaN3 (5 mmol·L−1), and p-quinone (5 mmol·L−1) on photodegradation kinetics of NP (C0 = 10 mg·L−1) under simulated sunlight

Environ Sci Pollut Res (2014) 21:7797–7804

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Table 2 Photodegradation kinetic parameters of NP (C0 =10 mg·L−1) in different aqueous solutions Solution conditions

k/(min−1)

R2

t1/2/ (min)

NP Isopropanol NaN3 p-quinone

0.0643±0.005 0.0634±0.007 0.0211±0.004 0.0439±0.003

0.968~0.983 0.960~0.976 0.976~0.990 0.985~0.998

10.8±0.1 10.9±0.1 32.8±0.6 15.7±0.1

N2 O2

0.0714±0.003 0.0384±0.004

0.986~0.999 0.980~0.993

9.7±0.04 18.1±0.2

1

To take O2 as an example, the proposed pathway for the photolysis of NP is shown in Fig. 4. From Fig. 4, one can obtain a simplified reaction scheme as follows: hv;Ia; ϕ

→ NPX 

NPX NPX 

k1



3

→ Aproduct k2

NPX þ O2

1

1

O2

k3

ð9Þ

k4

      d ½1 O 2 Š ¼ k 2 ½NPX Š 3 O2 −k 4 ½NPXŠ 1 O2 −k 3 1 O2 ¼ 0ð17Þ dt

1

 k 2 ½NPX Š½3 O2 Š O2 ¼ k 3 þ k 4 ½NPXŠ

Because the value of k3 for 1O2 is 2.5×105 s−1 (Buxton et al. 1988), k3 will be much greater than k4. The initial concentration of NP in this study was 10 mg·L−1 (i.e., 0.043 mM); thus, k3 >>k4 [NP], and Eq. (18) can be simplified to: 1

 k 2 ½NPX Š½3 O2 Š O2 ¼ k3

→ NPX þ O2 1

ð11Þ

According to the Beer-Lambert law,   I a ¼ I 0 1−e−εb½NPXŠ

ð19Þ

ð20Þ

ð12Þ

→ Bproduct

ð13Þ

Because the initial concentration of NP in this study was 10 mg·L−1 (0.043 mM) and εb [NPX]

Photodegradation of naproxen in water under simulated solar radiation: mechanism, kinetics, and toxicity variation.

The main objective of this study was to investigate the degradation mechanism, the reaction kinetics, and the evolution of toxicity of naproxen in wat...
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