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Articles Environmental Chemistry

May 2010 Vol.55 No.15: 1495−1500 doi: 10.1007/s11434-010-3139-y

SPECIAL TOPICS:

Photodegradation of fluoroquinolone antibiotic gatifloxacin in aqueous solutions GE LinKe, CHEN JingWen*, ZHANG SiYu, CAI XiYun, WANG Zhuang & WANG ChunLing Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), Department of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China Received March 25, 2009; accepted July 13, 2009

Fluoroquinolone antibiotics (FQs) are frequently detected as emerging pollutants in aqueous environments. In this study, kinetics, influencing factors and mechanisms on the photodegradation of gatifloxacin, a representative FQ, were investigated. The photodegradation follows the pseudo-first-order kinetics. Gatifloxacin photodegrades with a quantum yield of (5.94 ± 0.95) × 10−3 in pure water and undergoes direct photolysis as well as self-sensitized photodegradation. The FQ photodegrades slower in freshwater and seawater than in pure water, which is attributed to the integrative effects of pH and the aqueous dissolved matter (e.g., humic acids and NO3−) on the photodegradation. A toxicity test using Vibrio fischeri revealed the formation of hazardous photoproducts. gatifloxacin, photodegradation, influencing factors, reactive oxygen species Citation:

Ge L K, Chen J W, Zhang S Y, et al. Photodegradation of fluoroquinolone antibiotic gatifloxacin in aqueous solutions. Chinese Sci Bull, 2010, 55: 1495−1500, doi: 10.1007/s11434-010-3139-y

Antibiotics have been frequently detected in environmental water bodies and identified as emerging pollutants with limited understanding of their environmental fate and toxicological effects [1]. Fluoroquinolones (FQs) are a large class of antibiotics widely used in human and veterinary medicines, and aquaculture [2,3]. FQs have been detected in water bodies with their concentrations ranging from 2.30 ng L−1 to 405 μg L−1 in China [4−6], the United States [7] and Europe [8−10]. Therefore, the fate and effects of FQs need to be investigated so as to assess their ecological risks. The environmental fate of FQs is influenced mainly by two mechanisms, photodegradation and adsorption [11,12]. The photodegradation of FQs was the primary focus of most previous investigations. Knapp et al. [11] found that enrofloxacin photodegraded rapidly following first order kinetics and was transformed into ciprofloxacin in sunlit mesocosms. Araki et al. [13] found that sitafloxacin photo-

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2010

degradation also followed first-order kinetics, and the photolytic rate was affected by the matrix pH and Cl−. In previous studies, such aqueous photoreactive dissolved matter as Fe(III), NO3− and humic acids (HA) was shown to have significant impacts on the photochemical reaction kinetics and pathways of organic pollutants in surface waters [14−16]. These dissolved species coexist in waters and interact, so they may behave with multivariate effects on the photochemical reactions of pollutants. As for FQs, no research concerning the multivariate effects of water constituents on photochemical behavior has been reported. It was reported that certain organic pollutants absorbed actinic photons and transferred energy or electrons to other chemicals with the formation of reactive oxygen species (ROS), which subsequently oxidized and degraded the pollutants [17,18]. The process was designated as the selfsensitized photodegradation of pollutants. Previous studies described the generation of ROS (e.g., ·OH and 1O2) in irradiated FQ solutions [19−21]. However, it is not clear whether the generated ROS initiated the degradation of FQs [22]. csb.scichina.com

www.springerlink.com

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Gatifloxacin is a fourth-generation fluoroquinolone, which is photodegraded and transformed into products via decyclopropyl and decarboxylation [23]. This study selected gatifloxacin as a model compound, investigated its photodegradation kinetics and quantum yield in pure water and under simulated solar irradiation, and examined whether the FQ underwent self-sensitized photodegradation. The multivariate effects of Fe(III), NO3−, HA and salinity (NaCl) on the photolytic kinetics were assessed and the toxicity evolution of the photomodified FQ was investigated using Vibrio fischeri.

1 Materials and methods 1.1

Materials

Gatifloxacin was provided by Hubei Biocause Pharmaceutical Co., Ltd. with 99.0% purity. Humic acids (HA, Fluka No. 53680) were purchased from Sigma Aldrich, Inc. Vibrio fischeri was obtained from the Institute of Soil Science, Chinese Academy of Sciences (Nanjing, China). Acetonitrile, methanol, isopropanol and formic acid were of HPLC grade. Other reagents were of analytical grade. Ultra pure water was obtained with a Millipore-Milli Q system. Freshwater and seawater were collected from the Dalian Xishan reservoir and the Yellow Sea, respectively, filtered through 0.22-μm filters and stored at −20°C until use. Local humic acids (L-HA) were extracted from freshwater following the method recommended by the International Humic Substances Society [24]. Aquatic humic substances containing L-HA and fulvic acids (FA) were preconcentrated from freshwater by lowering the pH to 2 and adsorbing both components on a DAX-8 resin column. The L-HA and FA were extracted from the resin with a strong base followed by lowering the pH to 1 to precipitate the L-HA. The L-HA were centrifuged and then dialyzed to remove Cl−. 1.2

Photodegradation and analysis

An XPA-1 merry-go-round photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China) and a simulated solar light source (1 kW xenon lamp with 290 nm filter, λ>290 nm) were used to perform the photodegradation experiments in a chamber. The irradiance spectrum of the light source (Figure 1) was measured with a monochromator (Acton, SP-300). The light intensity (290−420 nm) at the reaction solutions was 0.83 mW cm−2. The chamber was ventilated to maintain the reaction temperature at (25 ± 1)°C. To test whether ·OH and 1O2 were involved in the photodegradation of gatifloxacin, isopropanol and NaN3 were added to the photoreaction solutions. Quantum yield (Φ) for gatifloxacin photodegradation in pure water was measured using p-nitroanisole/pyridine as a chemical actionmeter [25]. A central composite design was used to invest-

Figure 1 Structures of gatifloxacin, ultraviolet absorption spectra of 5 μmol L−1 gatifloxacin in three waters and humic acids (HA and L-HA) in pure water, and the irradiance spectrum of the light source.

tigate the multivariate effects of main water constituents on the photodegradation kinetics of gatifloxacin. Factors and their concentration levels are shown in Table 1. The concentrations are representative of those commonly encountered in the environment [15,26]. The design layout and data analysis were performed using Design Expert (ver. 7.1.3, Stat-Ease Inc.). Dark controls were performed simultaneously under the same conditions. In addition, the solutions of the FQ in pure water, freshwater and seawater were preserved in the dark for up to 5 d at room temperature to examine their aqueous stability. All the experiments were carried out at least in triplicate, and the results were reported as mean ± 95% confidence interval when available. An Agilent 1100 HPLC with a Hypersil BDS column (250 mm × 4.6 mm, 5 μm; 35°C) and a diode array detector (DAD) was used to analyze gatifloxacin. The mobile phase was a 10:15:75 (V:V:V) mixture of methanol, acetonitrile and acidified water (1% formic acid, pH 3.0 adjusted by NaOH) with a flow rate of 1.0 mL min−1. The DAD detection wavelength and injection volume were 293 nm and 50 μL, respectively. The retention time for gatifloxacin was 9.58 min. 1.3

Acute toxicity test

To indicate the toxicity variation during the photodegradation of gatifloxacin, a 15-min duration Microtox bioassay Table 1 Factors and concentration levels in the central composite design Factors (units)

Concentration levels −2

−1

0

1

2

Fe(III) (µmol L−1)

0.00

1.00

2.00

3.00

4.00

NO3− (µmol L−1)

0.00

10.0

20.0

30.0

40.0

HA (mg (C) L−1)

0.00

2.50

5.00

7.50

10.0

Cl− (mol L−1)

0.00

0.125

0.250

0.375

0.500

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using Vibrio fischeri was carried out according to the China 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 (L = luminescence): I % = (1 −

Lsample Lblank

) × 100%.

(1)

photolysis. The contribution of self-sensitized photolysis to the overall photodegradation of gatifloxacin is estimated as follows

R·OH = R1 O = 2

k·OH (PW)

2.1

Results and discussion Photodegradation in pure water

Under dark conditions, degradation loss of gatifloxacin in pure water, freshwater and seawater was observed to be less than 2.9%. A compound is considered to be hydrolytically stable if its residual concentration is 90% or more of its initial concentration after a 5-day stability test [27]. Therefore, the FQ is hydrolytically stable in the three waters. When exposed to simulated sunlight, gatifloxacin was photodegraded. Correlation coefficients (r2) for the linear regression of ln(C/C0) vs time (t) were greater than 0.95, which showed that photodegradation reactions followed pseudo-first-order kinetics. The determined photodegradation rate constant (k) in pure water was (4.94 ± 0.59) × 10−3 min−1 (± error represents the 95% confidence interval). As shown in Figure 2, the addition of isopropanol or NaN3 in pure water inhibited the photodegradation. As isopropanol is the quencher of ·OH [28], the inhibitive effect indicated that gatifloxacin photoreactions in pure water involved self-sensitized photodegradation via ·OH. NaN3 is the quencher of ·OH and 1O2 [28,29], and the inhibitive effect of NaN3 was more significant than that of isopropanol (Figure 2), which suggested that the FQ underwent 1O2-mediated self-sensitized photolysis in addition to ·OH-mediated



kPW

k1O

2

(PW)

kPW

where R·OH and R1 O

2

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2



kPW − kPW + isopropanol kPW

kPW + isopropanol − kPW + NaN3 kPW

(2)

,

(3)

,

are the contribution rates of self-

sensitized photodegradation via ·OH and 1O2, respectively; k·OH (PW) and k 1 O (PW) are rate constants (k) for the self2

sensitized photodegradation via ·OH and 1O2, respectively; kPW is the k value for the photodegradation in pure water; kPW + isopropanol and kPW + NaN3 correspond to the k values for

the addition of isopropanol and NaN3 in pure water, respectively. Based on eqs. (2) and (3), R·OH and R1 O were 2

calculated to be 64.8% and 9.5%, respectively. The determined quantum yield (Φ) for gatifloxacin photolysis in pure water was (5.94 ± 0.95) × 10−3. The environmental rate constant (k) and half-life (t1/2) for solar photodegradation of the FQ in surface waters are calculated according to the following eqs. k = 2.303Φ∑(Zλελ),

t1/ 2 =

ln 2 , k

(4) (5)

where Zλ stands for the photon flux rate of specific-wavelength (λ) sunlight at sea level [30,31] and ελ is the molar extinction coefficient (L mol−1 cm−1). The calculated t1/2 for gatifloxacin in surface waters at 45°Ν latitude is 11.0 min at mid-summer noon and 51.8 min at mid-winter noon. These t1/2 values are short compared to that of the FQ hydrolysis, thus photodegradation is a central factor in determining the FQ fate in sunlit surface waters. 2.2 Photodegradation in natural waters and the pH effect

Figure 2 Photodegradation of gatifloxacin (C0 = 5 μmol L−1) in three waters and the effects of isopropanol (100 mmol L−1) and NaN3 (5 mmol L−1). Error bars indicate one standard deviation.

As shown in Figure 2, the photolytic rate of gatifloxacin in freshwater and seawater was lower than that in pure water. At λ > 290 nm, the absorption spectra of gatifloxacin varied little in the three waters (Figure 1), suggesting that solution pH and dissolved matter might be the main influencing factors affecting the photodegradation kinetics. Results shown in Figure 3 indicate that pH significantly affected the photolytic rate constants (k). When the matrix pH rose from 5 to 11, the k of gatifloxacin increased first and then decreased with a peak value at pH = 8. The isoelectric point (pHiso) of gatifloxacin is 7.6 [32,33]. Therefore, the zwitter ionic form of gatifloxacin photodegrades faster than the

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(Figure 3), the FQ was expected to photodegrade faster in freshwater and seawater than in pure water. However, the experimental results (Figure 2) showed slower photodegradation in freshwater and seawater, which predicted that aqueous dissolved matter might affect photodegradation more significantly than pH. To verify the prediction, the effects of the main water constituents on photodegradation were studied. 2.3 Multivariate effects of HA, NO3−, Fe(III) and Cl− on gatifloxacin photodegradation

Figure 3 Effect of pH on the photodegradation rate constants (k) of gatifloxacin (C0 = 5 μmol L−1). Error bars represent the 95% confidence interval.

other styles, including the acidic and basic forms. A similar pH effect was also reported for the photodegradation of sitafloxacin [13]. The solution pH values for gatifloxacin dissolved in pure water, freshwater and seawater were determined to be 6.41, 8.31 and 7.63, respectively. Based on the k–pH profile

The pseudo-first-order rate constants (k) for gatifloxacin photodegradation in the central composite experiments are listed in Table 2. The relationship between k and the four variables (x1–x4) was evaluated by fitting a full quadratic expression [15]: k = β0 + β1x1 +β2x2 + β3x3 + β4x4 + β12x1x2 + β13x1x3+ β14x1x4 + β23x2x3 + β24x2x4 + β34x3x4 + β11x12 + β22x22 + β33x32 + β44x42, (6) where x1–x4 denote the levels of the four factors Fe(III) NO3−, HA and Cl− (Table 1) and βx (β0–β44) are the coefficients of eq.(6). The βx values were generated during the

Table 2 Solution conditions and rate constants (k) for gatifloxacin (C0 = 5 μmol L−1) photodegradation in the central composite experiments Run

Fe(III) (µmol L−1)

NO3− (µmol L−1)

HA (mg(C) L−1)

Cl− (mol L−1)

k (10−3 min−1)

1

1.00

10.0

2.50

0.125

1.38 ± 0.15

2

3.00

10.0

2.50

0.125

1.19 ± 0.25

3

1.00

30.0

2.50

0.125

1.04 ± 0.14

4

3.00

30.0

2.50

0.125

1.14 ± 0.21

5

1.00

10.0

7.50

0.125

0.86 ± 0.11

6

3.00

10.0

7.50

0.125

0.91 ± 0.02

7

1.00

30.0

7.50

0.125

0.80 ± 0.10

8

3.00

30.0

7.50

0.125

0.86 ± 0.30

9

1.00

10.0

2.50

0.375

1.23 ± 0.06

10

3.00

10.0

2.50

0.375

1.25 ± 0.04

11

1.00

30.0

2.50

0.375

0.85 ± 0.20

12

3.00

30.0

2.50

0.375

0.94 ± 0.12

13

1.00

10.0

7.50

0.375

0.79 ± 0.24

14

3.00

10.0

7.50

0.375

0.84 ± 0.01

15

1.00

30.0

7.50

0.375

0.86 ± 0.18

16

3.00

30.0

7.50

0.375

0.80 ± 0.09

17

0.00

20.0

5.00

0.250

1.00 ± 0.36

18

4.00

20.0

5.00

0.250

0.98 ± 0.09

19

2.00

0.00

5.00

0.250

1.15 ± 0.03

20

2.00

40.0

5.00

0.250

0.99 ± 0.15

21

2.00

20.0

0.00

0.250

1.98 ± 0.13

22

2.00

20.0

10.0

0.250

0.50 ± 0.07

23

2.00

20.0

5.00

0.00

1.08 ± 0.11

24

2.00

20.0

5.00

0.500

1.00 ± 0.20

25

2.00

20.0

5.00

0.250

0.95 ± 0.08

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Chinese Sci Bull

fitting process (Table 3). The contribution of each factor to photodegradation was evaluated by statistical analysis of the βx coefficients. As shown in Table 3, if the significance level P < 0.05, the corresponding βx value is significantly different from zero at the 95% level of confidence. Thus, HA is a significant contributing factor. In the case of the P < 0.1 level, NO3− is also a significant factor. The terms β2 and β3 are both negative, indicating that NO3− and HA both inhibited the photodegradation of gatifloxacin. Fe(III), Cl− and all the two-factor interactions had no significant impact on photodegradation (Table 3). As HA and Cl− are prominent water constituents in freshwater and seawater, respectively, their individual effect on gatifloxacin photodegradation was investigated. It was observed that HA (10–30 mg(C) L−1) and L-HA (10 mg(C) L−1) both inhibited photodegradation, while Cl− (0.1–1.0 mol L−1) did not significantly affect photodegradation (P > 0.05), which validated the results from the central composite experiments. Given enhanced understanding of HA and NO3−, it is possible to propose an interpretation for their inhibitive effects. HA (Figure 1) and NO3− [34] showed strong photoabsorption at λ = 290–370 nm, so they displayed lightfiltering effects and retarded the photodegradation of gatifloxacin. Moreover, HA scavenged the ROS (e.g., ·OH and 1 O2) [15,35] and thus partially inhibited the FQ self-sensitized photolysis. Although the solution pH in freshwater and seawater rather than in pure water was closer to the FQ pHiso, abundant HA and NO3− (TOC and NO3− in freshwater were 125.3 mg(C) L−1 and 454.8 μmol L−1, respectively; and in seawater, 3.5 mg(C) L−1 and 282.3 μmol L−1, respectively) in natural waters inhibited the photoz-degradation. Table 3 Parameter estimates for the βx coefficients of the quadratic equation in the central composite experiments

βx

βx key

β0

intercept

β1

Fe(III)

βx value 9.45× 10

3.34 × 10−6



β2

NO3

β3

HA

β4

Cl−

P

−4



0.919

−6.18 × 10−5

0.074

−2.20 × 10−4

< 0.0001

−3.23 × 10−5

0.332

−5

0.686

β12

Fe(III)-NO3

β13

Fe(III)-HA

7.03 × 10−6

0.861

β14

Fe(III)-Cl−

4.55 × 10−6

0.910

β23

NO3−-HA

6.24 × 10−5

0.135

β24

NO3−-Cl−

−9.91 × 10−6

0.805

2.11 × 10

−5

0.601

−6.30 × 10

−6

0.837

β34



HA-Cl

1.63 × 10

β11

Fe(III)

β22

(NO3−)2

1.37 × 10−5

0.656

β33

(HA)2

5.74 × 10−5

0.077

β44

− 2

−6

0.825

2

(Cl )

6.80 × 10

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Figure 4 Bioluminescent inhibition of photolyzed gatifloxacin (C0 = 100 μmol L−1 in pure water) to Vibrio fischeri. Error bars indicate one standard deviation.

2.4 Photomodified toxicity of gatifloxacin to Vibrio fischeri As shown in Figure 4, the luminescence inhibition rate (I%) of gatifloxacin to Vibrio fischeri was (39.6 ± 0.8)%. During photodegradation, the inhibition rate of the photolyzed FQ solution first decreased, then increased, and finally decreased. The toxicity evolution revealed the generation of several hazardous products and the photomodified toxicity of the FQ to Vibrio fischeri. These products might be stemmed from gatifloxacin with some groups lost, which resulted in lower steric resistance and easier penetration into the cells of luminescent bacteria, subsequently leading to the enhancement of toxicity [23,36,37].

3

Conclusions

When exposed to simulated sunlight, gatifloxacin underwent direct photolysis as well as self-sensitized photolysis via ·OH and 1O2. The observed photodegradation followed pseudo-first-order kinetics, and was affected by solution pH. The fastest photodegradation occurred around the FQ isoelectric point. The FQ photodegraded slower in freshwater and seawater than in pure water, which was attributed to the light-filtering effects and/or ROS scavenging effects of HA and NO3−. During photodegradation, gatifloxacin showed photomodified toxicity to Vibrio fischeri. This work was supported by the National Basic Research Program of China (Grant No. 2006CB403302), National Natural Science Foundation of China (Grant No. 20777010) and the Program for Changjiang Scholars and Innovative Research Team in University of China (Grant No. IRT0813).

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