Photocatalytic degradation of ciprofloxacin drug in water using ZnO nanoparticles

Journal of Luminescence 130 (2010) 2327–2331

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Photocatalytic degradation of ciprofloxacin drug in water using ZnO nanoparticles Maged El-Kemary n, Hany El-Shamy, Ibrahim El-Mehasseb Photo- and Nanochemistry Laboratory, Chemistry Department, Faculty of Science, Kafrelsheikh University, 33516 Kafr ElSheikh, Egypt

a r t i c l e in fo

abstract

Article history: Received 25 January 2010 Received in revised form 15 July 2010 Accepted 19 July 2010 Available online 3 August 2010

We report the synthesis of nanostructure ZnO semiconductor with  2.1 nm diameter using a chemical precipitation method. The resulting nanoparticles were characterized by X-ray diffraction analysis (XRD), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The optical properties were investigated by UV–vis and fluorescence techniques. The absorption spectra exhibit a sharp absorption edge at  334 nm corresponding to band gap of  3.7 eV. The fluorescence spectra displayed a near-band-edge ultraviolet excitonic emission at  410 nm and a green emission peak at  525 nm, due to a transition of a photogenerated electron from the conduction band to a deeply trapped hole. The photocatalytic activity of the prepared ZnO nanoparticles has been investigated for the degradation of ciprofloxacin drug under UV light irradiation in aqueous solutions of different pH values. The results showed that the photocatalytic degradation process is effective at pH 7 and 10, but it is rather slow at pH 4. Higher degradation efficiency ( 50%) of the drug was observed at pH 10 after 60 min. Photodegradation of the drug follows a pseudo-first-order kinetics. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.

Keywords: Nanoparticles ZnO Fluorescence Drug Ciprofloxacin Photocatalyst

1. Introduction Recently, a lot of studies have been concentrated on the degradation of toxic organic compounds in waste water via photocatalysis of various semiconductors [1,2]. The fact that, among other common water pollutants, pharmacy waste water pollutants such as ciprofloxacin (CF; Fig. 1) [3] undergoes photocatalytic degradation to produce less harmful products indicates some potential for photocatalytic treatments of waste water utilizing semiconductor and UV light. Semiconductor photocatalysis has attracted interest in the recent years due to its potential contribution to environmental problems. There has been a great deal of interest in ZnO semiconductor nanoparticles as one of the most important photocatalysts [4] widely used in the degradation and complete mineralization of environmental pollutants [5–7]. It has been widely used to deal with waste water, such as pharmacy waste water, printing, dyeing wastes and papermaking waste water [8]. It is relatively inexpensive and provides a photo-generated hole with high oxidizing power due to its wide band gap energy. Its large exciton binding energy (60 meV) could lead to lasing action based on exciton recombination even above room temperature [9]. In addition, ZnO has much simpler crystal-growth technology,

n

Corresponding author. Tel.: +20 100 297 421; fax: + 20 473 223 415. E-mail address: [email protected] (M. El-Kemary).

resulting in a potentially low cost for ZnO-based devices. It showed wide application in dye-sensitized solar cells [10–12], anti-ultraviolet-radiation cosmetics, light-emitting device, as well as cancer detecting biosensors, gas sensors and degradation of organic toxins [13–15]. Because of the quantum confinement effects, ZnO nanoparticles have some unique optical properties [16]. Recently, we have described the synthesis, characterization and optical properties of nanostructured CdS semiconductor [17] and organic nanoparticles based on piroxicam drug [18], as well as photocatalytic characteristics of ZnS nanoparticles [19]. Here, we report the synthesis, characterization and optical properties of nanostructured ZnO semiconductor and their influence on photodegradation efficiency of ciprofloxacin antibiotic drug in aqueous solutions with different pH values.

2. Experimental 2.1. Reagents Ciprofloxacin (abbreviated as CF; Fig. 1) purchased from Sigma, zinc acetate dihydrate (Aldrich), triethylamine (Aldrich), ethanol (Aldrich) and diethylether (Aldrich) were of analytical grade and were used without further purification. Double distilled water was used to prepare the buffer solution by adding appropriate amounts of NaOH (0.1 mol/L) or HCl (0.1 ml/L) from Merck.

0022-2313/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.07.013

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O

products. The 1570 and 1412 cm  1 peaks are attributed to asymmetric and symmetric CQO stretching of zinc acetate. The peak located at 463 cm  1 is due to the stretching mode of ZnO [20,21].

O F

HO N

N NH

Fig. 1. Chemical structure of ciprofloxacin.

2.2. Equipments

3.2. Optical properties Fig. 5 shows the UV–vis absorption spectra of ZnO nanoparticles. It is apparent that the spectra exhibit a sharp absorption edge at 334 nm. The corresponding value of band gap is 3.7 eV. Band gap of the nanoparticles is calculated from the following equation:

UV–vis absorption spectra were measured on a Shimadzu UV2450 spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-5301PC spectrofluorometer. Fourier- transform infrared (FT-IR) spectra were measured with a JASCO spectrometer 4100. X-ray diffraction (XRD) measurements were conducted using a Rigaku 2550D/max VB/PC X-ray diffractometer ˚ using CuKa radiation (l ¼ 0.154056 A). Transmission electron micrographs (TEM) were obtained using a JEOL 1230 microscope operating at an accelerating voltage of 120 kV. Scanning electron microscopy was carried out using JSM-6390.

E ¼ hc=l

2.3. Synthesis of ZnO nanoparticles

2R ¼

Zinc acetate dihydrate (ZnAc2  2H2O) and capping agent (triethylamine, TEA) were mixed at a molar ratio of 5:3 in ethanol. The mixed solution was stirred for 60 min at 50–60 1C in a condensation system until it turned white cloudy. The resulting cloudy solution indicates the growth of ZnO nanoparticles. The solution was centrifuged for 10 min to separate the precipitate from the ethanol layer. The ethanol portion was removed and the precipitate was washed with ethanol and was dried in vacuum.

where 2R is the diameter of the particles. An average diameter of 1.79 nm was estimated, which is slightly smaller than that calculated by XRD (1.97 nm). Fig. 6 shows the excitation wavelength dependence fluorescence emission spectra of ZnO nanoparticles in water solution of pH 7. The spectra observed under 334 nm excitation exhibit a near-band-edge ultraviolet emission at  410 nm and a relatively strong green broad emission at  525 nm. The UV emission is originated from excitonic recombination corresponding to the band-edge emission of ZnO and the green one is due to a transition of a photo-generated electron from the conduction band to a deeply trapped hole [24]. In ZnO nanoparticles along with the visible green emission, UV excitonic emission is also reported [23,25]. It is apparent that, the emissions display a strong dependence on excitation wavelength (Fig. 6). As the excitation wavelength decreased, for the green emission band, the peak position shows a blue-shift, the width turns less broad, the intensity decreases first and then increases. For the UV emission band, the peak position exhibits a blue-shift, and the intensity increases. This observation has been attributed to the presence of energetically different associated forms of the constituent molecules and slow rate of the excited state relaxation process [26].

2.4. Photodegradation experiments Photodegradation experiments were performed with a solution of 20 mg/L ZnO nanoparticles and 5 mg/L CF. The pH of both solutions was adjusted to 7 and 3 ml of the sample was transferred into 1-mm path length quartz cuvette. The solution was then exposed to ultraviolet light (365 nm wavelength) from a xenon lamp for variable time intervals and then the sample was quickly subjected to absorption or emission measurement.

3. Results and discussion 3.1. Structure characterization Fig. 2 shows the XRD pattern of the investigated ZnO nanoparticles, which shows the crystalline nature of ZnO nanomaterial. The peak broadening in the XRD pattern clearly indicates that very small nanocrystals are present in the samples suggesting that the nanoparticles exhibit a narrow size distribution. In order to elucidate the ZnO nanoparticles morphologies, scanning electron microscopy (SEM) was performed, Fig. 3A, which demonstrates clearly the formation of ZnO nanoparticles. It can be clearly seen that the nanoparticles have a flower-like shape. Fig. 3B shows the transmission electron microscopy (TEM) image of ZnO nanoparticles. This image shows that the size of ZnO nanoparticle is very consistent. The average particle size was estimated to be  2.1 nm. Fig. 4 shows the FT-IR spectra of ZnO nanoparticles (powders). The absorption band observed at 3438 cm  1 is due to O–H stretching of hydroxyl group. The peaks corresponding to symmetric and asymmetric C–H bonds are observed at 2924 and 3004 cm  1, respectively. C–H bonds are present in monoacetate groups as intermediate

ð1Þ

where E is band gap energy, h is Planck’s constant, c is the velocity of light and l the wavelength of absorption edge in absorption spectra. It should be noted that the calculated band gap (3.7 eV) for ZnO nanoparticles is blue-shifted from that for bulk ZnO (3.2 eV), which suggests the influence of quantum confinement. From the position of the spectral absorption edge (350 nm), the average particles size can be determined using Henglein’s empirical relation between particles size and absorption onset (l) [22,23] according to the following equation: 0:1 ð0:1380:0002345lÞ nm

ð2Þ

3.3. Photocatalytic activity of ZnO nanoparticles Fig. 7A shows the variations of absorption spectrum of CF during photocatalytic degradation by ZnO nanoparticles. The intensities of absorbance peaks at 270 and 330 nm significantly decreased while the absorbance shoulders at 240, 290 and 360 nm increased at a slow rate. In order to ensure that degradation of CF by ZnO was the dominant mechanism, 4 mg/L of CF was irradiated without ZnO for 30 min in 5 min increments. It was shown that there was no statistical change in concentration due to the direct photolysis of CF with 365 nm lamps based on a 95% confidence interval. The decrease of absorption spectra and therefore absorbance of samples at lmax of CF is indicated by degradation of CF in the applied conditions. As a consequence, the decrease of sample absorbance due to decrease of drug concentration is recorded for kinetic study of catalytic photodegradation of the drug. The degradation efficiency (E%) has been calculated using   C0 C E% ¼ 100x ð3Þ C0

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d=7.12819

M. El-Kemary et al. / Journal of Luminescence 130 (2010) 2327–2331

600

d=17.68063

400

O

O

O

300

Zn

Zn

Zn

--

d=2.15555

d=2.31498

d=2.68988

d=3.24640 d=3.11772

d=3.68293 d=3.54282

d=4.57172 d=4.40319 d=4.07809 d=3.99392

d=5.86481 d=5.40383

200

d=7.48341

ZnO

d=9.21796

X-ray intensity (a.u.)

500

100

0 4

10

20

30

40

2θ (degree) Fig. 2. XRD patterns of ZnO nanoparticles.

80

Transmittance (%)

70 60

2924

50

3004

40 1026

30

463

20

680 3438

10 0 4000

1412 1570

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 4. FT-IR spectra of ZnO nanoparticles.

1.6

Absorbance

1.2

0.8

0.4 Fig. 3. (A) SEM and (B) TEM images of ZnO nanopaticles.

0.0 where C0 is the initial concentration of the drug and C the concentration of the drug after irradiation in selected time interval. After 60 min, the observed maximum degradation efficiency of CF is about 18% at pH 4, 42% at pH 7 and 50% at pH 10. Therefore, nano-ZnO has more photocatalysis efficiency for degradation of CF at pH 10. The photocatalytic degradation of CF follows a pseudo-firstorder kinetics, which is generally expected from a ZnO photocatalytic system [27–29]. The pseudo-first-order rate constant, k,

250

300

350

400

450

500

Wavelength / nm Fig. 5. UV–vis absorption spectra of ZnO nanoparticles in aqueous solution of pH 7.

was determined from the slope of ln(C/C0) versus irradiation time, according to the following relation: lnðC=C0 Þ ¼ kt

ð4Þ

M. El-Kemary et al. / Journal of Luminescence 130 (2010) 2327–2331

200 Fluorescence Intensity /a.u.

λex / nm

280 305 334

150

100

50

0 350

400

450 500 550 Wavelength / nm

600

650

Fig. 6. Fluorescence spectra of ZnO nanoparticles as a function of excitation wavelength in aqueous solution of pH 7.0.

0.6

time (min) 0 5 10 15 20 25 30 35 40 45 50 60

Absorbance

0.5 0.4 0.3 0.2 0.1

500 Time (min.)

0.0 280

240

320

360

and the peak position shifted to a considerable longer wavelength from  421 to 442 nm as a function of degradation time. The influence of pH on catalytic efficiency of ZnO nanoparticles was examined by adjusting the pH of the reaction mixture at values of 4, 7 and 10. pH can alter the surface charge properties of the photocatalyst and probably the chemical structure of the drug; therefore photocatalyst reaction is pH-dependent. Fig. 7B shows the effect of pH on photodegradation of CF in the absence and presence of ZnO on irradiation with UV light of 365 nm. It is apparent that ZnO nanoparticles showed better degradation of CF at pH 10. As shown in Fig. 7B, the stability of CF increased considerably when the pH was lowered towards 4. In Section 3.3.1, it was observed that after 1 h exposure to radiation, photodegradation increased from 18% loss of parent compound at pH 4 to 42% at pH 7 and 48% at pH 10. (Fig. 8B). The results show that the maximum degradation was obtained at pH values of 7 and 10 under UV light, where the available hydroxyl ions can react with holes (h + ) to form hydroxyl radicals (OHd), which have high oxidation capability, subsequently enhancing the photodegradation rate of CF, as mentioned earlier for degradation of aromatic contaminants by ZnO nanoparticles [30]. At low pH values (pH¼4) the photodegradation of CF was retarded under UV light by the high concentration of protons, which hold high affinity for the hydroxyl anion, preventing the formation of hydroxyl radicals.

400

440

Wavelength (nm)

0.0

Fluorescence Intensity (a.u.)

2330

0 5 10 15 20 25 30 35 40 45 60

400

300

200

100

0 ln (C/C0)

-0.2

360

400

440 Wavelength /nm

480

520

-0.4

1.2 -0.6

1.0 -0.8 10

20

30

40

50

60

time (min) Fig. 7. (A) Change of absorption spectra of CF solution of pH 7 during photocatalytic degradation by ZnO naoparticles and (B) pseudo-first order plot for the kinetic photodegradation of CF in the presence of ZnO nanoparticles.

0.8 I / I0

0

0.6 pH=7, without ZnO pH=4 pH=7 pH=10

0.4 where C is the concentration at time t and C0 the initial concentration. The rate constant was calculated to be 0.0117 70.0029 min  1 at pH 4, 0.004870.0027 min  1 at pH 7 and 0.004370.0023 min  1 at pH 10. The plot used to determine the rate constant for the photocatalytic degradation of CF in the presence of ZnO at pH 7 can be seen in Fig. 7B. Fig. 8A shows the change in fluorescence emission spectra of CF solution in the presence of ZnO nanoparticles at pH 7 on UV irradiation. It is apparent that the emission intensity decreased

0.2 0

20

40 Time (min)

60

Fig. 8. (A) Quenching effects of CF drug (4 mg/L) in the presence of ZnO nanoparticles (20 mg/L) irradiated with UV light of 365 nm and (B) effect of pH on photodegradation of CF in the absence and presence of ZnO on irradiation with UV light of 365 nm.

M. El-Kemary et al. / Journal of Luminescence 130 (2010) 2327–2331

O2 et -

O 2-

-

CB

ht

R-COOH

+ hν ν

+

VB OH.

CO2 + H2O OH

-

Scheme 1. . Principle of ZnO nanoparticles assisting the photocatalytic degradation of CF (R–COOH).

As there are no free hydroxyl ions, the formation of hydroxyl radicals is not possible. Therefore photodegradation decreased at low pH or decrease in photodegradation may also be due to dissolution of ZnO at highly acidic conditions. Similar results were also reported in the photocatalysed degradation of azo dyes [31,32]. However, CF is an ampholytic compound with pKa value of 6.09 for the carboxylic group and 8.74 for the nitrogen on the piperazinyl ring [33]. The isoelectric point of zwitterions is at pH 7.4. CF seemed to be most sensitive to photodegradation in zwitterionic form at slightly basic pH. The maximum stability of the drug was observed in solutions at pH 4.0, where the COOH group is not ionized and the basic nitrogen is completely protonated. From the pharmaceutical point of view, stability of CF in acidic media is important because pH of liquid pharmaceutical formulations varies between 3.5 and 5.5 [34]. Based on the experimental results and from the earlier reports on photocatalytic degradation of organic molecules, we assumed that on irradiation with UV light, the ZnO nanoparticles generate an excitation of electrons in valence band into the conduction band (electron–hole separation). The generated electron (hole) transfer to the adsorbed drug on the nanoparticle surface has always been considered as the first step of the photocatalytic action of semiconductor nanoparticles [35,36]. Photo-generated holes as well as hydroxyl radicals oxidize the drug adsorbed at ZnO surface, as shown in Scheme 1. The high oxidative potential of holes can lead to direct and indirect oxidation of drug. In the indirect oxidation process of drug molecules, hole at the valence band generates the hydroxide reactive radicals (OHd) via reaction with water and/or hydroxide anions (OH  , Eqs. (6) and (7)). Primary photoproducts resulting from interfacial electron (hole) transfer, i.e. radical ions, undergo further transformations, leading to the formation of final photoproducts, Eq. (8) [37]. The proposed mechanism for drug degradation using photocatalysts (P) was suggested as follows: þ

P þhv-h VB þ e CB

ð5Þ

þ

ð6Þ

þ

ð7Þ

OH þh VB-OH d þ H þ H2 Oþ h VB-OH d þ H þ þ

Drugþ h VB-drug

dþ

-final species

ð8Þ

Five compounds were identified as probable products of photo-defluorination, decarboxylation and loss of piperazin [38].

4. Conclusions The diameter of the synthesized ZnO nanoparticles has been estimated (1.79 nm) from the spectral absorption edge, which is close to that obtained from TEM ( 2.1 nm). The fluorescence emission spectra from ZnO nanoparticles in water solution display an excitonic UV emission and green emission peaks. The observed

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emissions display a strong dependence on excitation wavelength. Photodegradation of CF as a model drug of environmental pollutants in the presence of ZnO nanoparticles follows pseudo-first-order kinetics with a degradation efficiency of 48% at pH 10. The photocatalytic degradation of CF by ZnO photocatalyst could be used as a practical technique for the removal of environmental pollutants, which contributes to drug waste-water treatment. However, introducing metal ions could enhance the photocatalytic activity of ZnO in some cases. Therefore, further studies on the effect of transition metal doping on the photocatalytic activity of ZnO will be carried out in our laboratory to be published later.

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