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Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles a
a
a
Mohammad Shokri , Akbar Jodat , Nasser Modirshahla & Mohammad A. Behnajady
a
a
Department of Applied Chemistry , Faculty of Science, Tabriz Branch, Islamic Azad University , Tabriz , Iran Accepted author version posted online: 15 Nov 2012.Published online: 19 Nov 2012.
To cite this article: Mohammad Shokri , Akbar Jodat , Nasser Modirshahla & Mohammad A. Behnajady (2013) Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles, Environmental Technology, 34:9, 1161-1166, DOI: 10.1080/09593330.2012.743589 To link to this article: http://dx.doi.org/10.1080/09593330.2012.743589
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 9, 1161–1166, http://dx.doi.org/10.1080/09593330.2012.743589
Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles Mohammad Shokri∗ , Akbar Jodat, Nasser Modirshahla and Mohammad A. Behnajady Department of Applied Chemistry, Faculty of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran
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(Received 29 December 2011; final version received 10 October 2012 ) In this work, silver-doped TiO2 (Ag/TiO2 ) nanoparticles were synthesized using a photodeposition technique. The prepared Ag/TiO2 nanoparticles were characterized using TEM, SEM, XRD, and EDX techniques. The characterization of Ag/TiO2 nanoparticles using SEM and EDX techniques revealed the dispersion of Ag metal on the surface of TiO2 . The photocatalytic activity of Ag/TiO2 and bare TiO2 in the presence of ultraviolet irradiation was investigated in the removal of chloramphenicol (CAP) as an antibiotic. CAP is a broad-spectrum antibiotic exhibiting activity against both Gram-positive and Gram-negative bacteria, as well as other groups of microorganisms. However, it is, in certain susceptible individuals, associated with serious toxic effects in humans including bone marrow depression, particularly severe in the form of fatal aplastic anaemia. The effects of the operational factors, such as doping content of Ag, photocatalyst dosage and calcination temperature were evaluated in the catalytic activity of Ag/TiO2 . The results showed that the photocatalytic efficiency of TiO2 nanoparticles for the degradation of CAP, can be significantly improved by deposition an optimum amount of Ag nanoparticles (0.96 wt%) in the calcination temperature 300◦ C. It was found that 900 mg/L of Ag/TiO2 is the optimum dosage in the removal of CAP with 20 mg/L initial concentration. The highest removal efficiency of CAP (∼100%) at the optimum conditions was observed in 20 min. A mineralization study under optimum conditions showed about 88% reduction in total organic carbon after 120 min of irradiation time. Keywords: silver-doped TiO2 ; photodeposition; photocatalytic degradation; chloramphenicol
Introduction In recent years the presence of drugs, especially antibiotics, in the aquatic environment and thus its potential adverse effects have become of increasing concern [1]. Antibiotic substances are not readily biodegradable and either pass through biological treatment plants intact, or adsorb to the active sludge with subsequent desorption, accumulating in the environment [2,3]. The accumulation of antibiotics in organisms may cause arthropathy, nephropathy, damage to the central nervous system and spermatogenesis, mutagenic effects, and light sensitivity [4]. A further and possibly far greater threat is, however, the possible development of antibiotic-resistant microorganisms, including pathogens [5]. Furthermore, the presence of antibiotics in wastewaters has also increased and their abatement will be a challenge in the near future [6]. During the past years many investigations on chemical and biological technologies have been reported for the decomposition of organic pollutants in aqueous matrices. In this context, various advanced oxidation processes (AOPs) have been successfully employed for the degradation of a wide range of organic pollutants in water and wastewater [7]. Among the various AOPs, heterogeneous semiconductor photocatalysis using TiO2 as the photocatalyst has been ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
found capable of achieving the complete oxidation of the organic pollutants via hydroxyl radicals HO• and/or valence band holes h+ generated when the semiconductor is exposed to UV irradiation [8]. The main advantages of the process include a lack of mass transfer limitations and operation at ambient conditions. The catalyst itself is inexpensive, commercially available, non-toxic and photochemically stable [9]. However, the wide band gap of TiO2 (>3.0 eV) and the high recombination rate of the photo-induced electron– hole pairs formed in photocatalytic processes limit the efficiency of the photocatalytic degradation of pollutants [10,11]. Doping TiO2 with noble metals, such as Pt, Pd and Au can overcome the aforementioned deficiency. However, these noble metals are too expensive to be utilized on an industrial scale. Compared with Au, Pd and Pt, Ag is for less expensive and therefore deserves further investigation. Ag can act as an electron trap and promote the interfacial charge transfer processes in the composite systems, which reduces the recombination of the photo-induced electron–hole pairs, thus improving the photocatalytic activity of TiO2 [12,13]. Chloramphenicol (CAP) is a broad-spectrum antibiotic exhibiting activity against both Gram-positive and Gramnegative bacteria, as well as other groups of microorganisms. However, CAP is, in certain susceptible individuals,
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associated with serious toxic effects including bone marrow depression, and particularly severe in the form of fatal aplastic anaemia [14]. Degradation of CAP using TiO2 as a catalyst has been studied by several researchers [14–16]. However, to the best of our knowledge no systematic study has been reported on the photocatalytic degradation of CAP with the silver-doped TiO2 (Ag/TiO2 ) nanoparticles. In the present study, the photocatalytic activity of Ag/TiO2 and bare TiO2 nanoparticles in the presence of ultraviolet (UV) irradiation has been investigated in the removal of CAP. Ag/TiO2 nanoparticles were synthesized using a photodeposition method, and characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) techniques. Along the way, the effects of various operational parameters (doping content of Ag, photocatalyst dosage and calcination temperature) were evaluated to maximize the degradation of CAP under investigation. The extent of photodegradation and mineralization was measured by using a UV-visible spectrometer and total organic carbon (TOC) analyzer, respectively.
Preparation of the Ag/TiO2 catalyst The Ag/TiO2 photocatalyst was prepared based on the reduction of AgNO3 in a TiO2 suspension. Metallic Ag was deposited onto the TiO2 surface using a photodeposition method. A predetermined amount of TiO2 was added to 100 mL of deionized water. Then the required amount of AgNO3 for doping was added into the suspension of TiO2 . The pH of the TiO2 suspension was adjusted to 3. Then the mixture was irradiated with UV light (30 W, λmax = 254 nm, manufactured by Philips, Holland) for 3 h and then dried at 100◦ C for 12 h. The dried solid was calcined at 300◦ C for 3 h in a furnace.
Material and methods Reagents
Evaluation of photocatalytic activity For the photocatalytic degradation studies, the CAP solution containing an appropriate dosage of Ag/TiO2 powder was prepared in water in darkness during the experiment. The solution was stirred for 30 min to reach the adsorption equilibration in the dark before irradiation. Afterwards, 100 mL of the above suspension was transferred into a borosilicate Petri dish (12 cm diameter and 2.5 cm height) as the photoreactor. The reaction mixture was stirred vigorously under the illumination of UV-C light from top of the solution using a UV lamp (15 W, UV-C, λmax = 254 nm, manufactured by Philips, Holland). The distance of the lamp from the solution was adjusted in such a way to obtain a light intensity of 50 W/m2 on the surface of the solution, which was measured by a Lux-UV-IR meter from Leybold Company. For the kinetic studies, at certain reaction intervals, 5 mL of the sample was withdrawn. To remove the Ag/TiO2 particles from the reaction media, the solution was centrifuged for 15 min at 5000 rpm. The variation in the CAP concentration was observed from its characteristic absorption band at 275 nm by means of a UV-visible spectrophotometer using a calibration curve.
TiO2 -P25 (Degussa), (80% anatase, 20% rutile; BET area 50 m2 /g; primary size 21 nm) was used as a supporting material. AgNO3 (99.9%) and H2 SO4 were obtained from Merck. CAP, as a model antibiotic pollutant, was purchased from Panreac. Table 1 shows the chemical structure and other characteristics of this antibiotic. Table 1.
Structure and characteristics of CAP. OH HO
O
H
Cl H
NH Cl
NO2
Molecular formula Trade names
IUPAC name
CAS number λmax (nm) Molecular mass (g mol−1 )
C11 H12 N2 O5 Cl2 Alficetyn, Amphicol, Biomicin, Chlornitromycin, Chloromycetin, Fenicol, Phenicol, Medicom, Nevimycin, Vernacetin, Veticol. 2,2-dichloro-N -[(1R,2R)-2hydroxy-1-(hydroxymethyl)2-(4-nitrophenyl)ethyl] acetamide 56-75-7 275 323.13
Characterization methods The crystalline phase of the nanoparticles were analyzed by XRD measurements by using Siemens XRD-D5000 (λ = 0.154 nm). SEM analysis was performed on Au coated samples using a Philips apparatus model XL30, equipped with a probe for the EDX microanalysis. The morphology of the Ag/TiO2 nanoparticles was characterized by TEM (PHILIPS CM 10 −100 keV).
Analytical methods A UV-visible spectrophotometer (Ultrospec 2000, England) was used to measure the concentration of CAP. The variations in the concentration of CAP with reaction time were measured using the absorption peak intensity at wavelength of 275 nm. The concentration of TOC in the solution was measured using a TOC analyzer (Shimadzu, TOC-VCSH ).
Environmental Technology 300
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A : Anatase R : Rutile
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200 150
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A
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R A
A
R
R (b)
100 50
(a) 0 20
30
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2q
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Figure 1. X-Ray diffraction patterns of: (a) undoped TiO2 and (b) 0.96 wt% Ag/TiO2 nanoparticles.
Results and discussion The characterization of the Ag/TiO2 nanoparticles The prepared Ag/TiO2 nanoparticles were characterized using XRD, SEM, EDX, and TEM techniques. Figure 1(a) and (b) shows the XRD pattern of pure and doped TiO2 powders by the optimum amounts of Ag prepared by photodeposition method. The XRD pattern shown in the figure consists of mainly anatase with a minor rutile phase (80 : 20). All the photocatalysts show the dominant anatase phase peaks at 2θ = 25.2, 38, 48.2, 55 and 62.5◦ and the small fraction of the rutile phase with peaks at 2θ = 27.5, 36, 54 and 69◦ . The XRD patterns of the Ag/TiO2 samples almost coincide with that of pure TiO2 showing no diffraction peaks due to Ag doping thus suggesting that the Ag dopants are merely placed on the surface of the crystals [17]. A comparison of the SEM micrographs of pure TiO2 and the Ag/TiO2 nanoparticles proved that the loading of Ag nanoparticles did not affect the spherical shape of the TiO2 particles (Figure 2). The EDX analysis (Figure 3) also confirmed the loading of Ag on TiO2 . The TEM image of the Ag/TiO2 nanoparticles prepared using photodeposition is shown in Figure 4. The shape and size of the titania crystallites were unchanged as a result of surface modification by Ag particles. The Ag particles (black dots indicated) were located on the surface of the individual TiO2 nanoparticles. Also, there is a possibility for the Ag to be incorporated into the interstitial positions of the semiconductor particles [18]. The estimated range of the deposited Ag nanoparticles was 2– 10 nm. Agglomerates of metallic Ag were also observed on TiO2 in addition to small Ag particles. The effect of doping content of Ag When compared to pure TiO2 , the TiO2 loaded with 0.32, 0.64, 0.96, 1.28 and 1.6 wt% Ag exhibited a significant increase in the CAP degradation rate as shown in Figure 5. It was observed that the degradation efficiency increases with an increase in the Ag loading up to 0.96 wt% (optimum
Figure 2. SEM micrograph of: (a) TiO2 -P25 and (b) 0.96 wt% Ag/TiO2 nanoparticles.
Element wt% at% OK 36.53 63.43 Ti K 62.72 36.38 Ag L 0.75 0.19 Total 100 100
Figure 3.
EDX analysis of 0.96 wt% Ag/TiO2 nanoparticles.
metal loading) and then decreases. The Ag nanoparticles deposited on the titanium dioxide surface can act as electron–hole separation centres. This results in the formation of a Shottky barrier at the metal–semiconductor contact region, which improves the charge separation and thus enhances the photocatalytic activity of TiO2 . In contrast, at an Ag amount above its optimum value, the Ag nanoparticles can also act as recombination centres, which results in decreasing the photocatalytic activity of TiO2
M. Shokri et al.
Figure 4.
TEM micrograph of 0.96 wt% Ag/TiO2 nanoparticles prepared by photodeposition method.
100
100
80
80 - TiO2-P25
60
D%
D%
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0.32 wt% Ag-TiO2 0.64 wt% Ag-TiO2 0.96 wt% Ag-TiO2 + 1.28 wt% Ag-TiO2 D 1.6 wt% Ag-TiO2
40 20
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0 0
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100 mg/L 300 mg/L 500 mg/L 700 mg/L 900 mg/L 1100 mg/L
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Time (min)
Figure 5. Degradation efficiency (%) of CAP using Ag/TiO2 catalysts with different Ag loading as a function of irradiation time to UV light (I0 = 50 W/m2 , [CAP]0 = 20 mg/L, [Catalyst] = 300 mg/L).
0
6
12 18 Time (min)
24
30
Figure 6. Degradation efficiency (%) of 20 mg/L solution of CAP using different dosages of 0.96 wt% Ag/TiO2 catalyst versus UV irradiation time (I0 = 50 W/m2 ).
[19]. Sano et al. [20] observed that the increase of silver amount resulted in Ag aggregation leading to decrease in photoactivity. For the detrimental effect of increasing Ag loading on the photocatalytic activity of TiO2 , several reasons have been mentioned in the literature: (1) the probability for the hole capture is increased by the large number of negatively charged Ag particles on TiO2 at a high Ag content, which reduces the efficiency of charge separation [21] and (2) the excessive coverage of the TiO2 catalyst with Ag nanoparticles limits the amount of light reaching to the TiO2 surface, reducing the number of photogenerated e− –h+ pairs and consequently, lowering the TiO2 photoactivity [17].
The degradation efficiency of the photocatalyst exhibits an increase by increasing the catalyst dosage up to 900 mg/L and above this dosage the removal efficiency decreases. The enhancement of the CAP removal rate is due to the increase in active sites available for photocatalytic reaction as the loading of the catalyst is increased. It is obvious that the rate increases with an increase in the amount of catalyst up to a level corresponding to the optimum light absorption. Above this value the suspended particles of the catalyst block the UV light passage and increase the light scattering [22]. Thus, any further increase of the catalyst dosage above 900 mg/L has inverse effect on the photocatalytic reaction. Hence, the optimum amount of the catalyst dosage (900 mg/L, 0.96wt% Ag/TiO2 ) was used for the photocatalytic reactions.
The effect of catalyst dosage To determine the effect of the catalyst dosage, a series of experiments was carried out by varying the amount of catalyst with optimum Ag loading (0.96 wt%), meanwhile the other reaction conditions were kept the same. The degradation efficiency of CAP using various catalyst dosages is depicted in Figure 6.
The effect of calcination temperature In order to study the influence of the calcination temperature on the photocatalytic activity of the catalyst, the 0.96wt% Ag/TiO2 was calcined at 300, 450 and 600◦ C for 3 h, while other experimental conditions were kept constant. As it can be seen in Figure 7, the results of the photocatalytic degradation experiments using these samples on the solutions with
Environmental Technology 100
D%
80 60 300 °C 450 °C 600 °C
40 20 0
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0
6
12 18 Time (min)
24
30
Figure 7. Degradation efficiency (%) of CAP solution using of 0.96wt% Ag/TiO2 catalyst calcined at different temperatures versus UV irradiation time (I0 = 50 W/m2 , [CAP]0 = 20 mg/L, [Catalyst] = 900 mg/L).
20 mg/L of CAP showed that with increasing the calcination temperature, the photocatalytic activity of the catalysts were decreased significantly and the highest degradation efficiency was obtained using catalyst calcined at 300◦ C. It is due to the fact that the increase in calcination temperature can promote the transformation of anatase to rutile phase, which has a lower photocatalytic activity [23].
Mineralization of CAP In order to assess the degree of mineralization obtained during the photocatalytic treatment, the formation of CO2 and inorganic ions is generally determined. The term ‘photocatalytic degradation’ usually refers to complete photocatalytic oxidation or photomineralization, essentially to CO2 , H2 O, 3− NO− 3 , PO4 and halide ions [24]. Mineralization of CAP in this process was studied by TOC loss at different Ag loadings and calcination temperatures after 120 min irradiation time at the optimum conditions. TOC values have been related to the total concentration of organics in the solution and the decrease of TOC reflects the degree of mineralization. The results in Table 2 indicate that the reduction in the TOC value in the presence of 0.96% Ag/TiO2 calcined at 300◦ C is significantly higher than other catalysts.
Table 2. Reduction in the TOC value (%) of CAP solution in the presence of various Ag/TiO2 catalysts (I0 = 50 W/m2 , [CAP]0 = 8 mg/L, [Catalyst] = 500 mg/L). Catalyst 0.32 wt% Ag/TiO2 0.64 wt% Ag/TiO2 0.96 wt% Ag/TiO2 1.28 wt% Ag/TiO2 1.6 wt% Ag/TiO2
Calcination temperature (◦ C)
Reduction in the TOC value (%)
300 300 300 450 600 300 300
66 72 88 63 49 61 55
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Conclusion Ag/TiO2 nanoparticles were successfully prepared using a photodeposition method. The characterization of prepared Ag/TiO2 photocatalysts using XRD, SEM, EDX and TEM techniques revealed the presence and dispersion of silver nanoparticles on the surface of TiO2 . According to the TEM micrographs, Ag particles with a size of around 2–10 nm were located on the surface of the TiO2 nanoparticles. It was found that the 0.96 wt% Ag loading was the optimum doping content to achieve the highest efficiency of CAP photodegradation. Also, the results showed that the maximum degradation rate (or the optimum conditions to obtain the highest photocatalytic activity) of the 20 mg/L CAP solution has been observed for experiments carried out using 900 mg/L of 0.96wt% Ag/TiO2 photocatalyst calcined at a temperature of 300◦ C. The highest removal efficiency of CAP (D% ≈ 100) at the optimum conditions was observed in 20 min. For the photodegradation reactions carried out at optimum conditions about 88% reduction in TOC was measured after 120 min of reaction time. Acknowledgement The authors would like to thank the financial support of the Islamic Azad University – Tabriz branch and the Iranian Nanotechnology Society.
References [1] D. Klauson, M. Krichevskaya, M. Borissova, and S. Preis, Aqueous photocatalytic oxidation of sulfamethizole, Environ. Technol. 31 (2010), pp. 1547–1555. [2] T. Herberer, Occurrence, fate, and removal of pharmaceutical residues in the aquatic environment: A review of recent research data, Toxicol. Lett. 131 (2002), pp. 5–17. [3] B. Halling-Sørensen, S. Nors Nielsen, P.F. Lanzky, F. Ingerslev, H.C. Holten Lützhøft, and S.E. Jørgensen, Occurence, fate and effects of pharmaceutical substances in the environment – a review, Chemosphere 36 (1998), pp. 357–393. [4] K. Kümmerer, A. Al-Ahmad, and V. Mersch-Sundermann, Biodegradability of some antibiotics, elimination of their genotoxicity and affection of wastewater bacteria in a simple test, Chemosphere 40 (2000), pp. 701–710. [5] W. Baran, J. Sochacka, and W. Wardas, Toxicity and biodegradability of sulfonamides and products of their photocatalytic degradation in aqueous solutions, Chemosphere 65 (2006), pp. 1295–1299. [6] E.S. Elmolla and M. Chaudhuri, Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2 O2 /TiO2 photocatalysis, Desalination 252 (2010), pp. 46–52. [7] E. Hapeshi, A. Achilleos, M.I. Vasquez, C. Michael, N.P. Xekoukoulotakis, D. Mantzavinos, and D. Kassinos, Drugs degrading photocatalytically: Kinetics and mechanisms of ofloxacin and atenolol removal on titania suspensions, Water Res. 44 (2010), pp. 1737–1746. [8] A. Fujishima, X. Zhang, and D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008), pp. 515–582. [9] E.S. Elmolla and M. Chaudhuri, Comparison of different advanced oxidation processes for treatment of antibiotic aqueous solution, Desalination 256 (2010), pp. 43–47.
Downloaded by [USC University of Southern California] at 04:20 20 August 2013
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M. Shokri et al.
[10] C. Kormann, D.W. Bahnemann, and M.R. Hoffmann, Preparation and characterization of quantum-size titanium dioxide, J. Phys. Chem. B 92 (1988), pp. 5196–5201. [11] A.L. Linsebigler, G. Lu, and J.Y. Yates, Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results, Chem. Rev. 95 (1995), pp. 735–758. [12] B.F. Xin, L.Q. Jing, Z.Y. Ren, B.Q. Wang, and H.G. Fu, Effects of simultaneously doped and deposited Ag on the photocatalytic activity and surface states of TiO2 , J. Phys. Chem. B 109 (2005), pp. 2805–2809. [13] X. Li, L. Wang, and X. Lu, Preparation of silver-modified TiO2 via microwave-assisted method and its photocatalytic activity for toluene degradation, J. Hazard. Mater. 177 (2010), pp. 639–647. [14] A. Chatzitakis, C. Berberdou, I. Paspaltsis, G. Kyriakou, T. Sklaviadis, and I. Poulios, Photocatalytic degradation and drug activity reduction of chloramphenicol, Water Res. 42 (2008), pp. 386–394. [15] W. Bahnemann, M. Muneer, and M.M. Haque, Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions, Catal. Today 124 (2007), pp. 133–148. [16] J. Zhang, D. Fu, Y. Xu, and C. Liu, Optimization of parameters on photocatalytic degradation of chloramphenicol using TiO2 as photocatalyist by response surface methodology, J. Environ. Sci. 22 (2010), pp. 1281–1289. [17] M.A. Behnajady, N. Modirshahla, M. Shokri, and B. Rad, Enhancement of photocatalytic activity of TiO2 nanoparticles by silver doping: Photodeposition versus liquid impregnation methods, Global Nest J. 10 (2008), pp. 1–7.
[18] S. Anandan, P. Sathish Kumar, N. Pugazhenthiran, J. Madhavan, and P. Maruthamuthu, Effect of loaded silver nanoparticles on TiO2 for photocatalytic degradation of Acid Red 88, Sol. Energy Mater. Sol. C 92 (2008), pp. 929–937. [19] A. Zielinska, E. Kowalska, J.W. Sobczak, I. Łacka, M. Gazda, B. Ohtani, J. Hupka, and A. Zaleska, Silver-doped TiO2 prepared by microemulsion method: Surface properties, bio- and photoactivity, Sep. Purif. Technol. 72 (2010), pp. 309–318. [20] T. Sano, N. Negishi, D. Mas, and K. Takeuchi, Photocatalytic decomposition of N2 O on highly dispersed Ag+ ions on TiO2 preapared by photodeposition, J. Catal. 194 (2000), pp. 71– 79. [21] N. Sobana, K. Selvam, and M. Swaminathan, Optimization of photocatalytic degradation conditions of Direct Red 23 using nano-Ag doped TiO2 , Sep. Purif. Technol. 62 (2008), pp. 648–653. [22] N. Daneshvar, D. Salari, and A.R. Khataee, Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2 , J. Photochem. Photobiol. A: Chem. 162 (2004), pp. 317–322. [23] U.G. Akpan and B.H. Hameed, Parameters affecting the photocatalytic degradation of dyes using TiO2 -based photocatalysts: A review, J. Hazard. Mater. 170 (2009), pp. 520–529. [24] I.K. Konstantinou and T.A. Albanis, TiO2 -assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetic and mechanistic investigations, Appl. Catal. B: Environ. 49 (2004), pp. 1–14.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles a
a
a
Mohammad Shokri , Akbar Jodat , Nasser Modirshahla & Mohammad A. Behnajady
a
a
Department of Applied Chemistry , Faculty of Science, Tabriz Branch, Islamic Azad University , Tabriz , Iran Accepted author version posted online: 15 Nov 2012.Published online: 19 Nov 2012.
To cite this article: Mohammad Shokri , Akbar Jodat , Nasser Modirshahla & Mohammad A. Behnajady (2013) Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles, Environmental Technology, 34:9, 1161-1166, DOI: 10.1080/09593330.2012.743589 To link to this article: http://dx.doi.org/10.1080/09593330.2012.743589
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 9, 1161–1166, http://dx.doi.org/10.1080/09593330.2012.743589
Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles Mohammad Shokri∗ , Akbar Jodat, Nasser Modirshahla and Mohammad A. Behnajady Department of Applied Chemistry, Faculty of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran
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(Received 29 December 2011; final version received 10 October 2012 ) In this work, silver-doped TiO2 (Ag/TiO2 ) nanoparticles were synthesized using a photodeposition technique. The prepared Ag/TiO2 nanoparticles were characterized using TEM, SEM, XRD, and EDX techniques. The characterization of Ag/TiO2 nanoparticles using SEM and EDX techniques revealed the dispersion of Ag metal on the surface of TiO2 . The photocatalytic activity of Ag/TiO2 and bare TiO2 in the presence of ultraviolet irradiation was investigated in the removal of chloramphenicol (CAP) as an antibiotic. CAP is a broad-spectrum antibiotic exhibiting activity against both Gram-positive and Gram-negative bacteria, as well as other groups of microorganisms. However, it is, in certain susceptible individuals, associated with serious toxic effects in humans including bone marrow depression, particularly severe in the form of fatal aplastic anaemia. The effects of the operational factors, such as doping content of Ag, photocatalyst dosage and calcination temperature were evaluated in the catalytic activity of Ag/TiO2 . The results showed that the photocatalytic efficiency of TiO2 nanoparticles for the degradation of CAP, can be significantly improved by deposition an optimum amount of Ag nanoparticles (0.96 wt%) in the calcination temperature 300◦ C. It was found that 900 mg/L of Ag/TiO2 is the optimum dosage in the removal of CAP with 20 mg/L initial concentration. The highest removal efficiency of CAP (∼100%) at the optimum conditions was observed in 20 min. A mineralization study under optimum conditions showed about 88% reduction in total organic carbon after 120 min of irradiation time. Keywords: silver-doped TiO2 ; photodeposition; photocatalytic degradation; chloramphenicol
Introduction In recent years the presence of drugs, especially antibiotics, in the aquatic environment and thus its potential adverse effects have become of increasing concern [1]. Antibiotic substances are not readily biodegradable and either pass through biological treatment plants intact, or adsorb to the active sludge with subsequent desorption, accumulating in the environment [2,3]. The accumulation of antibiotics in organisms may cause arthropathy, nephropathy, damage to the central nervous system and spermatogenesis, mutagenic effects, and light sensitivity [4]. A further and possibly far greater threat is, however, the possible development of antibiotic-resistant microorganisms, including pathogens [5]. Furthermore, the presence of antibiotics in wastewaters has also increased and their abatement will be a challenge in the near future [6]. During the past years many investigations on chemical and biological technologies have been reported for the decomposition of organic pollutants in aqueous matrices. In this context, various advanced oxidation processes (AOPs) have been successfully employed for the degradation of a wide range of organic pollutants in water and wastewater [7]. Among the various AOPs, heterogeneous semiconductor photocatalysis using TiO2 as the photocatalyst has been ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
found capable of achieving the complete oxidation of the organic pollutants via hydroxyl radicals HO• and/or valence band holes h+ generated when the semiconductor is exposed to UV irradiation [8]. The main advantages of the process include a lack of mass transfer limitations and operation at ambient conditions. The catalyst itself is inexpensive, commercially available, non-toxic and photochemically stable [9]. However, the wide band gap of TiO2 (>3.0 eV) and the high recombination rate of the photo-induced electron– hole pairs formed in photocatalytic processes limit the efficiency of the photocatalytic degradation of pollutants [10,11]. Doping TiO2 with noble metals, such as Pt, Pd and Au can overcome the aforementioned deficiency. However, these noble metals are too expensive to be utilized on an industrial scale. Compared with Au, Pd and Pt, Ag is for less expensive and therefore deserves further investigation. Ag can act as an electron trap and promote the interfacial charge transfer processes in the composite systems, which reduces the recombination of the photo-induced electron–hole pairs, thus improving the photocatalytic activity of TiO2 [12,13]. Chloramphenicol (CAP) is a broad-spectrum antibiotic exhibiting activity against both Gram-positive and Gramnegative bacteria, as well as other groups of microorganisms. However, CAP is, in certain susceptible individuals,
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associated with serious toxic effects including bone marrow depression, and particularly severe in the form of fatal aplastic anaemia [14]. Degradation of CAP using TiO2 as a catalyst has been studied by several researchers [14–16]. However, to the best of our knowledge no systematic study has been reported on the photocatalytic degradation of CAP with the silver-doped TiO2 (Ag/TiO2 ) nanoparticles. In the present study, the photocatalytic activity of Ag/TiO2 and bare TiO2 nanoparticles in the presence of ultraviolet (UV) irradiation has been investigated in the removal of CAP. Ag/TiO2 nanoparticles were synthesized using a photodeposition method, and characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) techniques. Along the way, the effects of various operational parameters (doping content of Ag, photocatalyst dosage and calcination temperature) were evaluated to maximize the degradation of CAP under investigation. The extent of photodegradation and mineralization was measured by using a UV-visible spectrometer and total organic carbon (TOC) analyzer, respectively.
Preparation of the Ag/TiO2 catalyst The Ag/TiO2 photocatalyst was prepared based on the reduction of AgNO3 in a TiO2 suspension. Metallic Ag was deposited onto the TiO2 surface using a photodeposition method. A predetermined amount of TiO2 was added to 100 mL of deionized water. Then the required amount of AgNO3 for doping was added into the suspension of TiO2 . The pH of the TiO2 suspension was adjusted to 3. Then the mixture was irradiated with UV light (30 W, λmax = 254 nm, manufactured by Philips, Holland) for 3 h and then dried at 100◦ C for 12 h. The dried solid was calcined at 300◦ C for 3 h in a furnace.
Material and methods Reagents
Evaluation of photocatalytic activity For the photocatalytic degradation studies, the CAP solution containing an appropriate dosage of Ag/TiO2 powder was prepared in water in darkness during the experiment. The solution was stirred for 30 min to reach the adsorption equilibration in the dark before irradiation. Afterwards, 100 mL of the above suspension was transferred into a borosilicate Petri dish (12 cm diameter and 2.5 cm height) as the photoreactor. The reaction mixture was stirred vigorously under the illumination of UV-C light from top of the solution using a UV lamp (15 W, UV-C, λmax = 254 nm, manufactured by Philips, Holland). The distance of the lamp from the solution was adjusted in such a way to obtain a light intensity of 50 W/m2 on the surface of the solution, which was measured by a Lux-UV-IR meter from Leybold Company. For the kinetic studies, at certain reaction intervals, 5 mL of the sample was withdrawn. To remove the Ag/TiO2 particles from the reaction media, the solution was centrifuged for 15 min at 5000 rpm. The variation in the CAP concentration was observed from its characteristic absorption band at 275 nm by means of a UV-visible spectrophotometer using a calibration curve.
TiO2 -P25 (Degussa), (80% anatase, 20% rutile; BET area 50 m2 /g; primary size 21 nm) was used as a supporting material. AgNO3 (99.9%) and H2 SO4 were obtained from Merck. CAP, as a model antibiotic pollutant, was purchased from Panreac. Table 1 shows the chemical structure and other characteristics of this antibiotic. Table 1.
Structure and characteristics of CAP. OH HO
O
H
Cl H
NH Cl
NO2
Molecular formula Trade names
IUPAC name
CAS number λmax (nm) Molecular mass (g mol−1 )
C11 H12 N2 O5 Cl2 Alficetyn, Amphicol, Biomicin, Chlornitromycin, Chloromycetin, Fenicol, Phenicol, Medicom, Nevimycin, Vernacetin, Veticol. 2,2-dichloro-N -[(1R,2R)-2hydroxy-1-(hydroxymethyl)2-(4-nitrophenyl)ethyl] acetamide 56-75-7 275 323.13
Characterization methods The crystalline phase of the nanoparticles were analyzed by XRD measurements by using Siemens XRD-D5000 (λ = 0.154 nm). SEM analysis was performed on Au coated samples using a Philips apparatus model XL30, equipped with a probe for the EDX microanalysis. The morphology of the Ag/TiO2 nanoparticles was characterized by TEM (PHILIPS CM 10 −100 keV).
Analytical methods A UV-visible spectrophotometer (Ultrospec 2000, England) was used to measure the concentration of CAP. The variations in the concentration of CAP with reaction time were measured using the absorption peak intensity at wavelength of 275 nm. The concentration of TOC in the solution was measured using a TOC analyzer (Shimadzu, TOC-VCSH ).
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A : Anatase R : Rutile
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200 150
A
A
R
R A
A
R
R (b)
100 50
(a) 0 20
30
40
2q
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Figure 1. X-Ray diffraction patterns of: (a) undoped TiO2 and (b) 0.96 wt% Ag/TiO2 nanoparticles.
Results and discussion The characterization of the Ag/TiO2 nanoparticles The prepared Ag/TiO2 nanoparticles were characterized using XRD, SEM, EDX, and TEM techniques. Figure 1(a) and (b) shows the XRD pattern of pure and doped TiO2 powders by the optimum amounts of Ag prepared by photodeposition method. The XRD pattern shown in the figure consists of mainly anatase with a minor rutile phase (80 : 20). All the photocatalysts show the dominant anatase phase peaks at 2θ = 25.2, 38, 48.2, 55 and 62.5◦ and the small fraction of the rutile phase with peaks at 2θ = 27.5, 36, 54 and 69◦ . The XRD patterns of the Ag/TiO2 samples almost coincide with that of pure TiO2 showing no diffraction peaks due to Ag doping thus suggesting that the Ag dopants are merely placed on the surface of the crystals [17]. A comparison of the SEM micrographs of pure TiO2 and the Ag/TiO2 nanoparticles proved that the loading of Ag nanoparticles did not affect the spherical shape of the TiO2 particles (Figure 2). The EDX analysis (Figure 3) also confirmed the loading of Ag on TiO2 . The TEM image of the Ag/TiO2 nanoparticles prepared using photodeposition is shown in Figure 4. The shape and size of the titania crystallites were unchanged as a result of surface modification by Ag particles. The Ag particles (black dots indicated) were located on the surface of the individual TiO2 nanoparticles. Also, there is a possibility for the Ag to be incorporated into the interstitial positions of the semiconductor particles [18]. The estimated range of the deposited Ag nanoparticles was 2– 10 nm. Agglomerates of metallic Ag were also observed on TiO2 in addition to small Ag particles. The effect of doping content of Ag When compared to pure TiO2 , the TiO2 loaded with 0.32, 0.64, 0.96, 1.28 and 1.6 wt% Ag exhibited a significant increase in the CAP degradation rate as shown in Figure 5. It was observed that the degradation efficiency increases with an increase in the Ag loading up to 0.96 wt% (optimum
Figure 2. SEM micrograph of: (a) TiO2 -P25 and (b) 0.96 wt% Ag/TiO2 nanoparticles.
Element wt% at% OK 36.53 63.43 Ti K 62.72 36.38 Ag L 0.75 0.19 Total 100 100
Figure 3.
EDX analysis of 0.96 wt% Ag/TiO2 nanoparticles.
metal loading) and then decreases. The Ag nanoparticles deposited on the titanium dioxide surface can act as electron–hole separation centres. This results in the formation of a Shottky barrier at the metal–semiconductor contact region, which improves the charge separation and thus enhances the photocatalytic activity of TiO2 . In contrast, at an Ag amount above its optimum value, the Ag nanoparticles can also act as recombination centres, which results in decreasing the photocatalytic activity of TiO2
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Figure 4.
TEM micrograph of 0.96 wt% Ag/TiO2 nanoparticles prepared by photodeposition method.
100
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80 - TiO2-P25
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D%
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0.32 wt% Ag-TiO2 0.64 wt% Ag-TiO2 0.96 wt% Ag-TiO2 + 1.28 wt% Ag-TiO2 D 1.6 wt% Ag-TiO2
40 20
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0 0
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Figure 5. Degradation efficiency (%) of CAP using Ag/TiO2 catalysts with different Ag loading as a function of irradiation time to UV light (I0 = 50 W/m2 , [CAP]0 = 20 mg/L, [Catalyst] = 300 mg/L).
0
6
12 18 Time (min)
24
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Figure 6. Degradation efficiency (%) of 20 mg/L solution of CAP using different dosages of 0.96 wt% Ag/TiO2 catalyst versus UV irradiation time (I0 = 50 W/m2 ).
[19]. Sano et al. [20] observed that the increase of silver amount resulted in Ag aggregation leading to decrease in photoactivity. For the detrimental effect of increasing Ag loading on the photocatalytic activity of TiO2 , several reasons have been mentioned in the literature: (1) the probability for the hole capture is increased by the large number of negatively charged Ag particles on TiO2 at a high Ag content, which reduces the efficiency of charge separation [21] and (2) the excessive coverage of the TiO2 catalyst with Ag nanoparticles limits the amount of light reaching to the TiO2 surface, reducing the number of photogenerated e− –h+ pairs and consequently, lowering the TiO2 photoactivity [17].
The degradation efficiency of the photocatalyst exhibits an increase by increasing the catalyst dosage up to 900 mg/L and above this dosage the removal efficiency decreases. The enhancement of the CAP removal rate is due to the increase in active sites available for photocatalytic reaction as the loading of the catalyst is increased. It is obvious that the rate increases with an increase in the amount of catalyst up to a level corresponding to the optimum light absorption. Above this value the suspended particles of the catalyst block the UV light passage and increase the light scattering [22]. Thus, any further increase of the catalyst dosage above 900 mg/L has inverse effect on the photocatalytic reaction. Hence, the optimum amount of the catalyst dosage (900 mg/L, 0.96wt% Ag/TiO2 ) was used for the photocatalytic reactions.
The effect of catalyst dosage To determine the effect of the catalyst dosage, a series of experiments was carried out by varying the amount of catalyst with optimum Ag loading (0.96 wt%), meanwhile the other reaction conditions were kept the same. The degradation efficiency of CAP using various catalyst dosages is depicted in Figure 6.
The effect of calcination temperature In order to study the influence of the calcination temperature on the photocatalytic activity of the catalyst, the 0.96wt% Ag/TiO2 was calcined at 300, 450 and 600◦ C for 3 h, while other experimental conditions were kept constant. As it can be seen in Figure 7, the results of the photocatalytic degradation experiments using these samples on the solutions with
Environmental Technology 100
D%
80 60 300 °C 450 °C 600 °C
40 20 0
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0
6
12 18 Time (min)
24
30
Figure 7. Degradation efficiency (%) of CAP solution using of 0.96wt% Ag/TiO2 catalyst calcined at different temperatures versus UV irradiation time (I0 = 50 W/m2 , [CAP]0 = 20 mg/L, [Catalyst] = 900 mg/L).
20 mg/L of CAP showed that with increasing the calcination temperature, the photocatalytic activity of the catalysts were decreased significantly and the highest degradation efficiency was obtained using catalyst calcined at 300◦ C. It is due to the fact that the increase in calcination temperature can promote the transformation of anatase to rutile phase, which has a lower photocatalytic activity [23].
Mineralization of CAP In order to assess the degree of mineralization obtained during the photocatalytic treatment, the formation of CO2 and inorganic ions is generally determined. The term ‘photocatalytic degradation’ usually refers to complete photocatalytic oxidation or photomineralization, essentially to CO2 , H2 O, 3− NO− 3 , PO4 and halide ions [24]. Mineralization of CAP in this process was studied by TOC loss at different Ag loadings and calcination temperatures after 120 min irradiation time at the optimum conditions. TOC values have been related to the total concentration of organics in the solution and the decrease of TOC reflects the degree of mineralization. The results in Table 2 indicate that the reduction in the TOC value in the presence of 0.96% Ag/TiO2 calcined at 300◦ C is significantly higher than other catalysts.
Table 2. Reduction in the TOC value (%) of CAP solution in the presence of various Ag/TiO2 catalysts (I0 = 50 W/m2 , [CAP]0 = 8 mg/L, [Catalyst] = 500 mg/L). Catalyst 0.32 wt% Ag/TiO2 0.64 wt% Ag/TiO2 0.96 wt% Ag/TiO2 1.28 wt% Ag/TiO2 1.6 wt% Ag/TiO2
Calcination temperature (◦ C)
Reduction in the TOC value (%)
300 300 300 450 600 300 300
66 72 88 63 49 61 55
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Conclusion Ag/TiO2 nanoparticles were successfully prepared using a photodeposition method. The characterization of prepared Ag/TiO2 photocatalysts using XRD, SEM, EDX and TEM techniques revealed the presence and dispersion of silver nanoparticles on the surface of TiO2 . According to the TEM micrographs, Ag particles with a size of around 2–10 nm were located on the surface of the TiO2 nanoparticles. It was found that the 0.96 wt% Ag loading was the optimum doping content to achieve the highest efficiency of CAP photodegradation. Also, the results showed that the maximum degradation rate (or the optimum conditions to obtain the highest photocatalytic activity) of the 20 mg/L CAP solution has been observed for experiments carried out using 900 mg/L of 0.96wt% Ag/TiO2 photocatalyst calcined at a temperature of 300◦ C. The highest removal efficiency of CAP (D% ≈ 100) at the optimum conditions was observed in 20 min. For the photodegradation reactions carried out at optimum conditions about 88% reduction in TOC was measured after 120 min of reaction time. Acknowledgement The authors would like to thank the financial support of the Islamic Azad University – Tabriz branch and the Iranian Nanotechnology Society.
References [1] D. Klauson, M. Krichevskaya, M. Borissova, and S. Preis, Aqueous photocatalytic oxidation of sulfamethizole, Environ. Technol. 31 (2010), pp. 1547–1555. [2] T. Herberer, Occurrence, fate, and removal of pharmaceutical residues in the aquatic environment: A review of recent research data, Toxicol. Lett. 131 (2002), pp. 5–17. [3] B. Halling-Sørensen, S. Nors Nielsen, P.F. Lanzky, F. Ingerslev, H.C. Holten Lützhøft, and S.E. Jørgensen, Occurence, fate and effects of pharmaceutical substances in the environment – a review, Chemosphere 36 (1998), pp. 357–393. [4] K. Kümmerer, A. Al-Ahmad, and V. Mersch-Sundermann, Biodegradability of some antibiotics, elimination of their genotoxicity and affection of wastewater bacteria in a simple test, Chemosphere 40 (2000), pp. 701–710. [5] W. Baran, J. Sochacka, and W. Wardas, Toxicity and biodegradability of sulfonamides and products of their photocatalytic degradation in aqueous solutions, Chemosphere 65 (2006), pp. 1295–1299. [6] E.S. Elmolla and M. Chaudhuri, Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2 O2 /TiO2 photocatalysis, Desalination 252 (2010), pp. 46–52. [7] E. Hapeshi, A. Achilleos, M.I. Vasquez, C. Michael, N.P. Xekoukoulotakis, D. Mantzavinos, and D. Kassinos, Drugs degrading photocatalytically: Kinetics and mechanisms of ofloxacin and atenolol removal on titania suspensions, Water Res. 44 (2010), pp. 1737–1746. [8] A. Fujishima, X. Zhang, and D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008), pp. 515–582. [9] E.S. Elmolla and M. Chaudhuri, Comparison of different advanced oxidation processes for treatment of antibiotic aqueous solution, Desalination 256 (2010), pp. 43–47.
Downloaded by [USC University of Southern California] at 04:20 20 August 2013
1166
M. Shokri et al.
[10] C. Kormann, D.W. Bahnemann, and M.R. Hoffmann, Preparation and characterization of quantum-size titanium dioxide, J. Phys. Chem. B 92 (1988), pp. 5196–5201. [11] A.L. Linsebigler, G. Lu, and J.Y. Yates, Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results, Chem. Rev. 95 (1995), pp. 735–758. [12] B.F. Xin, L.Q. Jing, Z.Y. Ren, B.Q. Wang, and H.G. Fu, Effects of simultaneously doped and deposited Ag on the photocatalytic activity and surface states of TiO2 , J. Phys. Chem. B 109 (2005), pp. 2805–2809. [13] X. Li, L. Wang, and X. Lu, Preparation of silver-modified TiO2 via microwave-assisted method and its photocatalytic activity for toluene degradation, J. Hazard. Mater. 177 (2010), pp. 639–647. [14] A. Chatzitakis, C. Berberdou, I. Paspaltsis, G. Kyriakou, T. Sklaviadis, and I. Poulios, Photocatalytic degradation and drug activity reduction of chloramphenicol, Water Res. 42 (2008), pp. 386–394. [15] W. Bahnemann, M. Muneer, and M.M. Haque, Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions, Catal. Today 124 (2007), pp. 133–148. [16] J. Zhang, D. Fu, Y. Xu, and C. Liu, Optimization of parameters on photocatalytic degradation of chloramphenicol using TiO2 as photocatalyist by response surface methodology, J. Environ. Sci. 22 (2010), pp. 1281–1289. [17] M.A. Behnajady, N. Modirshahla, M. Shokri, and B. Rad, Enhancement of photocatalytic activity of TiO2 nanoparticles by silver doping: Photodeposition versus liquid impregnation methods, Global Nest J. 10 (2008), pp. 1–7.
[18] S. Anandan, P. Sathish Kumar, N. Pugazhenthiran, J. Madhavan, and P. Maruthamuthu, Effect of loaded silver nanoparticles on TiO2 for photocatalytic degradation of Acid Red 88, Sol. Energy Mater. Sol. C 92 (2008), pp. 929–937. [19] A. Zielinska, E. Kowalska, J.W. Sobczak, I. Łacka, M. Gazda, B. Ohtani, J. Hupka, and A. Zaleska, Silver-doped TiO2 prepared by microemulsion method: Surface properties, bio- and photoactivity, Sep. Purif. Technol. 72 (2010), pp. 309–318. [20] T. Sano, N. Negishi, D. Mas, and K. Takeuchi, Photocatalytic decomposition of N2 O on highly dispersed Ag+ ions on TiO2 preapared by photodeposition, J. Catal. 194 (2000), pp. 71– 79. [21] N. Sobana, K. Selvam, and M. Swaminathan, Optimization of photocatalytic degradation conditions of Direct Red 23 using nano-Ag doped TiO2 , Sep. Purif. Technol. 62 (2008), pp. 648–653. [22] N. Daneshvar, D. Salari, and A.R. Khataee, Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2 , J. Photochem. Photobiol. A: Chem. 162 (2004), pp. 317–322. [23] U.G. Akpan and B.H. Hameed, Parameters affecting the photocatalytic degradation of dyes using TiO2 -based photocatalysts: A review, J. Hazard. Mater. 170 (2009), pp. 520–529. [24] I.K. Konstantinou and T.A. Albanis, TiO2 -assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetic and mechanistic investigations, Appl. Catal. B: Environ. 49 (2004), pp. 1–14.
