Article pubs.acs.org/IECR
Photoelectrocatalysis of Cefotaxime Using Nanostructured TiO2 Photoanode: Identification of the Degradation Products and Determination of the Toxicity Level Vijay V. Kondalkar,† Sawanta S. Mali,‡,§ Rahul M. Mane,† P. B. Dandge,⊥ Sipra Choudhury,∥ Chang K. Hong,§ Pramod S. Patil,‡ Shivajirao R. Patil,# Jin H. Kim,¶ and Popatrao N. Bhosale*,† †
Materials Research Laboratory, Department of Chemistry, ‡Thin Film Materials Laboratory, Department of Physics, ⊥Department of Biochemistry, and #Fluorescence Spectroscopy Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India ∥ Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India § School of Applied Chemical Engineering and ¶Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea S Supporting Information *
ABSTRACT: Nanostructured TiO2 thin films were fabricated via a facile, economical, and energy-efficient microwave-assisted dip-coating (MWDC) technique. Further, the resulting TiO2 films were characterized by means of X-ray diffraction, highresolution transmission electron microscopy, selected-area electron diffraction, Fourier transform Raman spectroscopy, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy techniques for their phase structure, morphology, and optical and surface properties. TiO2-mediated photoelectrocatalytic degradation of the antibiotic cefotaxime (CFX) in an aqueous solution was studied by varying the pH under UV illumination. The degradation intermediates and possible reaction degradation path of CFX were analyzed by electrospray ionization time-of-flight mass spectrometry (MS). The MS spectra revealed that degradation of CFX occurs through β-lactum corresponding to the cleavage of the cephem nucleus. Moreover, the antibacterial activity of CFX prior to and after photoelectrocatalytic degradation was carried out to analyze the toxicity against Staphylococcus aureus and salmonella typhi bacteria. Interestingly, it was observed that the antibiotic activity was drastically inhibited after photoelectrocatalytic degradation of the CFX solution. The photoelectrocatalytic stability of a nanostructured TiO2 electrode was evaluated by recycling the degradation experiments. It was observed that there was no significant decrease in the catalytic activity, indicating potential applications of the TiO2 electrode prepared by the MWDC method.
1. INTRODUCTION Over the last 30 years, pharmaceuticals have been widely used in human, aquaculture, and veterinary applications for disease treatment and improved quality of human life.1−3 Pharmaceuticals constitute an emerging environmental problem because of enormous use of antibiotics that can be easily found in the aqueous environment. These compounds have been observed in sewage effluent,4 surface water,5 groundwater,6 and even drinking water.7 The pharmaceutical compounds can reach the aquatic environment through various sources such as effluents from conventional wastewater treatment plants,8,9 pharmaceutical manufacturing wastewater,10 hospital effluents,11 and excretion from humans and livestock.1,2 Among the various pharmaceutical drugs, antibiotics have been of great environmental interest because of the development of antibioticresistant bacteria that cannot be treated with presently known drugs and the potential increase in chemical toxicity.12−14 Antibiotics have an adverse effect on humans and the aquatic ecosystem. A typical example of antibiotics is cefotaxime (CFX), which is a synthetic antibiotic. It is a third-generation cephalosporin antibiotic.15 CFX has broad spectral activity against Grampositive and Gram-negative bacteria as well as other groups of microorganism. CFX is low cost and readily available so it is extensively used. Its structure is as follows.16 © 2014 American Chemical Society
Thus, the abatement of antibiotics in the environment is a challenging task in the near future. Various technologies have been used to solve this environmental issue such as conventional methods like adsorption, membrane technique, biosorption, filtration, coagulation, flocculation, and sedimentation.17−20 Because of the low efficiency of these techniques and sometimes their lack of usefulness, new alternative advance oxidation processes (AOPs) have emerged.21 AOPs include the different processes of chemical oxidation (O3, O3/H2O2, and H2O2/Fe2+), photochemical (UV, UV/O3, and UV/H2O2), and photocatalysis (Fe2+/H2O2/UV, TiO2/ZnO/UV) for generating the hydroxyl radical (OH•), which is highly reactive and less Received: Revised: Accepted: Published: 18152
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selective, having standard oxidation potential E° = 2.8 V.22−25 AOPs, which have been successfully applied to eliminate various pharmaceutical compounds, still it have more complexity, high chemical consumption, and complicated usage and are inapplicable to industrial scale.26 In a particulate photocatalysis system, photocatalytic (PC) oxidation and reduction occur on the photocatalyst surface, leading to the rapid recombination of a photogenerated electron and hole pairs and lower photocatalysis efficiency.27−30 However, in commercial application, separation and recycling of the ultrafine catalyst from the treated wastewater is difficult, which may repollute the treated water and is time-consuming and expensive. These problems can be solved by immobilization of the photocatalyst on the solid support, which exhibits an excellent ability in the separation of nanosized materials and is cost-effective.31 There is renewed interest in the development of a wastewater treatment system that inevitably involves photoelectrocatalysis (PEC) based on a metal oxide semiconductor, in which TiO2, WO3, ZnO, ZrO2, Fe2O3, and BiVO4 were used as photocatalysts.32−34 Among all of these, TiO2 is still one of the most studied because it offers a number of advantages such as chemical stability, low-cost, nontoxicity, long durability, a high refractive index, a wide band gap, and high activity.35 PEC is a combination of PC and electrochemical processes, which are reliable for the remediation of pharmaceuticals in wastewater. PEC is an example of an electrochemical advance oxidation process (EAOP) capable of preventing the recombination of photogenerated electron−hole pairs on the PC surface and thus significantly improving the degradation of pharmaceuticals. The PEC approach is highly valuable because the oxidation halfreaction is physically separated from the reduction half-reaction, which allows the reaction to be studied separately.36−40 In this work, nanostructured TiO2 thin films (64 cm2) were prepared using a microwave-assisted dip-coating (MWDC) method. The resulting TiO2 thin films were characterized by various characterization techniques. The objectives of this investigation are (i) to gain a mechanistic pathway in PEC for the use of CFX as a relevant model antibiotic, (ii) to evaluate PEC by TiO2 electrodes deposited at different microwave radiation times and the effect of the pH on the degradation, and (iii) to analyze the antibacterial activity of the residual solution during PEC. To the best of our knowledge, we report herein for the first time PEC of CFX and the antibacterial activity of the residual solution.
varied as 4, 8, 12, and 16 min, which were designated as T4, T8, T12, and T16 samples, respectively. After microwave irradiation, the resulting solution was a slightly deeper yellow colloidal and remained clear of precipitate. Upon a further increase in the microwave irradiation time, the solution became highly turbid and precipitate formed. For dip coating, the colloidal solution was transferred to a narrow and tall 100 cm3 specially designed beaker. This confirms that most of the slide should be immersed in the colloidal solution. The TiO2 electrode was deposited by dipcoating clean F:SnO2 (FTO) glass substrates into the colloidal solution, withdrawing the substrate from the sol at a steady rate of 3 cm3/min. Finally, the films were dried at ambient temperature and annealed at 200 °C for 2 h. 2.3. TiO2 Electrode Characterization. Crystallographic characterization of the TiO2 electrode was carried out using Xray diffraction (XRD; D/MAX Uitima III XRD spectrometer, Rigaku, Japan) with Cu Kα radiation (λ = 1.542 Å). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) observation, and selected-area electron diffraction (SAED) were performed with a transmission electron microscope (Philips TECNAI F20, Amsterdam, The Netherlands) operating at an acceleration voltage of 200 kV. Raman spectra of the films were conducted on a Raman spectrophotometer (Bruker MultiRAM, German made) in the spectral range of 100−800 cm−1. UV−visible absorbance spectra of the TiO2 electrode were recorded in the wavelength range 300−1100 nm on a UV−visible spectrophotometer (Shimadzu UV-1800, Kyoto, Japan). The chemical state of the elements and surface chemistry were analyzed by an X-ray photoelectron spectrometer (Thermo Scientific VG Multilab2000, K-alpha, Waltham, MA). Photoluminescence (PL) spectra were recorded at room temperature. The fluorescence spectrometer (Shimadzu RF-5301 PC, Kyoto, Japan) used a 520 nm excitation wavelength. 2.4. PEC Experiments. The PEC reactor module consisted of a large-area 100 cm2 TiO2 thin film as the photoanode and a stainless steel disk as the counter electrode separated by a distance of 1 mm and facing toward the TiO2 photoanode. The photoactive area (64 cm2) of the TiO2 electrode was illuminated from the back side using UV (5 mW/cm2) light as the source. An external bias voltage of 1.5 V was used in order to reduce the recombination of the electron−hole pair and enhance the rate of the reaction using an Autolab PGSTAT100 FRA-32 potentiostat/1.5 V alkaline battery. An aqueous solution of 50 ppm of CFX antibiotic was used as the model organic pollutant. A 100 mL solution of CFX was inserted into the reservoir and recirculated through a specially designed single-cell PEC reactor with a constant flow rate of 14.4 L/h using a Reval peristaltic pump with silicon tubing. The experimental setup of the photoelectrocatalytic reactor module with a single cell is shown (Figure S1 in the Supporting Information). A specific amount of pollutant was taken from the reservoir at specific intervals of reaction time. 2.5. Analytical Procedures. The degradation intermediate product identification done by an ACQUITY UPLC system having a binary pump, sample manager, and column oven hyphenated to a high-resolution mass spectrometer (QTOFMS) system (Synapt G2, Waters Corp., Milford, MA). The samples were maintained at 10 °C in an autosampler. UPLC separation was performed by injecting 5 μL through the autosampler on a BEH C18 column (100 × 2 mm, 1.7 μm, Waters India Pvt. Ltd., Bangalore, India) maintained at 40 °C at
2. EXPERIMENTAL SECTION 2.1. Materials. Titanium tetraisopropoxide (TTIP, 99%), acetylacetone (AcAc), concentrated hydrochloric acid (HCl, 36%), and ethanol were of analytical reagent grade and were purchased from Sigma-Aldrich, and CFX standards with purity >99% were purchased from Alkem. All of the chemicals were used without further purification. 2.2. TiO2 Electrode Fabrication. Synthesis of the TiO2 electrode through a MWDC technique is typically achieved by chelating TTIP (14.2 g, 0.05 mol) with AcAc (2.5 g, 0.025 mol) in ethanol (74.4 g, 0.35 mol), forming a clear homogeneous yellow solution. The solution was stirred for 10 min before the addition of water (3.66 g, 0.2 mol). Hydrolysis of the yellow solution was done by the dropwise addition of water. The pH of the solution was adjusted to 2 by HCl and further stirred for 10 min. Controlled hydrolysis and polycondensation reactions were carried out by microwave irradiation at 50% microwave power (960 W, at 2.45 GHz). The duration of irradiation was 18153
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a mobile phase flow rate of 0.4 mL/min. The mobile phase was composed of (A) methanol/water (10:90, v/v), 5 mM ammonium formate, and 0.2% formic acid and (B) methanol/water (90:10, v/v), 5 mM ammonium formate, and 0.2% formic acid. A gradient program was 0−0.5 min/90% A, 0.5−4.5 min/90−50% A, 4.5−8 min/50−2% A, 8−11 min/2% A, 11−12 min/2−90% A, and 12−14 min/90% A. The ACQUITY UPLC system was coupled with a QTOF-MS system having an electrospray ionization (ESI) source. The ESI source parameters like capillary voltage (3 kV), sampling cone voltage (35 V), and extraction cone voltage (4 V) were optimized. The source temperature was 135 °C with a desolvation temperature of 500 °C. The cone and desolvation gases were 50 and 1000 psi, respectively. Data acquisition was performed in resolution and sensitivity modes with a MSE scan mode in the range of 100−1200 amu. The MSE scan mode provides MS data at low energy (4 V) and MS/MS data highenergy ramping (15−45 V). 2.6. Antimicrobial Susceptibility Test. The antibacterial susceptibility was determined by the agar well diffusion method, using Gram-positive Staphylococcus aureus (SA) and Gramnegative salmonella typhi bacteria. The nutrient agar medium contained 10 g of peptone, 10 g of beef exstract, 5 g of NaCl, and 23 g of agar in 1 L. The medium and Petri dish were sterilized by autoclaving at 121 °C at 15 lb. for 20 min. The nutrient agar medium was poured into the Petri dish and allowed to solidify for a time such that the temperature was not high enough to kill the bacteria. The bacterial suspension (115 μL) was then spread over the nutrient medium plates, and 4 mm wells were made in the agar plates. Each well was filled with 40 μL of standard CFX and a photoelectrocatalytically degraded solution at different time intervals. The plates were then incubated at 37 °C for 24 h prior to the observation of growth of the inhibition zone.
Figure 2. (a) TEM images of nanostructured TiO2 samples T4, T8, T12, and T16. (b) SAED pattern of T16. (c) HRTEM image of the T16 sample.
3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction (XRD). The XRD patterns of the T4−T16 sample were investigated to examine the crystal
Figure 3. FT-Raman spectrum of the T16 sample in the wavenumber range of 100−700 cm−1.
(200), (211), (204), (220), (215), and (101) reflections in all of the T4−T16 samples, and no other characteristic peak was detected, revealing that TiO2 is in a pure anatase phase. The crystallite sizes of the TiO2 thin films were estimated using Scherrer’s equation (1) for the (101) peak
Figure 1. XRD patterns of nanostructured TiO2 thin films at different microwave irradiation times for the T4, T8, T12, and T16 samples.
structure and phase purity. Figure 1 shows the XRD patterns of the T4−T16 samples synthesized at different microwave irradiation times. The diffraction peaks of the thin films were in good agreement with tetragonal anatase-type TiO2 (JSPDS-01071-1168). All of the diffraction peaks were indexed to (004),
D=
kλ β cos θ
(1)
where k is the dimensionless constant (0.94), λ is the wavelength of X-rays (1.5406 Å), β is the full width at half18154
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Figure 5. UV−visible absorption spectra of TiO2 thin films at different microwave irradiatin times for samples T4, T8, T12, and T16. Inset: Plot of (αhν)1/2 versus hν.
Figure 6. PL spectra of TiO2 thin films at different microwave irradiation times for samples T4, T8, T12, and T16.
Figure 4. XPS spectrum of the T16 sample: (a) survey spectrum of TiO2; (b) Ti 2p core level spectrum. (c) Deconvolution of the O 1s core level.
Figure 7. I−V plots of the TiO2 thin films used in the PEC reactor module at different microwave irradiation times for samples T4, T8, T12, and T16.
maximum (fwhm) intensity, θ is the diffraction angle, and D is the crystallite size. The microwave irradiation time increases as the width of the (101) reflection becomes narrower from the T4 sample to the T16 sample, implying a larger crystal size range of 8−14 nm. The overall XRD patterns of the T4−T16 samples
remain unchanged, but there was a slight increase in the peak intensity, indicating improvement in the crystallinity. The XRD result reveals that MWDC is a promising technique for the synthesis of TiO2 thin films to enhance the optoelectronic 18155
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3.3. Fourier Transform (FT)-Raman Spectroscopy. Figure 3 illustrates the FT-Raman spectrum of the T16 sample. The four Raman bands were observed at 144, 396, 516, and 639 cm−1 in the spectrum, corresponding to the phonon vibration modes of the anatase phase. The Raman bands at 144 and 639 cm−1 were assigned to the Eg vibration mode, which is due to the symmetric stretching vibration of O−Ti−O in the TiO2 octahedron. The Raman bands at 396 and 516 cm−1 are assigned to the B1g and (A1g, B2g) modes, respectively. The peak at 396 cm−1 is due to the antisymmetric bending vibration of O−Ti−O, and the B1g peak is caused by the symmetric bending vibration of O−Ti−O.41 The peak intensity of the Raman modes was high, revealing that the surface defect content of TiO2 nanoparticles decreases upon an enhancement in the crystallinity. 3.4. X-ray Photoelectron Spectroscopy (XPS). XPS is a highly surface-sensitive technique for surface analysis. XPS was imploded to analyze the surface oxidation state and chemical composition of TiO2 thin films. Figure 4a shows the XPS survey spectrum of sample T16. It reveals that the surface was composed of titanium, oxygen, and a trace amount of carbon, which was allocated to adventitious carbon. The Ti 2p core level of the XPS spectrum of the TiO2 thin film is shown in Figure 4b, with the doublet consisting of two broad peaks at 458.75 and 464.54 eV estimated for Ti 2p3/2 and Ti 2p1/2, respectively. The splitting between the two core levels, i.e., Ti 2p3/2 and Ti 2p1/2, is 5.79 eV, indicating a 4+ valence state of titanium and its octahedral coordination with oxygen.42 Figure 4c shows the O 1s core level spectrum, which exhibits a broad peak with a high-binding-energy side, suggesting that several kinds of oxygen species are present. The O 1s core level can be deconvoluted to yield three Gaussian curves. The main curve at a binding energy of 530.06 eV is assigned to oxygen in the TiO2 lattice in the manner of O−Ti−O. The second curve at a binding energy of 531.80 eV is due to the hydroxyl group (OH−) on the TiO2 surface. The third peak at 533.12 eV is assigned to oxygen of an atmospheric water molecule adsorbed on the TiO2 thin films.43 3.5. Optical Absorption. Figure 5 shows the UV−visible absorption spectra of the T4−T16 samples recorded in the wavelength range of 300−1100 nm. The absorbance edge of the TiO2 samples showed a red shift with increasing microwave irradiation time. This is attributed to the quantum size effects of the semiconductor, in which the band gap decreases with an increase in the crystallite size. The light absorption efficiency increases with the addition of crystalline porous TiO2 thin films because the interconnecting nanoparticles form a highway network for the flow of electrons.44 Optical data were analyzed using a classical absorption equation in order to confirm the band-edge transition in the entire sample. The band-gap energies (Eg) of all samples were estimated from the Tauc plots, i.e., variation of (αhν)1/2 versus hν. The Eg value was obtained by extrapolating a portion of the higher photon energy to zero absorption coefficient (α = 0). It is observed that Eg varies from 3.3 to 2.8 eV for the T4−T16 samples.45 3.6. Photoluminescence (PL) Spectra. PL spectroscopy is very useful to disclosing the efficiency of charge-carrier trapping, immigration, and transfer in TiO2. PL is the emission of light originating from the radiative recombination of photogenerated electrons and holes. Figure 6 shows room temperature PL spectra of the T4−T16 samples occurring from different microwave irradiation times. Typically PL emission signals were at 399, 439, and 472 nm, which were equivalent to
Figure 8. PEC of CFX using TiO2 electrodes deposited at different microwave irradiation times.
Figure 9. Effect of the initial solution pH on PEC of CFX for the T16 sample.
properties by minimizing the defect level and increasing the crystallinity. 3.2. High-Resolution Transmission Electron Microscopy (HRTEM). TEM images of samples T4−T16 are depicted in Figure 2a. A wormhole-like structure consisting of tightly interconnected nanocrystalline TiO2 particles with sizes of around 8−18 nm has been observed. Small particles aggregate to form large particles and increase the interconnectivity to form the porous nature with increasing microwave irradiation time. The defect level and number of grain boundaries were minimized because of good necking between particles, which suppresses electron recombination or the back-reaction process and enhances the electron-transport rate. The SAED pattern was used to characterize the crystallinity of the nanoparticles shown in Figure 2b. The ring pattern of SAED confirms that the TiO2 electrode is polycrystalline in nature and well-indexed to the anatase phase. The HRTEM image of the T16 sample is displayed in Figure 2c. The arrangement of the atoms to form an atomic plane with an interplanar distance of 0.35 nm corresponding to the (101) planes of anatase confirms the growth of high-quality nanocrystalline TiO2. The wellcrystalline anatase structure is highly desirable in PEC because ot the high mobility of photogenerated electron−hole pairs. 18156
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Figure 10. continued
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Figure 10. Positive ESI-TOF-MS of photoelectrocatalytic degradation of CFX: (a) CFX; (b) at 80 min; (c) at 160 min; (d) at 240 min.
crystals, which provides a highway network for charge carriers. Controlled-assembly MWDC provides a high surface area, which is beneficial for the intimate contact between the TiO2 surface and electrolyte. Diffusion of the electrolyte is preferred in porous TiO2 films for speeding up the interfacial reaction because of its large surface area. Hence, the T16 sample shows a higher photocurrent. 3.8. PEC of TiO2 Electrodes Deposited at Different Microwave Irradiation Times. The photoelectrocatalytic activities of the T4−T16 samples were judged in terms of the degradation of CFX. The results are demonstrated in Figure 8. The degradation ratio increased for all samples with increasing time. After 320 min, the degradation was 62, 74, 82, and 95% for samples T4, T8, T12, and T16, respectively. From the results, it was observed that the T16 photoelectrode possesses higher PC activity because of its higher photocurrent; hence, the electron−hole recombination is very low. Similar results were obtained by PL. The crystallinity of the T16 sample is high, which is favorable for minimizing the interfacial charge recombination; for this reason, the T16 electrode was preferred for all of the PEC experiments. 3.9. Effect of the pH on PEC. The pH of the solution was the most crucial factor in affecting PEC. Adsorption is a determinant step in heterogeneous photocatalysis, and the pHdependent adsorption was influenced by the properties of CFX and the photoelectrode. The removal kinetics of CFX at different pH values by using the T16 photoelectrode (Ianode = 1.181 mA/cm2) for 320 min is depicted in Figure 9. As can be seen, 88, 96, and 68% degradation were feasible after 320 min at pH 3, 7, and 10, respectively. It was observed that CFX degraded more quickly in acidic and neutral solutions than in a basic one. The effect of the pH on the degradation is a complex issue, and it is difficult to justify from the change in the electrostatic interactions between the TiO2 surface and CFX. For a TiO2 point of zero charge at pH 6.7, as the pH increases, the overall surface charge changes from positive to negative. The zwitterion of TiO2 is in equipoise with noncharged TiO2.55 The noncharged TiO2 is the main species in the pH range from 2 to 11, so adsorption is possible through various sorts of van der Waals or electrostatic interaction.56 CFX predominately existed as positively charged in acidic pH and negatively
3.1, 2.8, and 2.6 eV, respectively. The former emission band was attributed to band-to-band recombination because it was near the band-edge luminescence. The latter emission signal originated from a shallow trap near the band edge, a deep trap, and surface oxygen vacancies that formed on the TiO2 surface.46,47 It is well-known that the higher the PL intensity, the lower the electron−hole separation efficiency, resulting in lower photoelectrocatalytic activity.48,49 When the microwave irradiation time increases, the PL intensity decreases. This is because of a decrease in the shallow trap and surface vacancies. Among the TiO2 samples, the T16 sample shows the lowest PL intensity, indicating the longest lifetime of the photogenerated charge carriers. This result reveals the higher photoelectrocatalytic activity of the T16 sample. 3.7. Photoelectrochemical Behavior of TiO2 Electrodes. The I−V curves of TiO2 photoanodes (64 cm2) in 0.1 M NaOH using steel as the counter electrode at a distance of 1 mm under UV (25 mW/cm2) illumination are shown in Figure 7. The plot clearly reveals that the production of photocurrent increases linearly with the applied potential bias inside the small potential range. The photocurrent becomes a plateau at higher potential, which can be attributed to limiting the electron−hole pair generation, displaying a high PC activity at 1.5 V. The photocurrent increased to 0.740, 0.900, 1.00, and 1.181 mA/ cm2 for samples T4−T16, respectively. Kim et al. synthesized TiO2 nanorods using a simple hydrothermal method having Isc = 0.88 mA/cm2 under a 150 W xenon lamp (100 mW/cm2) as the light source.50 The photocurrent density of hydrothermally synthesized branched TiO2 nanorods is 0.83 mA/cm2 for under UV (88 mW/cm2) illumination using a xenon arc lamp.51 A nanocoral TiO2 thin film prepared by Mali et al. using a hydrothermal method shows a maximum photocurrent of 0.154 mA/cm2 under UV illumination (25 W).52 The mesoporous TiO2 thin film prepared by a sol−gel method exhibits a current density near about 0.2 mA/cm2 under UV illumination.53 A porous TiO2 thin film was synthesized by a sol−gel method, demonstrating a photocurrent density near about 0.15 mA/cm2 under UV light.54 The photocurrent was increased 2-fold from 0.740 to 1.181 mA/cm2 with an increase in the microwave irradiation time. This is attributed to the charge formation in a wormhole-like structure of tightly interconnected TiO2 nano18158
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Figure 11. Proposed photoelectrocatalytic degradation pathway of CFX and possible chemical structures of the intermediate products.
3.10. Identification of Intermediates and Proposed Degradation Mechanism. To analyze the reactive intermediates in the PEC system of CFX, electrospray ionization time-of-flight mass spectrometry (EIS-TOF-MS) was employed. The EIS-TOF-MS spectra of standard CFX and degradation at different time intervals of 80, 160, and 240 min are shown in Figure 10. Figure 10a illustrate the ESI-TOFMS spectrum of the standard CFX antibiotics. The peak at m/z 456 corresponds to the protonated molecule of CFX. The product ions at m/z 126, 241, 243, 277, 297, 324, 396, and 440 were detected along with main compound. The elaborated
charged in alkaline pH. The CFX ions are positively charged in an acidic solution, and TiO2 is present as a positive zwitterion. Because of the interaction between positively charged CFX and the zwitterion, degradation was achieved in acidic pH. The high degradation of CFX in neutral pH may be due to the adsorption of CFX increasing by 2-fold because of the greater variety of charge−charge interactions between the TiO2 surface and the CFX ion and smaller charge-transfer resistance. In basic pH, both TiO2 and CFX species may become negatively charged; hence, the interaction between TiO2 and CFX ions was much less, so the degradation of CFX was less. 18159
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possible mechanism of CFX degradation by the PEC process is depicted in Figure 11. 3.11. Reusability and Stability of the TiO2 Photoelectrode. The stability of the photoelectrode was vital to its practical application for pollutant degradation. The TiO2 photoelectrode via the MWDC method exhibits not only good activity but also high stability. The stability of the nanostructured TiO2 photoelectrode under CFX photoelectrocatalytic degradation was examined using recycling tests after 1 month and repeated up 5 months. Figure 12 displays good photoelectrode stability; only the 5% decrease in the activity after the first cycle and no further significant loss of the activity of the TiO2 photoelectrodes are observed. The high stability is attributed to the wormhole-like structured nanocrystalline TiO2 particles immobilized on the FTO, preventing the nanostructure, leading to loss of the working catalyst, and retaining the desirable stability after every recycling test. The excellent repetition of degradation was also confirmed by photocurrent measurements (Figure S2 in the Supporting Information), which changed very little, indicating its potential application for environmental abatement. 3.12. Evolution of the Residual Antibacterial Activity. The efficiency of the photoelectrocatalytic treatment of antibiotics is defined by not only chemical degradation but also the residual antibacterial activity of the antibiotics. The antibacterial activity of the degraded product was studied for SA and salmonella typhi bacteria using the agar well diffusion method. Activity analysis was carried out on a CFX solution at different photoelectrocatalytic time intervals and a standard CFX solution under the same concentration. The area of the inhibition zone around the well can be regarded as a measurement of the antibacterial activity of the residual solution in the well. It was found that, with an increase in the photoelectrocatalytic time, there was a decrease in the residual antibacterial activity of the treated solution. The residual percentage of the inhibition zone based on the selected bacteria at pH 3, 7, and 10 during the PEC of CFX is shown in Table 1. The inhibition zones for SA and salmonella typhi were investigated and photographed at different photoelectrocatalytic times at various pH values (Figure S3 in the Supporting Information). After 80 min of PEC of CFX at pH 7, no residual antibacterial activity was found for all investigated bacteria. The lower residual antibacterial activity is still observed at 80 min for a photoelectrocatalytic solution under both acidic and basic conditions, perhaps slower degradation rate, and after 160 min, no inhibition zone was observed in both acidic and basic conditions. This clearly indicates that the degradation product was completely independent of the antibacterial activity of
Figure 12. Reusability and stability of a nanostructured TiO2 photoanode (T16) to multiple cycles of photoelectrocatalytic degradation of CFX.
pathway of the intermediate product in the CFX degradation process on TiO2 at 80 min is illustrated Figure 10b. Allylic cleavage with loss of the acetyl group attached to the methyl observed in position 3 of the cephem ring gives the main fragment at m/z 353. The second cleavage was typical of fourmembered β-lactum and the corresponding cleavage of the cephem nucleus and formation of 2-(2-aminothiazol-4-yl)-2(methoxyimino)-N-(2-oxoethyl)acetamide at m/z 243. The formation of intermediate 2-(2-aminothiazol-4-yl)-2(methoxyimino)acetamide at m/z 201 was intended to occur via loss of acetaldehyde at m/z 243. Further, this leads to the formation of m/z 126 with loss of the oxime function and primary amine of the five-membered ring. Figure 10c describes CFX degradation at 160 min because breaking of the β-lactum ring with the formation of 2-(2-aminothiazol-4-yl)-2-(methoxyimino)-N-(1-methyl-2-oxoethyl)acetamide confirmed a peak ion at m/z 255. The degradation of an ion at m/z 255 leads to generation of the intermediate with 2-(2-aminothiazol-4-yl)-2(methoxyimino)-N-methylacetamide at m/z 213 via loss of methyl and carbonyl groups from the parent ion. Figure 10d depicts the extended treatment time of 240 min. The intense prominent ion at m/z 243 of the β-lactum ring breaks, and this results in fragments at m/z 231, 199, 172, 155, 127, and 112, which correspond to the cephem ring, methyl group of amidic NH, methoxy group of the oxime function, amidic NH2, and carbonyl group linked to the oxime and primary amine of the five-membered ring, respectively. Finally, the intermediate products would be degraded to small inorganic materials. The
Table 1. Percentage of Residual Antibacterial Activity Based on the Inhibition Zone Diameter versus Photoelectrocatalytic Degradation at Varying pH 3.0, 7.0, and 10.0 for SA and Salmonella Typhi Bacteriaa degradation time (min) species SA
salmonella typhi
pH
0 3 7 10 3 7 10
100 100 100 100 100 100
20
40
60
80
87.5 85.14 83.33 91.66 87.25 88.01
75.24 76.21 66.61 87.53 71.22 80.31
56.2 53.28 55.65 66.63 58.11 68.54
18.75 0 16.66 59.02 0 56.37
160
240 0 0 0 0 0 0
320 0 0 0 0 0 0
0 0 0 0 0 0
a
The time 0 represents the starting point of the PC reaction. The zone of inhibition of the sample prior to treatment (0) is considered to be 100%, and other zones of inhibition at different intervals of time are calculated with respect to that 100%. 18160
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CFX. Overall, observations reveal that the photoelectrocatalytic product has no toxic effect on the environment.
4. CONCLUSIONS Nanostructured TiO 2 thin films (64 cm2 ) have been successfully synthesized by a green and facile MWDC technique. The prepared TiO2 electrode exhibited superior phoelectrocatalytic activity toward the CFX antibiotic drug under UV illumination. EIS-TOF-MS revealed that degradation of CFX occurs through β-lactum, corresponding to cleavage of the cephem nucleus. The antibacterial susceptibility test was analyzed to judge the final toxicity level of the residual solution, which revealed the safe and ecofriendly discharge of the antibacterial activity of CFX antibiotic after photoelectrocatalytic degradation. The TiO2 photoelectrode showed no change in the PC activity after recycles of photodegradation, indicating that TiO2 films display long-term stability and do not suffer from photocorrosion. The outcomes of these results indicate that a nanostructured TiO2-based photoelectrocatalytic system has high potential to be utilized as a sustainable treatment system for pharmaceutical wastewater containing CFX antibiotics.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental setup, I−V plots, and an antimicrobial susceptibility test. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: p_n_bhosale@rediffmail.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS V.V.K. acknowledges the UGC, New Delhi, and DAE-BRNS, Mumbai, India, for financial support through UGC Major Research Project 41-301/2012(SR) and DAE-BRNS Major Research Project 2012/34/51/BRNS/2036. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2009-0094055).
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