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Efficient Photocatalytic Degradation of Norfloxacin in Aqueous Media by Hydrothermally Synthesized Immobilized TiO/Ti Films With Exposed {001} Facets 2
Murtaza Sayed, Luqman Ali Shah, Javed Ali Khan, Noor Samad Shah, Jan Nisar, Hasan M.Khan, Pengyi Zhang, and Abdur Rahman Khan J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016
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The Journal of Physical Chemistry
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Efficient Photocatalytic Degradation of Norfloxacin In Aqueous Media by Hydrothermally Synthesized Immobilized TiO2/Ti Films with Exposed {001} Facets
3 4
1,2,3
Murtaza Sayed*,3Luqman Ali Shah,3Javed Ali Khan,3,4Noor S. Shah, 3Jan Nisar, 3Hasan M. Khan*,1Pengyi Zhang*, 2Abdur Rahman Khan
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1
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State Key Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 10084, China. Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, 22060, Pakistan. National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120, Pakistan.
Department of Environmental Sciences, COMSATS Institute of Information Technology, Vehari, 61100 Pakistan.
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*Addresses of the corresponding Authors;
24 25 26 27 28 29 30 31 32 33 34
Murtaza Sayed, Ph. D Assistant Professor, Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad,22060, Pakistan Email: -
[email protected] Pengyi Zhang, Ph.D Email Address: -
[email protected] Hasan M. Khan, Ph.D Email Address: -
[email protected] 1 ACS Paragon Plus Environment
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Abstract
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In this study, a novel immobilized TiO2/Ti film with exposed {001} facets was prepared via a
37
facile one-pot hydrothermal route for the degradation of norfloxacin from aqueous media. The
38
effects of various hydrothermal conditions (i.e., solution pH, hydrothermal time (HT) and HF
39
concentration) on the growth of {001} faceted TiO2/Ti film were investigated. The maximum
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photocatalytic performance of {001} faceted TiO2/Ti film was observed when prepared at pH
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2.62, HT of 3 hrs and at HF concentration of 0.02M. The as-prepared {001} faceted TiO2/Ti
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films were fully characterized by field-emission scanning electron microscope (FE-SEM), X-ray
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diffraction (XRD), high resolution transmission electron microscope (HR-TEM) and X-ray
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photoelectron spectroscopy (XPS). More importantly, the as-prepared {001} faceted TiO2/Ti
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film exhibited excellent photocatalytic performance towards degradation of norfloxacin in
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various water matrices (Milli-Q water, tape water, river water and synthetic wastewater). The
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individual influence of various anions (SO42‒, HCO3‒, NO3‒, Cl‒) and cations (K+, Ca2+, Mg2+,
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Cu2+, Na+, Fe3+) usually present in the real water samples on the photocatalytic performance of
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as-prepared TiO2/Ti film with exposed {001} facet was investigated. The mechanistic studies
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revealed that •OH is mainly involved in the photocatalytic degradation of norfloxacin by {001}
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faceted TiO2/Ti film. In addition, norfloxacin degradation byproducts were investigated, on the
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basis of which degradation schemes were proposed.
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1. Introduction
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Norfloxacin is a family of flouroquinolone drug and has been widely identified in surface
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water 1, ground water 2, sea water
3
and drinking water 4. It has also been detected in highest
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concentrations (over 100 µg L‒1) in hospital wastewater 5. The presence of norfloxacin in aquatic
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environment causes bacterial drug resistance and this is a big issue of concern for the clinicians
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and pharmaceutical industry vis-à-vis hazardous effects on human and aquatic life 5. Therefore, it
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necessitates the need for developing fast and proficient oxidation technologies for the efficient
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removal of norfloxacin from wastewater. Numerous investigations have been made to improve
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the degradation of norfloxacin in water including biodegradation 6, ozonation 7, and chlorination
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8
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to be a complicated technique in which the micro-organisms that causes the degradation of target
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contaminant are affected by various environmental factors in aquatic media. On the other hand,
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degradation of norfloxacin by chlorination leads to the formation of chloride resulting into
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incomplete degradation of target contaminant 9.
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processes (AOPs) have gained much attention due to high effectiveness and production of
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powerful oxidizing specie, •OH, that can react non-selectively with wide range of pollutants
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causing their complete mineralization 10-13.
, however, they are suffered with many drawbacks. For example, biodegradation is considered
In the recent years, advanced oxidation
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UV/TiO2 photocatalysis, an AOPs, has extensively been investigated as a promising
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treatment option for the removal of variety of organic pollutants from aqueous media 14-16. It has
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been found that UV photolysis can effectively degrade norfloxacin in the aqueous media either
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alone or in the presence of TiO2 powders, however, the toxicity of the degradation products
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produced during degradation of norfloxacin has not been discussed
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Chu performed the degradation of norfloxacin in aqueous media by using the solar-light-
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mediated Bi2WO6 process and observed that the degradation is dependent on initial pH value of
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norfloxacin solution
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excellent properties for environmental remediation and energy conversion
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crystal facets with more undercordinated atoms as well as high surface energy reveal high
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photocatalytic reactivity. However, the reported anatase TiO2 nanocrystals are dominated by
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thermodynamically stable {101} facets except the more reactive {001} or {100} facets (0.90 J
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m‒2 for {001}; > 0.53 J m‒2 for {100}; > 0.44 J m‒2 for {101}), due to minimization of surface
95
energy during the crystal growth mechanism
19
17-18
. Similarly, Chen and
. Recently, anatase TiO2 crystals with exposed {001} facets has shown
23
20-22
. Preferably, the
. It has been investigated that anatase TiO2 3
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photocatalyst with defined {001} facets are generally more reactive towards photocatalytic
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applications. Using hydrogen fluoride (HF) as capping agent, initially, Lu and co-workers
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prepared anatase TiO2 crystals with exposed 47 % {001} facets 24. Yang and co-workers 25 later
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on, enhanced the exposure percentage of {001} facets to 64 % by applying 2- propanol as
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capping agent. The exposure percentage of {001] facets was further enhanced up to 98.7 % by
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reaction of TiF4 powder with 1-butanol
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various hydrothermal times was also investigated during the preparation of multifaceted titania
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spheres and their growth formation was explained26. Furthermore, Wu and Tang
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titania microspheres exposing nearly 100 % {001} facets by hydrothermal treatment of a thermal
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sprayed TiN/Ti coated with HF aqueous solution containing chromium powders for enhanced
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solar absorption.
23
. The influence of HF on etching of titania surface at 27
prepared
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However, most of the prepared anatase TiO2 crystals with exposed {001} facets are in
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powder form and involves expensive catalyst – recovery procedures after photocatalytic reaction
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28
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facets have not been reported yet. In our previous study, we have synthesized TiO2/Ti thin films
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with exposed {001} facets by applying 2-propanol as capping agent, which stabilized the TiO2
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film, however, the synthesis parameters which can affect the morphology, size and
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photocatalytic properties of {001} facets were not discussed
. Moreover, the photocatalytic degradation of norfloxacin by TiO2/Ti films with exposed {001}
29-30
.
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So, in this study we report a one pot facile hydrothermal technique for directly growing
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up of TiO2 films with exposed {001} facets on Ti-substrate with well-behaved morphology and
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size of {001} facets. The metal Ti was applied both as the conductive supporting material, as
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well as precursor to supply the required titanium amount for the growth of TiO2 film. The key
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hydrothermal parameters like hydrothermal time (HT), HF concentration and solution pH were
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investigated. The photocatalytic degradation of norfloxacin by TiO2/Ti film with exposed {001}
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facets was investigated in Milli-Q water, tape water, river water and synthetic wastewater of
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similar composition to wastewater from pharmaceutical industry. Furthermore, the photocatalytic
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degradation of norfloxacin in the presence of individual commonly found anions (SO42‒, HCO3‒,
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NO3‒, Cl‒) and cations (K+, Ca2+, Mg2+, Cu2+, Na+, Fe3+) in real water was also investigated. The
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scavenger studies revealed that mainly •OH is involved in the degradation of norfloxacin by
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{001} faceted TiO2/Ti films. In addition, the degradation products of norfloxacin were also
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investigated by UPLC-Ms/Ms and accordingly degradation schemes were suggested. The 4 ACS Paragon Plus Environment
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toxicity results showed that the as-prepared photocatalyst provide safe and non-toxic route for
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the degradation of norfloxacin from aqueous media.
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2. Experimental
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2.1.Materials
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The substrate, Ti-plate (99.5 % in purity) of size 180 mm × 120 mm × 0.15 mm, on to which
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TiO2 film was grown hydrothermally was supplied by Baoji Shengrong Titanium Corporation,
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Shaunxi Province, China. Norfloxacin was provided by Sigma-Aldrich. The molar mass,
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molecular formula and other properties are shown in table 1. Methanol and acetonitrile for HPLC
135
analysis were provided by Sigma-Aldrich. All other reagents used in the present study such as
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NaCl, NaNO3, Na2SO4, NaHCO3, NH4Cl, KCl, CaCl2, MgCl2, CuCl2, FeCl3 of analytical grade
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were purchased from Nanjing Chemical Reagent Co., Ltd (Nanjing, China). For the preparation
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of standard solutions, de-ionized water from Milli-Q water purification system (Millipore, USA)
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was used.
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2.2.Photocatalyst preparation
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In a typical fabrication procedure, metallic titanium (Ti) plate (99.5 % purity) of above stated
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size was first etched in boiled 10 % oxalic acid solution for 2 hrs. to remove the adsorbed oxide
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layer on the surface of Ti-plate and then was applied both as supporting material as well as
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starting material for the growth of TiO2 film. The pretreated Ti-plate was then immediately
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immersed in Teflon – lined autoclave having solvent made up of water/isopropanol (the volume
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ratio of which was 2.4) along with different concentrations of HF aqueous solution of 0.01‒ 0.04
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M. The pH of the hydrothermal solution was optimized from 2.62 ‒ 5.38 by adjusting with
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NaOH (0.01M), similar to the method described by Zhang and co-workers 31. The hydrothermal
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treatment was then carried out at 180 ˚ C for 3, 5, 8 and 15 hrs. Subsequently, the obtained
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samples were rinsed with Milli-Q water, dried in air and then calcined at 600 ˚C for 2hrs. for
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getting fluorine free surface.
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2.3.Characterization of photocatalyst
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The as-prepared TiO2 films with exposed {001} facets were characterized by field emission
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scanning electron microscope (FESEM, S-5500, Hitachi) for morphology investigations. The
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high resolution transmission electron microscopy (HR-TEM) analyses was conducted using a
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JEM – 2011F electron microscope (JEOL, Japan). The phase parameters and purity of as-
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prepared material (X-ray diffraction measurements) were recorded on a Rigaku D/max-RB with 5 ACS Paragon Plus Environment
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Cu Kα radiation (λ = 0.15418 nm), operated at 40 kV and 100 mA. The surface environment of
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{001} faceted TiO2/Ti films were investigated by X-ray photoelectron spectroscopy (XPS, PHI-
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5300, ESCA) at a pass energy of 50 eV, using Al Kα as an exciting X-ray source. All the binding
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energies were compared to C 1s peak at 284.8 eV. A 10 W low-pressure mercury lamp (λmax =
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254 nm) was applied as source of UV-irradiation.
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2.4.Photocatalytic measurements
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The photocatalytic measurements were performed by taking norfloxacin (10 mg L‒1) aqueous
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solution in a 150 mL cylindrical vessel with inner diameter of 30 mm and length of about 300
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mm. A low pressure mercury lamp (λmax = 254 nm) of 10 W, employed as source of UV-
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irradiation was introduced in the middle of photoreactor with quartz envelope, the inner diameter
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of which was 18 mm (See Fig S1 in supplementary informations for experimental setup).
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Oxygen gas at a flow rate of 30 mL min‒1 was continuously purged from the bottom of the
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cylindrical vessel during the photocatalytic experiments in order to avoid the recombination of
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photogenerated electrons and holes. The TiO2 film was kept in dark for 30 min. to allow for
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adsorption-desorption equilibrium to be establish, after which UV-lamp was turned on to
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perform photo-degradation experiments. Subsequently, 5 mL of reaction sample was taken after
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every 15 minutes and the concentration of norfloxacin was evaluated by high performance liquid
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chromatography (HPLC, Shimadzu, LC-10 AD) equipped with a UV detector (SPD-10 AV) at
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280 nm and a Kromasil C18 column (250 mm × 4.6 mm) for separation. The mobile phase was a
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mixture of methanol and water (2: 1 v/v) at a flow rate of 1.5 mL min‒1.
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2.5.Preparation of norfloxacin solution in various water matrices
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Norfloxacin (10 mgL‒1) was spiked in Milli-Q water, tape water, river water and synthetic
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wastewater. The chemical composition of natural waters and synthetic wastewater is shown in
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table 2.
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2.6.Degradation product studies
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In this typical set of experiment, the degradation products of norfloxacin were identified by
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ultra-performance liquid chromatography tendem – mass spectrometry (UPLC-MS/MS, Waters
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Corp., USA) attached with a C18 column (2.1 × 50 mm, particle size 1.7 mm) and Quattro
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Premier XE tandem quadrupole mass spectrometer with electro-spray ionization source. The
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linear gradient mobile phase composition was applied and started with 15 % A (2 mM
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ammonium acetate/methanol) and 85 % B (2 mM ammonium acetate/water) to 85% A in first 10 6 ACS Paragon Plus Environment
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min, then reversed to 25% A in 5 min and continued for 5 min. The column was re-equilibrated
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for 10 min after every gradient elution. The degradation products were analyzed in an
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electrospray interface (ESI) with positive ion mode at full scan from 50 to 400 m/z. The other
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mass parameters were optimized as follows: source temperature, 120 °C; capillary voltage 2.1
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kV; desolvation gas flow rate, 650 L h−1; and desolvation temperature, 280 °C.
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The cationic/anionic degradation products of norfloxacin were qualitatively evaluated by
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Metrohm Ion-chromatograph 800 series (IC), attached with electrical conductivity (EC) detector.
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The columns, Metrosep C1 (125 × 4.6 mm) and Metrosep Asupp 5 (250 × 4.0 mm) were
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alternatively used for the identification of respective cations and anions. The mobile phases for
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anionic and cationic degradation product analysis were composed of 3.2 mM Na2CO3/1 mM
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NaHCO3/50 mM H2SO4/water and tartaric acid (C4H6O6)/1 mM dipiclonic acid (C7H5NO4)/24
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mM boric acid (H3BO3), respectively.
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2.7.Toxicity tests
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In order to assess the potential impact of norfloxacin and its degradation products to the
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aquatic life, toxicity tests on Escherichia coli (E. coli) were made according to earlier study 32. In
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a typical experiment, E. coli suspension was attuned with OD 600 to be 0.1 by means of 2 mM
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Na2CO3 buffer. Afterwards, 1 mL of norfloxacin solution (5 × 10‒3 mM) was mixed with 2 mL
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of diluted E. coli solution and incubated for 6 hrs. Then, the supernatant was collected in the tube
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by centrifuging the obtained mixture. Then, 0.5 mL of thawed stock 3-(4,5-dimethylthiazol-2-
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yl)-2,5-diphenyl tetrazolium bromide (MTT) solution to the sample and stored at 37 ˚C for 1hr.
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The sample was then mixed with 3 mL of isopropanol and 0.04 M HCl and thoroughly mixed at
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room temperature for 2 hr until all the dark blue crystals are completely dissolved.
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The absorbance of the sample was measured at wavelength of 570 nm using UV-Visible spectrophotometer. The % inhibition was measured by expression given in Eq. (1)
( A0 − A) ×100 A0
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% Inhibition =
(1)
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Where A0 is the original absorbance of E. coli suspension and A is the absorbance after
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incubation. The UV-irradiated solutions of norfloxacin (6.3 × 10‒2 mM) in the presence of as-
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prepared photocatalyst at different time intervals were sampled, diluted 13 times and was
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examined instead of norfloxacin.
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3.
Results and Discussion
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3.1.
Morphological and structural properties
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Fig. 1 illustrates the typical scanning electron micrographs of TiO2 film with exposed
221
{001} facets prepared at various pHs (pH 2.62, 3.04, 3.75, 4.52 and 5.38). As shown in Fig. 1(a),
222
at pH 2.62 the exposed surface of the film comprises of sheet-like geometry of TiO2
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nanocrystals. According to Wulff construction for anatase single TiO2 crystal, the one top square
224
surface of the pyramidal can be ascribed to {001} facet having average size of ca 350 nm and
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other four isosceles trapezoidal surfaces are {101} facets. When the pH of the solution was
226
increased to 3.04 (Fig. 1b), the average {001} facets size was decreased to ca. 120 nm. It can
227
also be seen clearly that the geometry of the TiO2 crystal changed from sheet-like structure to
228
truncated tetragonal pyramid. When pH was controlled at 3.75 and 4.52, the average size of
229
{001} facet was decreased to ca. 80 nm and 60 nm, respectively (Fig. 1c and 1d). High
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magnification SEM image (inset in Fig. 1c and 1d) reveals top square surface of {001} facets
231
with larger flat surface area and minor four isosceles trapezoidal surface areas. When pH of the
232
reaction solution was further increased to 5.38, the morphology of the TiO2 crystals differed
233
remarkably from truncated tetragonal pyramidal shaped TiO2 crystals with exposed {001} facets.
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More precisely, TiO2 crystals with dominated {101} surface were obtained when the pH of the
235
reaction media was increased to 5.38, and is in agreement with the earlier study by Zhang and
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co-workers 31.
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Since HF is a weak acid with pka value of 3.17 33, that exists in the molecular HF and F‒
238
at low and higher pH values, respectively. The FE-SEM results at low pH values of the
239
hydrothermal solution suggest that the surface fluorination by dissociative adsorption of HF
240
improves the growth of {001} facets. In addition, the existence of molecular HF rather than the
241
F‒ is necessary for the effective exposure of {001} faceted anatase TiO2. Similar results were
242
also obtained by Zhang and co-workers
243
various pH solutions.
31
for anatase TiO2 with exposed {001} facets at
244
Fig. 2 demonstrates the change in surface morphology of {001} facets TiO2/Ti film when
245
subjected to hydrothermal treatments at 180 ˚C for 3, 5, 8 and 15 hrs. Fig. 2 (a) shows the FE8 ACS Paragon Plus Environment
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SEM image of {001} facet TiO2/ Ti product from a solution with hydrothermal time of 3 hrs.
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The obtained morphology of {001} facet TiO2 particles have sheet-like geometry, and the
248
surfaces of {001} and {101} facets are clean and smooth. When the hydrothermal reaction time
249
was increased to 5 hrs., multi-facet microspheres were observed covering the surface of Ti-plate
250
(Fig. 2 (b)). For further extended reaction times (8 hrs.), the distance between neighboring TiO2
251
particles become larger. Furthermore, the surfaces of TiO2 particles becomes rough, and edges of
252
{001} facets and {101} facets become molt together (Fig. 2(c)). This may be attributed to the
253
etching of HF on the TiO2 facets with reaction as follows;
254 255
TiO2 + 4H+ + 6F‒ → TiF62‒ + 2H2O
(R1)
256 257
Moreover, further increase in hydrothermal time i.e., upto 15 hrs., the surface of Ti was observed
258
to be consisted of large TiO2 microspheres, which indicative of the fact that {001} facets TiO2
259
particles have been disengaged from the Ti-substrate, and self-assembled into TiO2 microspheres
260
(Fig. 2(d)). The reason for this evolution in surface morphology of TiO2 from sheet-like to
261
spherical geometry with the change in the hydrothermal time from 3 hr to 15 hrs may be
262
attributed to hydrolysis of Ti-F groups on the surface of TiO2, which can induce the combination
263
of adjacent TiO2 particles and thus leads to the minimization of high energy {001} surfaces
264
Similar results have also been reported by Wu and co-workers, where they concluded that the
265
growth of multifaceted TiO2 sphere from single crystal anatase TiO2 with the passage of
266
hydrothermal time is mainly progressed in a following way; (1) first the formation of truncated
267
bipyramid from the precipitation of single crystalline anatase occurs, these truncated bipyramids
268
then act as a trunk for secondary truncated bipyramids, afterwards, (2) the nucleation and growth
269
of secondary truncated bipyramids on the trunk arises with surface defects due to erosion of HF,
270
(3) then the outward growth of the secondary truncated bipyramids in different directions results
271
in the formation of microspheres and finally (4) etching of the periphery of the microsphere
272
happens to expose the thermodynamic stable {101} facets 26.
34
.
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Fig. 3 (a) shows that the multifaceted formation of TiO2 particles on Ti-substrate were
274
observed at HF concentration of 0.01M. From the appeared morphology, it can be observed that
275
the percentage of exposed {001} facets is very less while the growth along {101} axis is much
276
higher. When the HF concentration was increased to 0.02 M, the growth along {101} axis is 9 ACS Paragon Plus Environment
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retarded while the percentage of {001} facets increases (Fig. 3(b)). Almost identical morphology
278
was also obtained by Wu and Tang
279
not only initiated the formation of more Ti (IV)- hydrate in the solution but also retarded the
280
formation of facets other than {001} during crystal growth. Further enhancement in HF
281
concentration to 0.03 M and 0.04 M, TiO2 microspheres consisted of truncated bipyramids with
282
{001} facets exposed outside were observed with average size of ca. 8.5 and ca. 10.2 µm,
283
respectively.
35
, where it was reported that increase in HF concentration
284
Based on the earlier discussion in connection with pH controlled growth, the involvement
285
of HF in the structural formation processes can be attributed to two types. At one side, HF plays
286
the role of dissolution reagent by providing the basic building material for structural formation
287
36
288
{001} faceted surface 25. The mechanistic role of HF in the synthesis of {001} facets TiO2 / Ti
289
film can be expressed by following reactions;
290
Ti + 6 HF → H2TiF6 + 2H2↑
(R2)
291
H2TiF6 + 4H2O → Ti(OH)4 + 6HF
(R3)
292
Ti(OH)4 → TiO2 + 2H2O
(R4)
293
At low pHs, the fluorination of TiO2 surface through dissociative adsorption of HF makes the
294
{001} axis a favorable site for crystal progression due to reduced surface energy of {001} facet
295
37
. Secondly, HF plays the role of facet controlling agent, aiding the growth and stabilization of
.
296
Fig. 4 shows the images of transmission electron microscopy and the corresponding
297
selected area electron diffraction patterns (SAED) of as-prepared TiO2/Ti films with exposed
298
{001} and {101} facets. The formation of {001} faceted TiO2/Ti films with lattice spacing of
299
0.235 nm has been confirmed by SAED pattern and the corresponding HR-TEM image ((Fig 4
300
(a)), whereas the lattice spacing of 0.352 nm corresponds to {101} planes also confirmed by the
301
corresponding SAED pattern ((Fig. 4 (b)) 38.
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The crystallographic structure of as-prepared TiO2/Ti films with exposed {001} facets
303
has been confirmed by X-ray diffraction patterns as shown in Fig. 5. The as-prepared samples
304
obtained from the hydrothermal solution of pH ≤ 5.38 are tetrahedral in crystal system with
305
space group of I41/amd (JCPDS No. 12-1272) 25. It was found that the samples consisted of both
306
anatase and rutile in the main crystal phase. The rutile character may be due to thermal oxidation
307
of metallic Ti-substrate when calcinated at 600 ˚C in air
39
. The as-prepared TiO2/Ti films 10
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308
consisted of ca. 83 % anatase and 17 % rutile, as calculated by following Eqs. (4) and (5),
309
respectively;
310
Xa = [1+1.26(Ir/Ia)] ‒1
(2)
Xr = 1‒ Xa
(3)
314
% anatase = Xa × 100
(4)
315
% rutile = Xr × 100
(5)
311 312
and
313
So,
316
Where Xa, Xr, Ia and Ir represents the rate of anatase in mixture, rate of rutile in the same
317
mixture, integrated intensity (101) reflection of anatase and the integrated intensity (110)
318
reflection of rutile, respectively.
319
Fig. 6 represents high resolution spectra of O 1s and Ti 2p. The XPS spectra of O 1s shows
320
two clearly visible peaks at 530.22 and 531.67 eV ((Fig. 6 (a)), which corresponds to lattice
321
oxygen of the Ti-O bond and surface hydroxyl groups, respectively
322
groups could be attributed to the formation of Ti-OH on the surface of TiO2
323
surface hydroxyl groups, playing the role of electron donors for photo-generated holes (h+), are
324
then oxidized to hydroxyl radicals (•OH), accordingly improving the separation of photo
325
generated species and photocatalytic performance of TiO2/Ti film. Fig. 6 (b) depicts XPS spectra
326
of Ti 2p with two peaks, which corresponds to Ti 2p1/2 and Ti 2p3/2 at binding energy (B.E)
327
values of 464.97 and 459.07 eV, respectively. Furthermore, the B.E values of Ti 2p3/2 and O 1s
328
are slightly higher than the reported values for Ti 2p3/2 and O 1s
329
B.E of Ti 2p3/2 and O 1s in current study compared to reported work suggests that relative less
330
electron density exists on Ti and O atoms. It is assumed that O atoms adjacent to oxygen defect
331
sites of the as-prepared TiO2/ Ti film shifts some of their electron density towards Ti atoms that
332
are not fully coordinated. This contribution of electron density towards Ti atoms is mainly being
333
liable for shifting the O 1s peak towards higher B.E values 44.
40
24, 42-43
. The surface hydroxyl 41
. These formed
. This slight increase in
334
3.2.UV-photocatalytic performance of as- prepared TiO2/Ti films
335
In order to assess the photocatalytic performance of as-prepared TiO2/Ti film with exposed
336
{001} facets, photocatalytic degradation of norfloxacin under UV-irradiation at various
337
fabrication conditions was performed. The hydrothermal solution pH plays a vital role in the
338
photocatytic performance of as-prepared {001} faceted TiO2/Ti films. The photocatalytic 11 ACS Paragon Plus Environment
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339
degradation efficiency of as-prepared TiO2/Ti films with exposed {001} facets follows the order
340
of pH 2.62 > pH 3.04 > pH 3.75 > pH 4.52 > pH 5.38 as displayed in Fig. 7 (a). The higher
341
degradation performance of as-prepared TiO2/Ti films synthesized at lower pH values can be
342
contributed to higher fractions of {001} facets. It is reported that anatase TiO2 with exposed
343
{001} facets possess higher photocatalytic activity 45. The values of observed pseudo-first order
344
rate constants (kobs.) are 0.0504 min‒1 for pH 2.62, 0.0181 min‒1 for pH 3.04, 0.0117 min‒1 for pH
345
3.75, 0.0097 min‒1 for pH 4.52 and 0.0081 min‒1 for pH 5.38, respectively (see Fig. 7(a) inset);
346
thus, indicating the effective degradation of norfloxacin by {001} faceted TiO2/Ti films when
347
synthesized under acidic conditions.
348
Furthermore, the hydrothermal time also has a pronounced impact on photocatalytic
349
efficiency of {001} faceted TiO2/Ti films. As depicted in Fig. 7 (b), the TiO2/Ti film synthesized
350
at hydrothermal time of 3 hrs showed best degradation efficiency, which then decreased with
351
increase in the hydrothermal time. It can be seen that increase in hydrothermal time affected the
352
photocatalytic performance of as-prepared TiO2/Ti films in the order of 3 hrs (kobs. = 0.0504 min‒
353
1
354
good photocatalytic efficiency of TiO2/Ti films synthesized at 3 hrs of hydrothermal time is
355
associated with higher exposure of {001} facets with clean surface. This also confirms that
356
norfloxacin is mainly degraded through oxidation reactions, as it has been reported that oxidation
357
reactions are favored at {001} facets while reduction reactions at {101} facets of TiO2 46.
) > 5 hrs (kobs. = 0.0139 min‒1) > 8 hrs (kobs. = 0.0056 min‒1) > 15 hrs (kobs. = 0.0042 min‒1). The
358
Fig. 7 (c) illustrates the comparison of degradation curves of norfloxacin by {001} faceted
359
TiO2/Ti films, synthesized at varying concentrations of HF in the hydrothermal solution. The
360
photocatalytic performance of as-prepared {001} faceted TiO2/Ti films was in the order: HF =
361
0.02 M (kobs. = 0.0504 min‒1) > HF = 0.04 M (kobs. = 0.0127 min‒1) > HF = 0.01 M (kobs. = 0.0111
362
min‒1) > HF = 0.03 M (kobs. = 0.00704 min‒1), which is comparable to a previously reported work
363
47
364
length along {101} axis is large, at HF concentration of 0.01 M. Due to this decreased length
365
along {001} axis the photocatalytic degradation of norfloxacin was observed to be relative
366
lower. The photocatalytic efficiency of as-prepared TiO2/Ti film with exposed {001} facets
367
reached its maximum value when HF concentration was maintained at 0.02 M due to highest
368
obtained percentage of {001} facets 46. Moreover, with further increase in the HF concentration
369
to 0.03 M or 0.04 M, the photocatalytic degradation of norfloxacin was remarkably decreased.
. As already discussed, the percentage exposure of {001} facets are relatively less, while the
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370
This decrease in the photocatalytic performance of as-prepared TiO2/Ti film with exposed {001}
371
facets at HF = 0.03M or 0.04 M could be attributed to the introduction of recombination
372
sites/trapping sites for photogenerated electrons (e‒) and holes (h+), and thus inhibit the charge
373
separation. This can be attributed to increase in the exposure ratio of {001} faceted anatase TiO2
374
particles, which results in the electron overflow effect on {101} facets due to their lower
375
percentage (see Table S1 in the supporting information). Due to this, the electrons on the {001}
376
facets and within the interior are hardly transferred to {101} facets and easily recombine with
377
holes on the {001} facets due to their dominance
378
performance of {001} faceted TiO2/Ti films prepared at HF = 0.03 M and 0.04M. Thus it can be
379
concluded that the relative percentage of {001} and {101} facets is a very important parameter
380
for the effective separation of photo-generated e‒ and h+ and obviously has significant influence
381
on the photocatalytic performance.
48
, causing decrease in the photocatalytic
382
The cyclic degradation performance of {001} faceted TiO2/Ti film prepared at hydrothermal
383
conditions of HF = 0.02 M, pH=2.62 and HT = 3hrs. was also investigated for eight continuous
384
cyclic runs (Fig. 7(d)). The results showed that % degradation of norfloxacin was almost same
385
till the 5th cyclic run, however after 5th run it was slightly decreased from 97 % to 89 % at the
386
end of 8th cyclic degradation experiment. This decrease in photocatalytic performance of {001}
387
faceted TiO2/Ti film after 5th cyclic run may be due to the formation of degradation products of
388
norfloxacin and their adsorption on the surface of TiO2/Ti film and thus blocking the active sites.
389
However, the photocatalytic performance can be recovered back by just heating the photocatalyst
390
at 100 ˚C to remove the adsorbed species.
391 392
3.3.Effect of water matrix and anions/cations on the photocatalytic performance of asprepared TiO2/Ti film
393
To examine the efficiency of as-prepared {001} faceted TiO2/Ti film, it was crucial to study
394
the viability of this oxidative technique in real water samples as well. For this purpose, the
395
photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film was checked in Milli-Q
396
water, tape water, real water and synthetic waste water. The photocatalytic degradation results at
397
norfloxacin initial concentration of 10 mg L‒1, and TiO2/Ti film synthesized at conditions of pH
398
=2.6, HT = 3hrs. and HF = 0.02M are shown in Fig. 8. The concentrations of ionic species in
399
different water matrices are presented in table 2. It can be seen that the % degradation of
400
norfloxacin in Milli-Q water was higher (70.5 %, 0.0504 min‒1) than tape water (55.1 %, 0.03 13 ACS Paragon Plus Environment
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401
min‒1) followed by river water (44.95 %, 0.009 min‒1) and then synthetic wastewater (39.89 %,
402
0.005 min‒1). This variation in the photocatalytic performance of {001} faceted TiO2/Ti film in
403
various water matrices could be attributed to the abilities of anions/cations to compete with the
404
norfloxacin for the reactive species
405
photocatalyst depending on the ionic strength
406
efficiency of the photocatalyst.
49
and/or to adsorb on the active surface sites of the 50
and thus affecting the overall degradation
407
Table 3 depicts the role of various anions (SO42‒, HCO3‒, NO3‒, Cl‒) and cations (K+, Ca2+,
408
Cu2+, Na+, Fe3+) on the photocatalytic degradation of norfloxacin by TiO2/Ti film. It can be seen
409
that both SO42‒ and HCO3‒ inhibited the degradation of norfloxacin probably due to scavenging
410
ability of these ionic species for •OH as shown by reactions (R5) and (R6), respectively 51;
411
SO42‒ + •OH + H+ → •SO4‒ + H2O
(R5)
412
HCO3‒ + •OH → CO3•‒ + H2O
(R6)
413
In the above reactions, sulphate and carbonate radicals were formed as a result of •OH
414
consumption by SO42‒ and HCO3‒, respectively. Sulphate radical underwent selective oxidation
415
and having a larger molecular structure (than •OH), resulting into decrease in photocatlytic
416
degradation of norfloxacin due to hindrance of its reaction with norfloxacin molecule. However,
417
NO3‒ and Cl‒ on the other hand did not considerably affect the photocatalytic degradation of
418
norfloxacin. The relatively less degradation in the presence of Cl‒ could be due to formation of
419
Cl• and Cl•‒ by reaction of Cl‒ with holes (h+) and Cl•, as shown below 49;
420
h+ + Cl‒ → Cl•
421
Cl• + Cl‒ → Cl2•‒ •
•‒
(R7) (R8) (EoSHE
•
‒
Cl /Cl = 2.41V;
EoSHE
•‒
Cl2 /2Cl‒ = 2.09
422
Cl and Cl2 are considered as strong oxidants
423
V) and have the ability to oxidize a variety of organic pollutants
424
Similar results were also observed by Ge and co-workers 53, who showed that Cl‒ has little effect
425
on the photocatalytic degradation of enrofloxacin.
52
, including norfloxacin.
426
The influence of frequently existing cations, such as K+, Ca2+, Mg2+, Cu2+, Na+ and Fe3+
427
in real water on the photocatalytic degradation of norfloxacin was also examined (table 3).
428
Interestingly, monovalent cations (K+, Na+) showed less inhibition in photocatalytic degradation
429
of norfloxacin than divalent alkaline earth metal cations (Ca2+, Mg2+). This higher inhibition of
430
divalent alkaline earth metal cations could be attributed to filtering the radiation required for the
431
activation of {001} faceted TiO2/Ti film. Table 3 also depicts that the addition of transition metal 14 ACS Paragon Plus Environment
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432
cations (Cu2+, Fe3+) greatly inhibited the photocatalytic degradation of norfloxacin. These
433
transition metal cations have the ability to form complexes with the organic molecule, and {001}
434
faceted TiO2/Ti film may react slowly with transition metal complexes due to which the
435
photocatalytic degradation of norfloxacin was inhibited to greater extent in the presence of Cu2+
436
and Fe3+. In addition, Cu2+ can scavenge the photoelectrons produced by irradiated photocatalyst
437
as given by reactions (R9) and (R10) 54;
438
Cu2+ + e‒ → Cu+ + e‒ → Cu
(R9)
439
Cu + h+ → Cu+ + h+ → Cu2+
(R10)
440
In conclusion, the difference in photocatalytic degradation of norfloxacin in various water
441
matrices could be attributed to the combined effects of co-existence of these cations/anions,
442
which needs to be considered carefully for practical application of {001} faceted TiO2/Ti film as
443
advanced oxidation technique. 3.4.Mechanistic investigations of photocatalytic degradation of norfloxacin by TiO2/Ti
444
film
445 446
The comparison of photocatalytic properties of TiO2/Ti films with exposed {001} facets
447
synthesized under various fabrication conditions showed that TiO2/Ti film when synthesized by
448
taking HF = 0.02 M, pH = 2.6 and treating it hydrothermally for 3 hrs showed best photocatalytic
449
behavior. Thus by taking the {001} faceted TiO2/Ti film with best photocatalytic abilities we
450
further investigated the mechanism involved in the degradation of norfloxacin by TiO2/Ti film
451
with exposed {001} facets. TiO2 photocatytic processes involves the production of various
452
reactive species (ROS) like, hydroxyl radical (•OH), electron (e‒cb), hole (hvb+), and singlet
453
oxygen (1O2). The production and role of these ROS in the degradation of norfloxacin by as –
454
prepared {001} faceted TiO2/Ti film was investigated by applying various scavengers, as shown
455
in Table 4. Molecular oxygen and 0.1 M Cu2+ ion were used to scavenge e‒cb ; 0.1 M tert-BuOH
456
was employed as •OH scavenger ; NaN3 (sodium azide) was utilized to scavenge both •OH and
457
1
O2 and HCO2H (formic acid) was applied to scavenge h+vb.
458
Fig. 9 depicts the photocatalytic degradation of norfloxacin by TiO2/Ti film with exposed
459
{001} facets in the presence and absence of scavengers. The photocatalytic degradation of
460
norfloxacin was higher in O2 saturated media than air and N2-saturated media (Fig. 9(a)),
461
suggesting that O2 plays an important role in the photocatalytic degradation of norfloxacin by
462
scavenging e–cb and thus avoids the recombination of e–cb/h+vb pair. The role of e–cb in the 15 ACS Paragon Plus Environment
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463
photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film was further explored by
464
the addition of Cu2+ in the norfloxacin sample solution and then purging with N2 gas. The results
465
indicated that photocatalytic degradation of norfloxacin in the presence of Cu2+ ions purged with
466
N2-gas was almost similar with the case when no scavenger was added, but saturated with O2-gas
467
(Fig. 9(b)). This suggests that the inhibition of the recombination of e‒cb and h+vb by Cu2+ and O2-
468
gas is critical in the photocatalytic degradation of norfloxacin by TiO2/Ti film with exposed
469
{001} facets.
470
Tert-BuOH, a scavenger for •OH, was found to considerably restrain the degradation of
471
norfloxacin (Fig. 9 (b)), suggesting that •OH is the most predominant specie involved in the
472
degradation of norfloxacin by TiO2/Ti photocatalysis. In order to assess the possible role of 1O2
473
in the degradation of norfloxacin, NaN3 was used which scavenges both •OH and 1O2 55,54. The
474
photo degradation of norfloxacin was not very much affected by the presence of sodium azide as
475
compared to tert-BuOH (Fig. 9 (b)). Which suggests that 1O2 plays a minimal role in the
476
degradation of norfloxacin.
477
Furthermore, in order to investigate the role of h+vb in the degradation process, HCO2H was
478
used to quench h+vb mediated reactions during photocatalytic degradation of norfloxacin by
479
{001} faceted TiO2/Ti film. The results in Fig. 9 (b) showed that degradation of norfloxacin was
480
almost completely inhibited in the presence of 0.1 M HCO2H. This finding can be contributed to
481
the fact that the production of h+vb plays a key role in the production of •OH and thus in
482
degradation of norfloxacin. The scheme 1 presents the formation of various ROS and their
483
involvement in the photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film.
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The Journal of Physical Chemistry
484
485
Scheme 1: Photocatalysis mechanism of {001} faceted TiO2/Ti film
486
It should also be mentioned that besides exposure of {001} facets, the charge transfer
487
between the TiO2 film and the metal Ti-substrate be another reason of this enhanced
488
photocatalytic performance of as-prepared TiO2/Ti film with exposed {001} facets, since the
489
charge separation is more efficient in such system56-58. In addition, anatase/rutile interaction may
490
also be responsible for this enhance in the photocatalytic performance of as-prepared
491
photocatalyst 59.
492
It is also of great concern to identify the degradation products (DPs) resulted from
493
photocatalytic degradation of norfloxacin by TiO2/Ti film with exposed {001} facets, because
494
some of these DPs may be more hazardous to the aquatic environment. The identified DPs are
495
summarized in Table 5, with their corresponding formulae, m/z values and their retention times.
496
The photo-catalyzed norfloxacin samples were also analyzed by IC for resulting anions
497
(CH3COO‒, F‒, NH4+, and NO3‒).
498
Based on the identified DPs, the degradation schemes can be separated into two sub-schemes
499
and they are initiated by the attack of •OH on quinolone moiety and/or piperazinyl substituent. In 17 ACS Paragon Plus Environment
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500
scheme 2, •OH attacks on piperazinyl substituent resulting in the formation of DP1a or DP1b
501
with m/z of 336.1, by various re-arrangement and elimination reactions. Degradation production
502
DP1b has also been reported in previous research 60
503
Klieser et al.
504
degradation product DP7 with m/z of 350.3.
54
. Based on the mechanism proposed by
, it is suggested that further oxidation of DP1b caused the formation of
505
The degradation product DP9 with m/z of 322.3 is obtained by the net loss of CHO at the
506
piperazinyl substituent of DP7. Nearly, identical path has also been suggested in the formation of
507
desethylene ciprofloxacin from ciprofloxacin 61. The formation of DP3 with m/z of 294.2 can be
508
ascribed to decabonylation of amide with group NH-CH=O. The formation of DP4 with m/z of
509
251.1 from degradation product DP3 may be suggested as a further oxidation at piperazinyl
510
substituent of DP3 with net loss of ‒CH2‒CH2‒NH2 group. DP5 was obtained by conversion of
511
amino group of DP4 to ammonium (NH4+) and/ or nitrate ions (NO3‒).
512 513 514 515 516
517 518 519 520 521 522
Scheme 2: Attack of •OH on piperazinyl substituent of norfloxacin
523
Compared to scheme 2, scheme 3 is the alternate route, in which the complete removal of
524
piperazinyl substituent is not done. Degradation product DP6 with m/z of 318.1 is produced as a 18 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
525
result of substitution of F atom with •OH at quinolone moiety with the release of F‒ ions.
526
Degradation products DP2a and DP2b with m/z of 334.1 are originated as a results of attack of
527
•
528
of DP2a and DP2b occurs after the attack of •OH on the piperazinyl substituent of DP6 with ring
529
opening and simultaneous amide formation, according to the mechanism previous reported by
530
Kleiser et al.605.
OH at piperazinyl substituent of DP6 and /or followed by reaction with oxygen. The formation
531 532 533 534 535 536 537 538 539 540
Scheme 3: Attack of •OH on quinolone moiety and partial elimination of piperazinyl
541
substituent of norfloxacin
542
In the present study, acetate ions (CH3COO‒) was also formed in addition to NH4+ and NO3‒
543
ions as detected by ion-chromatography. •OH is a strong oxidizing specie with oxidation
544
potential of E˚ = 2.8 V/SHE and has the ability to react non-selectively with all organics causing
545
their complete mineralization
546
react with norfloxacin and results in the formation of a number of DPs. Subsequently these DPs
547
further on reaction with •OH, leads to the formation of CO2 and inorganic ions as follows;
62
. Therefore, the generated •OH in {001} faceted TiO2/Ti film
548
Norfloxacin + •OH →→→ DPs
549
DPs + •OH →→→ CO2 + H2O + NH4+/ NO3‒ + CH3COO‒ + F‒
550
3.5.Toxicity Assessment 19 ACS Paragon Plus Environment
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551
Fig. 10 depicts the toxicity results of norfloxacin alone and photocatalysed in the presence of
552
TiO2/Ti film with exposed {001} facets at different intervals of time. Norfloxacin is a
553
flouroquinolone antibiotic with antibacterial activities especially towards gram negative bacteria,
554
such as Escherichia coli (E.coli). The % inhibition of E.coli activity by norfloxacin alone was
555
32, which was almost same (33 %) when TiO2/Ti film with exposed {001} facets was added to
556
norfloxacin solution. Our results demonstrate that as the photocatalytic reaction time of
557
norfloxacin went on, the % inhibition of E.coli decreased from 33 to less than 10, suggesting that
558
the toxicity of degradation products of norfloxacin is much less than pure norfloxacin. Briefly,
559
as-prepared TiO2/Ti film with exposed {001} facets provide a safe and less toxic way for the
560
degradation of norfloxacin from aquatic environment.
561
4. Conclusions
562
In conclusion, immobilized {001} faceted TiO2/Ti film have been successfully prepared by a
563
facile one-pot hydrothermal route for efficient photocatalytic degradation of norfloxacin. The
564
transformation of TiO2 particles on Ti - substrate at different pH, hydrothermal time and HF
565
concentrations have been explored. The photocatalytic performance of as-prepared TiO2/Ti film
566
with exposed {001} facets was enhanced when prepared at pH = 2.62, HT of 3 hrs and at HF =
567
0.02M. The highest photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film is
568
achieved in Milli-water followed by tape water, river water and then synthetic wastewater. The
569
presence of multivalent cations (Ca2+, Mg2+, Cu2+ and Fe3+) significantly inhibited the
570
photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film. The scavenger studies
571
demonstrate that •OH plays a major role in the photocatalytic degradation of norfloxacin by
572
{001} faceted exposed TiO2/Ti film while 1O2 plays a minimal role. The proposed degradation
573
schemes of norfloxacin by {001} faceted TiO2/Ti film were supported by identification of nine
574
DPs. Furthermore, the identified degradation products were less toxic than original norfloxacin
575
solution.
576 577
Supporting information Physiochemical properties of {001} faceted exposed TiO2/Ti films prepared at various
578
HF concentrations and Experimental setup for evaluating the photocatalytic activity. This
579
material is available free of charge via the internet at http://pubs.acs.org.
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The Journal of Physical Chemistry
Acknowledgement
581
This work was funded by National Basic Research Programme of China (2013CB632403),
582
Science Fund for Creative Research Groups (21221004), and the collaborative Innovation Centre
583
for Regional Environmental Quality. Murtaza Sayed is thankful to Higher Education
584
Commission of Pakistan for financial support of this project under start up research grant (SRGP
585
# 730).
586 587
References
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49. Kanakaraju, D.; Motti, C. A.; Glass, B. D.; Oelgemöller, M. TiO2 Photocatalysis of Naproxen: Effect of the Water Matrix, Anions and Diclofenac on Degradation Rates. Chemosphere 2015, 139, 579-588. 50. Dionysiou, D. D.; Suidan, M. T.; Bekou, E.; Baudin, I.; Laı ̂né, J.-M. Effect of Ionic Strength and Hydrogen Peroxide on The Photocatalytic Degradation of 4-Chlorobenzoic Acid in Water. Appl. Catal. BEnviron. 2000, 26, 153-171. 51. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (•OH/•O−) in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17, 513-886. 52. Sirtori, C.; Zapata, A.; Oller, I.; Gernjak, W.; Agüera, A.; Malato, S. Decontamination Industrial Pharmaceutical Wastewater by Combining Solar Photo-Fenton and Biological Treatment. Water Res. 2009, 43, 661-668. 53. Ge, L.; Chen, J.; Wei, X.; Zhang, S.; Qiao, X.; Cai, X.; Xie, Q. Aquatic Photochemistry of Fluoroquinolone Antibiotics: Kinetics, Pathways, and Multivariate Effects of Main Water Constituents. Environ. Sci. Technol. 2010, 44, 2400-2405. 54. Tang, L.; Wang, J.; Zeng, G.; Liu, Y.; Deng, Y.; Zhou, Y.; Tang, J.; Wang, J.; Guo, Z. Enhanced Photocatalytic Degradation of Norfloxacin in Aqueous Bi2WO6 Dispersions Containing Nonionic Surfactant Under Visible Light Irradiation. J. Hazard. Mater. 2016, 306, 295-304. 55. Fotiou, T.; Triantis, T. M.; Kaloudis, T.; O'Shea, K. E.; Dionysiou, D. D.; Hiskia, A. Assessment of the Roles of Reactive Oxygen Species in the UV and Visible Light Photocatalytic Degradation of Cyanotoxins and Water Taste and Odor Compounds Using C–TiO2. Water Res. 2016, 90, 52-61. 56. Ma, Y.; Qiu, J.-b.; Cao, Y.-a.; Guan, Z.-s.; Yao, J.-n. Photocatalytic Activity of TiO2 Films Grown on Different Substrates. Chemosphere 2001, 44, 1087-1092. 57. Diebold, U. The Surface Science of Titanium Dioxide. Surface science reports 2003, 48 (5), 53229. 58. Tan, B.; Zhang, Y.; Long, M. Large-Scale Preparation of Nanoporous TiO2 Film on Titanium Substrate with Improved Photoelectrochemical Performance. Nanoscale Res. Lett. 2014, 9, 1-6. 59. Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M. Synergism between Rutile and Anatase TiO2 Particles in Photocatalytic Oxidation of Naphthalene. Appl. Catal. A-Gen. 2003, 244, 383-391. 60. Kleiser, G.; Frimmel, F. H. Removal of Precursors for Disinfection By-Products (Dbps) — Differences between Ozone- and OH-Radical-Induced Oxidation. Sci. Total. Environ. 2000, 256, 1-9. 61. Ou, H.-s.; Ye, J.-s.; Ma, S.; Wei, C.-h.; Gao, N.-y.; He, J.-z. Degradation of Ciprofloxacin by UV and UV/H2O2 Via Multiple-Wavelength Ultraviolet Light-Emitting Diodes: Effectiveness, Intermediates and Antibacterial Activity. Chem. Eng. J. 2016, 289, 391-401. 62. Liu, Y.; Chen, L.; Yuan, Q.; He, J.; Au, C.-T.; Yin, S.-F. A Green and Efficient Photocatalytic Route for the Highly-Selective Oxidation of Saturated Alpha-Carbon C–H Bonds in Aromatic Alkanes Over Flower-Like Bi2WO6. Chem. Commun. 2016, 52, 1274-1277. 63. Lalumera, G. M.; Calamari, D.; Galli, P.; Castiglioni, S.; Crosa, G.; Fanelli, R. Preliminary Investigation On The Environmental Occurrence and Effects of Antibiotics Used in Aquaculture in Italy. Chemosphere 2004, 54, 661-668. 64. Lin, C.-E.; Deng, Y.-J.; Liao, W.-S.; Sun, S.-W.; Lin, W.-Y.; Chen, C.-C. Electrophoretic Behavior and Pka Determination of Quinolones with a Piperazinyl Substituent by Capillary Zone Electrophoresis. J. Chromatogr. A 2004, 1051, 283-290. 65. Chen, L.; Zhao, C.; Dionysiou, D. D.; O’Shea, K. E. TiO2 photocatalytic degradation and detoxification of cylindrospermopsin. J. Photoch. Photobio. A 2015, 307, 115-122. 66. Sayed, M.; Khan, J. A.; Shah, L. A.; Shah, N. S.; Khan, H. M.; Rehman, F.; Khan, A. R.; Khan, A. M. Degradation of Quinolone Antibiotic, Norfloxacin, in Aqueous Solution using Gamma-Ray Irradiation. Environ. Sci. Pollut. R 2016, 1-14.
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762 763 764 765 766 767 768 769 770 771
Tables
772 773 774
775
Table 1 Physiochemical characteristics of norfloxacin Name Molecular formula Molecular weight (g/mol)
Norfloxacin C16H18FN3O3 319.33
Water solubility (mg L‒1)
400 63
pKa log Kow Molecular structure
6.22, 8.38 64 1.03 60
pKa = acid dissociation constant; log Kow= octonol-water partition coefficient
776 777 778
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779 780 781 782 783 784 785 786
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Table 2 Chemical composition of tape water (TW), river water (RW) and synthetic wastewater (SWW).
[pH]0 SO42‒ (mg L‒1) TW RW SWW
7.9 7.3 8.1
5.2 7.5 12.5
NO3‒ (mg L‒1) 0.32 0.71 3.2
Cl‒ (mg L‒1) 0.5 0.63 1.31
K+ (mg L‒1) 0.03 0.07 9.5
Ca2+ (mg L‒1) 1.51 1.92 0.1
Mg2+ (mg L‒1) 0.53 0.78 12.5
Cu2+ (mg L‒1) ND ND 22.0
Na+ (mg L‒1) 0.39 0.53 1200
Fe3+ (mg L‒1) ND ND 50
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Table 3 Effect of inorganic anions and cations on the photocatalytic degradation of norfloxacin. Experimental conditions; [norfloxacin]0 = 10 mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, HF = 0.02 M and HT = 3hrs. Exp. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
[pH]0 6.5 7.3 7.7 8.6 9.0 10.5 10.6 7.5 7.8 7.2 7.4 7.8 8.1 6.5 6.8 5.5 6.2 7.2 7.5 6.5 6.8
SO42‒ (mM) 0 2 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
HCO3‒ NO3‒ (mM) (mM) 0 0 0 0 0 0 2 0 5 0 0 2 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Cl‒ (mM) 0 0 0 0 0 0 0 2 5 0 0 0 0 0 0 0 0 0 0 0 0
K+ (mM) 0 0 0 0 0 0 0 0 0 2 5 0 0 0 0 0 0 0 0 0 0
Ca2+ (mM) 0 0 0 0 0 0 0 0 0 0 0 2 5 0 0 0 0 0 0 0 0
Mg2+ (mM) 0 0 0 0 0 0 0 0 0 0 0 0 0 2 5 0 0 0 0 0 0
Cu2+ (mM) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 5 0 0 0 0
Na+ (mM) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 5 0 0
Fe3+ (mM) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 5
% kapp Deg. (min‒1) 98 0.0504 78 0.0193 71 0.0174 89 0.0252 85 0.0218 94 0.0346 92 0.0304 95 0.0372 93 0.0321 96 0.0490 94 0.0345 67 0.0136 62 0.0129 71 0.0153 68 0.0146 73 0.0160 69 0.0151 90 0.0305 87 0.0237 55 0.0106 50 0.0099
t1/2 (min.) 13.75 35.91 39.83 27.51 31.79 20.03 22.80 18.63 21.59 14.15 20.10 50.96 53.73 45.30 47.48 43.32 45.90 22.73 29.25 65.39 70.02
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1
Table 4
2 3 4
Scavenger studies on photocatalytic degradation of norfloxacin by TiO2/Ti film with exposed {001} facets. Experimental conditions; [norfloxacin]0 = 10 mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, HF = 0.02 M and HT = 3hrs.
5
Reactive specie e‒cb (electron)
Scavenger O2
e‒cb (electron) • OH (hydroxyl radical)
Cu2+ tert-BuOH (tertbutanol)
1
N3‒ (sodium azide)
O2 (Singlet oxygen)
h+vb (hole)
HCO2H (formic acid)
Reaction e‒cb + O2 → O2‒ k = 7.6 × 107M‒1s‒1 e‒cb + Cu2+ → Cu+ • OH + tert-BuOH → H2O + • CH2C(CH3)2OH k = 5.0 × 108 M‒1s‒1 N3 + O2‒ k =2.0 × 109 M‒1s‒1 2hvb+ + 2HCO2‒ → CO2 + 2H+
Reference 60
65 66
55
67
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
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Table 5 UPLC-Ms/Ms retention times, chemical structures, and chemical formulas of norfloxacin and its degradation products produced during TiO2/Ti photocatalysis with exposed {001} facets. Experimental conditions; [norfloxacin]0 = 10 mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, HF = 0.02 M and HT = 3hrs.
No.
Retention time (min)
Chemical Structure
Proposed Formula [M+H]+
Observed m/z
Calculated m/z
[M+H]+
[M+H]+
----
10.6
C16H19FN3O3
320.1
320.1
DP 1
12.5
C15H15FN3O5
336.1
336.1
DP 2
16.8
C15H16N3O6
334.1
334.1
C14H17FN3O3
294.1
294.1
O F
DP 3
8.2
N NH2
O OH
N H H3C
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DP4
7.3
C12H12FN2O3
251.1
251.1
DP5
4.2
C12H11FNO4
252.1
252.1
DP6
11.2
C16H20N3O4
318.1
318.1
DP7
13.2
C16H17FN3O5
350.3
350.3
DP8
9.8
C14H15F3NO4
308.2
308.2
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Figures
32 33
(a)
(b)
(c)
(d)
34 35 36 37 38 39 40 41 42 43 44 45 46
(e)
47 48 49 50 51 52 53 54 55 56 57 58
Fig.1 FE-SEM images of as-prepared TiO2/Ti film with exposed {001} facets at various pH of the hydrothermal solution. The insets in a, b, c, d and e corresponds to high – magnification FESEM images. (a) pH 2.62; (b) pH 3.04; (c) pH 3.75; (d) pH 4.52; (e) pH 5.38. Other conditions: HT = 3h, hydrothermal temperature = 180 ˚C and HF = 0.02M.
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Fig.2 FE-SEM images of as-prepared TiO2/Ti film with exposed {001} facets after hydrothermal treatments at 180 ˚C for different hydrothermal times. The insets in a, b, c, and d corresponds to high – magnification FE-SEM images. (a) 3 hrs; (b) 5 hrs; (c) 8 hrs and (d) 15 hrs. Other conditions: pH = 2.62, hydrothermal temperature =180 ˚C and HF = 0.02M.
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Fig. 3 FE-SEM of {001} facet TiO2 films prepared at various HF concentrations. The insets in a, b, c, and d corresponds to high – magnification FE-SEM images. (a) 0.01 M, (b) 0.02 M, (c) 0.03 M, (d) 0.04 M. Other conditions: pH = 2.62, hydrothermal temperature = 180 ˚C, HT = 3hrs.
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Fig 4. TEM images of as-prepared TiO2/Ti films with exposed {001} facets (a) and {101} facets (b). The insets in a and b corresponds to SAED patterns (top) and HR-TEM images (bottom). Other conditions: pH = 2.62, hydrothermal temperature = 180 ˚C, HT = 3hrs, HF = 0.02M.
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Fig. 5 XRD patterns of as-prepared TiO2/Ti films at 180 ˚C for 3 hr in 0.02 M HF solutions with different pH values (pH = 2.62, 3.04, 3.75, 4.52 and 5.38). Other conditions: HF = 0.02M, hydrothermal temperature = 180 ˚C, HT = 3hrs.
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Fig. 6 High resolution XPS spectra of (A) O 1s and (B) Ti 2P. Other conditions: pH = 2.62, HF = 0.02M, hydrothermal temperature = 180 ˚C, HT = 3hrs.
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Fig. 7 Photocatalytic performance of as-prepared TiO2/Ti film with exposed {001} facets (a) at various pHs, (b) at different hydrothermal time, (c) at different concentrations of HF and (d) cyclic photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film prepared at pH = 2.62, HF = 0.02M and HT = 3 hrs.
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Fig 8. Degradation of norfloxacin in various aquatic media by {001} faceted TiO2/Ti film. Experimental conditions: [norfloxacin]0 = 10mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, hydrothermal temperature = 180 ˚C, HF = 0.02M and HT =3 hrs.
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Fig.9 Photocatalytic degradation of norfloxacin by {001} faceted TiO2/Ti film in (a) O2saturated media, air-saturated media, N2-saturated media and (b) in the presence of various ROS scavengers. Experimental conditions: [norfloxacin]0 = 10mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, hydrothermal temperature = 180 ˚C, HF = 0.02M and HT =3 hrs.
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Fig. 10 Changes in % inhibition of norfloxacin alone and in the presence of TiO2/Ti film with exposed {001} facets for E. coli at various intervals of UV-irradiation. Experimental conditions: [norfloxacin]0 = 10mg L‒1, TiO2/Ti film with exposed {001} facets prepared at pH = 2.62, hydrothermal temperature = 180 ˚C, HF = 0.02M and HT =3 hrs.
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