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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

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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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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;

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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

36

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

43

diffraction (XRD), high resolution transmission electron microscope (HR-TEM) and X-ray

44

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

66 67

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

69

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

72

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

75

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

78

incomplete degradation of target contaminant 9.

79

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

81

causing their complete mineralization 10-13.

, however, they are suffered with many drawbacks. For example, biodegradation is considered

In the recent years, advanced oxidation

82

UV/TiO2 photocatalysis, an AOPs, has extensively been investigated as a promising

83

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

86

produced during degradation of norfloxacin has not been discussed

87

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

89

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

94

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

99

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

101

reaction of TiF4 powder with 1-butanol

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various hydrothermal times was also investigated during the preparation of multifaceted titania

103

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

105

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

109

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

147

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

161

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

190

for 10 min after every gradient elution. The degradation products were analyzed in an

191

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

200

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

203

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

205

Na2CO3 buffer. Afterwards, 1 mL of norfloxacin solution (5 × 10‒3 mM) was mixed with 2 mL

206

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-

208

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

213

% 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

217

examined instead of norfloxacin.

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3.

Results and Discussion

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3.1.

Morphological and structural properties

220

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

223

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

230

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.

234

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

236

co-workers 31.

237

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.

247

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

.

273

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.

302

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


Page 13 of 46

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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.


Page 17 of 46

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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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Page 18 of 46

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

Page 19 of 46

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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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Page 20 of 46

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.


Page 21 of 46

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580


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

588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619

1. Zhao, S.; Liu, X.; Cheng, D.; Liu, G.; Liang, B.; Cui, B.; Bai, J. Temporal–Spatial Variation and Partitioning Prediction of Antibiotics in Surface Water and Sediments from the Intertidal Zones of the Yellow River Delta, China. Sci. Total Environ. 2016, 569, 1350-1358. 2. Zhang, W.; Gao, H.; He, J.; Yang, P.; Wang, D.; Ma, T.; Xia, H.; Xu, X. Removal of Norfloxacin using Coupled Synthesized Nanoscale Zero-Valent Iron (nZVI) with H2O2 System: Optimization of Operating Conditions and Degradation Pathway. Sep. Purif. Technol. 2017, 172, 158-167. Liu, X.; Zhang, H.; Li, L.; Fu, C.; Tu, C.; Huang, Y.; Wu, L.; Tang, J.; Luo, Y.; Christie, 3.

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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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760 761


67. Marks, R.; Yang, T.; Westerhoff, P.; Doudrick, K. Comparative Analysis of The Photocatalytic Reduction of Drinking Water Oxoanions Using Titanium Dioxide. Water Res. 2016, 104, 11-19.

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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779 780 781 782 783 784 785 786


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

22 23 24 25

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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


Page 31 of 46

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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

26 27 28 29 30 31 ACS Paragon Plus Environment


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31

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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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59 60 61 62

(a)

(b)

63 64 65 66 67 68 69 70

(c)

(d)

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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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Efficient Photocatalytic Degradation of Norfloxacin ... - ACS Publications

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