Accepted Manuscript

Title: Effects of Titanate Nanotubes Synthesized by a Microwave HydrothermalMethod on Photocatalytic Decomposition of Perfluorooctanoic Acid

Authors: Ying-Chu Chen, Shang-Lien Lo, Jeff Kuo

PII: S0043-1354(11)00283-1

DOI: 10.1016/j.watres.2011.05.020

Reference: WR 8608

To appear in: Water Research

Received Date: 13 December 2010

Revised Date: 7 May 2011

Accepted Date: 21 May 2011

Please cite this article as: Chen, Y.-C., Lo, S.-L., Kuo, J. Effects of Titanate Nanotubes Synthesized bya Microwave Hydrothermal Method on Photocatalytic Decomposition of Perfluorooctanoic Acid, WaterResearch (2011), doi: 10.1016/j.watres.2011.05.020

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service toour customers we are providing this early version of the manuscript. The manuscript will undergocopyediting, typesetting, and review of the resulting proof before it is published in its final form. Pleasenote that during the production process errors may be discovered which could affect the content, and alllegal disclaimers that apply to the journal pertain.

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

1

Effects of Titanate Nanotubes Synthesized by a Microwave Hydrothermal 1

Method on Photocatalytic Decomposition of Perfluorooctanoic Acid 2

3

Ying-Chu Chena,b, Shang-Lien Loa,*, and Jeff Kuoc 4

5

aGraduate Institute of Environmental Engineering, National Taiwan University, Taipei, 6

106, Taiwan, 7

bDepertment of Environmental Monitoring & Information Management, 8

Environmental Protection Agency, Taiwan (R.O.C.), 9 cDepartment of Civil and Environmental Engineering, California State University, 10

Fullerton, CA 11

12

*Corresponding author: Shang-Lien Lo 13

14

E-mail address: sllo@ntu.edu.tw 15

Phone: +886-2-2362-5373 16

Fax: +886-2-2392-8821 17

Mailing address: Graduate Institute of Environmental Engineering, National Taiwan 18

University, 71, Chou-Shan Rd., Taipei 106, Taiwan (R.O.C.) 19

20

Abstract 21

Titanate nanotubes (TNTs) were used to remove perfluorooctanoic acid (PFOA) 22

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

2

from aqueous solutions in this study. Direct photolysis of PFOA by a 254-nm UV light 23

(400 W) was found effective to decompose PFOA without presence of photocatalysts. 24

Shorter-chain perfluorocarboxylic acids (PFCAs) and fluoride ions were formed 25

during photodecomposition. Addition of TNTs as photocatalysts did not greatly 26

enhance photocatalytic decomposition of PFOA. TNTs mainly act as adsorbents to 27

adsorb PFOA and form TNT-PFOA complexes. It suggested that sodium ions and 28

oxygen atoms on the surfaces of TNTs play important roles in PFOA adsorption. X-ray 29

Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy 30

(FTIR) analyses indicated that ion-exchange, electrostatic interaction, and 31

hydrophobic interaction all participated in the photocatalytic reaction of PFOA by 32

TNTs. 33

34

Keyword: Microwave; Perfluorooctanoic acid; Photocatalysis; Titanate nanotube; 35

X-ray Photoelectron Spectroscopy 36

37

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

3

1. Introduction 1

Perfluorocarboxylic acids (CnF2n+1COOH, PFCAs) and their derivatives are 2

anthropogenic organic compounds that have a wide range of applications. PFCAs can 3

be used as industrial surfactants, additives, firefighting foams, and lubricants (Lin et al., 4

2010a). Some perfluorinated acids, especially perfluorooctanoic acid (PFOA, 5

C7H15COOH), have garnered concerns because of their ubiquitous usage, persistence, 6

and bioaccumulation in the environment. PFOA is exceptionally inert and persists 7

indefinitely in the environment (Musijowski et al., 2007), even in remote polar areas 8

(Qu et al., 2010). Some evidences show that PFOA can accumulate in organisms and 9

pose a potential human health risk (Hinderliter et al., 2006; Potera, 2009). The Science 10

Advisory Board of the United States Environmental Protection Agency (U.S. EPA) 11

reported that PFOA is likely a carcinogen (Hogue, 2006). Furthermore, U.S. EPA 12

launched a program to reduce 95 % PFOA emissions from manufacturers by 2010 and 13

to eliminate the use of the chemical by 2015 (Qu et al., 2009). Development of 14

effective treatment methods to convert PFOA into harmless species is desirable. 15

Due to its inherent resistance to chemical and microbiological treatment, many 16

technologies have been developed to decompose PFOA, including advanced oxidation 17

processes (AOPs), such as UV-H2O2, persulfate, photo-Fenton, ozonation and 18

sonolysis (Cheng et al., 2008; Hori et al., 2004; Hori et al., 2008b; Moriwaki et al., 19

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

4

2005; Vicitis et al., 2008). Direct photolysis (Cao et al., 2010; Chen and Zhang, 2006; 20

Chen et al., 2007; Hori et al., 2004) and photocatalysis (Qu et al., 2010) in aqueous 21

solutions are alternatives that are being researched in recent years. PFOA is sensitive to 22

light with wavelengths from deep UV-region to 200 nm (Cao et al., 2010), thus can be 23

efficiently decomposed by direct UV irradiation. TiO2-mediated heterogeneous 24

photocatalysis of PFOA was found to involve generation of electron/hole pairs that led 25

to better photocatalytic efficiencies (Dillert et al., 2008; Doll and Frimmel, 2005; 26

Estrellan et al., 2010; Panchangam et al., 2009; Ravichandran et al., 2006; 27

Ravichandran et al., 2009). The -Gallium oxide (Zhao and Zhang, 2009) and ZnO 28

(Ravichandran et al., 2007) were also used as photocatalysts to assist 29

photodecomposition of PFOA. Decomposition of PFOA into fluoride ions, which can 30

readily react with Ca2+ to form CaF2, a raw material in global demand for production of 31

hydrofluoric acid (Hori et al., 2009; Hori et al., 2010; Ravichandran et al., 2006). 32

In addition to direct photolysis and photocatalysis, adsorption has been 33

demonstrated to be an effective and economical method to remove polar organic 34

pollutants from aqueous solutions (Senevirathna et al., 2010; Yu et al., 2009), but only 35

a few papers were found in our literature survey that studied the removal of PFOA by 36

adsorbents such as active carbon (Lampert et al., 2007; Ochoa-Herrera et al., 2008; 37

Senevirathna et al., 2010; Yu et al., 2009; Qu et al., 2009), polymers (Senevirathna et 38

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

5

al., 2010), zeolite and sludge (Ochoa-Herrera et al., 2008). PFOA is moderately 39

adsorbable with active carbon, and the adsorption efficiency is greatly influenced by 40

particle sizes of adsorbents. The powdered activated carbon (PAC) achieved an 41

adsorption equilibrium within 4 h (Senevirathna et al., 2010; Yu et al., 2009), while 42

granular activated carbon (GAC) required over 168 h to reach the equilibrium (Yu et al., 43

2009). 44

Titanate nanotubes (TNTs) synthesized by microwave hydrothermal (M-H) 45

methods have some desired properties, such as high specific surface area, 46

photocatalytic properties, and ion-exchangeable capabilities (Ou and Lo, 2007). In our 47

previous study, the specific surface area of microwave-assisted TNTs was 150 m2 g-1 48

and could adsorb 2,000 mg Pb(II) per gram of TNTs at pH of 4 (Chen et al., 2010a). 49

Compositing TNTs with semiconductors, such as cadmium sulfide (CdS), can enhance 50

their photocatalytic capability to eliminate 52.3 % of ammonia in water (Chen et al., 51

2010b). TNTs synthesized by M-H methods have great potentials to be photcatalysts 52

and adsorbents in various applications. 53

This study aimed at investigating adsorption and photocatalytic behaviors of TNTs 54

synthesized by a M-H method for their removal of PFOA from aqueous solutions. The 55

sorption isotherms were developed and effects of solution pH were evaluated; while 56

effects of UV light, loading concentrations, and solution pH on photocatalysis were 57

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

6

also studied. A detailed reaction mechanism was proposed based on the experimental 58

results and the instrumental (X-ray Photoelectron Spectroscopy (XPS) and Fourier 59

Transform Infrared Spectroscopy (FTIR)) analyses. Findings of this study provide 60

insights of using TNTs to remediate environmental media that are contaminated by 61

PFOA. 62

63

2. Materials and methods 64

2.1. Materials 65

All chemicals used in the experiments were of reagent grade. Perfluorooctanoic 66

acid (PFOA, C7F15COOH, 96 % purity) was from Aldrich (USA). Perfluoroheptanoic 67

acid (PFHpA, C6F13COOH, 98 % purity), perfluoropentanoic acid (PFPeA, 68

C4F9COOH, 97 % purity), and heptafluorobutyric acid (HFBA, C3F7COOH, 99 % 69

purity) were from Alfa Aesar (USA). Perfluorohexanoic acid (PFHxA, C5F11COOH, 70

97 % purity) was purchased from Fluka (USA). Methyl alcohol anhydrous (CH3OH, 71

99.9 % purity, Mallinckrodt Chemicals, USA), boric acid (H3BO3, 99.5 % purity, 72

Nacalai Tesque Inc., Japan), and sodium hydroxide (NaOH, 99.7 % purity, Alps 73

Chem Co. Ltd., Taiwan) were used to prepare mobile phases for high performance 74

liquid chromatography (HPLC) analyses. Sulfuric acid (H2SO4, 97 % purity, Nacalai 75

Tesque Inc., Japan) was used to regenerate the suppressor column of HPLC. Also, 76

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

7

sodium bicarbonate (NaHCO3, 99.6 % purity, Nacalai Tesque Inc., Japan), sodium 77

carbonate (Na2CO3, 99.0 % purity, Nacalai Tesque Inc., Japan), and H2SO4 were used 78

to prepare the mobile phase for ion chromatography. 79

Titanium dioxide (TiO2 Degussa P25, 99.5 % purity, Simakyu Pure Chemicals, 80

Japan), NaOH, and hydrochloric acid (HCl, 70 % purity, Fisher Scientific, USA) 81

were used to fabricate TNTs. Nitric acid (HNO3, 60.0 % purity, Yakuri Pure 82

Chemicals Co. Ltd., Japan) and NaOH were used for pH adjustment. All solutions 83

were prepared with Milli-Q ultrapure water (18 M-cm resistivity). 84

2.2. Synthesis of TNTs 85

Titanate nanotubes were synthesized by the M-H method described elsewhere 86

(Chen et al., 2010a). The microwave digestion system (Ethos Touch Control, 87

Milestone Corporation, Italy) consists of a double-walled vessel with an inner Teflon 88

liner and an outer shell of high strength ULTEMpolyetherimide. The Teflon liner 89

can resist reaction conditions employed in this study and no cross-contamination from 90

the Teflon liner was observed. Briefly, a mixture of 600 mg TiO2 and 70 mL NaOH (10 91

N) was stirred for 40 min in a Teflon container which was then sealed before the 92

microwave heating process. The container was then placed into the microwave 93

digestion system under 400-W irradiation at 403 K for 3 h. After the treatment, the 94

solution was washed three times with 0.5 N HCl and four times with deionized water, 95

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

8

followed by centrifugation at 800 rpm for 5 min (Kubota 6800, Kubota Co., Japan), 96

and vacuum dried at 214 K for 24 h (FD3-12P, Kingmech Co., Ltd., Taiwan). 97

2.3. Batch experiments 98

All the experiments were carried out in polypropylene (PP) bottles to avoid 99

potential interferences from sample containers. The experimental apparatus for 100

photocatalysis contains a cooling water jacket to maintain the temperature at 298 K 101

(B204, Firstek Scientific Co., Ltd., Taiwan) and a UV lamp (254 nm, 400 W, Philips, 102

Holland). Known amounts of photocatalysts (TNTs or TiO2) were dispersed into 2-liter 103

PFOA solutions (50 mg L-1). During UV irradiation, 20 mL aliquots were taken at 0, 1, 104

2, 3, 6, 9, 24 h, and photocatalysts were immediately removed by filtration through 105

0.22-m filters (MillexGS, Millipore, Ireland) for subsequent analyses. The 106

experiments were carried out twice and mean values were reported here. 107

Adsorption experiments were performed under ambient conditions. Analytical 108

grade PFOA was used to prepare a stock solution of 1,000 mg L-1, which was further 109

diluted with deionized water to different concentrations for adsorption experiments. 110

Adsorption isotherm studies were conducted with initial PFOA concentration ranging 111

from 5 100 mg L-1 together with 20 mg TNTs or TiO2 in 100 mL solutions. Solutions 112

were then shaken at 298 K for 24 h at a speed of 150 rpm in a reciprocal shaker bath 113

(BT-350, Bersing Co., Ltd., Taiwan). The pH values of initial solutions were adjusted 114

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

9

by using HNO3 or NaOH to reach the desired values. Samples were taken from each 115

solution and then analyzed by HPLC. All experiments were conducted twice and 116

average values were used in the data analyses. 117

2.4. Analytical Methods 118

Concentrations of PFOA and shorter-chain PFCAs were analyzed by HPLC 119

(Dionex, UltiMate 3000, USA) coupled with a conductivity detector (ED-50, Dionex, 120

USA) and an anion self-regenerating suppressor (ASRS 300 2-mm, USA). The 121

conductivity detector was used in conjunction with a Dionex micromembrane 122

suppressor column. The suppressor column was constantly regenerated using 0.5 mM 123

H2SO4 at a flow rate of 1 mL min-1. The PFCAs were extracted by a 150 2.1 mm, 124

3.5m column (AcclainPolar AdvantageII C18, Dionex, USA) maintained at 303 K. 125

A trinary gradient was employed with mobile phase A containing 100 % methanol, 126

mobile phase B containing Milli-Q water, and mobile phase C containing 9 mM NaOH 127

and 100 mM H3BO3 in deionized water. The flow rate of the solvents was 0.3 mL min-1. 128

The gradient was operated at 20 % phase A, 40 % phase B for the initial 5 min; 20 % 129

phase A increased to 60 % and 40 % phase B decreased to 0 % for the next 10 min (5 130

15 min); Maintaining 60 % phase A, 0 % phase B during 15 20 min; 60 % phase A 131

decreased to 20 %, 0 % phase B increased to 40% for final 5 min. The phase C was 132

maintained at 40 % during the total running time, 25 min. All calibration curves for 133

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

10

PFCAs were linear over the 0.5 50 mg L-1 range. The limits of detection (LODs) 134

using 50 L samples, based on a signal-to-noise (S/N) ratio of 3, were 1 mg L-1 for 135

PFCAs. Degradation ratios were calculated as follows: 136

1000

0

=

CCCR % (1) 137

where C is concentration of PFOA (mM) and 0C is initial concentration of PFOA 138

(mM). 139

Concentrations of aqueous fluoride ions were determined by an ion 140

chromatography (Metrohm 790 Personal IC, Metrohm Ltd., Switzerland) with a 141

column of CH-9101 Herisan (Metrohm Ltd., Switzerland). A mixture of 1.8 M 142

Na2CO3/1.7 M NaHCO3 (1:1) was used as the mobile phase at a flow rate of 1.0 mL 143

min-1. Defluorination ratios were calculated as follows: 144

100150

=

CC

R FF % (2) 145

Where FC is concentration of fluoride ions (mM), 0C is initial concentration of 146

PFOA (mM), and the factor of 15 corresponds to the number of fluorine atoms in one 147

PFOA molecule. 148

2.5. Characterization Methods for TNTs 149

The TNTs used in this study were fabricated in our research lab. The physchemcial 150

properties, such as BET surface area and zeta potential, of TNTs synthesized by the 151

M-H method can be found in Chen et al. (2010a). Surface functional groups of TNTs 152

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

11

were characterized by FTIR and XPS. The IR scanning range was 4000 650 cm-1 153

with 4 cm-1 resolution using a mercury-cadmium-telluride detector in a Nexus 470 154

spectrometer (Thermo Nicolet, USA). KBr powder was used to record the reference 155

spectrum. Chemical bonding of PFCAs onto surfaces of TNTs/TiO2 was investigated 156

by XPS using an ESCA PHI 1600 (Physical Electronics, USA). The XPS spectra were 157

recorded with a monochromatized Mg (K) source of 1253.6 eV energy (15 kV, 400 158

W). The bonding energy scale for final calibration was corrected by the C1s peak of 159

284.6 eV. 160

161

3. Results and discussion 162

3.1. Direct photolysis 163

Aqueous PFOA solutions (50 mg L-1) were irradiated with a 254-nm UV light (400 164

W), and the extents of photolysis were shown in Fig. 1(a). Control tests (without UV 165

irradiation) were carried out and no significant changes in PFOA concentration were 166

observed. The degradation rate was the fastest at pH 4, followed by pH 7, and then pH 167

10; 98% of initial PFOA was photodecomposed at pH 4 after 48-h irradiation. It was 168

reported that PFOA has a weak absorption of UV light longer than 200 nm, and only 169

9% of initial PFOA was decomposed with a 254-nm UV light after 2-h irradiation (23 170

W low-pressure mercury lamp) (Cao et al., 2010), and 89.5% of initial PFOA was 171

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

12

decomposed by a xenon-mercury lamp after 72-h irradiation (wavelengths were 172

mainly in the range of 220 460 nm) (Hori et al., 2004). It implies that irradiation 173

intensity of lights plays an important role in affecting photolytic efficiency of PFOA. 174

PFOA can be efficiently photodecomposed under a light emitting higher irradiation 175

intensity than that with weaker irradiation intensity. Photodecomposition of PFOA and 176

formation of shorter-chain PFCAs, bearing C6-C7 perfluoroalkyl groups were 177

quantified by HPLC. The C5-C4 perfluoroalkyl groups were hardly detected within 178

24-h irradiation, while C3-C2 perfluoroalkyl groups were known to be easily 179

evaporated under ambient conditions. At pH 4, the concentration of PFHpA increased 180

with irradiation up to 24 h and then decreased, concentration of PFHxA increased up to 181

36 h and then decreased, while fluoride concentrations increased continuously with 182

extended irradiation time (Fig. 1(b)). It indicates that photodecomposition of PFOA 183

was achieved by a step-by-step removal of the CF2 units. Besides irradiating with a 184

254-nm UV light, photolysis of PFOA was also effective in VUV system (185 nm) 185

(Cao et al., 2010; Chen and Zhang, 2006; Chen et al., 2007). Therefore, direct 186

photolysis of PFOA was mainly depended on irradiation intensity of lights when the 187

wavelength were the same; otherwise, longer wavelength of lights needs higher 188

irradiation intensity to assist photolytic efficiency of PFOA without catalysts. 189

After 24 h of irradiation, defluorination of PFOA at pH 4, 7, and 10 were 85 %, 68 190

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

13

%, and 55 %, respectively. The acid condition (pH 4) was found more favorable for 191

photodegradation (98 %) and defluorination (85 %) of PFOA. Although initial pH 192

value of solutions were adjusted by HNO3 and/or NaOH to reach the desired values, 193

pH value of solutions would gradually decrease during 24-h UV irradiation and finally 194

reached the equilibrium at pH 4, which is the natural pH of 50 mg L-1 PFOA under 195

ambient conditions. HNO3 has been demonstrated to have minor effects on degradation 196

and defluorination of PFOA (Ravichandran et al., 2006), whereas presume of sodium 197

ions may have retardation effects on defluorination and decomposition of PFOA since 198

dissolved metal ions were found to affect photocatalytic efficiencies of PFCAs 199

(Ravichandran et al., 2006). Therefore, using NaOH to increase initial pH value of 200

solutions to 7 or 10 might not be favorable in photodecomposition of PFOA. Released 201

fluoride ions can bind with sodium ions to form sodium fluoride (NaF), causing a 202

lower defluorination at pH 7 and 10. In addition, more anionic fluoride ions released 203

into solutions would also decrease pH value of solutions during photolysis. Solution 204

pH can be an indicator to observe if experiments reached the equilibrium when it 205

becomes steady. Consequently, this study suggests not to adjust pH value of the 206

solutions and maintain it at pH 4 (the highest photolytic efficiency) in the subsequent 207

experiments. 208

3.2. Adsorption of PFOA onto TNTs 209

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

14

Adsorption isotherms of PFOA onto TNT or TiO2 are shown in Fig. 2. TiO2 has a 210

much lower adsorption capacity for PFOA than TNTs at pH 4; it is as expected be due 211

to the fact that TNTs have a surface area that is three times larger, 140 v.s. 50 m2 g-1 212

(Chen et al., 2010a). The length of PFOA molecules is 1 nm (Yu et al., 2009), thus 213

PFOA can easily diffuse into inner pores of TNTs (pore diameters of TNTs were 9 nm 214

(Chen et al., 2010a)), and adsorb onto inner as well as outside walls of TNTs. 215

Fig. 2 also shows the extents of adsorption decrease with an increase in solution pH. 216

Due to the low pKa of PFOA, 2.5 (US EPA, 2002), more PFOA will be in ionic form as 217

solution pH increases. Electrostatic attraction between negatively charged PFOA and 218

positively charged TNT surfaces would promote surface reactions with valence band 219

holes (h+), which are favorable in PFOA photodecomposition (Bahnemann et al., 1997; 220

Estrellan et al., 2010). Higher adsorption efficiency achieved at lower solution pH was 221

due to electrostatic forces of attraction (Panchangam et al., 2009). Besides electrostatic 222

interaction, hydrophobic interaction was also important for adsorption of PFOA 223

(Higgins et al., 2005; Higgins and Luthy, 2006; Yu et al., 2009). Hydrophobic PFOA 224

and TNTs had a hydrophobic interaction during the adsorption process (Yu et al., 2009). 225

The formation of PFOA-TNTs complexes could enhance oxidative power of TNTs 226

(Dillert et al., 2008). 227

3.3. Photocatalysis induced by TNTs 228

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

15

First-order kinetics were used to simulate photodecomposition of PFOA at pH 4 by 229

a 254-nm UV light, and the results were shown in Fig. 3. The pseudo first-order rate 230

constants were calculated to be 0.0230 h-1, 0.0337 h-1 and 0.0271 h-1, for 0.05, 0.125 231

and 0.25 g L-1 TNTs, respectively. The remaining PFOA concentration at 24-h 232

photocatalytic reactions decreased as amounts of TNTs increased from 0.05 to 0.125 g 233

L-1; when amounts of TNTs further increased 0.25 g L-1, the remaining PFOA 234

concentration started to increase. It is apparent that amounts of TNTs have great 235

influences on photodecomposition of PFOA and a suggested loading amount was 236

0.125 g L-1. An adequate loading concentration increases the generation rate of 237

electron/hole pairs that promote photodecomposition of PFOA, while excessive 238

amounts of TNTs in solutions would decrease the light penetration (Zhu et al., 2005). 239

The superior photocatalytic performance of TNTs than TiO2 might be due to the larger 240

surface area of TNTs to receive more UV irradiation to excite PFOA adsorbed on the 241

surfaces than TiO2 particles (Estrellan et al., 2010; Panchangam et al., 2009). 242

Consequently, TNTs synthesized by raw materials of TiO2 can enhance their 243

photocatalytic efficiency to decompose PFOA. 244

Concentrations of remaining PFOA gradually decreased while those of fluoride 245

ions increased as the photocatalytic reaction proceeded; however, the reaction rates 246

with the presence of TNTs were much slower than that of direct photolysis. In the 247

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

16

presence of 0.125 g L-1 TNTs, only 59% of the initial PFOA was degraded, whereas the 248

corresponding yield for direct photolysis was almost 100% after 24-h irradiation. Thus, 249

an addition TNTs or TiO2 as photocatalysts would retard photodecomposition of PFOA. 250

This phenomenon can be explained in terms of heterogeneous reactions. TNTs, which 251

are intended as photocatalysts, act as adsorbents to adsorb PFOA before mineralization 252

occurs. Photocatalysts were also found to scatter UV lights (Zhu et al., 2005) and 253

suppress formation of shorter-chain PFCAs during photocatalytic reactions (Hori et al., 254

2004). Generating active hydroxyl radicals ( OH ) in TiO2 system has poor 255

photoreactivity for PFCAs (Cao et al., 2010; Hori et al., 2004). Slower degradation 256

rates of PFOA in a TNT system may also be attributed to release sodium ions, that 257

were reported to affect photodecomposition of PFCAs. 258

Multi-walled TNTs were synthesized by scrolling up four layers of TiO6 259

octahedrons (Scheme S1 in the online Supplementary Material) (Chen et al., 2002), 260

thus the inner surface area of TNTs is much larger than outside walls. Researchers 261

found that the non-planar structure provided a scattering center for incident light, thus 262

elongated the path to promote the photocatalytic capability of photocatalysts (Lin et al., 263

2010b). Outer surfaces of TNTs could harvest the lights from any directions in the 264

surrounding that could be helpful for increasing the photocatalytic efficiency; therefore, 265

when PFOA diffused into pores and adsorbed onto inner walls of TNTs, its opportunity 266

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

17

to receive UV lights was greatly reduced. 267

Defluorination ratios of PFOA were much smaller than the PFOA decomposition 268

ratios. It implies that intermediate products formed during photocatalysis (Chen and 269

Zhang, 2006). Fluoride ions can adsorb onto surfaces of photocatalysts, leading to less 270

available surface active sites and smaller defluorination ratios (Estrellan et al., 2010; 271

Panchangam et al., 2009). TNTs have a high affinity to adsorb fluoride ions (Fig. A1 in 272

the online Supplementary Material). Positive charges on TNT surfaces under acidic 273

conditions (pH 4) cause more electrostatic attractions of fluoride ions. From the above 274

results, it can be deduced that smaller defluorination ratios of photocatalytic reactions 275

was caused by adsorption of fluoride ions onto TNT surfaces since fluoride ions were 276

continuously released during photodecomposition of PFOA; the other possibility was 277

that PFOA was directly adsorbed onto inner surfaces of TNTs without mineralization. 278

3.4. FTIR and XPS analyses of PFOA-TNTs/TiO2 279

Figure 4 shows XPS results of TNT or TiO2 before and after photocatalytic 280

reactions. The O1s, C1s, F1s, Na1s, and Ti2p3 peaks were investigated in XPS 281

wide-scans (Fig. 4(a)). Intensities O1s peaks at 532 eV were obviously decreased after 282

photocatalytic reactions (Fig. 4(b)). It reveals that oxygen-terminated functional 283

groups on TNT surfaces react with PFOA during photocatalytic reactions that led to a 284

decrease in oxygen concentrations after photocatalytic reactions (Guan et al., 2007). 285

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

18

The exciting peaks at 287 eV and 292 eV corresponded to bulk carbon signals (Fig. 286

4(c)) (Guan et al., 2007). Enhanced intensities of C1s peaks suggest that PFOA and 287

shorter-chain PFCAs were directly adsorbed onto TNTs/TiO2 surfaces. If 288

mineralization of PFOA continued, peaks corresponded to fluoride ions should appear 289

at 689 eV (Fig. 4(d)) (Guan et al., 2007; Hori et al., 2006; Hori et al., 2008a); however, 290

no peaks corresponded to fluoride species were found. Most of PFOA molecules were 291

directly adsorbed onto surfaces of TNTs without mineralization during UV irradiation 292

since the adsorption rate of TNTs is very fast (Chen et al., 2010a). Intensities of Na1s 293

peaks apparently decreased after photocatalysis (Fig. 4(e)). These results support the 294

previous ones that TNTs mainly act as adsorbents to adsorb PFOAs rather than as 295

photocatalysts. The adsorption mechanism of TNTs is that sodium ions on TNT 296

surfaces react with PFOA and less sodium ions would be detected on the surfaces of 297

TNTs. No sodium ions were detected while the reactions were photocatalyzed by TiO2. 298

The binding energies for Ti(IV) (467 eV) and Ti(III) (461 eV) on TNTs or TiO2 were 299

similar after photocatalytic reactions (Fig. 4(f)). The XPS spectra support the proposed 300

photocatalytic mechanisms that PFOA firstly adsorbed onto surfaces of TNTs, and the 301

remaining PFOA in solutions or PFOA adsorbed on the outside walls of TNTs are 302

mineralized by UV irradiation and formed shorter-chain PFCAs. 303

Figure 5 shows FTIR spectra of TNTs/TiO2 photocatalysts and PFOA- TNTs/TiO2 304

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

19

complexes. The band at 1138 cm-1 was attributed to C-F bonds stretching vibrations 305

in OOCC7F15 groups on the surfaces of PFOA-TNTs/TiO2 complexes (Kutsuna and 306

Hori, 2008; Men et al., 2008; Xu et al., 2009). The bands at 1438 cm-1 and 1647 cm-1 307

were attributed to COO asymmetrical stretching vibrations and symmetrical 308

stretching vibrations, respectively (Xu et al., 2009). The wide band at 3400 cm-1 region 309

was attributed to presence of hydroxyl groups ( OH ) and traces of water in the KBr 310

pellet (Men et al., 2008). Intensities of hydroxyl groups ( OH ) after photocatalytic 311

reactions indicated that catalysts were excited by UV lights to form hydroxyl radicals 312

( OH ), which can degrade PFOA and prevent rapid recombination with electron/hole 313

pairs (Estrellan et al., 2010). However, intensities of hydroxyl groups ( OH ) after 314

photocatalytic reaction by TiO2 were ascribed to be the occurrence of oxygen 315

vacancies on the surfaces, which were also found to be beneficial to its photocatalytic 316

activity (Ismail et al., 2007). The above FTIR and XPS results indicated that the 317

removal of PFOA mainly occurred with the functional groups on the surface of 318

photocatalysts. 319

3.5. Mechanisms of PFOA decomposition 320

Based on our experimental results, it can propose the photodecomposition 321

mechanisms of PFOA by TNTs under UV irradiation (Fig. 6). PFOA exists as an 322

anionic compound when emerging in solutions (eq. (3)) (Burns et al., 2008). 323

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

20

+++ OHCOOFCOHCOOHFC 3-

1572157 (3) 324

Positive charges on TNT surfaces are favorable to adsorb anionic PFOA with or 325

without UV irradiation (eq. (4)). 326

COOFCTNTsCOOFCTNTsTNTsTNTs h 157157)( + + (4) 327

PFOA adsorbed on the outer surfaces of TNTs and the remaining PFOA in solutions 328

were excited by UV irradiation to form excited state of PFOA and photolyzed into 329

157FC and COOH radicals (eq. (5)). 330

+ COOHFCCOOHFCCOOHFC hh 157157157

(5) 331

The 157FC radical continue adsorb onto surfaces of excited TNTs to reach the 332

maximum adsorbility (eq. (6)). 333

157157 FCTNTsFCTNTsTNTsTNTsh

+ (6) 334

The C7F15 radicals also react with water (water acts as a nucleophilic reagent to attrack 335

the end carbon atom) to form OHFC 157 (Chen et al., 2007), an alcohol, which 336

undergoes HF elimination to form C6F13COF (eq. (7)) (Chen and Zhang, 2006; Hori et 337

al., 2004). C6F13COF was then hydrolyzed to form PFHpA with one less the CF2 unit 338

(Hori et al., 2004) and fluoride ions were releases into aqueous solutions (eq. (8)). 339

+ ++ FHCOFFCOHFC 136157 (7) 340

+ ++ FHCOOHFCCOFFC 136136 (8) 341

The C-C bond between C6F13 and COOH can be further cleaved to form C6F13 radicals, 342

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

21

which undergoes similar reaction routes as C7F15 radicals to give PFHxA. The bond 343

cleavage routes of PFOA were similar to the photo-Kolbe mechanism (Dillert et al., 344

2008; Ravichandran et al., 2006). In the same manner, PFOA bearing shorter-chain 345

PFCAs, such as PFHpA, PFHxA, PFPeA, and HFBA, were formed in a stepwise 346

manner as time progressed. 347

348

4. Conclusions 349

Titanate nanotubes (TNTs) synthesized by the M-H method with large surface 350

specific surface areas make them good candidates for removal of PFOA from aqueous 351

solutions. Direct photolysis, photocatalysis were conducted under different solution 352

pH, coupled with XPS and FTIR spectroscopies. The findings obtained in this study 353

have demonstrated the following: 354

(1) PFOA could efficiently be photodecomposed by a 254-nm UV light (400 W) 355

within 48 h without presence of photocatalysts. Also, shorter-chain 356

perfluorocarboxylic acids (PFCAs) and released fluoride ions were formed during 357

photodecomposition. 358

(2) TNTs are good adsorbents than TiO2 for PFOA removal. The maximum adsorption 359

capacity can be as high as 50 mg PFOA /g TNT at acidic pH of 4. 360

(3) XPS and FTIR analyses indicated that interactions for the removal of PFOA mainly 361

occurred on the surfaces of TNTs. 362

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

22

363

Acknowledgements 364

The authors would like to thank the National Science Council of the Republic of 365

China, for their financial support under Contract No. NSC 98-2221-E-002-040-MY3. 366

367

Appendix. supplementary data 368

Supplementary data associated with this article can be found in the online versions, 369

at doi:10.1016/j.watres.2011.00.000. 370

371

References 372

Bahnemann, D.W., Hilgendorff, M., and Memming, R. (1997) Charge carrier 373

dynamics at TiO2 particles: reactivity of free and trapped holes. J. Phys. Chem. B 374

101(21), 4265-4275. 375

Burns, D.C., Ellis, D.A., Li, H., Mcmurdo, C.J., and Webster, E. (2008) Experimental 376

pKa determination for perfluorooctanoic acid (PFOA) and the potential impact of 377

pKa concentration dependence on laboratory-measured partitioning phenomena 378

and environmental modeling. Environ. Sci. Technol. 42(24), 9283-9288. 379

Cao, M.H., Wang, B.B., Yu, H.S., Wang, L.L., Yuan, S.H., and Chen, J. (2010) 380

Photochemical decomposition of perfluorooctanoic acid in aqueous periodate with 381

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

23

VUV and UV light irradiation. J. Hazard. Mater. 179(1-3), 1143-1146. 382

Chen, J. and Zhang, P. (2006) Photodegradation of perfluorooctanoic acid in water 383

under irradiation of 254 nm and 185 nm light by use of persulfate. Water Sci. 384

Technol. 54(11-12), 317-325. 385

Chen, J., Zhang, P., and Liu, J. (2007) Photodegradation of perfluorooctanoic acid by 386

185 nm vacuum ultraviolet light. J. Environ. Sci. 19(4), 387-390. 387

Chen, Q., Du, G.H., Zhang, S., and Peng, L.M. (2002) The structure of trititanate 388

nanotubes. Acta. Cryst. Sect. B 58(4), 587-593. 389

Chen, Y.C., Lo, S.L., and Kuo, J. (2010a) Pb(II) adsorption capacity and behavior of 390

titanate nanotubes made by microwave hydrothermal method. Colloids Surf. A: 391

Physicochem. Eng. Asp. 361(1-3), 126-131. 392

Chen, Y.C., Lo, S.L., Ou, H.H., and Chen, C.H. (2010b) Photocatalytic oxidation of 393

ammonia by cadmium sulfide/titanate nanotubes synthesized by microwave 394

hydrothermal method. Water Sci. Technol., article in press. 395

Cheng, J., Vecitis, C.D., Park, H., Mader, B.T., and Hoffmann, M.R. (2008) 396

Sonochemical degradation of perfluorooctane sulfonate (PFOS) and 397

perfluorooctanoate (PFOA) in landfill groundwater: environmental matrix effects. 398

Environ. Sci. Technol. 42(21), 8057-8063. 399

Dillert, R., Bahnemann, D., and Hidaka, H. (2008) Light-induced degradation of 400

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

24

perfluorocarboxylic acids in the presence of titanium dioxide. Chemosphere 67(4), 401

785-792. 402

Doll, T.E. and Frimmel, F.H. (2005) Cross-flow microfiltration with periodical 403

back-washing for photocatalytic degradation of pharmaceutical and diagnostic 404

residues-evaluation of the long-term stability of the photocatalytic activity of TiO2. 405

Water Res. 39(5), 847-854. 406

Estrellan, C.R., Salim, C., and Hinode, H. (2010) Photocatalytic decomposition of 407

perfluorooctanoic acid by iron and niobium co-doped titanium dioxide. J. Hazard. 408

Mater. 179(1-3), 79-83. 409

Guan, B., Zhi, J., Zhang, X., Murakami, T., and Fujishima, A. (2007) Electrochemical 410

route for fluorinated modification of boron-doped diamond surface with 411

perfluorooctanoic acid. Electrochem. Commun. 9(12), 2871-2821. 412

Higgins, C.P. and Luthy, R.G. (2006) Sorption of perfluorinated surfactants on 413

sediments. Environ. Sci. Technol. 40(23), 7251-7256. 414

Higgins, C.P., Field, J.A., Criddle, C.S., and Luthy, R.G. (2005) Quantitative 415

determination of perfluorochemicals in sediments and domestic sludge. Environ. 416

Sci. Technol. 39(11), 3946-3956. 417

Hinderliter, P.M., DeLorme, M.P., and Kennedy, G.L. (2006) Perfluorooctanoic acid: 418

relationship between repeated inhalation exposures and plasma PFOA 419

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

25

concentration in the rat. Toxicology 222(1-2), 80-85. 420

Hogue, C. (2006) POFA called likely human carcinogen. Chem. Eng. News 83, 5. 421

Hori, H., Hayakawa, E., Einaga, H., Kutsuna, S., Koike, K., Ibusuki, T., Kitagawa, H., 422

and Arakawa, R. (2004) Decomposition of environmentally persistent 423

perfluorooctanoic acid in water by photochemical approaches. Environ. Sci. 424

Technol. 38(22), 6118-6124. 425

Hori, H., Murayama, M., and Kutsuna, S. (2009) Oxygen-induced efficient 426

mineralization of perfluoroalkylether sulfonates in subcritical water. Chemosphere 427

77(10), 1400-1405. 428

Hori, H., Murayama, M., Inoue, N., Ishida, K., and Kutsuna, S. (2010) Efficient 429

mineralization of hydroperfluorocarboxylic acids with persulfate in hot water. 430

Catal. Today 151(1-2), 131-136. 431

Hori, H., Nagaoka, Y., Sano, T., and Kutsuna, S. (2008a) Iron-induced decomposition 432

of perfluorohexanesulfonate in sub- and supercritical water. Chemosphere 70(5), 433

800-806. 434

Hori, H., Nagaoka, Y., Yamamoto, A., Sano, T., Yamashita, N., Taniyasu, S., and 435

Kutsuna, S. (2006) Efficient decomposition of environmentally persistent 436

perfluorooctanesulfonate and related fluorochemicals using zerovalent iron in 437

subcritical water. Environ. Sci. Technol. 40(3), 1049-1054. 438

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

26

Hori, H., Yamamoto, A., Koike, K., Kutsuna, S., Murayama, M., Yoshimoto, A., and 439

Arakawa, R. (2008b) Photocatalytic decomposition of a perfluoroether carboxylic 440

acid by tungstic heteropolyacids in water. Appl. Catal. B: Environ. 82(1-2), 58-66. 441

Ismail, A.A., Abboudi, M., and Holloway, P. (2007) Photoluminence from terbium 442

doped silica-titania prepared by a sol-gel method. Mater. Res. Bull. 42(1), 137-142. 443

Kutsuna, S. and Hori, H. (2008) Experimental determination of Henrys law constant 444

of perfluorooctanoic acid (PFOA) at 298 K by means of an inert-gas stripping 445

method with a helical plate. Atmos. Environ. 42(39), 8883-8892. 446

Lampert, D.J., Frisch, M.A., and Speitel Jr., G.E. (2007) Removal of perfluorooctanoic 447

acid and perfluorooctane sulfonate from wastewater by ion exchange. Periodical 448

Haz., Toxic, and Radioactive Waste Mgmt. 11(1), 60-68. 449

Lin, A.Y.C., Panchangam, S.C., and Ciou, P.S. (2010a) High levels of 450

perfluorochemicals in Taiwans wastewater treatment plants and downstream 451

rivers pose great risk to local aquatic ecosystems. Chemosphere 80(10), 452

1167-1174. 453

Lin, C.J., Chen, S.Y., and Liou Y.H. (2010b) Wire-shaped electrode of CdSe-sensitized 454

ZnO nanowire arrays for photoelectrochemical hydrogen generation. Electrochem. 455

Commun. 12(11), 1513-1516. 456

Men, X.H., Zhang, Z.Z., Song, H.J., Wang, K., and Jiang, W. (2008) Fabrication of 457

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

27

superhydrophobic surfaces with poly(furfuryl alcohol)/multi-walled carbon 458

nanotubes composites. Appl. Surf. Sci. 254(9), 2563-2568. 459

Moriwaki, H., Takagi, Y., Tanaka, M., Tsuruho, K., Okitsu, K., and Maeda, Y. (2005) 460

Sonochemical decomposition of perfluorooctane sulfonate and perfluorooctanoic 461

acid. Eviron. Sci. Technol. 39(9), 3388-3392. 462

Musijowski, J., Trojanowicz, M., Szostek, B., Lima, J.L.F.D.C., Lapa, R., Yamashita, 463

H., Takayanagi, T., and Motomizu, S. (2007) Flow-injection determination of total 464

organic fluorine with off-line defluorination reaction on a solid sorbent bed. Anal. 465

Chim. Acta 600(1-2), 147-154. 466

Ochoa-Herrera, V. and Sierra-Alvarez, R. (2008) Removal of perfluorinated 467

surfactants by sorption onto granular activated carbon, zeolite, and sludge. 468

Chemosphere 72(10), 1588-1593. 469

Ou, H.H. and Lo, S.L. (2007) Review of titania nanotubes synthesized via the 470

hydrothermal treatment: fabrication, modification, and application. Sep. Purif. 471

Technol. 58(1), 179-191. 472

Panchangam, S.C., Lin, A.Y.C., Tsai, J.H., and Lin, C.F. (2009) Sonication-assisted 473

photocatalytic decomposition of perfluorooctanoic acid. Chemosphere 75(5), 474

654-660. 475

Potera, C. (2009) Reproductive toxicology: study associates PFOS and PFOA with 476

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

28

impaired fertility. Environ. Health Perspect. 117(4), A148. 477

Qu, Y., Zhang, C., Li, F., Bo, X., Liu, G., and Zhou, Q. (2009) Equilibrium and kinetics 478

study on the adsorption of perfluorooctanoic acid from aqueous solution onto 479

powdered activated carbon. J. Hazard. Mater. 169(1-3), 146-152. 480

Qu, Y., Zhang, C., Li, F., Chen, J., and Zhou, Q. (2010) Photo-reductive defluorination 481

of perfluorooctanoic acid in water. Water Res. 44(9), 2939-2947. 482

Ravichandran, L., Selvam, K., and Swaminathan, M. (2007) Effect of oxidants and 483

metal ions on photodefluoridation of pentafluorobenzoic acid with ZnO. Sep. Purif. 484

Technol. 56(2), 192-198. 485

Ravichandran, L., Selvam, K., Krishnakumar, B., and Swaminathan, M. (2009) 486

Photovalorisation of pentafluorobenzoic acid with platinum doped TiO2. J. Hazard. 487

Mater. 167(1-3), 763-769. 488

Ravichandran, L., Selvam, K., Muruganandham, M., and Swaminathan, M. (2006) 489

Photocatalytic cleavage of C-F bond in pentafluorobenzoic acid with titanium 490

dioxide-P25. J. Fluorine Chem. 127(9), 1204-1210. 491

Senevirathna, S.T.M.L.D., Tanaka, S., Fujii, S., Kunacheva, C., Harada, H., Shivakoti, 492

B.R., and Okamoto, R. (2010) A comparative study of adsorption of 493

perfluorooctane sulfonate (PFOS) onto granular activated carbon, ion-exchange 494

polymers and non-ion-exchange polymers. Chemosphere 80(6), 647-651. 495

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

29

US EPA (2002) Revised draft hazard assessment of perfluorooctanoic acid and its salts. 496

United States Environmental Protection Agency Office of Pollution Prevention and 497

Toxics Risk Assessment Division: Washington, DC. Available from: 498

http://www.ewg.org/files/EPA_PFOA_110402.pdf. 499

Vecitis, C.D., Park, H., Cheng, J., Mader, B.T., and Hoffmann, M.R. (2008) Kinetics 500

and mechanism of the sonolytic conversion of the aqueous perfluorinated 501

surfactants, perfluorooctanoate (PFOA), and perfluorooctane sulfonate (PFOS) 502

into inorganic products. J. Phys. Chem. 112(18), 4261-4270. 503

Xu, X.H., Zhang, Z.Z., and Liu, W. (2009) Fabrication of superhydrophobic surfaces 504

with perfluorooctanoic acid modified TiO2/polystyrene nanocomposites coating. 505

Colloid. Surf. A: Phys. Eng. Asp. 341(1-3), 21-26. 506

Yu, Q., Zhang, R., Deng, S., Huang, J., and Yu, G. (2009) Sorption of perfluorooctane 507

sulfonate and perfluorooctanoate on active carbons and resin: kinetic and isotherm 508

study. Water Res. 43(4), 1150-1158. 509

Zhao, B. and Zhang, P. (2009) Photocatalytic decomposition of perfluorooctanoic acid 510

with -Ga2O3 wide bandgap photocatalyst. Catal. Commun. 10(8), 1184-1187. 511

Zhu, X., Castleberry, S.R., Nanny, M.A., and Butler, E.C. (2005) Effects of pH and 512

catalyst concentration on photocatalytic oxidation of aqueous ammonia and nitrite 513

in titanium dioxide suspersions. Environ. Sci. Technol. 39(10), 3784-3791. 514

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

31

0 10 20 30 40 50

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50

0

1

2

3

4

5

6(b)

C/C 0

Time (h)

pH 4 pH 7 pH 10 No UV

(a)

pH4 pH7 pH10 No UV0

20

40

60

80

100

Perc

enta

ge

Degradation Defluorination

Fluor

ide

io

n (m

M)

Time (h)

Fluoride ion PFOA PFHpA PFHxA Total fluoride ion

1

Fig. 1. (a) Direct photolysis of PFOA at different solution pH by 254 nm UV 2

irradiation (b) and formed shorter-chain PFCAs during photodecomposition. The 3

reaction conditions are [PFOA]initial = 50 mg L-1, T = 298 K in a 2 L solution. 4

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

32

20 40 60 80

-10

0

10

20

30

40

50

TiO2 (pH 4) TNTs (pH 4) TNTs (pH 7) TNTs (pH 10)

Q e (m

g g-1

)

Ce (mg L-1)

5

Fig. 2. Adsorption isotherms of PFOA onto TNTs/TiO2. The reaction conditions are 6

[PFOA]initial = 5 100 mg L-1, T = 298 K, reacted with 20 mg of TNTs or TiO2 in a 100 7

mL solution. Ce is the equilibrium concentration in solutions and qe is the amount of 8

PFOA adsorbed per unit weight of the adsorbent. 9

10

11

12

13

14

15

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

33

0 5 10 15 20 25

0.0

0.2

0.4

0.6

0.8

1.0

R2=0.9939

R2=0.9598

R2=0.9655

R2=0.9754

TiO2 (0.50 g) TNTs (0.50 g) TNTs (0.25 g) TNTs (0.10 g)

ln (C

0/C)

Time (h)

0.50g TiO2 0.50g TNTs 0.25g TNTs 0.10g TNTs0

10

20

30

40

50

60

49%

59%

44%

Perc

enta

ge

Degradation Defluorination

19%

16

Fig. 3. Pseudo first-order kinetics of photocatalytic decomposition of PFOA by 17

TNTs/TiO2 by 254 nm UV irradiation. The reaction conditions are [PFOA]initial = 50 18

mg L-1, pH 4, T = 298 K in a 2 L solution. The first-order rate reaction can be expressed 19

as 0t ln[C]-ktln[C] += , where t[C] is the concentration of the chemical of interest at 20

a particular time and 0[C] is the initial concentration. 21

22

23

24

25

26

27

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

34

1000 800 600 400 200 0 545 540 535 530 525

300 295 290 285 280 700 695 690 685 680

1085 1080 1075 1070 475 470 465 460 455

(b)

TNTs + PFOA

TNTs + PFOA

TNTs + PFOA

(a)

(e)

(d)(c)

Inte

nsi

ty (a.

u.)

(f)

TNTs + PFOA

TNTs + PFOA

TiO2 + PFOATiO2 + PFOA

TNTsTNTs

TNTs

TNTs

TNTs

TNTs

TNTs + PFOA

Binding Energy (eV)

Ti2p3Na1s

F1sC1s

O1s

TiO2 + PFOA

TiO2 + PFOA

TiO2 + PFOA

TiO2 + PFOA

28

Fig. 4. XPS spectra of virgin TNTs and PFOA-TNTs/TiO2 complexes after 29

photocatalysis. 30

31

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

35

4000 3500 3000 2500 2000 1500 1000-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

4000 3500 3000 2500 2000 1500 1000-1.0

-0.5

0.0

0.5

1.0

1.5TiO2+PFOA

TiO2

TNTs+PFOA

(b)

(a)TNTs

Abso

rbe

nce

Wavenumber (cm-1)

32

Fig. 5. The Fourier-transformed infrared spectra of virgin TNTs/TiO2 and 33

PFOA-TNTs/TiO2 complexes after photocatalysis. 34

35

36

MAN

USCR

IPT

ACCE

PTED

ACCEPTED MANUSCRIPT

36

Ionization

C7F15COOH

C7F15COO-

Adsorption Photocatalysis

TNT+ + C7F15COO-

TNTs

Ionization

TNT - C7F15COO

C7F15 + COOAdsorption Photocatalysis

TNT - C7F15

TNT - PFOA

C7F15OH

H2O

C6F13COF

H+F-

C6F13COOHF-

PFHpACF2

CF2

Mineralization

37

38

Fig. 6. Proposed PFOA decomposition mechanism by TNTs. 39

40

41

42

43

44

45