Photocatalytic decomposition of environmentally persistent perfluorooctanoic acid

Article abstract of DOI:10.1246/cl.2006.230

Perfluorooctanoic acid was photocatalytically decomposed by using TiO ₂/Ni-Cu, and a small bias potential (-0.1 V) applied on TiO ₂/Ni-Cu electrode greatly enhanced its decomposition. Copyright

Full text of DOI:10.1246/cl.2006.230

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Chemistry Letters Vol.35, No.2 (2006)
Photocatalytic Decomposition of Environmentally Persistent Perfluorooctanoic Acid
Ã
Jing Chen, Pengyi Zhang, and Li Zhang
Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, P. R. China
(Received November 24, 2005; CL-051454; E-mail: zpy@mail.tsinghua.edu.cn)
Perfluorooctanoic acid was photocatalytically decomposed
lytic decomposition reaction was conducted in a glass tubular
photoreactor. A low-pressure mercury lamp (23 W) with main
wavelength at 254 nm was used as UV source. The lamp was
placed in the center of the photoreactor with quartz tube protec-
tion (external diameter 25 mm). The gap between the inner wall
of the photoreactor and the external wall of quartz tube was
15 mm. An aqueous solution (250 mL) of PFOA (25 mg/L;
0.05 mM) was filled into the reactor. The TiO2/Ni–Cu photoca-
talyst sheet (75 Â 10 mm) was attached on the inner wall of the
photoreactor. A saturated calomel electrode (SCE) and a plati-
num sheet (25 Â 20 mm) were used as reference and counter
electrodes, respectively. And an electrochemistry workstation
(CHI660B) was used to control the potential at the surface of
the photocatalyst electrode. Before turning on the lamp, nitrogen
gas was bubbled into the reactor to remove the oxygen in water.
The concentrations of PFOA and photocatalytic products were
determined by LC-MS (ZQ 4000, Waters, U.S.A.). Fluoride
by using TiO2/Ni–Cu, and a small bias potential (À0:1 V)
applied on TiO2/Ni–Cu electrode greatly enhanced its decompo-
sition.
Perfluorocarboxylic acids (PFCAs) are a class of special
chemicals used as emulsifying agents in polymer synthesis and
as surface treatment agents in photolithography, paper coatings,
1
and waxes and polishes. Recently, perfluorinated acids espe-
cially perfluorooctanoic acid (PFOA, C7F15COOH) and per-
fluorooctanesulfonic acid (PFOS) have been widely identified
in the environment such as human and marine biota plasma,
2
blood and liver tissue, surface water, fresh water, air and dust,
3
sediments, and domestic sludge, and even in remote areas such
4
as the Arctic. These compounds are very stable due to C–F
bond, and do not decompose naturally. Thus, they persist and
bioaccumulate in the environment. The temperature required
À
ion (F ) was determined by ion chromatography (761 compact
IC, Metrohm).
ꢀ
for thermal decomposition is as high as 1200 C. Thus, it is
necessary to develop effective methods to decompose PFCAs
to harmless species under mild conditions.
Figure 1 shows the photocatalytic decomposition of PFOA
in the presence of TiO2/Ni–Cu. The direct photolysis of PFOA
under 254 nm UV light was very slow and negligible. However,
PFOA decomposed significantly in the presence of TiO2/Ni–Cu,
it almost disappeared after 6 h. And at the same time fluoride ion
in aqueous solution was detectable, after reaction of 6 h, the
defluorination was up to 40% (Figure 2), i.e. averagely 6 of 15
fluorine atoms in PFOA molecule were transformed into inor-
ganic fluoride ion. However, when we used P25 TiO2 powder
or sol–gel prepared TiO2 film as photocatalyst, no significant
decomposition of PFOA was observed.
Furthermore, if the TiO2/Ni–Cu photocatalyst electrode
was applied with À0:1 V bias potential vs SCE, and 0.1 mol/L
Na2SO4 was used as supporting electrolyte, the decomposition
of PFOA was greatly enhanced as compared to open circuit con-
dition. As shown in Figure 1, 92.5% PFOA was decomposed
within 2 h, and it completely decomposed after 4 h. Defluorina-
tion ratio was also greatly enhanced, after 6 h reaction up to
85.6%.
5
Recently, Hori et al. reported the photochemical decompo-
sition of PFCAs by use of persulfate (S2O8 ) as oxidant or
by a homogeneous photocatalyst i.e. tungstic heteropolyacid
2À
(
H3PW12O40) under UV–visible light irradiation (220–460
nm). They argued that PFCAs were oxidized by the photoexcited
ÁÀ
3ÀÃ
species (SO4 or [PW12O40] ), i.e. one electron was transfer-
6
red from PFCAs to the photoexcited species. Moriwaki et al.
also reported the sonochemical decomposition of PFOS and
PFOA under air or argon atmosphere. And they concluded that
PFOS and PFOA were mostly pyrolyzed at the interfacial region
between the bulk solution and the cavitation bubbles where the
temperature was as high as several thousands due to sonication.
Except the C atom of carboxyl in PFOA, other C atoms are
connected to F atoms. Because of high electronegativity of F
atom, the C atom is highly positive, and thus PFOA is very dif-
ficult to be oxidized. It has been demonstrated that PFCAs in-
cluding PFOA can not be decomposed by reaction with hydroxyl
radicals (OH
ꢁ
), which is a strong oxidant only secondary to F
The decomposition products of PFOA were also detected
atom.5 Though it was difficult to be oxidized, PFOA may be
decomposed through reduction. In general, electron–hole pair
is generated when the semiconductor photocatalyst is illuminat-
ed with UV light, and the photogenerated electron is highly
,6
100
8
6
4
2
0
0
0
0
0
7
reductive. It was well reported that CCl4 or perhalogenated
8
bezenes were reductively decomposed and dehalogenated in
photolysis
photocatalysis
photoelectrocatalysis
the photocatalytic process. Here, we firstly reported that PFOA
was photoreductively decomposed and defluorinated by use of
an immobilized TiO2/Ni–Cu photocatalyst.
The TiO2/Ni–Cu photocatalyst was prepared as follows.
Firstly, nickle was electrodeposited on the copper sheet. And
then a thin TiO2 film was coated on the above sheet by sol–gel
0
60
120
180
Time/min
240
300
360
ꢀ
method. Finally, it was heated at 450 C for 1 h. The photocata-
Figure 1. Decomposition of PFOA with reaction time.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.2 (2006)
231
1
00
60
50
PFBA
PFHxA
PFOA
PFPeA
photoelectrocatalysis
photocatalysis
photolysis
8
6
4
2
0
0
0
0
0
PHHpA
4
3
2
1
0
0
0
0
0
0
60
120
180
240
300
360
Time/min
0
60
120
180
240
300
360
Time/min
Figure 4. Changes in the concentrations of PFOA and inter-
mediates.
Figure 2. Defluorination of PFOA with reaction time.
electrons would initiate the PFOA decomposition and the nega-
tive bias potential (À0:1 V) effectively enhanced the decomposi-
tion. Hence, from our observations, C–F bond cleavage was ini-
tially induced by reduction mechanism. This mechanism was not
strange, because it is difficult for PFOA to donate an electron.
Fluoride atom in PFOA tends to accept an electron, which leads
to break of C–F bond.
6
C F13COOH
PFOA
24.39
23.75
1
00
%
0
C
5
F
11COOH
C F COOH
4 9
2
2.16
23.12
3 7
C F COOH
20.56
This work was supported by NSFC (No. 20577026) and
NCET (No. NCET-04-0090).
References
1
U.S. Environmental Protection Agency Office of Pollution
Prevention and Toxics Risk Assessment Division, 2003.
a) A. Karrman, B. van Bavel, U. Jarnberg, G. Lindstrom,
Anal. Chem. 2005, 77, 864. b) Z. Kuklenyik, L. L. Needham,
A. M. Calafat, Anal. Chem. 2005, 77, 6085. c) K. Kannan,
K. J. Hansen, T. L. Wade, J. P. Giesy, Arch. Environ
Contam. Toxicol. 2002, 42, 313.
20.00
22.00
24.00
Time (min)
2
Figure 3. TIC spectra of PFOA and its decomposition products.
and identified by LC-MS according to their mass spectra and
retention time (Figure 3). The main products were shorter
perfluorinated carboxylic acids containing 4–7 carbon atoms,
including perfluorobutyric acid (PFBA), perfluoropentanoic acid
3
4
a) M. Loewen, T. Halldorson, F. Wang, G. Tomy, Environ.
Sci. Technol. 2005, 39, 2944. b) M. Shoeib, T. Harner,
B. H. Wilford, K. C. Jones, J. Zhu, Environ. Sci. Technol.
2005, 39, 6599. c) C. A. Moody, W. C. Kwan, J. W. Martin,
D. C. G. Muir, S. A. Mabury, Anal. Chem. 2001, 73, 2200.
a) M. Smithwick, S. A. Mabury, K. R. Solomon, C. Sonne,
J. W. Martin, E. W. Born, R. Dietz, A. E. Derocher, R. J.
Letcher, T. J. Evans, G. W. Gabrielsen, J. Nagy, I. Stirling,
M. K. Taylor, D. C. G. Muir, Environ. Sci. Technol. 2005,
39, 5517. b) J. Verreault, M. Houde, G. M. Gabrielsen, U.
Berger, M. Haukas, R. J. Letcher, D. C. G. Muir, Environ.
Sci. Technol. 2005, 39, 7439.
(
PFPeA), perfluorohexanoic acid (PFHxA), and perfluorohepta-
noic acid (PFHpA). Small amount of trifluoroacetic acid (TFA)
and pentafluoropropionic acid (PFPA) were also detected. The
decomposition products are the same as those reported by Hori
et al. and Moriwaki et al. The change of concentrations of
PFBA, PFPeA, PFHxA, and PFHpA with reaction time in the
photoelectrocatalysis was shown in Figure 4.
5
6
For the mechanism of PFOA photochemical decomposition,
5
2À
Hori et al. proposed that oxidant (S2O8 ) or homogeneous
photocatalyst (H3PW12O40) was firstly photoexcited, and then
ÁÀ
PFOA was oxidized by the photoexcited species (SO4 or
[
3ÀÃ
PW12O40] ), i.e. PFOA donated one electron. In our experi-
5
a) H. Hori, E. Hayakawa, H. Einaga, S. Kutsuna, K. Koike,
T. Ibusuki, H. Kiatagawa, R. Arakawa, Environ. Sci. Tech-
nol. 2004, 38, 6118. b) H. Hori, A. Yamamoto, E. Hayakawa,
S. Taniyasu, N. Yamashita, S. Kutsuna, Environ. Sci. Tech-
nol. 2005, 39, 2383. c) H. Hori, Y. Takano, K. Koike, S.
Kutsuna, H. Einaga, T. Ibhusuki, Appl. Catal., B 2003, 46,
333. d) H. Hori, E. Hayakawa, K. Koike, S. Kutsuna, H.
Einaga, T. Ibhusuki, J. Mol. Catal. A: Chem. 2004, 211, 35.
H. Moriwaki, Y. Takagi, M. Tanaka, K. Tsuruho, K. Okitsu,
Y. Maeda, Environ. Sci. Technol. 2005, 39, 3388.
ment, PFOA decomposition was carried out under nitrogen at-
mosphere, and no extra oxidant was added. For the TiO2/Ni–
Cu photocatalyst, the open circuit potential in the dark was
À0:08 V vs SCE. When it was irradiated with 254 nm UV light,
an instant photogenerated potential of over than 100 mV was ob-
served. The potential of TiO2/Ni–Cu photocatalyst positively
increased to 0.02 V vs SCE. This suggests that photogenerated
electrons and holes were quickly separated, and photogenerated
electrons transferred to the surface of photocatalyst film and
accumulated, whereas holes transferred to metal substrate (Ni
deposited Cu). Because the surface of the photocatalyst was
in contact with PFOA aqueous solution, the photogenerated
6
7
8
J. Stark, J. Rabani, J. Phys. Chem. B 1999, 103, 8524.
H. Yin, Y. Wada, T. Kitamura, S. Yanagida, Environ. Sci.
Technol. 2001, 35, 227.
Published on the web (Advance View) January 21, 2006; DOI 10.1246/cl.2006.230