Journal of Hazardous Materials 192 (2011) 1869–1875
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Journal of Hazardous Materials
jou rn al h om epage: www.elsevier.com/loc ate/jhazmat
Photocatalytic decomposition of perfluorooctanoic acid (PFOA) by TiO
2
in the
presence of oxalic acid
Yuan Wang
∗
, Pengyi Zhang
State Key Joint Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University; Beijing 100084, PR China
a r t i c l e i n f o
Article history:
Received 27 December 2010
Received in revised form 1 July 2011
Accepted 6 July 2011
Available online 18 July 2011
Keywords:
Perfluorocarboxylic acids
Photocatalysis
Carboxyl anion radical
Hole scavenger
a b s t r a c t
Heterogeneous photocatalytic decomposition of perfluoroocatanoic acid (PFOA) by TiO
2
under 254 nm UV
light was investigated. Adding oxalic acid as a hole-scavenger significantly accelerated PFOA decomposi-
tion under nitrogen atmosphere. Fluoride ion, formic acid and six shorter-chain perfluorinated carboxylic
acids (PFCAs) bearing C
2
–C
7
were identified as intermediates. When using perchloric acid (HClO
4
) as a
replacement of oxalic acid to maintain the same pH of the reaction solution, PFOA did not decomposi-
tion efficiently. Compared with oxalic acid, potassium iodide (KI, another hole-scavenger) also led to a
slower PFOA decomposition, while the addition of an electron acceptor (potassium persulfate, K
2
S
2
O
8
)
obviously inhibited PFOA decomposition. This suggested that oxalic acid played more than one role in
PFOA decomposition rather than simply providing acidity and acting as a hole-scavenger. The electron
paramagnetic resonance (EPR) measurements confirmed the existence of carboxyl anion radicals (CO
2
•-
)
in the photocatalytic process, which was a result of the reaction between oxalic acid and photogener-
ated hole. These findings indicated that PFOA decomposition was primarily induced by CO
2
•-
radicals,
although photogenerated electron was also conducive to PFOA decomposition. A possible mechanism for
PFOA decomposition was proposed.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Perfluorinated compounds (PFCs) are one of anthropogenic
organic chemicals that are extensively used in industrial and
consumer products due to their unique nature [1]. Recently, perflu-
oroocatanoic acid (PFOA) as typical PFCs, has been found frequently
in a wide range of environmental water [2–6], and in blood
plasma/serum and breast milk of human being [7–9]. In surface
water, the detected concentration of PFOA ranged from pg L
-1
to
g L
-1
[10]. It was close to ng L
-1
in the water body near specific
point sources [11–13] and reached mg L
-1
to g L
-1
in some wastew-
aters [10]. Studies had shown that drinking water contaminated
with PFOA would end up with high levels of PFOA in blood plasma
and serum of local residents [7,9]. So far, PFOA has been regarded
as one of emerging pollutants due to its environmental persistence,
bioaccumulation and potential toxicity. Therefore, the removal of
PFOA is urgent for the safe of drinking water and human health.
It also brings the need to develop a feasible method or technology
under mild condition to fulfill this task.
Recent studies showed sonochemical treatment [14–16], sulfate
radical oxidation [17–19], electrochemical oxidation [20], pho-
∗
Corresponding author. Tel.: +86 10 62796840 603; fax: +86 10 62796840 602.
E-mail address: wang yuan@mail.tsinghua.edu.cn (Y. Wang).
tochemical oxidation [21,22] and photochemical reduction with
aqueous electrons [23] under irradiation of 220–460 nm UV light
were effective solutions for the decomposition of PFOA. TiO
2
pho-
tocatalysis have been extensively studied for the destruction of
various refractory organic compounds. Generally, it is understood
that most organics are destroyed by hydroxyl radicals (OH
•
) formed
in the photocatalytic process. It is well known that photogener-
ated holes (h
+
) in the valence band and photogenerated electrons
(e
-
) in the conduction band are formed respectively when TiO
2
is
irradiated with UV light:
TiO
2
+ hv → h
+
+ e
-
(1)
Dillert et al. [24] and Panchangam et al. [25] reported that
PFOA could be effectively decomposed by TiO
2
photocatalysis in
a highly acidic medium (HClO
4
, pH < 1) under oxygen atmosphere.
They argued that photogenerated holes were the oxidation sites
for perfluorinated carboxylic acids (PFCAs) decomposition. Though
photogenerated electrons had strong reduction potential (0.5 to
-1.5 V) [26], it was not reported that photogenerated electron of
excited TiO
2
could effectively decomposed PFOA. On the other
hand, it was well recorded that a strong reductant, i.e. carboxyl
anion radical (CO
2
•-
, normal potential E
0
(CO
2
/CO
2
•-
) = -1.85 V)
[27] could be formed when oxalic acid [28] or formic acid [29]
was employed to scavenge photogenerated holes of excited TiO
2
.
And it had been proposed that photoreduction of nitrate (NO
3
-
)
0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2011.07.026