Ultrasonics Sonochemistry 21 (2014) 1875–1880
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Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Enhancing decomposition rate of perfluorooctanoic acid by carbonate
radical assisted sonochemical treatment
Lan-Anh Phan Thi a, Huu-Tuan Do b, Shang-Lien Lo a,⇑
a
b
Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei, Taiwan, ROC
Faculty of Environmental Sciences, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Thanh Xuan Dist., Ha Noi, Viet Nam
a r t i c l e
i n f o
Article history:
Received 3 January 2014
Received in revised form 26 March 2014
Accepted 26 March 2014
Available online 4 April 2014
Keywords:
Perfluorooctane acid
Sonochemical
Ultrasonic reaction
Carbonate radical anion
a b s t r a c t
Perfluorooctanoic acid (PFOA) is a recalcitrant organic pollutant in wastewater because of its wide range
of applications. Technologies for PFOA treatment have recently been developed. In this study, PFOA
decomposition by sonochemical treatment was investigated to determine the effects of NaHCO3 concentrations, N2 saturation, and pH on decomposition rates and defluorination efficiencies. The results
showed that PFOA decomposition by ultrasound treatment only (150 W, 40 kHz), with or without saturated N2, was <25% after 4 h reaction. The extent and rate of PFOA decomposition and defluorination efficiencies of PFOA, however, greatly increased with the addition of carbonate radical reagents. PFOA was
completely decomposed after 4 h of sonochemical treatment with a carbonate radical oxidant and saturated N2. Without saturated N2, PFOA was also decomposed to a high (98.81%) degree. The highest PFOA
decomposition and defluorination efficiencies occurred in N2 saturated solution containing an initial
NaHCO3 concentration of 30 mM. Sonodecomposition of PFOA with CO
3 radical was most favorable in
a slightly alkaline environment (pH = 8.65). There isn’t any shorter-chain perfluorinated carboxylic acids
detected except fluorine ions in final reaction solution.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Perfluorocarboxylic acids (CnF2n+1COOH, PFCAs), including perfluorooctanoic acid (PFOA), are a family of anthropogenic fluorinated organic compounds with a wide range of applications [1].
They have been used in various commercial and industrial applications such as surface treatment, surfactant, polymers, metal coating and fire retardant [2]. The extremely strong carbon–fluorine
bonds (C–F, 116 kcal/mol) [1] in their structure gives the material
high thermal and acid resistance. Because of the widespread use of
these chemicals and the resistance to degradation, PFCAs have
been detected in drinking water [3] and wastewater treatment
plant effluents [4]. In addition, recent studies indicated that these
compounds are toxic and carcinogenic to animals such as rats,
fishes, monkeys, and even humans [5]; they are listed as persisted
organic pollutants (POPs) [6].
Technologies for PFOA treatment include photolysis, photochemical reaction, photocatalysis [7–10], sonochemical treatment
[11], electrochemical treatment [12], and adsorption [13,14].
⇑ Corresponding author. Address: Graduate Institute of Environmental
Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei 106, Taiwan,
ROC. Tel.: +886 2 2362 5373; fax: +886 2 2392 8821.
E-mail address: [email protected] (S.-L. Lo).
http://dx.doi.org/10.1016/j.ultsonch.2014.03.027
1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Among these treatment methods, sonochemical treatment is an
innovative technique that can enhance the degradation rate of
environmental contaminants in water. The effectiveness of sonochemical treatment can be further enhanced by combining it with
additives such as microwave [15], photocatalytic compounds [16],
persulfate [17–19], and carbonate [20].
The effects of sonochemical treatment are generally due to the
cavitation phenomenon; the formation, growth, and the sudden
collapse of cavitation bubbles in liquid gives rise to localized, transient high temperatures and high pressures [21,22]. The sonochemical decomposition of organic contaminants via reaction
with hydroxyl radicals (OH) formed from pyrolysis of water or
via direct pyrolysis. The ultrasonic reaction is powerful, but it is
relatively inefficient with respect to total input energy and is
therefore not economical when used alone. With additive-assisted
sonochemical treatment, the total input energy may be reduced
and the pollutant decomposition rate accelerated.
The carbonate radical anion (CO
3 ) is a secondary radical formed
by one-electron oxidation of carbonate or bicarbonate ions by OH
radical or sulfate radical (SO
4 ). It is a powerful oxidant with a oneelectron reduction potential of 1.59 V at pH of 12.5 and 1.78 V at
pH of 7 [23]. The radical is a strong acid (pKa < 0) [24]. In comparison to OH radical, CO
3 radical is more selective, but could
migrate towards the bulk solution and induce the decomposition
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of non-volatile pollutants [25]. It is hypothesized that carbonate
radicals are important in reducing the persistence of chemical pollutants, especially those that are electron-rich. In previous research
[26], CO
3 radical has been used successfully to decompose PFOA
under photolysis condition. In this study, we investigated the
decomposition of PFOA in aqueous solutions by sonochemically
induced CO
3 radical, and compared the efficiencies of decomposition and defluorination of PFOA under various conditions such as
pH values, NaHCO3 concentrations, N2 saturation, and reaction
time.
the reactor to saturate N2 with the aqueous solution at 200 cc/
min in 30 min. pH values were adjusted by using 1 N NaOH and/
or 1 N HCl solutions. The reactor was then placed in a cabinet. A
sonicator’s probe 22 mm diameter (Branson, 2000LPt, 150 W,
40 kHz, USA) was put into the aqueous solution to generate ultrasound. The experimental setup is shown in Fig. 1. The samples
were taken at various intervals and filtered by Millipore syringe filters with a 0.22 lm pore size. The PFOA concentration in the filtrate was determined by a HPLC and the F concentration was
measured by an IC system, respectively.
2. Materials and methods
2.3. Samples analysis
2.1. Instruments and chemicals
The samples were taken at various intervals and filtered by Millipore syringe filters with a 0.22 lm pore size. The PFOA concentration in the filtrate was determined by a high-performance liquid
chromatography (HPLC) (Dionex, UltiMate 3000, USA) equipped
with a conductivity detector (ED-50, Dionex, USA) and an anion
self-regenerating suppressor (ASRS 300 2-mm, USA). The PFOA
and other PFCAs (C3–C8) were extracted by a 150 2.1 mm,
3.5 lm column (AcclaimÒ Polar Advantage II, C18, Dionex, USA)
maintained at 30 °C. The ternary solution that included 70:30 (v/
v) acetonitrile/Milli-Q water (solution A), Milli-Q water (solution
B) and 9 mM NaOH and 100 mM H3BO3 in Milli-Q water (solution
C) was employed as an eluent at a flow rate of 0.3 mL/min. The gradient was operated at 20% solution A, 40% solution B, and 40 %
solution C for the first 5 min. For the next 15 min, it was operated
at 20–60% solution A, 40–0% solution B, and 40% solution C. For the
last 15–20 min, solution A was maintained at 60% and solution B at
0%. All calibration curves for PFCAs were linear over the 0.5–
50 ppm range. The decomposition ratio of PFOA was calculated
according to the following equation:
The perfluorooctanoic acid (PFOA, C7F15COOH, 96% purity) was
from Aldrich; the perfluoroheptanoic acid (PFHpA, C6F13COOH,
98% purity), perfluoropentanoic acid (PFPeA, C4F9COOH, 97% purity), and perflurobutyric acid (PFBA, C3F7COOH, 99% purity) were
from Alfa Aesar. The perfluorohexanoic acid (PFHxA, C5F11COOH,
97% purity) and perfluoropropionic acid (PFPrA, C2F5COOH, 97%
purity) were purchased from Fluka, and the trifluoroacetic acid
(TFA, CF3COOH) was from Riedel–deHaen. Sodium carbonate (Na2CO3, 100% purity) was from Nacalai Tesque, sodium bicarbonate
(NaHCO3, 99.6% purity) was from J.T. Baker, and nitrogen gas (N2,
99.99% purity) was from Chin-Fon, Co., Ltd., Taipei, Taiwan. The
fluorine standard was purchased from High-Purity Standards.
2.2. Reaction procedures
N2 gas
PFOA, at a concentration of 100 ppm, and NaHCO3, at a concentration of 0.5 N, were prepared and stored at 4 °C. In each experiment, solution containing 50 ppm of PFOA and different initial
concentrations of NaHCO3 was mixed by a magnetic stirrer. A 1liter closed double-layered glass reactor containing 1 L solution
was connected with circulating water bath (B204, Firstek Scientific
Co., Ltd., Taipei, Taiwan) to control the reactor’s temperature at
25 ± 1 °C. Air volume controller was connected with N2 tank and
Degradation ð%Þ ¼
C0 C
100
C0
ð1Þ
where C is the PFOA concentration (ppm), and C0 is the initial PFOA
concentration (ppm). The other byproducts of PFCA were calculated
based on the external calibration curves.
HPLC
IC
Sonicator
Stirrer
Water bath
Fig. 1. The sonochemical experiment setup schematic.
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The F concentration was measured by an ion-chromatograph
(IC) system. The system (Dionex, ICS-3000, USA) consists of an
automatic sample injector, a degasser, a pump, a guard column
(Ion Pac AS4A guard column, Dionex, USA), a separation column
(Ion Pac AS4A analytical column, Dionex, USA), and a conductivity
detector with a suppressor device. The mobile phase was an aqueous solution containing NaHCO3 (1.7 mM) and Na2CO3 (1.8 mM) at
a flow rate of 2 mL/min. The defluorination efficiency was calculated according to the following equation:
Defluorination ð%Þ ¼
CF
100
C 0 SF ð2Þ
where CF is the fluoride ion concentration (ppm), C0 is the initial
PFOA concentration (ppm) and SF is the stoichiometric factor of
the fluoride ions (15 for PFOA).
3. Results and discussion
Fig. 3. Defluorination of PFOA (50 ppm) at different conditions: (j) US only; (d) US
in saturated N2 gas; (N) US and NaHCO3 (30 mM); (.) US and NaHCO3 (30 mM) in
saturated N2 gas.
3.1. Decomposition and defluorination of PFOA
To identify the PFOA decomposition efficiencies under different
conditions, experiments were carried out under the following conditions: ultrasound only (US); US and N2 gas; US and NaHCO3; and
US, NaHCO3 and N2 gas. The decomposition of PFOA under these
conditions is shown in Fig. 2a and b. The results showed that PFOA
decomposed very slowly with US only and with US and N2 gas;
<25% of the PFOA was decomposed after 4 h.
However, when NaHCO3 (30 mM) was added to US, the rate of
PFOA decomposition increased rapidly and reached 83.58 & and
98.81% after 2 and 4 h reaction, respectively. The defluorination
efficiency was >90% after 4 h (see Fig. 3). The reason was OH,
formed from pyrolysis of water, reacted with bicarbonate (HCO
3)
to form CO
3 radical (Eq. (4)) [27] which could migrate towards
the bulk solution [25] and induce the decomposition of PFOA.
ÞÞÞ
H2 O ! H þ OH
ð3Þ
OH þ HCO3 ! H2 O þ CO
3
ð4Þ
Moreover, when NaHCO3 (30 mM) was combined with saturated N2 gas, the rate of PFOA decomposition was higher than that
achieved with only NaHCO3, as indicated by the rate constants in
Table 1. PFOA decomposed completely and the defluorination efficiency reached 90.07% and97.2% after 2 and 4 h reaction, respec-
Table 1
PFOA decomposition, defluorination efficiencies and pseudo-first-order rate constants
under different test conditions.
PFOA decomposition
in 4 h (%)
Defluorination in 4 h (%)
Rate constant k (h1)
R2
US only
US/N2
US/NaHCO3
US/NaHCO3/N2
24.35
24.87
98.8
100
5.8
0.071
0.978
6.6
0.073
0.86
90.2
1.161
0.987
97.2
1.439
0.987
tively. This was because N2 gas enhanced the formation of OH
radical in aqueous solutions under ultrasound condition [28]. The
additional OH radical resulted in the formation of more CO
3 radical which enhanced PFOA decomposition rate.
3.2. Effect of NaHCO3
Results from experiments with US, different NaHCO3 concentations, and saturated N2 gas are summarized in Table 2 and depicted
in Fig. 4a and b. The rate of PFOA decomposition was the highest
when NaHCO3 concentration was 30 mM. After 4 h, PFOA was
Fig. 2. Decomposition of PFOA (50 ppm) at different conditions: (j) US only; (d) US in saturated N2 gas; (N) US and NaHCO3 (30 mM); (.) US and NaHCO3 (30 mM) in
saturated N2 gas. (a): after 2 h, (b): after 4 h.
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Table 2
PFOA decomposition, defluorination efficiencies and pseudo-first-order rate constants at different initial NaHCO3 concentrations in saturated N2.
PFOA decomposition in 4 h (%)
Defluorination in 4 h (%)
Initial pH
Final pH
Rate constant k (h1)
R2
0 mM
10 mM
20 mM
30 mM
40 mM
50 mM
24.87
6.6
5.62
6.04
0.073
0.86
96.9
81.8
7.85
6.95
0.805
0.91
99.5
85.6
8.6
8.44
1.219
0.916
100
97.2
8.65
8.6
1.439
0.987
99.4
84.3
8.7
8.62
1.249
0.95
98.3
83.3
8.74
8.64
0.969
0.92
Fig. 4. Decompostion of PFOA (50 ppm) at different initial NaHCO3 concentrations in saturated N2: (a) decomposition efficiencies; (b) rate constant k.
Fig. 5. Decompostion of PFOA (50 ppm) at different initial concentrations of NaHCO3 with and without saturated N2 gas: (a) decompostion efficiencies; (b) rate constant k.
decomposed completely; the pseudo first-order rate constant was
1.439 h1. When NaHCO3 concentration increased from 10 to
30 mM, the decomposition rate of PFOA increased. However, when
bicarbonate ions (NaHCO3) were higher than the optimal levels (40
and 50 mM), the pH of the solution became elevated resulting in
the decay of CO
3 radical, as depicted by the following equations
[23,29]:
2
CO
3 þ CO3 ! CO2 þ CO4
H2 O
CO
3 þ CO3 ƒƒƒ! 2CO2 þ HO2 þ OH
ð5Þ
ð6Þ
3.3. Effect of N2
Effect of N2 gas on PFOA decomposition was examined at three
NaHCO3 concentrations (20, 30, and 40 mM), the results are plotted in Fig. 5a and b. The experiments without saturated N2 gas
was carried out by sonodecomposed PFOA 50 ppm in NaHCO3 solution. In these conditions, 87.82%, 98.81%, and 98.78% of PFOA was
decomposed at 20, 30, and 40 mM of NaHCO3, respectively after
4 h. With saturated N2, the PFOA decomposition efficiencies
increased slightly to 99.54%, 100%, and 99.43% at 20, 30, and
40 mM of NaHCO3, respectively. However, with saturated N2 gas,
the PFOA decomposition rate increased significantly, especially at
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30 mM of NaHCO3 (Fig. 5b). The presence of N2 gas enhanced OH
radical formation, which helped to produce more CO
3 radical
resulting in higher PFOA decomposition rate [28,30].
CF3
3.4. Effect of pH
CF2
The effect of pH on PFOA decomposition efficiencies was studied; the results are shown in Fig. 6. Decomposition of PFOA was
the fastest at pH value of 8.65. At this pH, the reaction rate constant
for PFOA decomposition was 1.439 h1, compared to 0.027 h1 and
1.1 h1 at pH values of 11 and pH 4.00, respectively. The results
showed that a slightly alkaline environment was the best for PFOA
decomposition. The amount of CO
3 radical formed at a pH of 4.00
was less than the amount formed at a pH of 8.65 because the formation of CO
3 radical decreased with decreasing pH, while the
radical–radical reaction rates do not change [24]. However, the
decay of CO
3 radical increased with the more alkaline environment, according to Eqs. (5) and (6).
In addition, it was observed that many tiny bubbles formed at
pH 11.00 in the aqueous solution (Fig. S3, Supplementary materials) in comparison to the amounts of bubbles formed at pH 4.00
and pH 8.65 (Figs. S1 and S2, Supplementary materials). These tiny
bubbles adhered to the sonicator’s probe and prevented the ultrasound energy from being distributed to the entire reactor. This may
partially explain why PFOA decomposition at pH 11.00 was low
(<10% after 4 h).
CF
CF2O
CFO
CO2
CFHO
CH2O
CHO
CH
CO
Fig. 7. The proposed scheme of C1 fluoro-radical transformation pathways occurs
in the transiently cavitating bubble vapor.
þH2 O
CF3 ðCF2 Þ5 CF2 ƒƒƒƒ! CF3 ðCF2 Þ5 CF2 H þ HO
D
CF3 ðCF2 Þ5 CF2 ƒƒ! CF3 ðCF2 Þ4 CF@CF2 þ F
D
From our results, it was observed that there isn’t any shorterchain perfluorinated carboxylic acids detected except fluorine ions.
Because PFOA is surfactant and low Henry’s constant [28] so they
was sonodecomposed at the bubble–water interface and in bulk
solution. In US only experiment, PFOA diffused and adsorbed on
cavitating bubble interface transiently and followed by pyrolytic
degradation at the cavitating bubble interface [31]. PFOA is ionized
and existed as an anionic compound (C7F15COO) in solution. Initial cleavage of the C–C bond between the perfluorinated tail and
the carboxylate group yields carbon dioxide (CO2) and a perfluoroalkyl anion (Eq. (7)). The perfluoroanion can form a 1H-perfluoroalkane by proton transfer (Eq. (8)), or eliminates a C–F bondbreaking step and circumvents the perfluoroolefin formation pathway (Eq. (9)).
D
C7 F15 COO ƒƒ! CF3 ðCF2 Þ5 CF2 þ CO2
ð7Þ
ð9Þ
The fluoro-intermediate will dissociate into C1 fluoro-radical
constituents prior to any intervening bimolecular reactions [31].
The stoichiometries for 1H-perfluoroheptane and perfluorooctene
decompositions are given in Eqs. (10) and (11), respectively. These
degradation unimolecular reactions are in a cavitating bubble and
the reaction time estimated about ns. Therefore, we did not detect
any shorter-chain perfluorinated carboxylic acids in the final
products.
CF3 ðCF2 Þ5 CF2 H ! CF3 þ 5CF2 þ CF2 H
3.5. Proposed mechanisms
ð8Þ
D
CF3 ðCF2 Þ4 CF@CF2 ! CF3 þ 5CF2 þ CF
ð10Þ
ð11Þ
HCO
3
When
added, it will accelerate the rate of reaction
through forming CO
3 radical as following Eqs. (3) and (4). The
CO
radical then reacts with anionic compound (C7F15COO)
3
through electron transfer.
2
CO
3 þ C7 F15 COO ! CO3 þ C7 F15 þ CO2
ð12Þ
(C7F15)
The perfluorinated alkyl radicals
then was sonodecomposed to CF3 and CF2 according to Eq. (13).
D
C7 F15 ! CF3 þ 6CF2
ð13Þ
The C1 fluoro-radicals are subsequently transformed into carbon monoxide (CO) and CO2. A series of bimolecular reactions with
H2O, H, HO, and O-atom are proposed in Fig. 7 for the conversion of
the C1 fluoro-radicals into CO, CO2 and HF [31].
4. Conclusions
Fig. 6. Decompostion efficiencies of PFOA (50 ppm) at different initial pH values.
PFOA decomposition by ultrasound treatment (150 W, 40 kHz),
with or without saturated N2, was <25% after 4 h reaction. The
extent and rate of PFOA decomposition and defluorination efficiencies of PFOA, however, greatly increased with the addition of carbonate radical reagents. PFOA was decomposed completely after
4 h of sonochemical treatment with a carbonate radical oxidant
and saturated N2. Without saturated N2, PFOA was also decomposed to a high (98.81%) degree. The highest PFOA decomposition
and defluorination efficiencies was observed in N2 saturated solution containing 30 mM of NaHCO3. Sonodecomposition of PFOA
with CO
3 radical was most favorable in a slightly alkaline environment (pH = 8.65) because the CO
3 radical concentration was low
under acidic conditions, and the CO
3 radical concentration could
decay quickly by second-order reactions in highly alkaline conditions. We did not detect any shorter-chain perfluorinated carboxylic acids in the final reaction solution and CO, CO2 and HF are
the final products.
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CO
3 radical is a selective radical, and it works efficiently in
decomposing PFOA. Based on the results from this study, sonochemical treatment using carbonate radicals has the potential of
removing PFOA and other similar pollutants in water and wastewater effluents.
Acknowledgement
The authors would like to thank the financial support from the
National Science Council (NSC) of Taiwan under project number of
NSC 100-2221-E-002-043.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ultsonch.2014.03.
027.
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