Mass Balance Analysis of Perfluorocompound
Decomposition by DC Plasma Generated in Gas Bubbles
Summary
The aqueous solution of perfluorooctanesulfonic acid (PFOS) (55.5 mg/L, 50 mL) was decomposed by a dc plasma generated in argon gas bubbles for 600 min and the
mass balances of carbon and fluorine were measured. The mass balances of carbon and fluorine were 57.0% and 72.3% at 600 min, respectively and didn’t reach 100%.
To detect unknown by‐products, the liquid phase and the gas phase products were qualitatively analyzed by high performance liquid chromatography mass
spectrometry (HPLC/MS) and Fourier transform infrared spectroscopy (FT‐IR). CmHF2mSO3H (m=2–8) and ClHF2lCOOH (l=2–7) were detected in the liquid phase and
fluorocarbon gases such as CHF3, C2HF5, and C2F6 were found in the gas phase. These results indicate that some PFOS is gradually degraded at the C–F, C–C, and C–S
bonds, by the collisions of high energy particles. Such processes take relatively long time for detaching F−. Other PFOS is thermally decomposed by the heat of the
plasma (1600 K) and F− are detached in a short period.
Objectives
Experimental Setup
Perfluorooctanesulfonic acid (PFOS, C8F17SO3H)
•
•
•

Surfactant
Chemically stable
Used in semiconductor process
Have carcinogenicity
PFOS degradation methods
Concentration, vol., W, %, hours
UV254 in 2‐Propanol solution
（National Inst. Environ. Studies, 2008）
Ultrasonic irradiation
（Caltech, 2010）
DC plasma in bubbles
(Tokyo Tech, 2010)
20 mg/L, 750 mL, 30W, 75%(9days)
13mg/L, 400 mL, 75W, 70%(2h)
60mg/L, 200 mL, 45W, 50%(1h)
 The plasma is generated inside bubbles in the solution.
 The high‐voltage power supply is connected to the plasma reactor through a ballast resistor which stabilizes the plasma current.
 The average current is regulated at 10 mA. Experimental condition
Experimental setup of the plasma reactor
Gas: Argon
Gas flow rate: 100 sccm
Current measurement resister
1 k
Gas outlet
Solution: PFOS 55.5 mg/L, 50 mL
Efficiency
[mg/kWh]
0.6
Acrylic
4.5
Ground
electrode
DC
power
supply
26
Cooling
water
Plasma
Ceramic
(Hole diameter ：0.3 mm)
PFOS can be efficiently decomposed by a dc plasma generated in gas bubbles.
However, the reaction process of PFOS decomposed by a plasma remains unknown.
Gas inlet
Ballast resistor
250 k
Stainless mesh
Ar
Discharge image in bubbles
Mass flow controller
Results and Discussions
1. Discharge characteristics
1
5
Reactor voltage
0
10
20
30
Time [ms]
Total ion current chromatogram of PFOS solution treated for 180 min
PFOS peak
0.5
0
0
40
306
The current fluctuated between 9.5 mA and 10.5 mA.
20
PFOS
0
TFA(n=1)
PFPrA(n=2)
PFBA(n=3)
PFPeA(n=4)
PFHxA(n=5)
PFHpA(n=6)
PFOA(n=7)
1
0
120 240 360 480 600
Time [min]
10
10
0
0
0
120 240 360 480 600
Time [min]
60
CO
40
PFCAs
20
CO2
PFOS
0
0
120 240 360 480 600
Time [min]
Mass balance of fluorine [%]
Mass balance of carbon [%]
Unknown
10
181
Unknown peaks 2
Retention Time [min]
Unknown
60
CHF3
C2HF5
0.002
231
PFOS solution that was
treated for 180 min
 CmHF2mSO3H (m=2–8, m/z=131, 181, … ,481) and ClHF2lCOOH
(l=2–7, 145, 195, …, 395) were detected.
20
30
0
1100
1200
W avenumber [cm-1]
1300
5. Proposed reaction process of PFOS decomposition in solution
Step 4
Cleavage of C－F
bond or C－C
bond or C－S bond
PFCAs
20
PFOS
0
120 240 360 480 600
Time [min]
• The mass balances of carbon and fluorine didn’t reach 100% during the degradation of PFOS.
−F
+H
− CF3
+H
− SO3−
+ COO−
F−
Unknown by‐products exist!
The unknown by‐products were detected as follows: CmHF2mSO3H (m=2–8) and ClHF2lCOOH (l=2–7) in the liquid phase and CHF3, C2HF5,and C2F6 in the gas phase C2 F 6
C2HF5
0.001
40
0
 Fluorocarbon gases such as CHF3, C2HF5,and C2F6 were detected.
Gradual degradation caused by the
collisions of high energy particles
100
80
Amount of CO [mol]
CO Concentration [ppm]
8.2 mol
20
3. Mass balances of carbon and fluorine
80
431 381 331 281
Unknown peaks 1
Remaining PFOA peak
0.003
20
30
120 240 360 480 600
Time [min]
100
Cm HF2m SO3H (m=2–8)
395 345 295 245 195
481
145
Infrared spectrum of the gas released from the plasma reactor
0
0
Amount of CO2 [mol]
CO2 Concentration [ppm]
0
0
CiHF2i COOH (i=2–7)
0
40
10
30
0
Concentrations of CO2 and CO in gas.
10
20
Gas phase analysis by FT-IR
120 240 360 480 600
Time [min]
40
Peaks and corresponding m/z
0.5
Absorbance
F-
20
10
Remove the peaks of PFOS and PFCAs.
Intensity [a.u.]
PFCAs Concentration [mg/L]
Concentration [mg/L]
40
20
2
1
2
15.2 mol
PFOS solution that was
treated for 180 min
4
Retention Time [min]
Concentrations of PFOS, PFCAs (CnF2n+1COOH, n=1–7),
and F− in solution.
30
PFCAs peaks
PFOA peak
0
0
308 310 312 314
W avelength [nm]
2. Degradation characteristics of PFOS
0
6
The gas temperature of the plasma was estimated at 1600 K.
60
• PFCAs peaks were detected at 8–12 min.
• PFOS peak was found at 12‐14 min.
8
Intensity [a.u.]
1000
Liquid phase analysis by HPLC/MS
Experiment
Simulation
Tr = 1600 K
Tv = 3000 K
Current
10
0
4. Qualitative analysis to detect the unknown products
Emission spectrum of
OH radicals
Intensity [a.u.]
2000
15
Current [mA]
Reactor voltage [V]
Waveforms
measured at 60 min
Step 5
Repetition of cleavage
of C－F bond or C－C bond
or C－S bond
C8HF16SO3−(aq)
…
C7HF14SO3−(aq)
…
C7F15COO −(aq)
…
CHF3(g)
+
C2HF5(g) + SO42−(aq) + F−(aq)
+
C2F6(g)
+
+
CO2(g)
Bulk Phase
C8F17SO3
+ H2O
− 2H+
−H
Gradual degradation
 A part of PFOS is gradually degraded by the collisions of high energy particles.
 Intermediates began to adsorb on the gas‐liquid interface and degrade after almost all of PFOS in solution is decomposed.
−
(adsorption)
Step 1
Transportation to
plasma phase
Plasma Phase
C8F17SO3−(g)
Immediate pyrolysis caused by
the heat of the plasma
1600 K
Step 2
Thermal
degradation
CO(g) + SO3(g) +
CF3(g)+ SO3(g)
H2O +
+
OH
CO2(g)
CF2(g)
H
Step 3
+
Reaction with
CF(g)
H O, OH, H
2
HF(g)
Immediate pyrolysis
 Other part of PFOS is decomposed by the heat of the plasma (1600 K) in the plasma phase.
 F− are generated soon after the decomposition of PFOS.
(K. Tachibana)