24pO5-7
Plasma Degradation of Perfluoro Compounds in Water
水中有機フッ素化合物のプラズマ分解
Koichi Yasuoka
安岡 康一
Tokyo Institute of Technology
2-12-1,S3-3,Ookayama,Meguro-ku, Tokyo 152-8552, Japan
東京工業大学 〒152-8552 目黒区大岡山2-12-1, S3-3
Environmentally harmful and persistent substances, such as perfluorooctanoic acid (PFOA) andperfluorooctanesulfonic acid (PFOS), have been successfully decomposed in solution using DC oxygen
plasmas generated within small gas bubbles. After 180 min of operation, 99% of fluorine atoms weredetached from the PFOA, and the energy efficiency of defluorination was 6.0 mg/kWh. Although almost
100% of PFOS was decomposed, only 68% of fluorine atoms were detected in the PFOS solution after 480
min of operation. The defluorination energy efficiency of PFOS was 2.2 mg/kWh. This lower efficiency
seems to be caused by the reformation of perfluoroalkyl sulfonates with the detached sulfate ions.

1. Introduction
Perfluorocarbons (PFCs), such as PFOA andPFOS, are widely used owing to their chemical
stability; however, unexpected toxicity oflong-chain PFCs has raised environmental
concerns [1]. Advanced oxidation technology
utilizing OH radicals is unable to decompose
PFCs. Methods developed for decomposing
PFOA include UV-induced photochemical [2]and sonochemical [3] techniques, VUVirradiation [4], S2O8-added hot water treatment at
80°C [5], and a microwave hydrothermal
technique [6]. PFOS is more persistent thanPFOA; therefore, PFOS degradation has beenreported in a limited number of studies, e.g.,using zero-valent iron in subcritical water [7] andsonochemical methods [3,8].
We have found that DC plasmas generated
within oxygen bubbles [9] can decompose
PFOA/PFOS in solution. The decomposition rates
and efficiencies were significantly higher thanthose of the aforementioned methods. In thispaper, we will show the differences in thedecomposition rates and mechanisms of PFOA
and PFOS degradation.
2. Experimental Setup
A schematic diagram of the plasma reactor isshown in Fig. 1. The container was filled with 50
mL of 50 mg/L PFOA (C7F15COOH) or PFOS
(C8F17SO3H) solutions, for which the molecularweights are 414 and 500, respectively. Both arefound in ionic states in the solution.
The electrode system consisted of ahigh-voltage electrode, a 1-mm-thick dielectricalumina spacer having a central hole (diameter,0.2-mm), and a grounded 0.2-mm-thick annular
metal plate immersed in the solution. Oxygen was
fed into the solution through the hole at a flow rateof 100 sccm. Gas bubbles (diameter, 4 mm) formed
and detached from the hole. The high-voltage
electrode was connected to a DC power supply
through a 200-k ballast resistor. DC plasmas
originated from the electrode and ended at the innersurface of the bubbles. The concentration of anions
was measured with a pH/ion meter (Mettler-Toledo,S80) and an ion chromatograph (Metrohm, 861).
The products in the solution were measured via
liquidchromatography-mass
spectrometry( LC-MS; Shimadzu, LCMS-2020).
Current monitor
1 kHigh-density
polyethylene

PFOA/PFOS
Solution
BubbleGroundedannularelectrode

H.V.
Electrode
200 k

PlasmaDielectricspacer
OxygenBallastresistor

Fig. 1. Schematic diagram of plasma reactor

3. Results and Discussion
The initial conductivity of the PFOA and PFOS
solutions were 26 and 29 S/cm, respectively. Thesolution conductivity increased with thedegradation of PFOA or PFOS, owing to theincrease of detached fluorine ions in the solution.

The plasma current was maintained at 10 mA. Fig.
2 shows the PFOA/PFOS decomposition ratio as afunction of input energy which included energy loss
with the ballast resistor and the electric resistanceof the solution. Ninety-seven percent of PFOA
molecules were decomposed after 180 min ofoperation and 99% of fluorine atoms contained inPFOA were detached into the solution. The energy
efficiency of defluorination was 6.0 mg/kWh. ForPFOS
degradation,
however,completedecomposition required a longer time (480 min),and around 70% of fluorine atoms detached fromPFOS. As shown in Fig. 2, the decomposition rateof PFOS changed at 11 Wh; 60% of PFOS wasdecomposed.
Decompsition ratio [%]10050PFOA
PFOS
0100

101102
Input energy [Wh]Fig. 2. Decomposition ratio vs. input energy

The degradation processes of PFOS were evaluatedby measuring the concentration of F-, SO42-,
C7F15COOH,
C6F13COOH,
C5F11COOH,
C4F9COOH, C3F7COOH, C2F5COOH，CF3COOH，
C4F9SO3H, PFOA, and PFOS.
The reaction processes for PFOA/PFOS
degradation are not fully understood; nevertheless,a possible explanation for the plasma degradation
processes is shown in the following equations thatrefer to atom-initiated processes [10].
C8F17SO3- + M → C8F17SO3･ + M + eC8F17SO3･ → C8F17･ + SO3
C8F17･ + H2O →C8F17 O-+ H++ H
C8F17 O- +H2O →C7F15COO- + 2H+ + 2FC7F15COO- + M + 2H2O →C6F13COO- + 2H+ + 2F- + H + CO2 + M
PFOS molecules ionize in the solution and reactwith M, which represents the plasma effect at thebubble and solution interface. The generated
radical, C8F17SO3•, is unstable and dissociates toC8F17• and sulfur trioxide, SO3. The sulfur trioxide
molecule thus formed reacts with a water moleculeand forms sulfuric acid, H2SO4. The fluorinatedradical C8F17• reacts with H2O to yield C8F17OH,
which is unstable and dissociates to C8F17COF, andthen reacts with H2O to yield a carboxyl substance:
C7F15COO–(PFOA). In the same way, by-productswith shorter carbon chains should be formed.
By-products with the sulfo group might begenerated through the following reaction by thedetached sulfur trioxide from PFOS. This reaction
may reduce the decomposing rate at 11 Wh of input
energy as shown in Fig. 2.
CnF2n+1･ + H2SO4 → CnF2n+1SO3－ + H+ + ･OH
Theenergy
consumptionofPFOA
decomposition was 58 mg/kWh to achieve 30%
detachment of fluorine atoms from the PFOA. ForPFOS decomposition, 26 mg/kWh was required todetach 54.8% of fluorine atoms. The energy
efficiency was higher compare to the 0.61 mg/kWhrequirement for PFOS photo degradation in
two-propanol solution, or the 1.2 mg/kWhrequirement under ultrasonic cavitation.
4. Conclusions
Thedegradation ofPFOA/PFOS
wassuccessfully demonstrated by using DC plasmas
generated inside oxygen bubbles, and thedecomposition efficiency was higher than that ofother methods. The lower decomposition rate for
PFOS may be attributed to the reaction associatedwith sulfuric acid, which was a by-product of PFOS
decomposition.
This work was supported by Grant-in-Aid for
Scientific Research (K2324).
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