Reaction Process of Perfluorooctanesulfonic Acid (PFOS) Decomposed by DC Plasma Generated in Argon Gas Bubbles

Perfluorooctanesulfonic acid (PFOS: C
₈
F
₁₇
SO
₃
H) is a bioaccumulative and environmentally persistent substance. PFOS cannot be decomposed by conventional water treatments; therefore, a new degradation method is required. PFOS can be efficiently degraded by dc plasma generated in gas bubbles. Since this process has yet to be elucidated, we investigated the by-products and the reaction process during the degradation of PFOS. Aqueous PFOS (initial concentration of 55.5 mg/L) was decomposed by dc plasma generated in argon gas bubbles for 600 min, and the concentrations of PFOS, perfluorocarboxylic acids (PFCAs: C
_n
F
_2n+1
COOH, n=1-7), carbon dioxides, carbon monoxides, and fluoride ions were measured, but we did not achieve mass balances for carbon or fluorine. To detect the unknown by-products, both aqueous PFOS that was treated for 180 min and the gas released from the reactor were analyzed. C
_m
HF
_2m
SO
₃
H (m=2-8) and C
_i
HF
_2i
COOH (i=2-7) were detected in the liquid phase, and CHF
₃
, C
₂
HF
₅
, and C
₂
F
₆
were found in the gas phase. Given these by-products, it has been proposed that some PFOS is immediately thermally decomposed to carbon dioxides, fluoride ions, and hence forth., and the remaining PFOS is gradually degraded by the collisions of high energy particles, leading to the final products that include fluorocarbon gases and fluoride ions.