## 3 Results and Discussion  
### PFOS Removal by CP  
Figure 2a illustrates the removal of PFOS over time during duplicate experiments. The initial concentration of PFOS (8.0 mg/L) sharply decreased to approximately 3.0 mg/L within 1 h (about a 62.5% removal efficiency). Subsequently, the rapid degradation of PFOS suddenly ceased, and its concentration very slowly decreased to around 2.5 mg/L (a 68.8% removal efficiency) on average, even after 10 h.  
The rapid removal efficiency of PFOS over time is likely associated with the generation of ROS/RNS by CP, as these strong oxidizing species can readily and quickly attack PFOS's sulfonyl group (-SO\({}_{3}\)H). When comparing CP's PFOS-degrading ability with other AOPs, [PERSON] et al. reported that applying UV radiation to a PFOS sample with an initial concentration of 40 \(\upmu\)mol over a 24 h period resulted in a removal efficiency of 27.2% [52]. According to [PERSON] [53], the electrochemical application of boron-doped diamond (BDD) to PFOS at an initial concentration of 4.6 \(\upmu\)mol resulted in a 98% removal efficiency after 43 h. [PERSON] [54] determined that the electrochemical application of a lead dioxide anode for 180 min led to a 41.6% removal efficiency for PFOS with an initial concentration of 0.0929 mmol/L. [PERSON] [55] found that applying an electron beam (900 kGy) to PFOS with an initial concentration of 100 mg/L resulted in a removal efficiency of 61.0%. Additionally, [PERSON] [56] reported that PFOS treated for 1 h by applying gliding arc plasma achieved a removal efficiency of 93.1%. [PERSON] [57] reported that an initial concentration of 8.3 mg/L of PFOS was treated for 1 h with a high-voltage (HV) electrode plasma reactor, achieving a removal efficiency of over 90%.  
Indeed, CP demonstrates a stable PFOS removal efficiency in a shorter timeframe compared to alternative AOPs. Furthermore, CP is relatively economical in its power usage (0.15 kW) and does not require additives such as ozone, hydrogen peroxide, or iron. As a result, CP represents a viable and cost-effective method for removing PFOS.  
Previous studies investigating the efficacy of AOPs in removing PFOS have examined their effects on the by-products of PFOS's degradation, including PFOA, perfluorobutanoic acid (PFBA), perfluorobutanesulfonate (PFBS), perfluoroheptanoic acid (PFHPA), perfluorohexanesulfonate (PFHxS), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), and others. This area represents a promising avenue for future research on the CP method. Figure 2b show the TOC analysis conducted during PFOS's degradation by CP. The initial TOC value was observed to be 1.45 mg C/L, with a removal efficiency of approximately 77% within 2 h, a trend that continued beyond that point. Therefore, the TOC analysis results suggest the potential for the effective removal of the organic carbon in PFOS by CP.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Carrier gas & Species & He (or N\({}_{2}\)) \\ \hline Flow rate & mL/min & 0.5–4 \\ \hline Column temp & \({}^{\circ}\)C & 40–310 \\ \hline Injector temp & \({}^{\circ}\)C & 150–300 \\ \hline Injection volume & \(\upmu\)L & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Analytical conditions of GC-ECD.