以二氧化錳催化過氧化氫程序降解全氟辛烷磺酸：探討氫氧自由基氧化與超氧自由基還原之角色

Abstract

全氟辛烷磺酸(Perfluorooctanesulfonic acid, PFOS)因具有優異之界面活性劑特性與抵抗氧化能力，而被廣泛運用在工業中。然而，過去毒性相關研究顯示其會對生態圈與人體造成負面危害。本研究調查使用二氧化錳/過氧化氫(MnO2/H2O2)程序產生具活性之自由基降解全氟辛烷磺酸。在三種不同型態之二氧化錳(α-, β-, and γ-MnO2)中，γ-MnO2可最有效催化過氧化氫降解PFOS。整體降解效果在較高H2O2濃度、γ-MnO2劑量與初始反應pH值會有所提升，而初始濃度為0.25 μM之PFOS亦可在最佳條件下於15分鐘內完全降解完畢([H2O2] =1 M, [γ-MnO2] = 20 mg/L, 初始pH = 7)。在MnO2/H2O2程序中，電子順磁共振光譜儀(Electron paramagnetic resonance, EPR)驗證降解反應中氫氧自由基(Hydroxyl radical, OH•)與超氧自由基(superoxide radical, O2•-)的產生，而X射線光電子能譜儀(X-ray photoelectron spectroscopy, XPS)則顯示二氧化錳表面之錳價態變化與氧之錯合鍵結(coordination environment)變化。由自由基捕捉實驗與副產物鑑定結果可知在PFOS之降解過程中，其碳-硫鍵結(C-S bond)會優先被OH•氧化斷鍵。後續反應還包括其碳-碳鍵結(C-C bond)被OH•氧化斷鍵生成短碳鏈副產物，以及其碳-氟鍵結(C-F bond)被O2•-還原斷鍵生成脫氟副產物。另外，有些長碳鏈之副產物亦有被偵測到，其生成原因可能為各類有機自由基之重組。

Perfluorooctanesulfonic acid (PFOS) has been used in various industries due to its excellent surfactant property and oxidative resistance. However, toxicology studies have shown that PFOS can cause adverse effects in ecological and human health. This study investigated the use of MnO2/H2O2 process to generate reactive radicals for PFOS removal. Our results indicated that among the three MnO2 polymorphs (α-, β-, and γ-MnO2), γ-MnO2 was more effective in catalyzing H2O2 for PFOS degradation and the degradation was enhanced by a higher H2O2 concentration, MnO2 loading and initial pH. PFOS (0.25 μM) could be completely degraded in 15 min under the optimal condition ([H2O2] =1 M, [γ-MnO2] = 20 mg/L, initial pH = 7). Electron paramagnetic resonance (EPR) verified the formation of hydroxyl radical (OH•) and superoxide radical (O2•-) and X-ray photoelectron spectroscopy (XPS) revealed the evolutions of Mn oxidation state and O coordination environment on the MnO2 surfaces in this process. Radical quenching experiments and intermediates identification indicated that the degradation of PFOS was initiated by OH• oxidation to break the C-S bond. Subsequent cleavages of the C-C bond by OH• oxidation and the C-F bond by O2•- reduction could lead to the formation of short-chain and defluorinated intermediates. Long-chain byproducts were also identified, likely due to the recombination of organic radicals.