全氟辛烷磺酸與全氟辛酸於使用氧化石墨烯二氧化鈦電極的光電化學系統之降解機制及競爭抑制行為

PFOS and PFOA degradation mechanisms and their associated competitive inhibition via photoelectrochemical system using GOTiO2 photoelectrodes

楊政憲(Jheng-Sian Yang)

指導教授 ： 林郁真

國立臺灣大學/工學院/環境工程學研究所/博士(2022年)

https://doi.org/10.6342/NTU202201196

摘要

全氟烷基酸 (Perfluorinated alkyl acids, PFAAs) 為一廣泛分布於全球各環境中的持久性有機污染物，其中的全氟辛烷磺酸 (perfluorooctane sulfonic acid, PFOS) 和全氟辛酸 (perfluorooctanoic acid, PFOA) 因其在環境中流佈廣泛且濃度較高，為當今國際間所重視的重要環境議題之一。儘管目前有許多研究致力於PFOS及PFOA的去除及降解後副產物的鑑定，然而至今仍僅發現由PFOA降解後所生成部分的全氟羧酸 (Perfluorinated carboxylic acid, PFCAs) 副產物。此外，亦有研究發現，PFAAs在高級氧化處理系統中，所產生之副產物在水中存在不同基質影響下，不會僅產生簡單的短鏈PFAAs。本研究之結果不僅重新定義PFOS和PFOA的降解機制與途徑、鑑定新的副產物、更深入探討混合PFAAs於系統中降解可能受到之影響。
本研究成功製造出應用於光電化學系統中降解PFOS及PFOA的氧化石墨烯二氧化鈦光電極。研究結果發現5 wt. %的氧化石墨烯二氧化鈦光電極具有最佳的光電化學效能：能隙為2.42 eV；比表面積為72.6 m2 g−1；比電容為4.63 mF cm−2。在應用此光電極的光電化學系統中，PFOS及PFOA可以在4小時內於各最佳化條件下（包含電流密度，電極距離，溶液pH值，PFOS與PFOA初始濃度及電解質濃度）達去除率98.2及100 %，其擬一級速率常數分別為0.80及0.74 hour−1。在光電化學系統中造成PFOS與PFOA降解的主要原因為經電子轉移，以及與羥基自由基、超氧自由基和活性氯物種反應。另外，在使用氯化鈉做為電解質的光電化學系統中，氯化副產物的發現可以證明系統中所產生的活性氯物種參與了PFOS與PFOA的降解。
本研究共鑑定出30種PFOS降解之副產物，並定義其在系統中的降解途徑，亦首次發現全氟烷烴磺酸鹽、全氟醛類及氫氟碳化物的產生。PFOS在系統中降解的第一步為脫磺化，然後氧化接著脫氟、脫羧、脫羰、磺化、脫氟和羥基化。另外， PFOA的降解途徑可能有以下四種：脫羧後氧化、脫氟、羥基化和氯原子取代。此外，本研究發現PFAAs的反應性與其碳鏈長度有關，例如較短鏈的PFAAs降解速率較長鏈的PFAAs為慢。在混合不同碳長度PFAAs於系統中之降解情況發現，較短鏈的PFAAs與較長鏈的PFAAs相比，降解速率下降較不明顯，其結果表示較短鏈的PFAAs在處理過程中具有更強的競爭抑制作用和更強的環境抵抗力，此結果有助於了解實際廢水處理過程中所存在之混合PFAAs的降解宿命。

關鍵字

光電化學系統 ； 全氟辛烷磺酸 ； 全氟辛酸 ； 活性氧物質 ； 活性氯物質 ； 副產物和反應途徑 ； 競爭抑制作用

並列摘要

The global distribution and environmental persistence of perfluoroalkyl acids (PFAAs) has been considered a critical environmental concern. Although many efforts have been made to identify perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) degradation byproducts, previous studies have only reported byproducts that are related to perfluorinated carboxylic acids (PFCAs). In addition, in the case of combinatorial advanced oxidation processes (AOPs), the decomposition products may not just be simple short-chain PFAAs, especially under the influence of a matrix. This is the first study to elucidate the new PFOS and PFOA degradation pathway and byproducts. This study further systematically explores how PFAA degradation may be affected in the mixture system.
In this work, a graphene oxide-titanium dioxide (GOTiO2) photoelectrode was successfully fabricated for PFOS and PFOA degradation in a photoelectrochemical (PEC) system. The results reveal that a 5 wt. % GOTiO2 anode possesses the optimal PEC performance, with a band gap (Eg) of 2.42 eV, surface area (SBET) of 72.6 m2 g−1 and specific capacitance (Cs) of 4.63 mF cm−2. In the PEC system, the process parameters, including current density, electrode distance, solution pH, PFOS and PFOA concentration and electrolytes concentration were optimized. PFOA was significantly reduced in 4 h (98.2 % removal); PFOS can be efficiently removed within 4 h of reaction time, with a pseudo-first-order rate constant of 0.80 and 0.74 hour−1. The transfer of electrons, hydroxyl radicals and superoxide radicals are all responsible for PFOS and PFOA decomposition/transformation using chloride anion (Cl−) as an electrolyte; the presence of chlorinated byproducts in PEC system using Cl− as electrolyte indicated that reactive chlorine species contributed to PFOS and PFOA degradation.
New degradation pathways were identified; a total of 30 PFOS byproducts are reported in this work; perfluoroalkane sulfonates (PFSAs), perfluorinated aldehydes (PFALs) and hydrofluorocarbons (HFCs) were identified for the first time. PFOS degradation involves the desulfonation pathway as the first step, followed by oxidation and subsequent defluorination, decarboxylation, decarbonylation, sulfonation, defluorination and hydroxylation. Four possible routes of PFOA decomposition, namely, decarboxylation followed by oxidation, defluorination, hydroxylation and Cl atom substitution, were determined in PEC system using Cl− as electrolyte. The results from this work also show that the reactivity of PFAAs is related to their carbon chain length, with shorter-chain PFAAs exhibiting a lower degradation rate. In a PFAA mixture, a decline in the degradation rate was observed for the shorter-chain-length PFAAs, suggesting stronger competitive inhibition and indicating stronger environmental recalcitrance during the treatment process. The results acquired in this study aid in comprehensively understanding the degradation and fate of PFAA cocktails during wastewater treatment.

並列關鍵字

Photoelectrochemical ； PFOS ； PFOA ； Reactive oxygen species ； Reactive chlorine species ； Byproducts and reaction pathways ； Competitive inhibition

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