鹼性條件下臭氧去除水中全氟辛酸

Abstract

全氟碳化物(perfluorinated compounds，PFCs)常被半導體與光電產業用來酸洗電子零件或晶圓表面，或當作光版程序的界面活性劑。PFCs因其強大的碳-氟鍵結(C-F bond)，在自然界中不易被光解、水解或生物降解，也導致其在環境中有持久性與生物累積性的特質。許多文獻已證實PFCs具有致突變腫瘤與致癌性，因此利用工程方法去除PFCs為近年來熱門的研究議題。其中，全氟辛酸(perfluorooctanoic acid，PFOA)為一種最被廣泛應用的人造全氟碳化物，且比起許多其他的PFCs更具毒性、更難以分解。 PFOA無法在中性或酸性環境下使用臭氧直接降解，因此本研究嘗試使用臭氧在鹼性條件下，產生氫氧自由基(hydroxyl radical，OH•)降解水中PFOA。實驗中改變之參數有：pH (5~6, 11)、臭氧濃度(0 wt%, 2 wt%, 2.5 wt%, 7 wt%, 9.5 wt%, 10.5 wt% (ozone/oxygen))，PFOA初始濃度(50 ppb, 5 ppm)、過氧化氫(H2O2/O3 =5, 10, 20 molar%)、腐植酸(15 mg/L)與外加超音波場源震盪影響。 當在鹼性條件(pH=11)下，使用臭氧能有效降解PFOA。使用2.5 wt%之臭氧可得到最佳處理效率(90 %)；若添加過氧化氫增加氫氧自由基之穩定態濃度，可使去除效率增加15 %~ 56 %；添加腐植酸抑制氫氧自由基之穩定態濃度，會使去除效率減少15 %~44%；外加超音波場源震盪則增加2 %去除效率，顯示降解機制為氫氧自由基攻擊目標污染物。此方法可將PFOA降解成毒性較低的中間產物如：全氟庚酸(PFHpA)、全氟己酸(PFHxA)、全氟戊酸(PFPeA)、全氟丁酸(PFBA)與氟離子，以減少PFOA在環境中的危害。 此外，本研究亦嘗試將最佳操作條件用於處理全氟辛烷磺酸(Perfluorooctanesulfonic acid, PFOS)與實廠廢水，結果顯示PFOS去除效率可達84 %，用於處理實廠廢水6小時可去除92 %之PFOA、4小時即可去除99 %之PFOS。

Perfluorinated compounds (PFCs) have been widely used as surfactants in photolithographic processes in semiconducting and optoelectronic industry. The highly stabilities in photodegradation, hydrolysis, and biodegradation of perfluorinated compounds is attributed to their strong C-F bond, resulting in persistence and bioaccumulation in the environment. PFCs have been proved to be a carcinogen to human. Perfluorooctanoic acid (PFOA), one of human-made PFCs, is more toxic and difficult to be degraded than many other PFCs. PFOA can not be degraded by direct ozone reaction in acidic/neutral condition. Consequently, this study aimes to investigate PFOA ozonation in alkaline condition due to the generation of hydroxyl radical to degrade PFOA. The degradation efficiency of PFOA varied with pH values (5~6, 11), ozone concentrations (0 wt%, 2 wt%, 2.5 wt%, 7 wt%, 9.5 wt%, 10.5 wt% (ozone/oxygen)), the initial PFOA concentrations (50 ppb, 5 ppm), the molar ratio of hydrogen dioxide to ozone (H2O2/O3 =5, 10, 20 molar%), presence of humic acid (15 mg/L) and with the assistant of ultrasound. In alkaline condition (initial pH=11), ozonation can effectively degrade PFOA and exhibits the best efficiency with 2.5 wt% (ozone/oxygen). Adding hydrogen dioxide to enhance the steady concentration of hydroxyl radical (OH•) increases 15~56% of the conversion. Adding humic acid to restrain the steady concentration of OH• decreases 15~44% of the conversion. Combining ultrasonic vibration increases 2% of the conversion. Those phenomena illustrate that the hydroxyl radical attacking the target compound is the main mechanism of degradation. In this study, PFOA can be degraded to the-lower-toxic intermediates such as PFHpA, PFHxA, PFPeA, PFBA, and fluoride ion. In addition, this technology was also applied in treating perfluorooctanesulfonic acid (PFOS) and industrial wastewater under the optimum operating condition. Result shows that 84% PFOS can be degraded, and 92% PFOA and 99% PFOS in the industrial wastewater can be degraded within 6 hours and 4 hours, respectively.