水體環境中全氟化合物之流佈與外加電場薄膜處理之研究

Abstract

本研究針對北部某科學園區工業廢水處理廠放流及其下游承受水體進行全氟化合物(perfluorinated compounds, PFCs)，包括三種perfluoroalkyl sulfonates (PFASs)：perfluorobutane sulfonate (PFBS (C4))、perfluorohexane sulfonate (PFHxS (C6))及perfluorooctane sulfonate (PFOS (C8))及七種perfluoroalkyl carboxylates (PFCAs)：perfluorohexanoic acid (PFHxA (C6))、perfluoroheptanoic acid (PFHpA (C7))、perfluorooctanoic acid (PFOA (C8))、perfluorononanoic acid (PFNA (C9))、perfluorodecanoic acid (PFDA (C10))、perfluoroundecanoic acid (PFUnA (C11))及 perfluorododecanoic acid (PFDoA (C12))之分析調查。水樣分析結果顯示十種目標PFCs皆於工業廢水處理廠放流中發現，PFOS (6930 ng/L)為PFASs中最主要之污染物，PFHxS (2662 ng/L)次之；而PFOA (3298 ng/L)則為PFCAs中最主要之污染物，於工業廢水處理廠放流至其下游及承受水體之PFCs濃度顯著的升高且保持一相近之數值，其可說明工業廢水處理廠所排放之廢水為其下游承受水體受PFCs汙染的主要來源。工業廢水處理廠下游及承受水體之PFBS與PFOS的濃度皆遠超過Avian Wildlife Value (AWV)之標準，顯示此二種化合物將可能對棲息於客雅溪之鳥類(食物鏈頂層之野生動物)造成不良之影響；另亦發現PFOS之濃度已超過Criteria Continuous Concentration (CCC)之標準，顯示將可能對承受水體中水生生物產生慢毒性之影響。 底泥樣品中可偵測到五種PFCAs (PFHxA、PFOA、PFDA、PFUA及PFDoA)及二種PFASs (PFHxS及PFOS)，其中以PFOS (1.5-78 ng/g)、PFOA (0.5-5.6 ng/g)及PFDoA (nd-5.4 ng/g)有相對較高之濃度。且發現Log (Csediment / Cwater)隨PFCAs之碳數增加而增加，其中PFOA、PFDA、PFUA及PFDoA之Log (Csediment / Cwater)分別為0.2-0.3、2.2-2.5、2.5-2.7及3.2-3.3，表示長碳鏈之PFCs (PFDA、PFUA及PFDoA)較易被吸附於底泥中。 於生物體中，PFOS之濃度比例皆為十種目標PFCs中最高者(肌肉組織：76-84%於客雅溪；55-64%於基隆河，肝臟組織：94-95%於客雅溪；44-51%於基隆河)，且PFCs濃度於肝臟組織大於肌肉組織(以吳郭魚A1生物體為例，其肝臟組織PFOS含28933 ng/g；肌肉組織PFOS含1386 ng/g)。Log (Cbiological tissue / Cwater)有隨PFASs及PFCAs之碳數增加而增加之趨勢，以肝臟組織為例，PFBS、PFHxS及PFOS之Log (Cliver / Cwater)分別為1.0-1.5、2.2-2.3及3.5-3.7；PFOA、PFNA、PFDA、PFUA及PFDoA之Log (Cliver / Cwater)分別為1.7-1.8、3.8-4.2、3.7-4.0、4.3-4.4及5.3-5.4，表示長碳鏈PFCs較短碳鏈PFCs易累積於生物體內，且客雅溪之生物體PFCs濃度高於基隆河約1-197倍。綜合水樣、底泥及生物體之分析結果，顯示該工業廢水處理廠對下游承受水體客雅溪的生態環境具有潛在之衝擊影響。 因此勢必要提升工業廢水處理廠對於PFCs之去除效率，才可有效控制PFCs排放至自然水體。近年來薄膜技術已普及應用於廢水處理，因此依PFCs在水中以負電形式(low pKa)存在之特性，利用microfiltration (MF)薄膜外加電場技術(electro-microfiltration, EMF)進行處理，發現可大幅提升PFCs之去除效率。 本研究探討改變不同操作參數，如電場強度的施加(0、29、43.5及58 V/cm)、溶液pH值(4、7及10)、離子強度(0.4-4.8 mM Na2SO4、NaCl、NH4Cl及CaCl2)及溶解性有機物(dissolved organic matter, DOM)之存在對於EMF去除PFOA及PFOS之影響。結果顯示僅利用MF薄膜過濾，幾乎無法去除PFOA及PFOS，但施加一電場穿透過薄膜能夠大幅增加PFOA及PFOS的去除效率，如溶液pH=10下，去除效率可從無外加電場之<3%，提升至施加58 V/cm電場強度的>84%。於EMF程序下，當溶液氫離子濃度及離子強度增加，PFOA及PFOS去除效率有降低之趨勢，可能因薄膜表面界達電位降低，減少薄膜與PFOA及PFOS之間的靜電排斥力所致。當施加電場強度小於臨界電場強度(critical electrical field strength, Ecritical, HA)時，可觀察到有腐植酸存在時PFOA及PFOS之去除效率較低，推測此狀況下HA會朝向薄膜表面傳輸並吸附於膜面，進而產生積垢，此積垢層會降低薄膜表面界達電位所致。因此本研究假設在電場施加下，上述之無機及有機基質存在會改變薄膜表面界達電位，因而影響PFOA及PFOS之去除效率。 EMF可有效去除PFOA、PFOS及其他三種PFCs (PFDA、PFHxS及PFHxA)共同存在之溶液(電場強度58 V/cm，去除效率範圍於溶液pH=7下為70-76%；於溶液pH=10下為81-86%)。EMF亦能夠有效去除實廠廢水中之PFOA及PFOS，電場強度為58 V/cm下，PFOA及 PFOS及DOM去除效率分別為70%及80%。

We investigated the occurrence of perfluorinated compounds (PFCs) in the effluents of an industrial wastewater treatment plant (IWWTP) and its receiving rivers. Ten target PFCs including three perfluoroalkyl sulfonates (PFASs)：perfluorobutane sulfonate (PFBS (C4)), perfluorohexane sulfonate (PFHxS (C6)), and perfluorooctane sulfonate (PFOS (C8)) and seven perfluoroalkyl carboxylates (PFCAs)：perfluorohexanoic acid (PFHxA (C6)), perfluoroheptanoic acid (PFHpA (C7)), perfluorooctanoic acid (PFOA (C8)), perfluorononanoic acid (PFNA (C9)), perfluorodecanoic acid (PFDA (C10)), perfluoroundecanoic acid (PFUnA (C11)), and perfluorododecanoic acid (PFDoA (C12)) were detected in the effluents of IWWTP; PFOS (6930 ng/L) and PFHxS (2662 ng/L) were the major constituents of PFASs; PFOA (3298 ng/L) was the major constituent of PFCAs. IWWTP effluent and its receiving rivers demonstrated to have similar PFCs distribution and concentrations, indicating that the IWWTP wastewater is the major source of PFCs to its receiving rivers. Compared to the reported Avian Wildlife Value (AWV), PFBS (392 ng/L) and PFOS (7165 ng/L) level detected showed to exceed the AWV 23 and 152 times, respectively, which may potentially result in adverse effects to birds and other wildlife. In addition, PFOS concentration found (7165 ng/L) at the receiving rivers was higher than the reported Criteria Continuous Concentration (CCC), indicating the potential chronic toxicity to aquatic organisms. In sediment samples, five PFCAs (PFHxA, PFOA, PFDA, PFUA and PFDoA) and two PFASs (PFHxS and PFOS) were detected. PFOS (1.5-78 ng/g), PFOA (0.5-5.6 ng/g), and PFDoA (nd-5.4 ng/g) were present at relatively higher concentration. Log (Csediment / Cwater) increases with an increasing carbon chain length of PFCAs; the Log (Csediment / Cwater) values for PFOA, PFDA, PFUA, and PFDoA were 0.2-0.3, 2.2-2.5, 2.5-2.7, and 3.2-3.3, respectively and therefore, PFCs with higher carbon numbers (PFDA, PFUA and PFDoA) are easier adsorbed onto sediments. In biological tissue, PFOS was presented at higher concentration among the target PFCs and was the predominant PFCs (PFOS % in total PFCs in muscle tissue：76-84% in Keya River；55-64% in Keelung River and in liver tissue：94-95% in Keya River；44-51% in Keelung River). The PFCs concentration in the liver tissue was higher than those found in the muscle tissue (e.g. in A1-mouthbeeder biological tissue, PFOS concentration was 28933 ng/g in liver tissue; 1386 ng/g in muscle tissue). Log (Cbiological tissue / Cwater) increases with an increasing carbon chain length of PFASs or PFCAs. In liver tissue, the Log (Cliver / Cwater) values for PFBS, PFHxS, and PFOS were 1.0-1.5, 2.2-2.3, and 3.5-3.7, respectively；the Log (Cliver / Cwater) values for PFOA, PFNA, PFDA, PFUA, and PFDoA were 1.7-1.8, 3.8-4.2, 3.7-4.0, 4.3-4.4, and 5.3-5.4, respectively. The results indicated that PFCs with higher carbon numbers are easier accumulated in biological tissues. In addition, PFC concentrations in biological tissues in Keya River were 1-197 times greater than levels found in Keelung River. These water, sediment and biota data together imply that the receiving aquatic environments were impacted by the industrial discharges. Therefore in order to reduce, and better yet, to eliminate PFCs discharge into aqueous environments, advanced treatment technologies appear viable for their removal. PFCs are negatively charged and have low pKa values in water; therefore, a laboratory-scale electro-microfiltration (EMF) unit that applies a direct-current (DC) electrical field across its membrane can greatly enhance their removal from aqueous systems. We examined the effects of electrical field strength (0, 29, 43.5 and 58 V/cm), an aqueous inorganic matrix (pH: 4, 7 or 10; ionic strength: 0.4-4.8 mM; ionic composition: Na2SO4, NaCl, NH4Cl or CaCl2) and an organic matrix such as dissolved organic matter (DOM) on the ability of EMF to remove PFOA and PFOS. In the absence of an electrical field, PFOA and PFOS removal efficiencies were low; however, the application of a DC electrical field through the membrane greatly enhanced PFOA and PFOS removal. PFC rejections increased from <3% (0 V/cm) to >84% (58 V/cm) at pH=10 solution. Decreases in PFOA and PFOS removal were observed as the increase of proton concentration and ionic strength, suggesting decreased membrane zeta potential, in turn reducing the electrostatic repulsion force between the membrane and PFOA and PFOS. At 29 V/cm, the electrical field was less than the critical electrical field strength (Ecritical, HA), conditions under which humic acid (HA) could be transported toward the membrane; thus, its adsorption would decrease the membrane zeta potential. HA adsorption would decrease the PFOA and PFOS rejection efficiency during EMF with an electrical field strength lower than its critical value. Therefore, we hypothesize that these matrices affect PFOA and PFOS rejection by altering membrane zeta potential during filtration in the presence of an electrical field. EMF was found to remove coexisting PFCs including PFOA, PFOS and three other PFCs (PFDA, PFHxS and PFHxA) effectively; at 58 V/cm, their removal efficiencies were 70-76% and 81-86% at pH=7 and pH=10, respectively. In addition, EMF was also able to remove effectively 70% PFOA and PFOS and 80% DOM from real industrial wastewaters at 58 V/cm.