改性活性碳去除短碳鏈全氟丁酸之研究

Abstract

全氟化合物(Perfluorinated chemicals, PFCs)是由碳鏈上氫被氟取代形成穩定性極強之C-F鍵，另一端由親水官能基團組成之化合物，所以兼具親水及疏水兩種特性，因此被廣泛應用於各類產品與其製程中。過去產業間大多使用長碳鏈PFCs，但隨後發現長碳鏈PFCs具有環境持久性、生物累積毒性會對環境造成危害影響，於是各國之間開始擬定相關政策，長碳鏈逐漸被短碳鏈PFCs取代，如全氟丁酸(Perfluorobutanoic acid, PFBA)，但對於短碳鏈PFCs之毒性、生物累積等相關知識不完善，相關研究已指出短探鏈PFCs可能導致人體以及環境暴露於相關風險之中。因此發展有效去除短碳鏈PFCs之技術，成為當今至關重要的議題。 本研究擬用活性碳(Activated Carbon, AC)吸附水中PFBA。首先，分析活性碳基本性質，如：界達電位、比表面積、組成元素、表面結構；比較PFBA初始濃度、pH值、AC劑量、攪拌速率等不同參數條件下比較去除效果影響，藉此找出最佳參數條件。進一步採用實驗所獲之最佳參數條件，以改性(陽離子界面活性劑、聚乙烯亞胺、硫酸) AC，探究改性AC對吸附PFBA產生之影響。研究結果顯示，在室溫、PFBA初始濃度為200 ppm、pH值為3.0、磁石長 =2.5 cm、AC劑量 = 8 g/L、600 rpm轉速攪拌250 mL水樣以及包含緩衝溶液(KH2PO4= 2.0 mM；H3BO3= 2.0 mM；KCl= 2.0 mM)的條件下反應六小時後，發現以陽離子界面活性劑的四丁基溴化銨(Tetrabutyl-ammonium bromide, TBAB)改性AC具有最佳吸附效果，去除率為96.72%，吸附量為24.23 mg/g。此外，在各pH值其他陽離子界面活性劑改性AC，均能增加對PFBA之吸附能力，但以聚乙烯亞胺或硫酸改性AC去除效果反而下降。研究吸附過程大多符合擬二階吸附動力模式。AC與改性TBAB之AC在濃度50~500 ppm範圍內吸附過程符合Langmuir等溫吸附模式。 另外，本研究探討競爭吸附行為，測試未改性於最佳吸附效果之AC吸附其他PFCs，比較PFBA、全 氟 己 酸 (Undecafluorohexanoic acid, PFHxA)、全氟丁基磺酸(Perfluorobutanesulfonic acid, PFBS)之競爭吸附行為。經本研究發現在合成廢水下使用未改性AC，PFHxA最容易被吸附，PFBA最不易被吸附；使用改性TBAB之AC，PFBS最容易被吸附，PFBA最不易被吸附。此外，改性TBAB之AC吸附PFBA、PFBS、PFHxA的吸附容量均高於未改性AC。最後研究為了驗證改性TBAB之AC可行性，以實廠廢水為基質與合成廢水進行比較，結果表明實廠廢水中存在有機物以及鹽類可能會與PFCs競爭吸附點位，造成吸附效果比研究合成廢水差。 經本研究證實改性TBAB之AC能比未改性AC更有效吸附短碳鏈之PFBA，但合成廢水未考慮實廠環境因素，因此未來可測試在不同實廠廢水基質下探討改性AC的可行性。

Perfluorinated chemicals (PFCs) are compounds composed of hydrogen substituted by fluorine in the carbon chain to form a highly stable C-F bond and hydrophilic functional groups, so that they are both hydrophilic and hydrophobic. They are widely used in various products and processes. PFCs with long carbon chains were widely used in the industry, but recently they were found to the environmental persistent with bioaccumulative toxicities. Countries began to formulate policies to replace long-chain PFCs with short-chain PFCs, such as perfluorobutanoic acid (PFBA). However, information about the toxicity and bioaccumulation of short-chain PFCs is scarce; studies have shown that short-chain PFCs might also pose human and ecological risks. Therefore, development of effective technologies for removal of short-chain PFCs from environmental media of importance. In this study, activated carbon (AC) was used for removal of PFBA from water through adsorption. First, basic properties of activated carbon, such as zeta potential, specific surface area, constituent elements, and surface structure, were analyzed. Effects of the initial PFBA concentration, pH value, AC dose and stirring rate were then compared to find out the optimal parameters for removal. Further, best experimental parameters were used to modify AC (cationic surfactant, polyvinylideneamine, sulfuric acid) to investigate the effectiveness of the modified ACs on the adsorption removal of PFBA.The results showed that after six hours of reaction at room temperature, initial PFBA concentration of 200 ppm, pH 3.0, AC dose = 8 g/L, stirring speed = 600 rpm ,a stir bar = 2.5 cm, in a 250 mL buffer solution (KH2PO4 = 2.0 mM; H3BO3= 2.0 mM; KCl= 2.0 mM), the best adsorption effect was achieved by AC modified by tetrabutyl-ammonium bromide (TBAB), a cationic surfactant, with a removal rate of 96.72% and an adsorption capacity of 24.23 mg/g. In addition, modifications of AC with other cationic surfactants at various pH values also increased the adsorption removal of PFBA. However, the removal effectiveness of AC modified with polyvinylideneamine or sulfuric acid decreased. The adsorption processes were mostly in accordance with the the pseudo-second-order adsorption kinetics model, and the adsorption processes of the AC and the TBAB-modified AC were in accordance with the Langmuir isothermal adsorption model. This study also investigated the competitive adsorption behavior by testing the adsorption of PFBA, undecafluorohexanoic acid (PFHxA), and perfluorobutanesulfonic acid (PFBS) by the AC. When the AC was used in synthetic wastewater, the extents of adsorption were PFHxA > PFBS > PFBA. When the TBAB-modified AC was used in synthetic wastewater, the extents of adsorption were PFBS> PFHxA > PFBA. In addition, the adsorption capacities of the TBAB-modified AC for PFBA, PFBS, and PFHxA were higher than those of the AC. Finally, in order to verify the applicability of the TBAB-modified AC, a comparison was made between real wastewater and synthetic wastewater. The results showed that the presence of organic matter and salts in the real wastewater might compete with PFCs for the adsorption sites, resulting in poorer adsorption. This study confirmed that the TBAB-modified AC could adsorb PFBA more effectively than the AC. However, its applications for removal of PFBA in real wastewaters need to be further investigated.