以碳酸自由基破壞去除水中全氟辛酸污染物之研究

Abstract

Perfluorooctanoic acid (PFOA) 因為其高生物累積性與化學穩定性的特性，被視為一持久性(persist)有機污染染物並常見於工業廢水之中。因為其具有高能量的C-F鍵結， PFOA廣泛地應用於含氟聚合物、油漆塗料、化裝品、紡織、造紙等領域及半導體工業、電子、電鍍及蝕刻等製程。在美國環保署發現在美國人民中之血液及環境中皆有低濃度的全氟辛酸，並且存於人體中的時間相當長。並在動物試驗的究中發現此物質對於動物之生長有很大的影響，其會促使老鼠肝臟引發腫瘤，導致肝臟癌、胰臟癌及乳房癌，致癌程度可能會遠比原預估的嚴重。 如何有效地將其從環境中去除目前仍是一個棘手的問題。PFOA具有優異的穩定性、低表面張力和疏水疏油的特性，近年來發展出PFOA處理方法並不多見，大部分降解PFOA的方法條件皆伴隨著高溫、高壓等高耗能的極端狀況，例如紫外光(UV) 光化學法、超音波(sonochemical)或高熱(Thermal)處理法。 在本論文中主要分別分為光降解(photodecomposition) 與超音波降解法(sonodecompostion)兩大部分主軸。在光降解部分實驗設計包括了背景實驗、PFOA水溶液於UV光照射下添加H2O2,、 NaHCO3在不同的反應溫度、初始濃度、pH等實驗條件下測試。而在超音波降解法的實驗亦同於不同的反應實驗條件下探討PFOA的去除率與反應機制等結果。 在254nm、400W的UV光源照射下，以碳酸自由基(CO3•–)氧化水中PFOA實驗。碳酸自由基是一種強氧化劑具有高選擇性，而被發現能夠有效去除水中PFOA污染物。在12小時的UV光源照射下反應時間後，碳酸自由基可以100％去水中的PFOA污染物，相對比較於只有使用UV光源照射下反應去除率只有52.1％的去除率。此外，最適合的反應條件是在pH=8.8的微鹼性下以40mM的劑量 NaHCO3進行，並在反應過程中發現了短鍊的perfluorinated carboxylic acids (PFCs)為中間產物，與氟離子為最終產物，佐證了整個PFOA去除降解反應的進行。 在超音波降解部分實驗設計包括了PFOA濃度、添加NaHCO3在不同的初始濃度、pH與通以飽和氮氣(N2)等實驗條件下測試探討PFOA的去除率與反應機制等結果。實驗結果發現在150W, 40KHz的超音波 (Ultrasound) 下，PFOA在添加NaHCO3後可以完全去除水中的PFOA 污染物，反應時間可以大幅縮短到4小時，並發覺在通以氮氣的反應反而會使去除率增加。最佳的反應條件是是在pH=8.65的微鹼性下以30mM的劑量 NaHCO3並通以氮氣進行，但整個反應的過程並無發現有短鍊的PFCs中間產物，只有最終產物氟離子出現。 碳酸自由基(CO3•–)是一高選擇性的氧化劑，可以有效的去除PFOA污染物，在一般的工業廢水中可以大量的用運，並且搭配去除其他類似水中不易降解的污染物，具有實廠廢水的運用性。

Perfluorooctanoic acid (PFOA) is a persisted organic pollutant and a common contaminant in wastewater because of its widespread occurrence in the environment and its ability to bioaccumulate. The extremely strong carbon–fluorine bonds in their structure result in the high resistance of PFCs and that characteristic makes it an important constituent for various commercial and industrial applications such as surface-treatment, surfactant, and fire retardant. Recent studies indicate that PFOA is toxic and carcinogenic to animals such as rats, fishes, monkeys, and even humans. Technologies for PFOA treatment have recently been developed. Some processes in critical conditions have been developed to decompose PFOA, such as photochemical, sonochemical treatment, thermal treatment, ultra violet (UV). In this study, the experiments of photodecomposition and sonodecompostion were carried out separately. The photodecomposition experiments were designed including background experiments, PFOA in aqueous carbonate solution with UV irradiation experiments were carried out by adding H2O2, NaHCO3 in different conditions of temperature, concentration, pH... The sonodecomposition experiments were designed simultaneously to identify the PFOA decomposition efficiencies under different conditions. PFOA was decomposed in water using carbonate radical anions (CO3–●) under 254 nm UV irradiation at 400W. CO3–● is a strong oxidizing and selective radical, however it works efficiently in decomposing PFOA solution. In this study, the results showed that PFOA was decomposed 100% after 12 h by using a combination of UV irradiation and CO3–●, while under only UV irradiation, 52.1% of PFOA was decomposed. In addition, the decomposition of PFOA with CO3–● under UV irradiation was more favorable in a slightly alkaline (pH = 8.8) solution and sodium hydrogen carbonate (NaHCO3) 40 mM. Moreover, the intermediates included the shorter-chain perfluorinated carboxylic acids and fluorine ions. PFOA decomposition by sonochemical treatment was investigated to determine the effects of NaHCO3 concentrations, N2 saturation, and pH on decomposition rates and defluorination efficiencies. The results showed that PFOA decomposition by ultrasound treatment only (150 W, 40 kHz), with or without saturated N2, was < 25 % after 4 h reaction. The extent and rate of PFOA decomposition and defluorination efficiencies of PFOA, however, greatly increased with the addition of carbonate radical reagents. PFOA was completely decomposed after 4 h of sonochemical treatment with a carbonate radical oxidant and saturated N2. Without saturated N2, PFOA was also decomposed to a high (98.81%) degree. The highest PFOA decomposition and defluorination efficiencies occurred in N2 saturated solution containing an initial NaHCO3 concentration of 30 mM. Sonodecomposition of PFOA with CO3•– radical was most favorable in a slightly alkaline environment (pH = 8.65). There isn’t any shorter-chain perfluorinated carboxylic acids detected except fluorine ions in final reaction solution. CO3•– radical is a selective radical, and it works efficiently in decomposing PFOA. The using carbonate radical has the potential of removing PFOA and other similar pollutants in water and wastewater effluents.