超音波促進光催化氧化法去除水中全氟辛酸

Abstract

本研究利用結合超音波與UV/TiO2光觸媒光催化氧化法(sonophotocatalysis)降解全氟辛酸(PFOA)，分為串聯(sequential)與同時(simultaneous)操作，以探討PFOA的去除效率。 自行合成的Sol-gel TiO2經BET分析得比表面積為38.54 m2/g，TEM圖顯示粒徑約為30至50 nm，XRD分析礦物晶型為Anatase與Rutile共存的結構，能隙為3.03 eV。 僅用光催化法之PFOA去除效率為22%，零階速率常數為0.0292 (hr-1)，當結合超音波與光催化系統於同時操作時，超音波震盪可使TiO2保持均勻分散，減少發生凝聚現象(agglomerate)，並使表面反應位置更新(refreshed)，以增加TiO2與反應物的質量傳輸效率，產生協同效應(synergistic effect)有效使PFOA去除效率提升至49%，擬一階速率常數為0.0866 (hr-1)，串聯操作則無提升效率，而將TiO2改為Sol-gel TiO2後，去除效率更提升至64%，擬一階速率常數為0.1309 (hr-1)。在不同pH環境時，去除效率為：鹼性(pH=10) > 酸性(pH=4) > 中性(pH=7)，因鹼性環境下有較多高反應性氫氧自由基(．OH)與PFOA進行降解反應。 PFOA可有效被降解成毒性較低的全氟庚酸(PFHpA)、全氟己酸(PFHxA)與氟離子之中間產物，減少PFOA在環境中的危害性。

The objective is to utilize ultrasonic irradiation assisted photo-catalysis which divided into sequential operation and simultaneous operation for perfluorooctanoic acid (PFOA) removal in this study. The process is called “sonophotocatalysis.” The specific surface area of synthesized sol-gel TiO2 was 38.54 (m2/g) by BET analysis and the particle diameter was about 30 to 50 nm by TEM. Its crystal structure was a mixture of anatase and rutile showed by XRD analysis. Also, it had 3.03 eV for energy band gap. The removal efficiency of PFOA was 22% and rate constant of zero-order equation, k, was 0.0292 (hr-1) for only photo-catalysis. It was increased to 49% and 0.0866 (hr-1) of pseudo first-order rate constant, k, due to the synergistic effect in simultaneous operation. Because ultrasonic irradiation can disperse TiO2 more evenly, prevent the agglomerate of TiO2, and keep the reactive surface of TiO2 being refreshed to increase mass transfer between TiO2 and reactant. However, the removal efficiency of PFOA was increased to 64% and 0.1309 (hr-1) of k by using sol-gel TiO2. But sequential operation was useless. In addition, the removal efficiency was better at pH=10 > pH=4 > pH=7 because there are much hydroxide radicals (．OH) to react with PFOA at pH=10. PFOA was also efficient to degrade to intermediates, such as PFHpA, PFHxA and fluoride ion to be harmfulless.