以過碘酸鹽結合超音波系統提升全氟辛酸降解之研究

Abstract

本研究利用超音波化學並添加過碘酸鹽針對全氟辛酸之降解進行探討。過碘酸鹽可透過超音波產生的空化作用發生熱裂解反應，而釋放出具有強氧化性的自由基，如碘酸根自由基（IO3．）、過碘酸根自由基（IO4．）及氫氧自由基（OH．）等，這些氧化性物質可促進全氟辛酸之氧化分解。本研究探討不同操作參數對於超音波降解全氟辛酸的效應影響，包括過碘酸鹽濃度效應、全氟辛酸溶液初始pH值效應、飽和氣體、鹽類添加以及自由基抑制劑的影響。 全氟辛酸在單一超音波系統反應下，當pH值為4，經過120分鐘其去除率僅有26%，但加入4.5 mM的過碘酸鹽其效率可達到62%。而隨著過碘酸鉀濃度的增加，全氟辛酸之降解率也隨之提高，當過碘酸鹽濃度為45 mM時，其降解率與脫氟率可高達93%以上，且全氟辛酸之降解動力符合擬一階反應動力模式，反應速率常數（kobs）為0.0222 min-1。此外，於酸性條件（pH=4）下之反應速率僅為鹼性條件（pH=11）下之1.05倍，因此無論在酸性或鹼性的環境下，其降解全氟辛酸之差異性並不明顯。比較不同飽和氣體對全氟辛酸降解的影響顯示，其反應速率大小為 kN2 > knon-aeration > kair > kO2，因此採用氮氣作為飽和氣體有利於全氟辛酸之降解，其降解率為70%左右；而氧氣會抑制活性自由基的產生，影響超音波結合過碘酸鹽系統降解全氟辛酸之效率。添加鹽類可改變全氟辛酸之物化特性，如蒸氣壓、表面張力等，可增強空化效應使更多的渦泡崩解。此外，當鹽類加入水溶液時，鹽離子會造成鹽析效應（salting-out effect )，促使全氟辛酸向氣液界面移動，與自由基反應發生。結果顯示，其反應速率之大小為 kKBr > kNaCl > kNa2SO4，且隨著溴離子濃度的增加，有助於促進全氟辛酸之降解，當濃度為35 mM時，其分解率與脫氟率可高達87%與89%。以第三丁醇（C4H9OH）作為自由基抑制劑，其會消耗碘酸根自由基（IO3．）與氫氧自由基（OH．），對全氟辛酸之去除有顯著的抑制作用。

In this study, the introduction of periodate (IO4－) on sonochemical decomposition of perfluorooctanoic acid (PFOA) in aqueous solution using ultrasonic (US) irradiation was investigated. This enhancement of PFOA decomposition is due to the degradation of IO4－ by pyrolytic cleavage in cavitation bubbles, which conducts to the release of oxidizing free radicals such as IO3．, IO4．and OH．that can accelerate the decomposition of PFOA. The effects of various parameters such as the concentration of IO4－, the initial pH of the solution, saturated gases, salts and free radicals inhibitors adding in US/IO4－ system have been investigated. The decomposition efficiency of PFOA is only 26% using ultrasonic (US) irradiation at pH 4.0 in 120 min but can be enhanced to 62% with adding IO4－ dosage of 4.5mM. Increasing of the concentration of IO4－will result in increase of decomposition and defluorination efficiency of PFOA which reached as high as 93% in the presence of 45mM IO4－. The PFOA decomposition reactions can be well simulated with pseudo first-order kinetics and the rate constant (kobs) is 0.0222 min-1. In addition, the decomposition rate under acidic condition (pH=4) is about 1.05 times faster than under alkaline condition (pH=11). There are insignificant differences between acidic and alkaline conditions on the decomposition of PFOA. Comparison on decomposition of PFOA with a variety of saturated gases shows that the reaction rate is kN2 > knon-aeration > kair > kO2. It would be beneficial to use nitrogen on enhancement of PFOA decomposition because of the decomposition efficiency 70%. However, the reaction of the US/ IO4－ system may be inhibited or terminated by oxygen due to inhibiting the generation of the free radicals. With dissolved salts as additives can alter the physicochemical properties such as vapor pressure and surface tension of PFOA facilitating generation of more violent collapse of the cavities. Moreover, ions present during sonication aqueous solution may induce an occurrence called the salting-out effect. This phenomenon pushes PFOA toward the bubble-bulk solution interface which is the main reaction sites. The results showed that the reaction rate is kKBr > kNaCl > kNa2SO4 and bromide ions exhibited a promoting effect to enhance the decomposition of PFOA with the increasing of the concentration. When the concentration of bromide ions is 35 mM, the decomposition and defluorination efficiency were 87% and 89%. The tert-butyl alcohol as the free radical scavengers can easily scavenge active species, such as IO3．and OH．, having strong oxidizing properties which will markedly inhibit the removal of PFOA in aqueous solution.