芬頓試劑

芬頓試劑(英語:Fenton’s reagent)是一種含有過氧化氫H2O2亞鐵離子 Fe2+(作催化劑,通常是硫酸亞鐵)的溶液。芬頓試劑被用於氧化有機污染物或廢水,是高級氧化技術英語Advanced oxidation process(AOPs)中常用的方法,其對有機廢水中的三氯乙烯(TCE)和四氯乙烯(PCE)等有機物的降解效果明顯。芬頓試劑是由H. J. H. Fenton英語Henry John Horstman Fenton在1894年發現的。[1][2][3]

機理

芬頓試劑的氧化性來源於過氧化氫在Fe2+ 催化下產生的羥基自由基 ·OH [4],首先亞鐵離子被過氧化氫氧化為鐵離子,在此過程中產生羥基自由基和氫氧根離子,而後鐵離子被另一個過氧化氫分子還原為亞鐵離子,產生過氧化氫自由基和氫離子,反應總和為亞鐵離子催化過氧化氫歧化為兩種自由基。

Fe2+ + H2O2 → Fe3+ + HO + OH 1
Fe3+ + H2O2 → Fe2+ + HO2 + H+ 2
2 H2O2 → HO + HO2 + H2O 1+2

此過程產生的自由基有著較高的氧化還原電位,是一種非選擇性強氧化劑[5],芬頓試劑與有機物的氧化反應迅速且放出大量熱,主要氧化產物為二氧化碳。反應(1)的機理由哈伯與韋斯在1932年提出,並囊括於哈伯 - 韋斯反應之中[6]

硫酸亞鐵是最常用的鐵催化劑,對於亞鐵離子氧化後還原重新切入催化循環的機制尚未有共識。Yamazaki等人報道利用順磁共振(ESR)方法以5,5-二甲基吡咯啉-1-氯氧化物(DMPO)作為自由基捕獲劑分析芬頓反應的機理[7],據稱反應是亞鐵離子被過氧化氫氧化成三價鐵後,由Fe2+和Fe3+共同催化產生羥基自由基的過程,且有部分鐵被氧化為Fe(IV)價態。也有研究認為芬頓反應中除了產生羥基自由基外,也有高價鐵中間體產生,並且在有機物的氧化過程中是Fe=O2+起主導作用[8]

影響因素

對於芬頓反應的速率(尤其是對光芬頓反應而言),pH是關鍵因素之一。在低pH值時,Fe2+水合物會與HO發生絡合,降低氧化效率[9],且較低pH值時體系中的多餘質子也會與產生的HO發生反應[10]。而pH值偏高時,鐵離子會形成沉澱,使鐵逐漸從催化體系中被去除,從而降低反應速率[11],並且在鹼性條件時H2O2也會自發地分解[12]。較高的pH值也會降低HO的氧化還原電位,從而降低其氧化效果[13][14]

pH對反應速率的影響
低pH 形成[Fe(H2O)6]2+水合物
OH被過量的H+消耗
高pH 降低OH的氧化還原電位
H2O2在鹼性條件下會自發分解
產生Fe(OH)3沉澱

應用

芬頓試劑被用作污水處理試劑[11][15],芬頓試劑在化學反應中可用作羥基供體或氧化劑,如[16]

芬頓反應在生物化學中有著不同的應用方法,通過在體內細胞中鐵的反應產生或消除自由基,雖然臨床上的用途和重要性尚不明確,但也是活動性感染時避免補鐵的可行方法之一,或是其他任何由鐵介導的感染[19]

拓展閱讀

外部連結

參考文獻

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