以芬頓反應降解乙醯胺酚之去除率與礦化率之機制研究

Abstract

隨著化學分析技術之進步，目前可以檢測到過去所無法檢測的低濃度污染物稱為新興污染物(ECs)，其對人類健康和生態都具有潛在的風險。乙醯胺酚(ACT)為一種解熱、鎮痛和抗炎藥物，被歸類為新興污染物中的藥物和個人護理產品(PPCPs)之一。乙醯胺酚是一種非常受歡迎的非處方用藥，其經過不當處理和處置進而進入自然水體中，對環境與健康造成危害。 本研究的目的在探討Fenton反應降解乙醯胺酚去除率與礦化率之機制，並研究Fe2+活化過硫酸鹽(PDS)在一樣操作條件下之結果，另外也研究了天然有機物質(包括腐殖酸和檸檬酸鹽)存在時對去除效率之影響。實驗過程中透過測量乙醯胺酚濃度、pH值、ORP電位和TOC濃度等實驗參數，瞭解兩種氧化反應之機制。 由於反應過程中添加之氧化劑濃度低，在Fenton反應和Fe2+活化過硫酸鹽降解乙醯胺酚實驗中皆發現pH值沒有顯著的改變。Fenton反應的第一分鐘內，ORP電位顯著地增加。由於目標污染物消耗了氫氧自由基，在兩分鐘後呈現逐漸下降之趨勢。而在Fe2+活化過硫酸鹽的實驗中，不同於Fenton反應，由於硫酸根自由基是經由電子移轉對有機污染物進行降解，ORP電位在前2分鐘內明顯增加，5分鐘後呈略微增加趨勢，並維持在高氧化還原電位。 在本研究中發現，當Fenton反應操作條件為FeSO4：H2O2 = 0.5mM：0.5mM進行試驗時，可以得到最佳之去除率約88％。經由測定時間段剩餘之氫氧自由基濃度，驗證氫氧自由基濃度會隨著過氧化氫濃度增加而上升，而去除率也是隨過氧化氫濃度增加而提高。且在反應5分鐘後，氫氧自由基消耗殆盡，此與去除率變化一致，當反應於5分鐘後去除率不再有提升。 在最佳操作條件下(FeSO4：H2O2 = 0.5mM：0.5mM)，去除率約為88%，但礦化率僅僅只有約9%。在反應初始階段上升，並在反應5分鐘後，趨於緩慢且未再有任何礦化發生，但礦化率並未等於去除率。 而Fe2+活化過硫酸鹽之降解過程中，其中Fe2+與過硫酸鈉的比例保持與Fenton反應試劑相同(即0.5mM：0.5mM)，可觀察到其去除率僅約39％，礦化率僅僅只有約2%。造成此結果的主要原因是，硫酸根自由基與乙醯胺酚或亞鐵離子的反應速率常數相接近。因此硫酸根自由基除了與乙醯胺酚反應外，也會同時與亞鐵離子反應，導致其去除率較Fenton反應來的低。另外由於硫酸根自由基與乙醯胺酚的中間產物的反應速率常數低於系統中其他物種與自由基的反應速率常數，導致礦化率僅只有約2%。 當溶液中存在天然有機物質(腐殖酸與檸檬酸鹽)時，對Fenton反應和Fe2+活化過硫酸鹽反應降解乙醯胺酚之去除率與礦化率並未有顯著的改變。腐植酸在Fenton反應中其生成與消耗之氫氧自由基的量相同，因此對於乙醯胺酚之去除率未有顯著的影響。檸檬酸鹽對於Fenton反應降解乙醯胺酚去除率之影響並不顯著。此外利用天然水體作為背景，進行Fenton反應降解乙醯胺酚，經降解30分鐘後所得之去除率與在純水中進行實驗相似。

With the advancement of chemical analytical technology, many emerging contaminants (ECs) which exhibit potential risks both to human health and ecology have been detected at different levels of concentration. Acetaminophen (ACT), an antipyretic, analgesic and anti-inflammatory drug, is classified as one of the pharmaceutical and personal care products (PPCPs) of ECs. The ACT is also an over-the-counter medicine with a high degree of popularity. Inappropriate treatment and disposal of such medicine cause its entrance to the water body impacting the environment and public health. The purpose of this study is to investigate the Fenton and Fe2+ activated peroxydisulfate (PDS) processes in the degradation of aqueous acetaminophen. Additionally, the removal efficiency at the presence of natural organic matter (NOM) including humic acid and citrate was explored. The experimental parameters of ACT concentrations, pH, ORP and TOC were measured. Since the concentration of the oxidant added during the reaction was low, the pH value with no significant variation in both of the Fenton reaction and the Fe2+ activated PDS degradation of the acetaminophen. The ORP increased significantly during the first minute of the Fenton process and exhibited a gradually-decreasing trend after two minutes due to •OH consumption. Different from the Fenton process, the ORP of Fe2+ activated PDS increased obviously during the first two minutes and displayed a slightly-increasing trend after five minutes since SO4•- reacts with organic compounds primarily through electron transfer. In the present study, when Fenton degradation was carried out with 0.5 mM ferrous sulfate and 0.5 mM hydrogen peroxide, the most effective removal rate was observed at about 88 %. By measuring the remaining concentration of hydroxyl radicals in each period, the remaining concentration of hydroxyl radicals increases as the concentration of hydrogen peroxide increases, this was the same as the removal efficiency of ACT. After 5 minutes of reaction, the hydroxyl radicals were consumed all, which was consistent with the variation in removal efficiency. Under the optimal operating conditions, the removal efficiency was about 88%, but the mineralization of ACT was only about 9%. The variation trend of mineralization of ACT is similar to the removal efficiency, which rising in the initial stage of the reaction, and after 5 minutes, it tended to be slow and no further mineralization occurs, but the mineralization efficiency is not equal to the removal efficiency. For the Fe2+ activated PDS degradation process in which, the ratio of persulfate to ferrous sulfate was maintained the same (i.e., 0.5mM to 0.5mM), the removal efficiency and the mineralization of ACT was only about 39% and 2%, respectively. The main reason for this result is that the reaction rate constant of sulfate radicals between ACT and Fe2+ is similar, therefore the sulfate radicals reacted with ACT and Fe2+, which resulted in a lower removal rate. Furthermore, since the reaction rate constant of the intermediate product of ACT reacts with sulfate radicals is lower than the reaction rate constant of other species and free radicals in the system, the mineralization rate is only about 2%. At the presence of natural organic matters (humic acid and citrate), there was no significant change in the removal efficiency and mineralization of the degradation of ACT by the Fenton and the Fe2+ activated PDS processes. The increase in •OH generation was equal with the •OH consumption by the humic acid presence, resulting in the observed no significant effect on ACT removal in the present study. The effect of citrate in the removal efficiency of the degradation of ACT by the Fenton process is not significant. In addition, the degradation of ACT by the Fenton process using natural water as a background, the removal efficiency of ACT was similar to that in pure water.