光還原法處理添加異丙醇之全氟辛酸水溶液

Abstract

全氟辛酸 (perfluorooctanoic acid, PFOA) 是全氟化物 (perfluorinated compounds, PFCs) 的一種，自1950年代被合成出來後，廣泛使用在聚合物、塗料等工業製程中。由於具有持久性有機汙染物的性質，近年來PFOA在環境中的流佈途徑、及生態毒理機制已被廣泛的研究與重視。因此，本研究嘗試利用光 (紫外線) 還原反應的處理方式，探討在水溶液為無氧狀態下，初始pH值、電子提供者 (異丙醇) 濃度與觸媒 (二氧化鈦) 添加量對PFOA降解的影響。 研究結果顯示，在不添加二氧化鈦觸媒的情況下，不論pH值變化與添加異丙醇濃度為何，PFOA在24小時反應時間的去除率都約在20% 左右，但氟離子產率則隨初始pH值與異丙醇濃度的增加而上升。根據質譜儀的分析結果，未添加異丙醇時PFOA紫外線反應之降解產物以短鏈全氟羧酸為主，而含異丙醇環境下之降解機制則明顯不同，其中m/z=235[C4F9O]-為可能的中間產物。推測未添加異丙醇時，PFOA以直接光解方式氧化成短鏈全氟羧酸，而在含異丙醇的環境下，短鏈全氟羧酸進一步產生自由基連鎖還原反應，因此產生更多的氟離子，尤其在鹼性環境下則有利於還原性更強的自由基產生。 在添加觸媒的光還原反應PFOA研究結果上，0.5 g/L 的二氧化鈦添加量有最佳的PFOA降解效果，然而在鹼性環境下PFOA降解不明顯，推測原因為二氧化鈦與PFOA在鹼性條件下均帶負電，在電性相斥的現象下，二氧化鈦無法發揮作用；在酸性溶液之不同異丙醇濃度環境下，PFOA在24小時反應時間的去除率在57至73% 之間，與未添加二氧化鈦觸媒的條件比較，已明顯提高甚多去除率。根據質譜儀的分析結果，未添加異丙醇時降解產物以短鏈全氟羧酸為主，而含異丙醇環境下之降解機制亦明顯不同，還原產物m/z=395[C7F14HCOO]-為可能的中間產物。推測未添加異丙醇時，PFOA與二氧化鈦產生的電洞產生氧化反應，生成短鏈全氟羧酸；在含異丙醇環境下，異丙醇與電洞反應產生自由基，進一步還原PFOA。綜合上述各反應條件，結果顯示添加0.5 g/L二氧化鈦、2 wt %異丙醇與初始pH3的反應條件下，PFOA有最佳的降解效果，24小時反應時間有73%的降解率。

Perfluorinated compounds (PFCs) were used as an industrial raw material since 1950s, Perfluorooctanoic acid (PFOA) was one of these PFCs. PFOA has properties of persistant organic pollutants (POPs) and the characteristics of environmental fate and bio-toxicology had investigated widely in recent years. Based on above, this study invesgated the photo-reduction (UV) of PFOA under various pH, electron donor (isopropanol, IPA) concentration and amount of catalyst (titanium dioxide, TiO2) in anaerobic aqueous solution. The experimental result showed that decomposition rate of PFOA was about 20% after 24 hours reaction time in all conditions of no titanium dioxide added. Moreover, higher pH value and isopropanol concentraton implied higher fluoride ion yield in aqueous solution. According to the MASS analysis, the formation of short chain perfluorocarboxylic acids (PFCAs) were main byproduct under no IPA condition, and the reaction mechanism was significantly different with IPA aqueous solution. Especially, the intermediate compound of m/z=235 [C4F9O]- was inferred in IPA aqueous solution. In no IPA condition, direct photolysis was suggested the reaction mechanism to form shorter chain PFCAs. However, the byproduct of shorter chain PFCAs would induce into radical chain reaction and yield more fluoride ion in IPA aqueous solution, especially in alkaline condition. In catalytic photo-reduction PFOA, the result exhibited that the 0.5 g/L of titanium dioxide (optimum dose) presented significantly higher PFOA removal rate than no TiO2 condition. The PFOA removal rate could be ranged from 57% to 73% during 24 hours reaction time under various IPA concentrations in acidic condition. However, in alkaline condition PFOA did not display same high removal rate. The reason could refer that electrical repulsion between TiO2 and PFOA caused by same negative surface charges in alkaline condition. Formation of short chain PFCAs were main byproduct under no IPA condition, and the reaction mechanism was also significantly different with IPA aqueous solution. The intermediate compound of m/z=235 [C7F14HCOO]- of reduced PFOA was inferred in IPA aqueous solution. PFOA react with electron hole which generated by UV/TiO2 to produce shoter chain PFCAs under no IPA added. In IPA aqueous solution, IPA reacts with electron hole to incur radical chain reaction and reduced PFOA. Summarizing, the highest PFOA removal efficiency of 73% during 24 hours reaction time was under the condition of 0.5 g/L TiO2, 2 wt% IPA and pH 3 in aqueous solution.