異相光解全氟羧酸化合物之反應機制研究

Heterogeneous Photocatalytic Decomposition of Perfluorocarboxylic Acids (PFCAs): Mechanism and Efficiency Studies

檀香(Sri Chandana Panchangam)

指導教授 ： 林正芳

國立臺灣大學/工學院/環境工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.00948

摘要

含氟有機物(PFCs)具有持久性及生物累積性為近年來重要的環境議題，本研究探討異相光催化及結合超音波對分解PFCs之效能。光催化氧化分解全氟辛烷磺酸化合物(PFOS)之反應性不佳，因此篩選全氟羧酸化合物(PFCAs)之全氟辛酸(PFOA)作為研究標的物質，進行各種背景之效能及機制研究。以二氧化鈦作為酸性水溶液之催化劑對三種全氟羧酸化合物(PFCAs)：全氟辛酸(PFOA)、全氟壬酸(PFNA)及全氟癸酸(PFDA)進行異相光催化分解，全氟羧酸化合物被分解為碳鏈較短的化合物及氟離子，以紫外光照射產生激發態二氧化鈦光穴為全氟羧酸化合物氧化作用的反應址，在酸性水溶液之介質提供此光穴可促進全氟羧酸化合物之分解，異相光解使全氟羧酸化合物在七小時內可達到高於99% 的分解及38%的完全礦化，全氟壬酸及全氟癸酸分別可達到54%及38%的去氟效率，其他全氟羧酸化合物可達到99%的分解，高氯酸鹽可促進全氟羧酸化合物之離子化而提升分解效率。
研究進一步結合光催化及超音波兩階段之分解全氟辛酸，首先以光催化結合30分鐘的超音波裂解，此為超音波輔助光催化作用(sonication-assisted photocatalysis)，另外以光催化及超音波裂解同時使用，為超音波光催化作用(sonophotocatalysis)。常溫常壓及接近環境pH值之狀態下，以商用及自製溶膠-凝膠二氧化鈦分別進行超音波輔助光催化分解全氟辛酸，此程序有效地將全氟辛酸分解為氟離子及較短鏈之全氟羧酸化合物如全氟庚酸(PFHpA)、全氟己酸(PFHxA) 、全氟丙酸(PFPA)及三氟乙酸(TFA)，效率達64%，以溶膠-凝膠二氧化鈦作為催化劑均較商用二氧化鈦效率為高，溶膠-凝膠二氧化鈦表面積高於商用二氧化鈦三倍，pH值在不同的控制條件下影響全氟辛酸分解及光催化，氟烷自由基可能經由二氧化鈦介質於pH 5及10之光催化反應中超氧及羥基自由基之生成而被氧化。超音波輔助光催化反應之超音波裂解為輔助而非主要作用機制，其經由物理擴散及質量傳輸活化二氧化鈦表面之反應址並促進光催化反應。超音波光催化為分解全氟辛酸及全氟癸酸之可行處理技術，有效分解效率可達到58%，反應動力常數隨著全氟辛酸之初始濃度增加而減小。

關鍵字

異相光解 ； 氟辛酸 ； 全氟羧酸化合物 ； 分解 ； 反應機制

並列摘要

The increasing worldwide contamination of environmental matrices with numerous industrial chemicals poses a significant threat to both aquatic and human life. Fluorine containing organics like perfluorochemicals (PFCs) are of significant environmental concern due to their persistent and bioaccumulative characteristics. The persistence of PFCs is due to the greater stability offered by the extremely strong carbon-fluorine bonds. PFCs resist several conventional treatment processes in the drinking water and wastewater treatment plants and demands more refined treatment techniques.
Advanced oxidation processes (AOPs) such as sonication, photocatalysis, ozonation, peroxone etc. are promising treatment technologies that have shown to be efficient in decomposing several organic contaminants. Before realizing the full advent of these processes towards the decomposition of PFCs, the complete mechanism of decomposition, treatment parameters like pH, and the energy requirements are need to be critically studied.
This study explores the capabilities of heterogeneous photocatalysis and combination of sonication and photocatalysis towards the decomposition of PFCs. Perfluorooctane sulfonate (PFOS) was found to be non-susceptible to the oxidative decomposition by photocatalysis and therefore further efficiency and mechanism studies were carried out on perfluorocarboxylic acids (PFCAs). Among PFCAs, perfluorooctanoic acid (PFOA) was chosen as a model compound for all the background and mechanism studies. Loss of PFOA and PFOS was not found due volatilization and adsorption onto glass or TiO2 but only a minor loss was observed during direct photolysis. Efficient decomposition of three important PFCAs such as PFOA, perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA) is studied by heterogeneous photocatalysis with TiO2 as a photocatalyst in acidic aqueous solutions. The PFCAs were decomposed into shorter carbon chain length PFCAs and fluoride ions. Photoholes of excited TiO2 generated upon UV-irradiation are found to be the oxidation sites for PFCAs. Therefore, creation and sustenance of these photoholes in the acidic aqueous medium has enhanced the decomposition of PFCAs. Heterogeneous photocatalytic treatment achieved more than 99% decomposition and 38% complete mineralization of PFOA in 7 h. The decomposition of other PFCAs was as high as 99% with a defluorination efficiency of 38% for PFDA and 54% for PFNA. The presence of perchloric acid was found to enhance the decomposition by facilitating the ionization of PFCAs. The oxygen present in the medium served both as an oxidant of perfluoroalkyl radicals and an electron acceptor. The mechanistic details of PFCA decomposition and their corresponding mineralization are elaborated. The PFCAs photocatalytic decomposition involves predominantly four steps: ionization, electron transfer, decarboxylation and oxidation. Initially PFCAs are ionized in the water and the corresponding anions will donate electron to the excited TiO2 forming a perfluoroperoxy radical. This radical undergoes immediate decarboxylation transforming to perfluoroalkyl radical. The perfluoroalkyl radicals thus generated are oxidized by either superoxide or hydroxyl radicals resulting in PFCA with one less carbon releasing two fluorides. This decomposition mechanism follows a cyclic order until all the fluoride ions are stripped off from the perfluorinated carbons. Mean while the oxygen being supplied to the system will act as electron acceptor and keeps the catalytic cycle of TiO2 active.
Further, decomposition of PFOA was studied by a combination of sonication and photocatalysis in two different strategies. At first, photocatalysis was coupled with half-an-hour of sonication which is called as sonication-assisted photocatalysis. Another strategic way of combination was concurrent use of sonication with photocatalysis called sonophotocatalysis. Sonication-assisted photocatalysis with commercial TiO2 (RdH) and home-made TiO2 (sol-gel) at ambient temperature, pressure and near neutral pH was studied for the decomposition of PFOA. The process was efficient to decompose PFOA into fluoride ions and to several perfluorinated carboxylic acids with a shorter carbon chain length such as perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), perfluoropropanoic acid (PFPA), and trifluoroacetic acid (TFA). The efficiency of sonication-assisted photocatalysis was found to be 64%. In all the cases, higher efficiencies were obtained when sol-gel TiO2 was used as a photocatalyst than the commercial RdH TiO2 catalyst. The specific surface area is three times higher for sol-gel TiO2 than commercial RdH TiO2 and appears to be the probable reason for the observed differences in the corresponding efficiencies. It is also interesting to note that pH plays a determining role in the decomposition of PFOA and correspondingly photocatalyses were carried out under different controlled pH. Perfluoroalkyl radicals are presumably oxidized by superoxide and hydroxyl radicals generated during the TiO2-mediated photocatalysis at pH 4 and 10, respectively. The role of sonication in sonication-assisted photocatalysis was construed to be an aid to photocatalysis than a tool itself. Sonication enhances photocatalysis through physical dispersion of TiO2 and eases mass transfer which keeps on rejuvenating the TiO2 surface.
Sonophotocatalysis was shown to be a promising treatment technology to decompose PFOA and PFDA. Significant synergistic efficiency of about 58% was observed when sonication is combined with photocatalysis. The kinetic rate constant was found to decrease with increase in PFOA initial concentration. The energy required by sonophotocatalysis was also three times less than the sum of energy required by individual processes. From energy requirement comparisons and efficiency wise, heterogeneous photocatalysis and combinatorial process (sonication-assisted and sono- photocatalysis) emerge to be viable treatment techniques as demonstrated in this study.

並列關鍵字

Heterogeneous photocatalysis ； perfluorooctanoic acid ； perfluorocarboxylic acids ； decomposition ； mechanism

參考文獻

3M. 2000. Sulfonated perfluorochemicals in the environment: sources, dispersion, fate and effects.

Abdullah M, Low G K C, Matthews R W. 1990. Effects of common inorganic anions on rates of photocatalytic oxidation of organic carbon over illuminated titanium dioxide. J. Phys. Chem. 94, 6820-6825.

Anipsitakis G P, Dionysiou D D. 2004. Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol, 38, 3705-3712.

Azrague K, Bonnefille E, Pradines V, Pimienta v, Oliveros E, Maurette M T, Benoit-Marquie F. 2005. Hydrogen peroxide evolution during V-UV photolysis of water. Photochem. Photobiol. Sci. 4, 406-408.

Becker A M, Gerstmann S, Frank H. 2008a. Perfluorooctane surfactants in wastewaters, the major source of river pollution. Chemosphere. 72, 115-121.