以超臨界流體萃取技術萃取都市下水污泥中鄰苯二甲酸-2-乙基己酯 (DEHP) 之研究

Abstract

鄰苯二甲酸-2-乙基己基酯 (Di-(2-ethylhexyl) phthalate, DEHP) 為聚氯乙烯 (PVC) 塑膠加工過程中之增塑劑 (plasticizer)，其用途與用量皆相當廣泛；DEHP 對於生物之影響以慢毒性為主要機制，對於水棲生物具有強烈之富集 (bioconcentration) 作用，極易分配 (partitioning) 至生物體內之脂肪層累積，同時對於鼠類與大部份的哺乳動物具有致突變性與致肝臟、腎臟毒害性 (heptatotoxic, nephrotoxic)，對人體亦具有致癌的潛勢；塑膠類製品之廣泛製造、使用，導致 DEHP 以各種不同之途徑進入污水廠並在污泥中累積，進一步降低污泥再利用之適用性與安全性。因此，污泥樣品中 DEHP 之萃取與鑑定實為污泥再利用與資源化 (農地利用) 之首要工作。 由於超臨界流體 (supercritical fluid) 具有低黏滯性、低表面張力、高擴散性、高溶解度與不具毒性等優點，近年來已成為取代傳統溶劑萃取程序之高效率技術，對於環境樣品之萃取與相關復育技術之應用更是發展迅速，並已成為分析前樣品製備之先驅。因此，本研究利用超臨界流體萃取法 (supercritical fluid extraction, SFE) 具備之優點，針對都市下水污泥中之 DEHP 進行萃取並建立此程序之最佳萃取參數，以期能在最短之時間內完成污泥中危害性有機物之萃取與鑑定並可達到最高之萃取效率，同時利用本研究建立之最佳化萃取參數 (optimal extraction parameters)，進一步做為日後探討污泥餅中 DEHP 分解行為之優良技術與參考準則。 本研究使用之超臨界流體為高純度之二氧化碳 (CO2)，並利用正己烷做為收集溶劑、樣品收集之型式為固相捕集管柱，分別探討 DEHP 在不同基質效應 (matrix effects) 環境下之參數變化，使用之基質種類為不含 DEHP 之石英砂、畜產污泥，最後則為含有 DEHP 之實廠都市污泥；主要探討之參數為萃取壓力、溫度、靜態與動態萃取時間、流體流速、樣品量、樣品含水率與修正劑 (modifier) 之使用或種類等。研究結果顯示，污泥量 0.2 g、萃取壓力 202 bar、萃取溫度 80oC、流體流速 2.0 mL/min 並添加 0.5 mL 之 methanol 做為修正劑、靜態萃取與動態萃取時間分別為 1 與 20 min 為本程序之最佳操作值，此時污泥中被萃出之 DEHP 含量為 142.86 mg/kg，同時其相對標準偏差值 (RSD) 為 0.52%；另一方面，利用傳統 24 h 索氏萃取程序獲得之 DEHP 濃度為 124.64 mg/kg、RSD 為 1.23%，因此就萃取效率與再現性而言，皆以本研究獲得之 SFE 參數較佳。 除了建立 SFE 最佳參數之外，本研究亦根據此階段建立之最佳值，針對採樣期間內 (87 年 7 月至 88 年 2 月) 收集之污泥餅進行萃取與分析，研究結果顯示民生廠污泥餅中之 DEHP 濃度與含量並無任何下降之跡象，相反的其濃度值於過去八個月中卻出現上升之趨勢，顯示 DEHP 無間斷的存在於污泥中，同時污水與污泥處理單元可能並無法有效的將之去除，因此可於污泥餅中偵測出大量 DEHP 之存在，其濃度變化範圍為 71.69 - 142.86 mg/kg，此結果明顯大於國外之安全限制濃度 (50 mg/kg)；另一方面，根據本研究進行 DEHP 降解之試驗結果發現，污泥中 DEHP 於 189 天之試驗期間內，其濃度變化趨勢相當緩慢、且最終仍有將近 70% 殘留在污泥餅中，由此結果可清楚得知 DEHP 相當穩定且不易被分解，即使經過如此長時間之試驗期間，其濃度亦無法降低至法規之限制範圍。 重金屬的部分則除了 Cd 之濃度略微偏高之外 (14.2 mg/kg)，其餘皆在國外污泥農地利用之限制值以下，因此若以危害性有機物與重金屬含量進行此污泥農地利用之適用性評估，則可明顯發現 DEHP 對於農地生態之影響可能大於重金屬。本研究結果顯示民生廠之污泥餅中確實含有危害性有機物 (DEHP)，欲以農地利用的方式進行污泥資源化與再利用之途徑時，則需詳細進行風險評估等相關研究，以期能降低農地環境之負荷並達到最有效的資源化目標，日後對於 SFE 應用於污泥農地利用前之相關研究與推廣，其成果亦指日可期。

Di-(2-ethylhexyl) phthalate (DEHP), a xenobiotic organic compound is found in the municipal sewage sludge. It has been identified that it is hepatocarcinigenic, nephrotoxic to rats and mice and toxic to mammalians. Since DEHP is a mainly used material to manufacture the polyvinyl chloride (PVC) derivatives, it becomes a ubiquitous pollutant and largely accumulates in the sewage sludge. DEHP in the municipal sewage sludge causes risks to the ecosystems when the sludge is disposed on the environment and utilized for the agriculture purposes. Thus, it is necessary to extract and determine the presence of DEHP in the municipal sewage sludge in order to enhance the feasibility and safety of sludge before agricultural application. Since supercritical fluid is low viscosity, low surface tensions, high diffusibility, high solubility and non-toxic, it is developed and established as high efficiency sample preparation technique to replace the conventional solvent extraction methods. In the present study, supercritical fluid extraction (SFE) method used to determine DEHP from the sewage sludge, and the extraction parameters were optimized for efficiently determine the concentration of DEHP presented in the sewage sludge. The matrix used in this study were non DEHP contained quartz sand, Stock Raising sludge and real municipal sewage sludge separately in order to discuss the differences of extraction parameters which should be adjusted during extraction process. High purity carbon dioxide is used as extraction fluid, and the analyte collection apparatus is solid-phase trap column. After extraction step completed, n-hexane is used as rinse solvent. The optimal extraction condition for the present study was the extraction pressure of 202 bar, temperature of 80oC, fluid flow rate of 2.0 mL/min and the static and dynamic extraction times were 1 and 20 min respectively with 0.5 mL methanol as the modifier. The amount of DEHP presented in the Min-Shen municipal sludge was 142.86 mg/kg and the RSD value was 0.52%. Comparison of the results with Soxhlet extraction method shows the extraction efficiency and the reproducibility of SFE are both better than Soxhlet method. In addition to establish the optimal operation parameters of SFE, the aims of this study also focused on the concentration variation of DEHP during the sampling periods. The results showed the amount of DEHP could be quantified largely in the final sludge cake, and the concentration range was 71.69 - 142.86 mg/kg. It showed that neither the wastewater nor the sludge treatment units could possible remove DEHP efficiently. It is noteworthy that the concentrations of DEHP decreased very slowly during 189 days of decaying experiment, and remained 70% in the testing sludge after 189 days and still over the restricted values applied by Danish Environment Protection Agency. The results revealed that DEHP presented in the sewage sludge was stable and difficult to degrade by microorganisms. Besides cadmium, the concentrations of heavy metals were all below the restricted values. Therefore, comparisons of the impact and the effect between the DEHP and heavy metals, it is clear that DEHP presented in the sewage sludge might caused higher risks than heavy metals. If this sludge will be reused directly for agricultural purpose, a detailed study of risk assessment is necessary to minimize the impact and to reduce the loading to the ecosystem, further achieved the goals of waste sludge minimization and recycling.