利用光 Fenton 法降解與去除水中加保扶毒性之研究

Abstract

加保扶為氨基甲酸鹽類之殺蟲劑，易從施作場址滲出進而污染地表水與地下水。由於加保扶之毒性不利於生物處理，僅高級氧化法可以在短時間內降解加保扶。因此，高級氧化法中之光化學氧化法 (例如光Fenton法)，被視為處理加保扶最具有潛力技術之一。本研究中將探討UV照射時間、初始加保扶濃度、pH值以及過氧化氫合三價鐵之加藥量等操作因子，對於光 Fenton 法處理受農藥加保扶污染水體之影響，並由加保扶以及溶解性有機碳 (dissolved organic carbon, DOC) 去除率評估光 Fenton 法之處理效率。實驗結果顯示除了初始加保扶濃度外，其餘操作因子對於加保扶以及 DOC 去除效率皆有正面影響。加保扶濃度 50 mg L-1 條件下，當過氧化氫和三價鐵加藥量分別大於4 mg L-1 min-1 以及 35 mg L-1 時，加保扶與 DOC 去除率並沒有顯著的提升。此外，光 Fenton 法降解與礦化加保扶之最佳pH 值為3。在 pH 值 3 以及過氧化氫和三價鐵加藥量分別控制在 4 mg L-1 min-1與 35 mg L-1 之條件下，濃度 50 mg L-1 之加保扶可以在30分鐘之內完全降解，同時反應 120 分鐘後礦化率可達 93%。 本研究主要目的為藉由二因子中央合成設計與反應曲面法，找出三價鐵 (1-100 mg L-1) 與過氧化氫 (1-10 mg L-1 min-1) 最適之加藥量，同時建立不同反應時間下，加保扶和 DOC 去除率以及 BOD5/DOC 比例等反應變數之二階多項式。實驗結果顯示，DOC 之去除率與系統中過氧化氫加藥量高度相關，如欲在短時間內達到較高之礦化率，則須添加較多之過氧化氫。由加保扶去除率與 BOD5/DOC 比例實驗結果分析發現，降解 100 mg L-1 加保扶最適之三價鐵與過氧化氫加藥量分別為 59 mg L-1 以及 5.4 mg L-1 min-1。在此操作條件下，加保扶可在 30 分鐘完全去除，同時反應 60 分鐘後 DOC 去除率約41%，過氧化氫與三價鐵之比例介於 0.75 到 9.04 之間。另外，Microtox® 毒性試驗與BOD5/COD 比例實驗結果顯示，在最適加藥條件下，45 分鐘為結合光 Fenton 法與生物處理之最適反應時間，暴露 15 分鐘之 Microtox® 毒性單位可下降至 10，且 BOD5/DOC 比例由初始 0 上升至 0.38。毒性單位之下降與生物降解性之提升，表示此時出流水水質為生物較易分解之水質特性。此外，平均氧化狀態(average oxidation state, AOS) 以及碳氧化狀態 (carbon oxidation state, COS) 兩指標分別由 -0.03 增加至 0.42 與 1.82，顯示在光 Fenton 法處理加保扶過程中，經氧化產生有機之中間產物。而中間產物分析結果顯示，有五種中間產物在光 Fenton 降解加保扶程序中產生，推論在氨基甲酸鹽之 C-O 鍵與呋喃環之 3-C 位置皆被氧化。綜合以上結果，對於受加保扶污染水體之處理，光 Fenton 法為結合生物處理程序極具潛力之前處理單元。

Carbofuran, a pesticide belong to the category of carbamate derivative, will contaminate surface and groundwater through leaching from applied site. Since carbofuran exhibits a special biorefractory character and require longer biodegradation time, its complete degradation in a shorter time could only be achieved by advanced oxidation processes. Therefore, photochemical oxidation i.e. photo-Fenton process has been considered as a promising technology for the degradation of carbofuran. The effects of irradiation time, carbofuran concentration, pH, H2O2 dosage rate and Fe3+ dosage on photo-Fenton treatment of carbofuran were evaluated in this study. The treatment efficiency was estimated based on the reductions of carbofuran concentration and dissolved organic carbon (DOC). The outcomes demonstrate that all the parameters had positive effect on carbofuran degradation except carbofuran concentration. With carbofuran concentration of 50 mg L-1, no significant improvement in the carbofuran and DOC removals was observed when the H2O2 dosage rate and Fe3+ dosage were increased beyond 4 mg L-1 min-1 and 35 mg L-1, respectively. Furthermore, the value of pH 3 was found to be the best suitable for carbofuran degradation and mineralization under the photo-Fenton process. At pH value of 3, carbofuran (50 mg L-1) was completely degraded within 30 min and 93% DOC removal was achieved after 120 min reaction under H2O2 dosage rate 4 mg L-1 min-1 and Fe3+ dosage 35 mg L-1. A two factor central composite design along with response surface methodology was employed to estimate the favorable H2O2 dosage rate (1-10 mg L-1 min-1) and Fe3+ dosage (1-100 mg L-1) in the photo-Fenton treatment of carbofuran and to develop the second-order polynomial equations in terms of carbofuran and DOC removals and BOD5/DOC ratio as responses with different reaction times. DOC removal is found to be highly related with H2O2 concentration in the system. Therefore, high loading of H2O2 dosage rate must be employed to achieve high carbofuran mineralization within a short reaction time. Based on the results of carbofuran removal and BOD5/DOC ratio, H2O2 dosage rate of 5.4 mg L-1 min-1 and Fe3+ dosage of 59 mg L-1 were found to be the favorable reagent dosages for carbofuran degradation. Carbofuran (100 mg L-1) was completely removed within 30 min reaction and 41% DOC removal was reached at these conditions. Moreover, the H2O2/Fe3+ ratio was found to be varied from 0.75 to 9.04 within 60 min reaction. Based on the results of Microtox® test and BOD5/COD ratio at the favorable reagent dosages, a reaction of 45 min was chosen as the appropriate time for coupling the photo-Fenton process with biological treatment. Under these conditions, 96% carbofuran removal was achieved, the toxicity unit (TU) measured with 15 min exposure was decreased to 10 and the biodegradability evaluated by BOD5/COD ratio was increased from 0 to 0.38. The reduction of TU and increase of biodegradability represented an easily biodegradable effluent. Moreover, average oxidation state and carbon oxidation state were increased from -0.03 to 0.42 and 1.82, respectively, which revealed the strong mineralization and generation of oxidized organic intermediates during the photo-Fenton treatment of carbofuran. Furthermore, five major carbofuran intermediates were identified, which indicate that C-O bond of the carbamate group and 3-C position of the furan ring were oxidized as a result of the photo-Fenton reaction. The results obtained in this study demonstrate that the photo-Fenton process is a promising pretreatment method before the application of biological treatment for carbofuran removal from contaminated water/wastewater.