地下水中污染物電化學降解之研究

電化學高級氧化法(electrochemical advanced oxidation process)為一種高級氧化技術(advanced oxidation processes, AOPs)，具有高效節能、操作簡單、環保及設備簡單，其中又以BDD (boron-doped diamond)做為陽極對於污染物之去除效率較高，相較於其它材料具有低成本與較高之污染物礦化效率等優點。本研究採用BDD電氧化受氯乙烯類及苯類污染之地下水樣本。藉由不同電化學參數(電流密度、時間、電極間距及電解質)來探討地下水中有機污染物降解之影響，找出較佳操作參數。敵避(N,N-diethyl-m-toluamide, DEET)，屬於新興污染物的一種，主要用途為驅逐蚊蟲藥劑，因此常存於地下水中。通過試驗中得出最佳操作參數於污染之地下水中添加DEET及配製溶液進行降解試驗，探討其電化學降解效率及有機物去除效率。利用螢光激發/放射光譜(EEM)、紫外光/可見分光光度計(UV-visible)進行分析，藉由液相層析儀(LC-MS)及高效能液相層析儀(HPLC)分析中間產物以探討DEET降解途徑。研究結果顯示，以化學探針進行氫氧自由基捕捉之螢光分析結果可知添加硫酸鈉後的螢光強度相對較高。水樣經2、4小時電化學試驗後BTEX及氯乙烯類處理後濃度均符合地下水污染物管制標準第二類。另外水樣S03與D01試驗中，大部分烷類、苯類、烯類及其他化合物的降解效率均達100%。較高初始濃度總石油碳氫化合物(Total Petroleum Hydrocarbons, TPH)的降解效率明顯較高>90%對應的TOC去除效率介於50%~97%。使用自製與國外商用電極分別進行4小時的電化學試驗，總有機碳去除效率為95%，且苯類污染物濃度皆符合第二類地下水污染物管制標準限值，而氨氮、亞硝酸鹽氮及硝酸鹽氮的濃度均符合法規標準。電化學降解0.05 M 硫酸鈉溶液的DEET，可將DEET濃度降至ND，其TOC去除效率可達98%。將DEET添加至受污染之地下水中，其降解效率為92%~99.5%:TOC去除效率為91%~94%，氨氮均降至ND，而亞硝酸鹽氮及硝酸鹽氮的濃度均符合法規標準。縮短電極間距無助於提升降解效率，但可降低能耗。於UV-Vis進行全波長掃描DEET分析中，於圖譜中出現2個吸收峰(210 nm 及230 nm)，而隨著電降解時間增加峰值皆有下降的趨勢，於240分鐘後接消失。依據螢光特性分析結果顯示，所配制含DEET的0.05M硫酸鈉溶液於電降解前在區域Ⅳ有一個波峰及有一個波峰介於Ⅰ、Ⅱ及Ⅳ之間(類酪氨酸&類色胺酸&可溶性微生物副產物)，而隨著電降解時間增加螢光強度會隨時間下降，於240分鐘消失，此結果與UV分析結果近似。DEET(m/z = 191)經電降解產生質荷比(m/z) =257、255、243、239、227、225、223、222、221、209、206、207、197、195、193、192、181、179、177、169、163、150、135、116、90、89及46。

關鍵字

地下水；電化學氧化法；摻硼鑽石電極；苯類；氯乙烯類；總石油碳氫化何物；敵避；紫外-可見分光光普分析；螢光激發/放射光譜；液相層析質譜儀分析

並列摘要

Electrochemical advanced oxidation process (EAOP) is one of advanced oxidation process (AOPs), with the advantages of good energy efficiency, easy operation, and environmental friendliness. Among them, BDD (boron-doped diamond) is used as the anode to remove pollutants with high efficiency. Compared with other materials, it has the advantages of low cost and high mineralization efficiency of pollutants. In this study, a BDD (boron-doped diamond) electrode is used as the anode to remove pollutants from contaminated groundwater samples. Different electrochemical parameters (current density, time, electrode distance, and electrolyte) were tested to find suitable operating parameters for this EAOP process. N, N-diethyl-m-toluamide (DEET), a type of emerging pollutant, is mainly used to expel mosquitoes and is therefore often found in groundwater. Degradation tests were also carried out by spiking DEET in preparing solutions or contaminated groundwater samples to explore the electrochemical degradation efficiency of DEET. Liquid chromatography-mass spectrometry (LC-MS), high performance liquid chromatography (HPLC), and ion chromatography (IC) analyses were performed to identify the intermediates (products) and pathways of TC electro-degradation. Moreover, Ultraviolet-visible (UV-Vis) and fluorescence excitation-emission matrix (EEM) tests were conducted to evaluate the electro-degradation characteristics of water matrices during operations. The results show that the fluorescence intensity after adding sodium sulfate in the hydroxyl radical capture test using terephthalic acid as the chemical probe is relatively higher. The concentrations of BTEX and vinyl chloride in contaminated groundwater samples after 2- and 4-hour electrochemical oxidation met the requirements of the Category 2 of Groundwater Pollution Control Standards. In addition, in the water samples S03 and D01, most of alkanes, benzenes, alkenes and other compounds, reached the degradation efficiency of 100%, while that of total petroleum hydrocarbons (TPH) was over 90% at higher initial concentration and the TOC removal efficiencies ranged between 50% and 97%. For 4 h electrochemical degradation using a prepared or commercial electrode, the TOC removal efficiency was 95%, and the concentrations of benzene pollutants, ammonia nitrogen, nitrite nitrogen and nitrate nitrogen were all in compliance with the regulations. The concentrations of DEET in 0.05‒1.00 M Na2SO4 solutions after electrochemical degradation were reduced to ND, with TOC removal efficiencies of ~98%. The degradation efficiency of DEET in DEET-spiked different contaminated groundwater samples were 92%‒99.5 %, and those of TOC ranged from 91% to 94%; the concentration of ammonia nitrogen was lowered to ND, and those of nitrite nitrogen and nitrate nitrogen were below the limits of regulations. Shortening electrode distance did not increase TOC removal efficiency but decrease energy consumption. In the full-wavelength scanning analysis of UV-vis spectrum, two absorption peaks (210 nm and 230 nm) appeared for the DEET in 0.05 M Na2SO4 solution; moreover, the intensities of these peaks decreased with the increase of electro-degradation time and finally the peaks disappeared after 4 h electro-degradation. In fluorescence analysis, the DEET-spiked sodium sulfate solution had a fluorescence peak in region IV and another one across regions I, II, and IV; the fluorescence intensities of these two peaks decreased with electro-degradation time and lastly disappeared at 240 minutes. This result was similar to that of UV analysis. According to LC/MS analysis, The intermediates of DEET electro-degradation in prepared solution were m/z = 257, 255, 243, 239, 227, 225, 223, 222, 221, 209, 206, 207, 197, 195, 193, 192, 181, 179, 177, 169, 163, 150, 135, 116, 90, 89 and 46。