以奈米金屬顆粒處理飽和多孔介質中比水重非水相液體之研究

Treatment of DNAPLs in Saturated Porous Media by Using Nanoscale Metal Particles

魏裕庭(Yu-Ting Wei)

指導教授 ： 吳先琪

國立臺灣大學/工學院/環境工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.01922

摘要

自1980年代發現含氯有機物釋放到地下水多孔介質中易形成比水重非水相液體（Dense Non-Aqueous Phase Liquids, DNAPLs）後，有關於如何處理DNAPLs一直是研究的焦點。以奈米金屬顆粒處理地下水中DNAPLs物質，可利用奈米金屬顆粒的高比表面積，促進反應速率及效率，降低處理成本。經由文獻回顧，可將本研究主題分為三部分再深入研究，作為論文的重點：第一，穩定奈米顆粒懸浮液的研發。奈米粒子之間的距離很小，顆粒間的吸引力使其極容易絮聚，而無法均勻分散於液相中，因而限制實用性。目前的研究多是使用表面改質劑，增加顆粒表面電荷斥力或是空間阻隔力，使奈米粒子穩定不易絮聚；第二，現地處理效能的研究。製備完成的奈米鐵顆粒用於處理含氯有機物已證實其可行性，但較缺乏現地處理的條件和參數，以及顆粒處理劑配佈後對現地影響的相關資訊；第三，奈米金屬顆粒傳輸性研究。奈米顆粒灌注後的分布範圍大小，會影響處理劑的用量以及灌注井的數量，是現地處理的重要參數，如果可以經由理論推導建立傳輸模式，則不僅可以用於整治場址的設計，亦可延伸應用於其他膠體顆粒之模擬預測。
有關穩定奈米顆粒懸浮液的研發及對含氯碳氫化合物反應性研究，係區分為親水性及疏水性奈米鐵兩類。親水性奈米鐵使用生物可分解之非離子介面活性劑作為分散劑，製作成為穩定懸浮水中的奈米鐵處理劑；疏水性奈米鐵，則是利用磷酸酯介面活性劑鍵結奈米鐵，使其具有疏水性表面，成為油溶性的處理劑，不過此疏水性處理劑尚未應用於現地處理。現地試驗是應用所研發的親水性奈米鐵懸浮液添加鈀做為催化劑，成為雙金屬複合材料，於一處地下水受1, 2-二氯乙烷及氯乙烯污染場址，進行為期約三個月的現地模廠處理試驗，並與以聚丙烯酸作為分散劑的商業化奈米鐵的現地模廠處理結果作比較。模廠試驗場地面積約為200 m2，場地內共設有13處井叢，以監測現地三維的處理參數及現地灌注的成效，試驗過程同時進行貫穿試驗，確認處理劑之移動性。實驗中發現，所選用的奈米鐵分散劑，因為具有生物可分解性，可以提供微生物碳源及氮源，會促進奈米鐵還原1,2-二氯乙烷。由過去文獻研究得知，氯乙稀可以被奈米鐵迅速反應還原為乙稀，如果在鈀金屬的催化下，更能加速其完全反應為乙烷，但是1,2-二氯乙烷則是一個難以被奈米鐵還原的污染物，即使添加鈀催化的狀態下，反應速率亦相當慢。所以現地在奈米鐵灌注後明顯觀測到1,2-二氯乙烷的降解，可以解釋為觸發生物反應的效果。和使用以聚丙烯酸作為分散劑的商業化奈米鐵灌注結果比較起來，由於研究中使用的奈米鐵配方經由30公分管柱顯示其貫穿比率達78%，現地貫穿率則和管柱試驗相當，均明顯具有傳輸性，而且使得現地產生較為均勻的還原狀態，氧化還原電位介於-450到-280 mV，處理效果較為顯著，未來應可推廣至含氯碳氫化合物污染場址之實廠整治應用。研究中整理包括對地質化學（geochemistry）特性的影響，以及對現地微生物反應觸發的效果，可以提供實廠處理之設計參考依據。
為預測改質後奈米顆粒能穿透之距離，提供工程設計所需參數，所以研究中亦開發新模式，作為預估奈米顆粒在多孔介質中的傳輸行為之模擬工具。本研究模式，克服尤拉推導的方法（Eulerian approach），在遇到改質後奈米顆粒表面具有電荷斥力（electrostatic）或空間阻隔力（steric repulsion）等能量屏障的情況下，不容易以控制方程式描述這些新增斥力，造成計算上的限制問題，故採用軌跡分析方法（trajectory analysis method）分析。此方法是直接運用牛頓第二運動定律描述顆粒受力移動的情況，只要能夠定義每項作用力，就能夠以Langevin方程式加以計算膠體顆粒的移動軌跡，並進而推算顆粒在多孔介質中過濾收集的效率（collection efficiency），以及黏合係數（sticky coefficient or attachment efficiency）。研究中配合壓縮管模型（constricted tube）設定孔隙中流場，將粒子置入單一壓縮管中，在加入布朗運動效應，以Langevin方程式計算包括凡得瓦爾力、電荷斥力、空間阻隔力等外力情況下，粒子被濾除收集的效率，然後推算黏合係數。經由撰寫Fortran程式，使用此軌跡模式，在不考慮電荷引斥力及空間阻隔力的情況下，模擬單顆粒介質的濾除率，其結果與Rajagopalan and Tien（1976）所使用的過濾模式及Tufenkji and Elimelech（2004）修正傳輸係數後模擬的結果相當，也比Nelson and Ginn（2005）使用軌跡模式搭配球形流場（Happel’s sphere-in-cell）所模擬的結果為佳，同時也使用無因次參數迴歸推算關連方程式 (correlation equation)，提供計算應用。另外模擬顆粒在具有斥力的情況，模式模擬結果，與四組不同奈米鐵的管柱試驗的數據相當接近，顯示模式模擬的有效性，也為新開發的奈米材料在多孔介質中傳輸，提供一個好的預測工具。

關鍵字

奈米零價鐵 ； 含氯碳氫化合物 ； 現地處理 ； 傳輸模式 ； 軌跡分析 ； 黏合係數 ； 多孔介質

並列摘要

With the findings of Dense Non-Aqueous Phase Liquids (DNAPLs), which are easily formed from the release of chlorinated hydrocarbons into groundwater aquifers, the challenge to clean up the contaminated sites have been arduous since the 1980’s. Cost-effective and reliable technologies are sought after to treat DNAPLs contamination. The use of nanoscale metal particles to treat DNAPLs or contaminants released from them is a novel scheme developed in recent years, involving the utilization of nanoscale zero valent iron (NZVI) or bimetallic nanoparticles in reducing chlorinated hydrocarbons. This research takes an in-depth look at three aspects: 1. Manufacturing stabilized NZVI. The proximity of nanoparticles makes them aggregate easily and harder to disperse. Surface modifiers such surfactant or polymer are needed to enhance stability and practicality. 2. Researching the in-situ application. In-lab studies of the effectiveness of NZVI in treating chlorinated hydrocarbons have been proven, but the in-situ applications are scarce. 3. Researching the transport of NZVI. The deploying range of NZVI after injection affects directly the amount of NZVI reagent used the number of injection wells applied. With the development of a model to predict the transport of NZVI, the design would allow the delivering of NZVI to be more applicable and affordable. .
With regard to the first aspect, the biodegradable nonionic-surfactant modified NZVI suspension was developed and deployed. These approaches are effective on handling dissolved contaminants from DNAPLs, however, are not effective in dealing with the main pollutant sources, residual saturation or DNAPLs pools. In this respect, this study has also included the preparation of hydrophobic NZVI suspension for environmental restoration. These agents are used to enhance the destruction of chlorinated DNAPLs in source zones by creating better contact between the DNAPLs and reducing NZVI.
A field investigation on the influence of NZVI on geochemical properties of groundwater and contaminants degradation was also conducted. A 200 m2 pilot-scale field test successfully demonstrated the effective remediation of groundwater contaminated with chlorinated organic compounds in Taiwan within six months by using NZVI. Both commercially available and on-site synthesized NZVI were used. A monitoring system allowing the collection of three-dimensional spatial data from 13 nested multi-level monitoring wells was established to monitor geochemical parameters in groundwater. The degradation efficiency of vinyl chloride (VC) measured at most of the monitoring points was 50-99%. A decrease in oxidation-reduction potential (ORP) values from about -100 to -400 mV after NZVI injection was observed. This revealed that NZVI is an effective means of achieving highly reducing conditions in the subsurface environment. Both VC degradation efficiency and ORP showed a correlative tendency as an increase in VC degradation efficiency corresponded to a decrease of ORP. This is in agreement with the previous studies suggesting that ORP can serve as an indicator for the NZVI reactivity. VC was degraded by NZVI quickly, with the process supposedly abiotic, while the 1,2-DCA degradation was relatively sluggish within three months. Nevertheless, as 1,2-DCA is known to resist chemical reduction by NZVI, the observation of 1,2-DCA degradation and hydrocarbons production suggests that biological processes have been involved. The bioremediation may be attributed to the production of hydrogen as electron donor from the corrosion of ZVI in the presence of water or the added biodegradable surfactant serving as the carbon source as well as electron donor to stimulate the microbial growth.
To have insight of the NZVI transport, a new trajectory simulation algorithm was developed to describe the efficiency of a single collector (pore) to catch submicrometer particles moving through saturated porous media. A constricted-tube model incorporating the deterministic (interception, hydrodynamic retardation, van der Waals force and gravitational sedimentation), stochastic (Brownian diffusion) and thermodynamic (electrostatic and steric repulsion force) mechanisms was established to predict the transport and deposition of surface modified NZVI particles by applying Lagrangian trajectory analytical approach. The simulation results show good agreement with the results predicted by existing energy-barrier-free models except for the particle size less than 100 nm at low approach velocity. The number of realizations per starting location could be decreased down to one hundred with the simulations still exhibiting acceptable relative standard deviation for engineering purposes. The correlation equations with dimensionless parameters fitting the simulation results were proposed to provide a quick estimation of the collection efficiency. With the consideration of energy barriers, the model successfully describes the breakthrough curve of polymer-modified NZVI in a bench-top soil column as well. The novel simulation scheme can be a useful tool for predicting the behavior of the nanoscale colloidal particles moving through filter beds or saturated soil columns under conditions with repulsion and attraction forces among surfaces.

並列關鍵字

nanoscale zero valent iron ； chlorinated hydrocarbons ； in-situ treatment ； transport model ； trajectory analysis ； attachment coefficient ； porous media

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