| 摘要: | 本研究利用管柱實驗模擬零價鐵反應牆，針對反應牆之鐵粉配置量、反應牆厚度、進流濃度及進流速度等物理因子，以及三氯乙烯(TCE)之還原脫氯反應進行研究，並探討零價鐵反應牆長期操作的可行性。 實驗結果顯示，TCE在管柱內之降解過程，可利用一階反應進行模擬，且TCE的反應速率常數會隨著鐵粉配置量的增加而提高，不過，鐵粉配置必須使鐵粉表面積/孔隙體積(S/V)低於747 m2/L，以使反應牆維持足夠之滲透率(>10-2 cm/sec)。另外，在高S/V值範圍，TCE去除速率會受到脫氯反應的限制，因而反應速率常數值無法與S/V值呈線性關係，但可利用K(S/V)/Ks+(S/V)的關係式加以修正，其中，K即為最大反應速率常數值，Ks則為反應速率常數值等於1/2K時的S/V值，其表示TCE與零價鐵表面反應的親和力。此外，當S/V值為121.3m2/L，進流水中的TCE濃度達72.5mg/L時，TCE之降解速率仍不會受到抑制，且表面流速對反應速率亦無明顯影響。 另一方面，研究結果顯示，在TCE降解過程中，當接觸時間達5.5hr時，即無法偵測到反應中間產物二氯乙烯(DCE)的生成，故推測TCE於鐵表面脫率為DCE及氯乙烯後，在很短時間內即會於鐵表面繼續發生脫氯反應，待完全脫氯後，其反應產物再由鐵表面脫附，而釋放於水溶液中。另外，對於不同含氯數的含氯烯類污染物而言，含氯數較高者，具有較佳的去除效率，主要由於其還原傾向較大。不過，對於三種含氯數相同的DCE而言，推測其分子結構特性及污染物與鐵表面接觸的方位，會使得三種DCE的反應速率有很大差異，其中，以cis-DCE的去除速率較低，其反應速率常數值僅為1,1-DCE反應速率常數值的1/4左右，故欲使cis-DCE達到特定去除率時，則需加大反應牆厚度或提高鐵粉配置量。另外，當TCE及四氯乙烯(PCE)於地下水中共同存在時， PCE的降解速率會受到TCE的競爭性抑制而降低，且受抑制的程度隨TCE濃度的提高，而逐漸增加，然而，由於PCE的反應速率常數值，仍會高於TCE的反應速率常數值，故競爭性抑制效應，對於反應牆設計並沒有明顯影響，仍須以TCE的去除效果作為設計依據。在90天的操作時間下，模擬管柱均可維持很高的TCE去除率，且未發現去除率有降低的情形，主要是由於進流水中僅含有微量的TCE，對鐵粉的消耗速率非常緩慢，因此零價鐵反應牆在長期操作下具有良好而穩定的處理效果。 The objective of this study was aimed to investigate the elimination of trichloroethylene (TCE) in contaminated water by the zero-valent iron reactive barrier. The column experiments were carried out to simulate the operation of reactive barrier. Physical factors including iron surface per porosity volume (S/V) or arranged amounts of zero-valent iron, the thickness of barriers, inlet concentration of TCE and superficial velocity were studied to understand the performance of reactive barrier. Moreover, the reaction mechanism of reductive dechlorination and the feasibility of long-term operation were also investigated in this study. Results indicated the removal of TCE could be simulated by the first-order equation. The reaction rate constant increased as S/V of iron were increased, and excellent linear relationship between them was obtained in this study. However, the value of S/V should be lower than 747 m2/L to maintain enough permeability(>10-2cm/sec) in the reactive barrier. Also, the increase extent of rate constant would decrease as higher S/V was arranged in the barriers. This fact suggested the removal of TCE would be limited by the rate of dechlorination. The relationship of rate constant and S/V could be modified to the model of K(S/V) / Ks + (S/V), where the K was the maximum rate constant and the Ks (S/V at 1/2 K)represented the affinity between the iron and TCE. Meanwhile, when the S/V was set to 121.3 m2/L, the reaction rate was similar even the inlet concentration of TCE was increased to be as high as 72.5 mg/L. Effect of superficial velocity was also minimal on the removal of TCE. In addition, by the analysis of intermediate and chlorine ion, experimental results also suggested the reactive products would release to the bulk solution until the TCE was dechlorinated completely in the grains of iron. Furthermore, the rate constant of chlorinated ethene increased generally with the increase of chlorine numbers or reduction potential. However, for the dichloroethylene (DCE), the removal efficiency of cis-DCE was lower than that of tran-DCE greatly and this fact was suggested due to the characteristics of molecular structure and the direction of collision between the DCE and iron surface. Therefore, in order to enhance the removal of cis-DCE in contaminated water, increasing the thickness of barriers or the arranged amounts of iron was necessary. On the other hand, the removal efficiency of PCE was reduced by the competition inhibition when PCE and TCE coexisted in the contaminated water. Since the rate constant of PCE was still greater than that of TCE, the effect of competition inhibition could be ignored for the design of reaction barriers. The performance of column experiments was excellent and stable during 90 days of period. Thus, the reactive barrier exhibited great potential in the long-term operation. |
