零價鐵反應牆應用於三氯乙烯還原脫氯之整合研究

Integrated Study on the Reductive Dechlorination of Trichloroethylene by Zero-Valent Iron Reactive Barrier

劉志忠(Chih-Chung Liu)

指導教授 ： 曾迪華

國立中央大學/工學院/環境工程研究所/博士(2006年)

摘要

本研究之目的，係探討零價鐵材料特性與不同水質條件對三氯乙烯(trichloroethylene, TCE)還原脫氯的影響，及提升零價鐵反應牆去除TCE的方法，最後結合生物反應牆之整合研究，以建立完整的零價鐵反應牆整治受TCE污染之地下水現地處理模式。
實驗結果顯示，經過酸洗前處理後之零價鐵，由XPS分析結果發現，鐵的主能階束縛能接近零價的狀況，顯示表面較易釋出電子。此外，可減少表面的不純物，且酸洗後的鐵表面鐵氫氧化物，轉變為易水解之α-FeOOH，同時也減少鐵表面γ-Fe2O3型態的鈍化膜，因而提升TCE的降解能力。當二價鐵離子存在於經酸洗前處理之零價鐵時，二價鐵離子形成氫氧化亞鐵沈澱物，覆蓋於鐵粉表面，降低TCE的降解速率，然而，若存在於未經酸洗前處理之零價鐵時，會吸附至鐵粉表面，將鐵表面鈍化膜轉化成具半導體特性的Fe3O4，允許零價鐵表面釋放之電子通過，與TCE發生氧化還原反應。硫酸根的影響除與二價鐵作用相似之外，還可能與氫氧化亞鐵迅速地發生反應，移除生成於鐵粉表面之氫氧化亞鐵沈澱物，提升TCE的降解速率；硝酸根優先被零價鐵還原，與TCE發生競爭抑制效應，直到硝酸根還原成氨氮；腐植酸對TCE降解速率的影響最大，係因為腐植酸持續累積在零價鐵表面，減少鐵表面可反應位置。
通電可促進零價鐵反應牆對TCE的還原脫氯反應，主要係因為陽極端水的電解反應發生，產生氫離子，對陽極附近及後端的鐵顆粒，產生酸洗的效果，改善零價鐵表面的反應性。就電極設置位置而言，陽極應設置在前端的鐵反應牆內，陰極則需設置於後端的石英砂段，以此方式，當電位梯度為1.0 V/cm，在25天的長期通電操作下，TCE的去除率維持100％，顯示此方法具有長時間操作效能。
採自疑受TCE污染場址的現地土壤，其中土壤原生菌可以甲苯為生長基質，好氧共代謝的方式去除TCE，且直接好氧氧化氯乙烯。生物管柱實驗結果顯示，當管柱操作至30天時，管柱前端5cm處，對污染物的去除率均可達99％以上，且未有其他含氯中間產物的生成或累積。土壤原生菌鑑定結果發現，Burkholderia cepacia ATCC及Burkholderia sp.菌種為優勢菌，此皆具有好氧共代謝TCE的能力。整合零價鐵反應牆及生物反應牆整治受TCE污染的方法，可以完全降解TCE及其含氯的中間產物，以確保處理後之地下水水質的穩定。

關鍵字

零價鐵 ； 三氯乙烯 ； 還原脫氯 ； 地下水水質 ； 通電 ； 生物反應牆

並列摘要

The purposes of this study were to investigate the characteristics of the zero-valent iron (ZVI) and various groundwaters quality that may affect the reductive dechlorination of trichloroethylene (TCE). Also, techniques, enhanced TCE removal by ZVI reactive barrier, and the combination of biobarrier would be examined to establish the treatment model of remediation of TCE-contaminated groundwater by ZVI reactive barrier at in-situ field.
Experimental results indicated that the binding energy of Fe 2p on the iron surface was close to the condition of zero valent after acid-washing by XPS analysis, meaning electron released easily from the iron surface. Also, the impurities on the iron surface were reduced and conformation of iron hydroxides was changed to more hydrated α-FeOOH form and reduced the passivation ofγ-Fe2O3 simultaneously, therefore, enhancing the reductive dechlorination of TCE. The efficiency of TCE degradation decreased with the increase in ferrous concentration due to the production of ferrous hydroxide on the surface of acid-washed ZVI. However, ferrous were adsorbed onto the surface of unacid-washed ZVI and transformed to magnetite (Fe3O4). Thus, these permitted electrons to pass through precipitates from iron surface and allowed reductive dechlorination of TCE to occur in bulk solution. The effect of sulfate was similar to ferrous function. But, the sulfate could also react with the ferrous hydroxides and remove the precipitates coated on the iron surface, enhancing the TCE degradation. Nitrate would be preferentially reduced by ZVI so that the competition effect with TCE would appear until nitrate was reduced to ammonium. Effect of humic acid on the TCE degradation was apparent because the humic acid continuously accumulated on the iron surface, therefore the reaction sites of iron surface were decreased obviously.
The direct current was externally supplied to ZVI reactive barrier was feasible to enhance TCE degradation because the surface of the iron particles was acid-washed by H+, owing to water oxidation around the anode and promoted the reactivity of the surface iron. An anode electrode should be placed in contact with the iron filling near the inlet of the column, and then a cathode was located on the top of the column in that the TCE removal efficiency reached 100% and maintained this high level after 25 days with 1.0V/cm of potential gradient applied, displaying excellent potential for development in long-term operations.
Results of batch tests indicated that aerobic co-metabolism with toluene as growth substrate could effectively biodegrade TCE by indigenous soil cells, which were sampled from a doubtful TCE-contaminated site. These indigenous microorganisms could biodegrade vinyl chloride (VC) directly under an aerobic condition. In addition, results of soil microcosm column tests demonstrated that the concentrations of TCE and VC in the column effluent decreased with the increase of operation time. The removal efficiencies of TCE and VC were greater than 99% after 30 days of operation time and no other chlorinated byproducts were detected while TCE and VC were biodegraded. The predominant species were identified as both Burkholderia cepacia ATCC and Burkholderia sp. that were ability to biodegraded TCE by aerobic co-metabolism. The process of iron wall associated with biobarrier could degrade TCE and their chlorinated intermediates completely, thus obtaining the stable groundwater quality.

並列關鍵字

TCE ； zero-valent iron ； reductive dechlorination ； groundwater quality ； electricity supplied ； biobarrier

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