| 摘要: | 六氯苯過去廣泛應用於農業作為制菌劑，具有難降解、生物累積毒性等特性，為疏水性物質，易和土壤行疏水性吸附而難以去除。本研究配置六氯苯污染土壤，以靜態及動態熱裂解兩種方式，分別探討土壤中六氯苯去除效率及氯苯分布，並嘗試添加零價鐵，探討其在熱裂解系統中對六氯苯去除效率之影響。靜態結果顯示未添加零價鐵，操作時間60分鐘，250oC、300oC、350oC及400oC之六氯苯去除效率為18%、42%、74%及96%，去除效率隨著溫度升高而提升，而各操作溫度隨著操作時間從30分鐘增加至60分鐘，去除效率皆未顯著提升；添加5%奈米零價鐵於操作時間60分鐘，250oC、300oC、350oC及400oC之六氯苯去除效率為則為34%、42%、67%及97%，250oC時去除效果優於未添加之情況，然而於350oC去除效率卻較未添加零價鐵為低，顯示加入零價鐵於不同溫度對六氯苯去除效率有不同程度之影響。動態系統未添加零價鐵時，操作時間60分鐘，250oC、300oC、350oC及400oC之六氯苯去除效率為22%、53%、66%及88%，添加5%奈米零價鐵之效率為36%、51%、65%及88%，去除效率隨著溫度升高而提升，和靜態系統之趨勢相同。然而兩系統於添加零價鐵後皆有其他氯數之氯苯生成，其中350oC生成最為顯著。氯苯生成量於400oC大幅降低，顯示此溫度之破壞效率已大於生成效率。零價鐵於熱裂解系統內釋出電子並對六氯苯進行降解，反應後Fe2+及Fe3+可能與土壤中之元素生成金屬催化物，例如FeCl2, FeCl3，催化致使氯苯生成。戴奧辛生成以350oC時最為顯著，以高氯數戴奧辛為主；毒性當量之貢獻以2,3,7,8-TeCDD、1,2,3,7,8-PeCDD及2,3,4,7,8-PeCDF最為顯著，顯示於缺氧情況下，以熱裂解去除土壤中六氯苯之過程仍有戴奧辛生成之潛勢能。然而400oC未見戴奧辛生成，顯示400oC之戴奧辛破壞效率高於生成效率。
Hexachlorobenzene (HCB) was widely used in agriculture as pesticides. Some of the important characteristics of HCB include low water solubility, bioaccumulation. It is easy to adsorb and difficult to remove from soil. In this study, pyrolysis with static system and dynamic system are applied to treat HCB-contaminated soil, the impact of nanoscale iron on HCB removal will also be evaluated. The results display the HCB removal efficiencies achieved with the static system with temperature varying from 250 to 300, 350oC and 400oC at 60 min without nZVI are 18%, 42%, 74% and 96%, respectively. It displays the benefit of HCB removal at a higher temperature. However, the removal efficiency does not change much as the treatment time is extended from 30 to 60 min. The HCB removal efficiencies achieved with temperature varying from 250 to 300, 350oC and 400oC at 60 min with 5%-nZVI are 34%, 42%, 67% and 97%, respectively. The HCB removal efficiency increases at 250oC if compared with the case without nZVI but decreases at 350oC. It displays that nZVI has different effects on HCB removal at different temperatures. The results obtained with the dynamic system indicate that HCB removal efficiencies achieved with temperature varying from 250 to 300, 350 and 400oC at 60 min without nZVI are 22%, 53%, 66 and 87%, respectively. In the presence of 5%-nZVI, the HCB removal efficiencies achieved are 36%, 51%, 65 and 87%, respectively. These trends are similar to that observed in static system. The results obtained from both static system and dynamic system indicate that CBs are generated when nZVI is added. CBs are generated significantly at 350oC but decreased at 400oC. It indicates that the destruction efficiency is increased significantly at 400oC. Possible reasons of CB generated are that nZVI release electrons and generate other iron-containing compounds, like FeCl2 or FeCl3. Precursors exist in soil may react with chloride to form chlorobenzenes through the catalysis of these iron-containing compounds. PCDD/Fs are generated significantly at 350oC and form higher chlorinated congeners. 2,3,7,8-TeCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF are the main species contributing to toxicity. On the other hand, formation of PCDD/Fs is not significant for the system operating at 400oC. It indicates that pyrolysis of HCB-contaminated soil also generates PCDD/Fs even oxygen is not provided to the system. |
