以低溫熱裂解程序降解土壤中五氯酚之研究;Investigation on Low-Temperature Pyrolysis of Pentachlorophenol-Contaminated Soil

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Title:	以低溫熱裂解程序降解土壤中五氯酚之研究;Investigation on Low-Temperature Pyrolysis of Pentachlorophenol-Contaminated Soil
Authors:	歐婷芳;Ngo,Thuan-Thi
Contributors:	環境工程研究所
Keywords:	dioxin formation;byproducts;soil;nano scale zero valent iron;low thermal;Pentachlorophenol;soil;nano scale zero valent iron;dioxin formation;byproducts;low thermal;Pentachlorophenol
Date:	2012-07-30
Issue Date:	2012-09-11 18:19:09 (UTC+8)
Publisher:	國立中央大學
Abstract:	焚化與燃燒之高能量消耗與戴奧辛之生成限制了台灣安順廠同時處理含高濃度PCP和PCDD/Fs汙染之土壤之能力。本研究探討低溫下(150-400°C)土壤中的PCP裂解和多氯聯苯和呋喃之生成特性。受PCP汙染土壤裂解時之脫氯與裂解在本研究中有詳細的探討。大部分的PCP (>90%)和PCP之副產物可在350°C經40分鐘處理被去除。PCP之衰減率在溫度從150°C升到400°C時明顯上升(0.20-1.96 min-1)。裂解過程中發現PCDD/F濃度很低，土壤中為0.38-2.48 ng TEQ/kg，氣相中為0.0015-0.0044 ng TEQ/Nm3。土壤中PCP去除率達到70%時，最高的PCDD/F生成在250°C，為1436±230 ng/kg。然而，最高的毒性濃度(4.20±0.62 ng TEQ/kg)是在PCP去除效率為80%且操作溫度在300°C時。本研究進一步指出OCDD是裂解PCP之主要生成物種，OCDF則是第二顯著物種，可能原因是因為PCP裂解的主要副產物2,3,4,5-TeCP反應所造成。溫度大於300°C時，少量的戴奧辛與呋喃會被偵測到是由於高氯數之戴奧辛與呋喃脫氯所造成，尤其是OCDD在350°C和450°C時。土壤脫附是PCDD/F在氣相中分布的主要機制，350°C和400°C的戴奧辛與呋喃濃度並無明顯的差別。為測試nZVI對於PCP的活性，熱促進處理方法(thermally enhanced pump-and-treatment method)與nZVI被用來移除土壤中的PCP和去除土壤中和液相之毒性。結果顯示土壤中與液相之PCP去除效果隨nZVI劑量增加而上升。利用nZVI復育土壤時PCP在水中的分布會增加。而pH降低會導致PCP在液相中分布減少但PCP脫氯增加。當溫度從25°C提升至85°C時，脫氯速率從2.26 h-1提高至6.84h-1。在85°C與pH = 1的條件下，脫氯提升了42%，PCP殘餘量則降低了6%。此系統中脫氯位置發生傾向於ortho>meta>para。基於此結果，本研究利用裂解結合nZVI去除土壤中之PCP。在150-300°C時，裂解結合nZVI之裂解速率為0.591-3.699 min-1，為未結合nZVI之四倍。活化能部分有nZVI為23.80 kJ/mol，無nZVI為36.98 kJ/mol。當nZVI劑量增加時，PCP之降解速率呈線性上升。衰減速率在200°C時nZVI劑量從0%到10%為0.21 min-1到1.56 min-1。PCP之脫氯在有無nZVI之情況下皆相同，但若有nZVI則裂解會更完全使最終產物變為phenol。溫度增加至300°C時土壤中主要為TeCP (0.4±0.1%)並且無其他副產物之生成，當溫度上升至300°C以上且時間在30分鐘以上情況亦同。特別的是在氣相中皆未偵測到PCP以及任何副產物。此研究提供了利用低溫熱裂解處理PCP汙染土壤之相關風險評估資訊。High energy cost and potential formation of dioxins during incineration/combustion of pentachlorophenol (PCP) have limited their application on simultaneous removal of highly contaminated soil of PCP, polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) at AnShun in Taiwan. In this dissertation, investigation of PCP pyrolysis in soil at a relatively low temperature range (150-400oC) and the behavior of PCDD/Fs formation, dechlorination and destruction during pyrolysis of PCP-contaminated soil have been examined in detail. Most PCP (>90%) and PCP byproducts can be removed from soil at 350oC for 40 min. The PCP decay rates from soil increased exponentially from 0.20 to 1.96 min-1 as temperature was increased from 150oC to 400oC. Very low levels of PCDD/Fs were found in soil (0.38-2.48 ng TEQ/kg) and gaseous phase (0.0015-0.0044 ng TEQ/Nm3) during pyrolysis of PCP-contaminated soil. 70% of PCP removal from the soil was achieved, resulting in 1436?230 ng/kg, the highest PCDD/F formation at 250oC; however, the highest toxic concentration was measured around 4.20 ? 0.62 ng TEQ/kg at 300oC with 80% PCP removal from the soil. Further analysis has revealed that OCDD is the most dominant congener supposed to form from the pyrolysis of PCP, while OCDF is the second prevailing congener, possibly due to 2,3,4,5-TeCP reaction which is a main byproduct of PCP pyrolysis. Detection of less chlorinated dioxins and furans over 300oC indicates the dechlorination of highly chlorinated dioxins and furans, especially OCDD at 350oC and 400oC. Desorption from soil was supposed as a main mechanism for the distribution of PCDD/Fs in the gas phase, and not much difference in dioxins and furan levels was observed at 350oC and 400oC in the gas phase. In order to test the nZVI reactivity with PCP, a thermally enhanced pump-and-treatment method coupled with nZVI was proposed to remove PCP from soil and to detoxify aqueous phase and soil. The results indicated that total PCP removal in soil and aqueous phases increased with increasing nZVI dose. The PCP distribution in aqueous phase was enhanced when PCP-contaminated soil was remediated with nZVI. In addition, decrease in pH resulted in decreasing PCP distribution in aqueous phase but increasing PCP dechlorination. Dechlorination rate was enhanced from 2.26 to 6.84 h-1 as the temperature was increased from 25oC to 85oC. Dechlorination and PCP residual in soil were increased to 42% and decreased to 6%, respectively, at 85oC and pH1. The dechlorination of PCP preferred to occur at ortho> meta> para positions in the respect of OH group. Based on the results of nZVI reactivity with thermal enhancement, the combination of pyrolysis and nZVI was proposed to investigate the PCP removal from soil. Consequently, the decay rate constant (k) of pyrolysis combined with nZVI increased exponentially from 0.59 to 3.67 min-1 which were 4 times higher than that without nZVI in the temperature range of 150°C -300°C. The activation energies of PCP removal from soil with and without nZVI are 23.80 and 36.98 kJ/mol, respectively. PCP degradation increases linearly with increasing nZVI dose. The rate decay constant increased from 0.21 min-1 to 1.56 min-1 as nZVI dose was increased from 0% to 10% at 200oC. The order of PCP dechlorination during pyrolysis coupled with nZVI is the same as that in the absence of nZVI but dechlorination process during pyrolysis with nZVI occurred more completely into the final product as phenol. Increasing temperature to 300oC resulted in the predominant TeCP ((0.4 ? 0.1) %) in soil and none byproducts was detected in soil as either temperature or time was increased above 300oC and 30 min, respectively. Especially, both PCP and byproducts were not detected in gaseous phase. This study provides relevant information for risk assessment for PCP contaminated soils when low thermal pyrolysis is applied for remediation of PCP contaminated soil.
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