介质非均质性对蒸汽强化SVE法修复TCE污染影响的二维土箱模拟研究

2D-Box Study on Effects of Media Heterogeneity on Remediation of TCE by Steam Enhanced Extraction

摘要: 蒸汽强化SVE法(Steam Enhanced Extraction)是一个新近发展起来的并具有较好应用前景的土壤包气带非水相液体(NAPLs)污染修复方法，是把蒸汽和空气的混合气体注入污染土壤促进污染物的挥发，利用气流将土壤中污染物带出的技术。其实质是在人为施加的工程条件下非饱和多孔介质中NAPLs的运移转化过程。介质非均匀性影响着NAPLs去除过程，对NAPLs去除效率存在一定的影响。本研究选取三氯乙烯(Trichloroethylene, TCE)为典型污染物，采用不同质地的石英砂(粗砂、细砂和粉砂)模拟介质非均匀性，通过二维土箱气体模拟实验，分析介质非均质性对蒸汽强化气体抽提修复中TCE去除效率和去除过程的影响。结果表明：蒸汽强化SVE法较好的改进了SVE法去除过程中的“拖尾”现象。粗砂实验、细砂实验、粉砂实验的去除率分别为89.5%、88.2%、85.2%，受介质非均质的影响，细砂、粉砂区域的气体通透性降低，气体流经该区域出现“绕流”现象，从而减少了TCE随空气的去除量。同时介质非均质性影响实验进程中热量的传递，使热在非均质砂土中前进不均匀，进而影响TCE的去除过程，降低其去除效率。在细砂实验和粉砂实验里，均出现污染羽积累与下渗的现象，进一步降低了TCE去除效率。

Abstract: Steam Enhanced Extraction is a newly developed and promising soil remediation technique for non-aqueous phase liquids (NAPLs) in vadose zone. Mixture of steam and air is injected into the contaminated soil to promote the volatilization of the pollutants. The remediation is a process of NAPLs transformation in unsaturated porous media un-der artificial engineering condition. Removal process and efficiency may be greatly impacted by medium heterogeneity. In this study, trichloroethylene (TCE) removal using Steam Enhanced Extraction was carried out in a 2-dimensional sandbox with three different layered sand structures under unsaturated conditions. Three kinds of sands (coarse, fine and silt) were used to simulate heterogeneous media. The results show that SEE perfectly improves “tailing” effect com-pared to soil vapor extraction (SVE). And the removal ratios from the three experiments in 100 minutes are 89.5%, 88.2% and 85.2%. Meanwhile, there were by-passing flows when gas through the coarse sand and fine sand area, which affected the total removal ratio and process. The layered structure with finer sand, in which the steam and air convection was faster, resulted in faster removal rate and larger removal ratio compared to that with clay sand. In the experiments packed with finer or clay sand, a small amount of NAPL phase TCE was cumulated, which also reduced the total re-move ratio.