依据挥发性污染物浓度变化划分土壤气相抽提过程的研究

土壤气相抽提（SVE）是去除包气带土壤中挥发性有机物（VOCs）经济快捷的原位土壤修复方法。VOCs饱和蒸汽压高,能在负压气流下被定向地带到地面收集处理。为了便于划分SVE过程,试验采用两种土壤污染方案：直接污染和间接污染。间接污染土壤的目的是为了避免在土壤中形成非水相液体（NAPLs）,方法是使用气相污染源长时间污染土壤。通过多次对土壤进行间接污染和通风净化,证明VOCs主要来源于NAPLs。试验表明,依据VOCs的浓度变化,SVE过程能被划分为两个阶段：（1）高效去除阶段,即污染土壤中含NAPLs阶段,液态的VOCs进入土壤间隙形成NAPLs或溶解在土壤水中,或被土壤和有机质吸附。SVE过程中VOCs气相浓度降低,停止抽提后浓度能够恢复;（2）低效率的拖尾阶段,即土壤中无NAPLs存在,通风能够快速地降低污染物浓度,并且VOCs浓度降低后不能恢复。试验同时显示出在不同的土质中VOCs浓度变化具有相似的规律。

Identifying Process of SVE Based on VOCs Concentration Variation

Soil Vapor Extraction （SVE） is a widely accepted and cost-effective technique in situ that is used to remediate unsaturated soil contaminated with volatile organic eompounds（VOCs）. The VOCs were taken out from the soil under the negative pressure of air flow and collected by activated carbon. The experiment was conducted by dumping liquid VOCs into the soil directly or VOCs equilibration between volatilization and adsorption could be achieved after 6 weeks. The VOCs concentration in the polluted soil declined sharply after short time venting when the VOCs vapor source was removed. After Re-contaminating the remediate column with VOCs vapor, the result showed that the VOCs were mainly come from Non Aqueous Phase Liquids（ NAPLs ） diffusion. Based on the existence or nonexistence of NAPLs source in the soil, the SVE process could be divided into two phases ： when there were some NAPLs in the soil, the venting gas had high and stable eon- centrat＇ion VOCs. It was so-called Phase I, that was a duration with effective removal rate. At phase I the concentration could be recovered by VOCs transfer after continuously venting or intermittent venting, it depended on VOCs seepage into soil and absorption of NAPLs on it, dissolution into the soil water; Phase II was well-known tailing phase. The phase II may be prolonged for a long time because there was no NAPLs in system, the VOCs were mainly from absorption phase. The results indicated that VOCs concentration in tailing gas had the same decline tendency for different type soils.