【作者】 王喜；

【导师】 陈鸿汉；

【作者基本信息】 中国地质大学（北京） ， 环境工程， 2009， 博士

【摘要】 土壤气相抽提(Soil Vapor Extraction)是一项被广泛接受经济有效的治理挥发性有机物(Volatile Organic Compounds)污染的非饱和带土壤的技术。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。在非饱和带土壤中,非水相液体(Non Aqueous Phase Liquids)、界面相和气相VOCs为主要存在形式。NAPL将随着时间的推移逐渐减少至消失。NAPL存在时,毛细作用和铺展作用时刻发生,并使其污染范围扩大,VOCs气体在土壤孔隙中不断扩散。VOCs的污染多发生在非饱和带土壤中,本研究探讨液态VOCs进入土壤后的存在相态、非气态和气态的运移规律、运移机理、拖尾机理。室内实验,汽油液气两相在土壤中的运移规律研究和吸附机理分析1.液态汽油运移:定量向土柱中加入液态汽油,测试数据显示汽油的运移距离平方和运移时间呈线性关系,证明了毛细作用的和铺展作用的存在,证实了液态的汽油运移可以用Washburn方程描述时间和运移距离关系;2.气态汽油扩散:使用稳定的气相污染源,测试了汽油气相在空气介质和土壤介质中的扩散。测试结果显示:在均匀的固相土壤介质中,扩散速度受到土壤含水率、土壤颗粒、土壤密度等条件的影响。污染物的浓度分布呈现由污染源向污染外围递减趋势,随着时间的增长固定空间的浓度趋于稳定;气相的汽油运移可以用Einstein-Smoluchowski方程描述时间和运移距离关系;3.汽油吸附:采用直接和间接两种污染方式。直接方式既直接向土柱中定量加入液态汽油,间接方式既是使用气相污染源对土柱进行污染,静止稳定10天。通风实验显示:直接污染的土柱中的浓度下降缓慢,停止通风后浓度可以得到恢复,说明了污染物主要来源于NAPL的挥发;间接污染的土柱经过通风后,浓度下降速度快,并且不能恢复,间接污染的浓度来源于孔隙中的污染物。实验说明汽油被土壤吸附,导致通风后期低的汽油浓度;4.拖尾分析:液态汽油的运移直接导致吸附,随着时间的增长,汽油被吸附的范围加大。铺展现象的发生,说明汽油同土壤之间发生了吸附作用,吸附作用改变了液态汽油表面张力,使被吸附的汽油焓降低。使用BET法测量了实验用土壤的表面积,定量计算出被吸附的汽油量的范围。按照吸附理论和分压定律,被吸附的物质的饱和蒸汽压将出现降低,既宏观表现为浓度降低。实验估算了降低后的浓度值。野外实验:验证气液两相的运移规律5.液相规律:使用气相色谱测试了不同深度土壤中的汽油浓度和组分,测试数据显示:相同深度相同土壤构成时汽油具有相似的组分构成和相近的浓度。接近地面的组分由于受到气相蒸发的影响,短碳链的组分偏少。6.气相规律:通过测试场地的压力场分布,浓度分布和通风后浓度变化,表明场地土壤具有较好的通透性,适用于SVE方法。TVOC浓度由污染区向地面呈逐渐递减趋势。使用气相色谱测试了不同空间位置的汽油组分分布,数据显示相同深度土壤具有相似的组分分布,不同深度的组分分布不同;理论分析,并综合实验的气液运移规律,获得下述结论:1.润湿是NAPL在土壤中的主要运移方式,扩散是气相运移方式。VOCs污染区域划分为毛细作用、铺展作用和气相扩散三个区域;2.润湿和气相吸附形成的界面相和NAPL为非气相VOCs;3.润湿和气相吸附降低VOCs的焓,引起VOCs的饱和蒸气压降低,导致拖尾。更多还原

【Abstract】 Soil vapor extraction (SVE) is a widely accepted and cost-effective technique that is used to remediate unsaturated soil contaminated with volatile organic compounds (VOCs). SVE is a simple process in which a vacuum induces advective airflow to enhance the volatilization of entrapped VOCs.VOCs’leakage usually happens in unsaturated zone. VOCs existing formats are non aqueous phase liquids,interface phase and vaporous phase. NAPL will gradually disappear with time elapsing. Capillary and wetting will enlarge the contaminated soil zone when NAPL is existing, VOCs vapor will transfer into soil porosity constantly.Transportation principle and mechanism, tailing mechanism of VOCs existing phase, non vapor and vapor VOCs when liquid VOCs goes into soil was studied.Laboratory Test: liquid and vapor transportation principle and absorption mechanism1. Liquid gasoline transportation: Quantitative liquid gasoline was added into the soil column. The relationship between square of liquid gasoline running distance vs. elapsed time is a liner one; it shows that the capillary and wetting exist between soil particle and liquid gasoline. The Washburn equation can be used to describe the running distance vs. elapsed time.2. Vaporous gasoline diffusion: Vaporous gasoline source in stable condition used to test vaporous gasoline diffuse in air and soil column. The results shows that diffusion velocity was affected by the soil water content, soil particle size, and soil density. The VOCs concentration is declining from the vaporous source. Concentration equilibrium will be built with the time elapsing. The relationship of vapor running distance vs. elapsed time can be described with Einstein-Smoluchowski equation.3. Gasoline absorption: Two methods were applied, for the direct method, adding quantitative gasoline into the column directly; the indirectly method, a stable gasoline vaporous source was applied and connected with the soil column. Venting the two columns after 10 days equilibrium, the venting shows that the concentration in the directly polluted column is declining slowly, and the concentration can be recovered after venting was stopped. It shows that the VOCs concentration comes from NAPL. For the indirectly polluted column, the concentration decline fast, and the concentration can’t be recovered after stop venting. The concentration came from the soil porosity. It also shows the low concentration comes from the desorption from the interface, which is one reason for low concentration in late venting period.4. Tailing: Sorption happened when the liquid gasoline impregnation into soil part. And the contaminated zone will be enlarged with the time elapsed. The wetting is a kind of interface sorption. The enthalpy will be low and the surface tension will be changed when the sorption happened between the soil and the gasoline. The quantitative gasoline was estimated based on the soil surface that was measured by BET. The sorpted VOCs vapor pressure will be low and the same for concentration based on adsorption theory and the law of partial pressure Field verification: liquid and vapor gasoline transporting principle5. Liquid principle: The concentration and components at different depth were measured. The results show that for the same depth and soil components, the gasoline components are similar and concentrations are closer. The evaporation affects the gasoline components close to ground surface; it contains less short carbon chain components.6. Gas phase principle: the pressure field, concentration field and variation of the concentration in field were measured. The results show the plot has permeability and it fits for SVE. TVOC concentration is declining from the polluted center to the ground surface. GC results show that the components are same at the same depth; the components are different at different depth.. Conclusions:1.Impregnation is the main transportation for NAPL in unsaturated soil and diffusion is the transportation for vapor. The VOCs contaminated zone should be defined as three zones: capillary zone, wetting zone and diffusion zone.2. Non gas phase VOCs should include NAPL and the new interface phase formed by wetting and vapor absorption.3. Wetting and vapor absorbing will reduce VOCs enthalpy, causing lowering of VOCs components vapor pressure, resulting in tailing.更多还原