Injecting n-BuOH to achieve density conversion of dense non-aqueous phase liquid: Pore-scale experimental simulation
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摘要: 密度大于水的重非水液相（DNAPLs）有机污染物在重力作用下向地下介质深部迁移从而增加污染范围。前人通过一维砂柱和二维砂箱试验发现利用密度调节技术可降低DNAPLs向下迁移的风险，但目前缺乏微观尺度上密度调节影响DNAPLs迁移的定量观测。本研究试验模拟丁醇注入微空隙调节四氯乙烯（PCE）的密度，通过建立非水相中染色PCE浓度、密度与灰度的定量关系，监测注入丁醇后空隙介质中非水相密度的动态变化，基于空隙中代表性非水相PCE受力情况分析其运移状态，揭示空隙尺度介质性质和密度调节程度对DNAPLs迁移的影响。试验结果表明：丁醇注入后，PCE浓度和密度迅速下降，离散状PCE与丁醇有效接触面积大且起效快；当非水相密度降至略大于水相密度时，非水相受毛细力和摩擦力的影响停止向下迁移；当非水相密度小于水相密度时，非水相才在注入压力与浮力的作用下克服毛细力、重力和摩擦力向上迁移；注入压力、摩擦力、毛细力、浮力与重力影响着空隙中非水相的迁移行为，空隙半径越大，毛细力对调节PCE向上迁移的影响越小；密度比水小的丁醇注入介质后向上迁移，因此丁醇从DNAPLs下端注入可提高修复效率。试验证实了向空隙介质中注入丁醇能够显著减小DNAPLs的密度从而降低其向下迁移的风险，为实际场地DNAPLs修复方案的制定提供微观机制方面的信息。Abstract: Dense non-aqueous phase liquids(DNAPLs) with a density greater than water have a tendency to migrate to the depth of the underground medium under natural conditions and under the action of gravity in the process of artificial remediation, thus increasing the risk of contamination.Previous experiments on one-dimensional sand column and two-dimensional sand box have found that the density-modified displacement(DMD) could reduce the risk of downward migration of DNAPLs, but there is a lack of quantitative observation on the effect of DMD on DNAPLs migration at the pore scale.In this study, simulated n-BuOH injection into microvoids was used to achieve PCE density conversion.By establishing the quantitative relationship between the PCE concentration, density and grayscale of dyeing PCE in non-aqueous phase(NAPL), the dynamic change of PCE-butanol density in microvoids after n-BuOH injection was quantitatively monitored.The migration state was analyzed based on the stress of representative PCE-butanol blobs in voids.The influence of the properties of void-scale media and density regulation on DNAPLs migration was revealed.The experimental results show that after n-BuOH injection, the concentration and density of PCE decreased rapidly, resulting in the reduction of the combined force of buoyancy and gravity and the friction of tube wall in the PCE-butanol blobs.The effective contact area between discrete PCE and n-BuOH is large and the effect is fast.When the density of PCE-butanol blobs decreases to slightly higher than that of water phase, the PCE-butanol blobs stop migrating downward under the influence of capillary force and friction.When the density of PCE-butanol blobs is less than that of water phase, the PCE-butanol blobs migrates upward, overcoming the capillary force, gravity and friction under the action of injection pressure and buoyancy.Injection pressure, friction, capillary force, buoyancy and gravity influence the migration behavior of PCE-butanol blobs in microvoid.The larger the microvoid radius, is the less influence the capillary force has on regulating the upward migration of PCE.n-BuOH with lower density than water migrates upward after being injected into the microvoids, so the n-BuOH injection from the lower end could improve the repair efficiency.The experiment confirmed that the injection of n-BuOH into the porous medium could significantly reduce the density of the DNAPLs and the risk of downward migration, providing information on the microscopic mechanism for the DNAPLs remediation program in the actual site.
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图 3 染色非水相中PCE体积分数与灰度和密度的关系图
a.灰度与染色非水相中PCE体积分数的关系; b.染色非水相密度与灰度的关系; c.比色卡
Figure 3. Relationship between PCE concentration and grayscale and density in dyeing PCE- butanol mixed phase
图 4 代表性染色PCE团位置图
Figure 4. Location of representative dyeing PCE blobs(n-BuOH injection on the right side and outflow on the left side)
图 5 A点染色非水相中PCE体积分数变化与迁移图(右侧注入丁醇左侧流出)
Figure 5. Variations and migration of PCE concentration dyeing PCE-butanol blobs at point A(n-BuOH injection on the right and outflow on the left)
图 6 B点染色非水相中PCE体积分数变化与迁移图(右侧注入丁醇左侧流出)
Figure 6. Variations and migration of PCE concentration dyeing PCE-butanol blobs at point B(n-BuOH injection on the right and outflow on the left)
图 7 A点和B点染色非水相受力随丁醇注入量变化图
Figure 7. Stress variation diagram of the dyeing PCE-butanol blobs at points A and B with the injection amount of n-BuOH
表 1 微空隙模板中裂隙特征
Table 1. Fracture properties in microvoids
裂隙名称 延伸长度/cm 角度/(°) 平均隙宽/mm N1 2.53 84.51 1.14 N2 1.81 21.58 0.86 N3 2.17 127.09 0.69 N4 3.06 59.49 0.50 N5 2.12 23.08 0.76 N6 2.65 143.60 0.65 N7 1.67 32.34 0.72 N8 2.05 76.40 0.89 
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