| The remediation of polluted groundwater depends on the permeability and homogeneity of aquifer media and the characteristics of target pollutants,among which the difficulty and bottleneck problem is the removal of heavy non-aqueous liquid pollutants（DNAPL）in heterogeneous aquifers.Steam injection remediation technology is an effective method to simultaneously remediate volatile and semi-volatile organic polluted aquifers.However,the application effect of steam injection technology is affected in low permeability and heterogeneous polluted aquifers.Therefore,how to improve the remediation effect of steam injection in heterogeneous aquifers is the critical problem at present.Although steam injection remediation technology is widely used in developed countries,there is a lack of systematic research on the influencing factors of remediation effect and remediation in heterogeneous aquifers.At present,the influence of heterogeneous aquifers on steam migration,temperature distribution and contaminant removal effect are not clear.There are few studies on the remediation effect of enhanced steam injection in heterogeneous aquifers.So how to improve the effect of steam injection remediation technology in the heterogeneous aquifer is still a key issue.This article mainly researched the influencing factors of steam injection remediation of polluted aquifers,such as steam flow,medium,pollutant types,temperature,etc.The effect of temperature on the physical and chemical properties of organic compounds was quantitatively described and the change in azeotropic temperature was found when alcohols were used as azeotropic agents.The steam migration and temperature distribution were studied in homogeneous and heterogeneous aquifers（layered heterogeneous aquifers and lens-Containing heterogeneous aquifers）,and the temperature distribution models were built.The method of adding alcohol into the heterogeneous aquifer to enhance the remediation effect of the contaminated aquifer,and the mechanism of alcohol enhanced steam remediation was expounded.The research results provided a theoretical basis for improving the steam injection technology and have practical application significance.This study not only improved the removal effect of organic compounds in heterogeneous aquifers,and shortened the repair time,but also reduced the repair cost,providing a new idea for the application of steam injection in heterogeneous contaminated aquifers.The main results are as follows:1.Considering various factors,especially considering the effects of four alcohols on reducing surface tension and azeotropic temperature,ethanol was selected as the reagent to strengthen steam remediation.The azeotropic temperatures of the binary mixture of chlorobenzene,nitrobenzene and water were 94.3℃and 97.4℃respectively.The azeotrope temperature decreased by 12.9℃and 14.2℃respectively after adding ethanol,which greatly reduced energy consumption and pollutant removal cost.2.Considering the remediation efficiency and repair cost comprehensively,it was determined that 30°C is the minimum temperature for chlorobenzene removal.When the temperature was higher than 30°C,chlorobenzene had a better removal effect in different media.It was found that the minimum removal temperature of naphthalene was 50℃.When the temperature was higher than the minimum temperature,the removal rates of chlorobenzene and naphthalene in different media were above 93.0%.The restoration area of steam was predicted according to the minimum temperature of chlorobenzene.The predicted restoration area was compared with the actual restoration area when the chlorobenzene removal rate was higher than 98.0%,and the difference was only 8.7%.Therefore,the minimum temperature of pollutants can be used to determine the steam remediation area and radius to a certain extent.3.In a homogeneous aquifer,when the hydraulic gradient of the aquifer was very small,the temperature presented symmetrical distribution after steam injection.When the groundwater velocity increased,due to the convection of water flow,the steam-affected area expanded downstream and no longer presented symmetrical distribution.Both the steam flow rate and the size of medium particles affected the shape of the heating zone distribution.It was found that when the hydraulic conductivity of the aquifer was lower than 1.6×10-3cm·s-1,the heating zone in the aquifer had an H-shaped distribution.When it was higher than 3.5×10-2 cm·s-1,the heating zone had a V-shaped distribution for a high steam injection flow rate(1.0 kg·h-1),and an H-shaped distribution for a low injection flow rate(0.5 kg·h-1).Due to the thermal conductivity,permeability and capillary force of the aquifer medium,the total area of the heating zone in the aquifers with different media was in the following order of sand particle size:coarse sand>fine sand>medium sand.Owing to the heat pipe and heat dispersion effects,the heating zone area in the fine sand aquifer was larger than that of the medium sand aquifer.When the capillary force was large,the condensate produced was not easy to flow back to the heating zone to reduce the temperature,and the heat transfer between water and medium was enhanced due to thermal dispersion.Therefore,the fine sand aquifer had a better heat transfer capacity,making it have a larger heating zone area.4.It was found that in the layered heterogeneous aquifer,the formation structure of the upper low permeability medium and the lower high permeability medium blocked the steam,forming a steam blocking interface,resulting in the accumulation of steam below the interface and the increase of the temperature in the lower medium.The larger the hydraulic conductivity is,the more obvious the vapor accumulation is.When the hydraulic conductivity ratio（lower layer/upper layer）R value increased by 20 times,the average temperature under the interface increased by 15℃.The concentration of nitrobenzene in the lower layer decreased by 47.0%,while the nitrobenzene concentration in the upper low permeability medium increased by 1 time.The uneven temperature distribution resulted from the interface effect increased the removal rate of the lower nitrobenzene,while the nitrobenzene accumulated in the upper medium,resulting in uneven removal of nitrobenzene and reduced remediation effect.In the low permeability medium,the heat convection was weakened and the heat conduction was dominant.The reason and mechanism of the reduction of steam remediation effect for layered heterogeneous aquifer were clarified.Due to the interface interception,the steam was unable to enter the upper medium,which weakened the heat convection in the upper low-permeability aquifer.It mainly depended on the heat conduction heat transfer caused by the temperature difference,resulting in slow heat transfer in the upper layer and reduced repair effect.Given the problem that the remediation effect decreased due to the layered heterogeneous interface,the effect of enhanced steam remediation was studied.In the layered heterogeneous polluted aquifer system with R=381,after adding ethanol,the average temperature of the simulated system increased by 12℃,and the steam influence area increased by 13.0%.The content of nitrobenzene in groundwater decreased by 16.1%,and that in the solid phase decreased by 14.0%.The addition of ethanol reduced the accumulation of pollutants in the upper medium and enhanced the removal effect of nitrobenzene.The addition of ethanol resulted in a significant decrease in surface tension,and the gradient of surface tension caused Marangoni convection,which enhanced heat and mass transfer processes.Azeotrope occurred in the steam zone,which enhanced pollutant removal and reduced repair costs.5.In the heterogeneous aquifer with low permeability lens,a steam blocking interface was formed below the lens,resulting in the flow of steam around the lens,and the remediation effect was not ideal.When the hydraulic conductivity ratio（background/lens）R increased from 88 to 190,the average temperature in the lens decreased by17℃,the area of the heating zone decreased by 42.0%,and the removal rate of nitrobenzene in the lens decreased by 31.0%.The larger the ratio of hydraulic conductivity,the less ideal the effect of steam injection technology on repairing nitrobenzene contaminated lens heterogeneous aquifer.Aiming at the problem of poor nitrobenzene repair effect caused by low permeability lens,the enhanced steam remediation effect was studied.When the hydraulic conductivity ratio R=88 and R=190,the average temperature and the area affected by steam in the simulation system increased by adding ethanol.The concentrations of nitrobenzene in groundwater decreased by 20.0%and 40.7%,and those in the lens decreased by 23.0%and 49.5%,respectively.At the end of the experiment,nitrobenzene in the solid phase was mainly concentrated in the low permeability lens.After adding ethanol,the content of nitrobenzene in the solid phase in the lens decreased by more than 30.0%.The addition of ethanol can not only improve the repair efficiency,and shorten the repair time,but also reduce the repair cost.Moreover,it is verified that the addition of ethanol in different formation structures can enhance the repair effect of steam injection technology,and reduce the application limitation of steam injection technology in heterogeneous aquifers,thus further expanding the application scope of steam injection technology.The main innovations of this paper are as follows:it revealed the migration and heat transfer mechanism of steam in homogeneous and heterogeneous aquifers and clarified the reasons and mechanism of the degradation of steam remediation effect in heterogeneous aquifers.The minimum temperature at which chlorobenzene removal rate was higher than 96.0%and naphthalene removal rate was higher than 93.0%was found,and the feasibility of predicting the remediation area by the minimum temperature was verified in the simulation system.A method of using ethanol to strengthen the remediation of heterogeneous aquifers was proposed and the mechanism of ethanol enhanced remediation was clarified.The research results provided theoretical support for the optimization and enhancement of steam remediation technology. |
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