Study On The In-situ Remediation Of Diesel Polluted Heterogeneous Aquifer By Thermal Conductive Heating Combined With Chemical Oxidation

For a long time,the pollution of low permeability zones has been a difficult problem internationally.Thermal Conductive Heating（TCH）as one of thermal remediation technologies can fast remove contaminants with high removal efficiency.Although this technology is energy-intensive and the cost of remediation is high,TCH is one of the few effective technologies for remediating polluted heterogeneous layers in subsurface,especially the polluted low permeability zones,because heat transfer is not restricted by the permeability or heterogeneity of the subsurface.In this study,the heat transfer,the remediation effect and the influencing factors in the process of TCH were studied in order to improve the application efficiency of the technology;for the perspective of reducing the remediation cost,it was proposed to combine In-Situ Chemical Oxidation（ISCO）in relatively high permeable zone of heterogeneous aquifer,which was relative low-cost.The residual heat in the TCH cooling process was used to activate persulfate（PS）,and the pollutants were oxidized with the wasted heat energy being reused,which realized the combination of TCH and ISCO in space and time and provided a new idea for the effective remediation of complicated heterogeneous aquifers.The thermal remediation technologies have successfully remediated multiple types of organic pollutants in actual sites,and satisfactory remediation results are achieved even in low permeability zones.Although scholars have conducted a large number of studies on TCH,most of them are from the perspective of engineering application and focus on the final removal efficiency of the pollutants.There are few studies on the change of the heated zone and the removal mechanism of pollutants in the process of remediation.Among them,temperature is an extremely important parameter in thermal treatment,the temperature and the area of the heated zone in aquifer being closely related to the removal efficiency of pollutants and the range of remediation zone.Therefore,understanding the change of heat transfer,the influence of the groundwater velocity and the aquifer medium on the temperature distribution of heated zone was very necessary,which can provide theoretical guidance for the analysis of pollutant removal effects.In addition,how to reduce the required temperature for pollutants removal was of great significance to lower the cost of TCH remediation.Firstly,the heat transfer law and the change of the heated zone by TCH were quantitatively studied,the heat transfer mechanism of TCH in the aquifer being clarified.Diesel with high boiling point was selected as the target pollutant,and the removal mechanism of diesel in the aquifer by TCH was elucidated theoretically and experimentally.It was studied to reduce the remediation temperature of diesel by the addition of azeotrope ethanol,which would reduce the energy consumption.Finally,the TCH and ISCO technologies were combined in space and time to treat the low-permeability and high-permeability zones,respectively,making full use of the waste heat in the cooling process of TCH,which realized the complementary advantages of different remediation technologies,significantly reduced the cost of remediation and provided a new perspective for the remediation of complex sites.The main results are as follows:1.The heat transfer in the aquifers by TCH（1）The experimental results showed that in homogeneous aquifers,the horizontal heat transfer velocity gradually decreased and the heated area finally tended to reach stability with the heating time extention.A mathematical model of heat transfer distance（y）and time（t）was established for a quantitative analysis,and the function of y-t showed a quadratic function relationship of one variable.The decreasing rate of heat transfer velocity and the stability time of the heated area could be obtained by the derivative of the function.With the increase of groundwater velocity and the power of the heating resistance,the heated zone reached stability faster.Under the condition of medium sand aquifer and the heating resistance of 500W,when the groundwater velocity increased from 0 to 1m/d,the decreasing rate of heat transfer velocity was increased from 0.0002 to 0.0004m/s²,and the time for the heated zone reaching the stability was shortened by 232min.（2）The thermal conductivity of the medium played a key role on the temperature change of the sand.Theoretically,the order of the heated area should be consistent with the thermal conductivity:coarse sand>medium sand>fine sand,while there was an abnormal phenomenon that the heated area of fine sand aquifer being larger than that of medium sand aquifer.Because the heat-pipe effect and thermal dispersion existed in the fine sand aquifer,the condensate would not drop back to the heated zone due to the large capillary force in the aquifer,which enhanced the heat transfer capacity of the aquifer.Theλ^*value was introduced to describe the heat transfer capacity of the aquifer,including the influence of heat conduction and convection on the heat transfer.The maximumλ^*of fine sand was 160 times of its thermal conductivity,which meant that the contribution of heat-pipe effect to heat transfer was much higher than heat conduction.Theλ^*value was positively correlated with groundwater velocity and the power of the heating resistance,and it theoretically explained that the heated zone presented an obvious V-shaped distribution under the condition of high power of heating resistance or at a large groundwater velocity.（3）In layered heterogeneous aquifer（medium sand-clay-medium sand）,the heat transfer capacity valueλ^*in clay layer was basically equal to the thermal conductivity value of clay,which showed that the heat conduction was dominant in clay layer.The thermal diffusivity of clay was small and the heat transfer rate was slow,resulting the isotherms in the clay layer being depressed;additionally,the average temperature of the heterogeneous aquifer contained clay layer was higher because of the larger thermal conductivity of clay.2.The diesel removal mechanism,influencing factors and the remediation effect of diesel by TCH（1）Evaporation/boiling is the main removal mechanism of diesel.According to Antoine equation and Raoult’s law,the mixture of water-diesel(C₁₀-C₂₀)started co-boiling at 100.3℃theoretically,which was below the boiling point of diesel（180-370℃）,and the mass of TPH in the vapor was 54.18%.To further lower the required temperature for diesel removal,the azeotrope ethanol was added.When the volume ratio of ethanol was 20%,the co-boiling temperature of the mixture was reduced to80.8-97.9℃,which was decreased by 2-20℃compared with no ethanol added.The addition of azeotrope could further lower the comprehensive energy consumption of remediation.Understanding the temperature range of pollutant removal from aquifer provided theoretical guidance for the remediation of practical contaminated sites.（2）The removal of diesel accorded with the first-order kinetic model.The higher the heating temperature,the larger the initial concentration of diesel,the greater the removal efficiency of pollutant.Temperature had a significant effect on the pollutants removal,the kinetic coefficient k being increased 8.3 times with the temperature rising from 20 to 80°C.（3）In different aquifer media,the remediation effects of TCH treating diesel contaminated aquifer was good and the remediation effect of coarse sand aquifer was better than that of the other two media,the average residual TPH concentration being0.23mg/g on solid phase.3.The remediation effect of diesel polluted heterogeneous aquifer by TCH combined with ISCO（1）The combination of TCH and ISCO technologies remediated the diesel polluted aquifer contained clay lens,the low-permeability and high-permeability layers being remediated by different technologies.When the heating time of TCH was 2h,the TPH removal efficiency in the low permeability lens was around 75.5%-89.6%.In the cooling process of TCH,the residual heat was fully utilized to activate PS,the TPH removal efficiency was increased to 80.3%-91.2%in the high permeability aquifer.The combination of the two technologies not only achieved complementary advantages,but also expanded the scope of remediation,the influence distance of the heating resistances being increased from 25.5cm to 42.5cm.This provided potential for reducing the consumption of energy and the remediation costs.（2）Comparing different heating methods,under the condition that the same energy consumption of a heating resistance of 800W and two 400W heating resistances,the latter was the optimal heating method,because of its higher temperature in lens and high permeability zone and its good remediation effect.The main innovations of this paper are as follows:The influence of different factors on the distribution of heated zone was systematically analyzed and the change of heat transfer by TCH was quantitatively described by mathematical model.It was found that the temperature of fine sand or clay was higher,which provided theoretical support for TCH to remediate the polluted heterogeneous aquifer.The removal mechanism of high boiling point pollutant diesel in the aquifer by TCH was clarified and the remediation temperature was significantly reduced with the addition of azeotrope,which provided a theoretical basis for saving the remediation cost.A new idea of combining of TCH and ISCO in space and time was proposed.TCH remediated the low permeability area and ISCO remediated the high permeability zone by making full use of the residual heat of TCH cooling process,realizing the complementary advantages of different technologies,making it possible to reduce the remediation cost of TCH.This was helpful to optimize the parameters of the thermal remediation technology for engineering application,reduce the comprehensive energy consumption and the remediation cost,which laid a foundation for the popularization and application of thermal remediation technology.