Study On The Effects Of Alcohol-enhanced Air Sparging Remediation In A Benzene-contaminated Aquifer

In recent years,with rapid social and economic development,soil and groundwater organic pollution has become increasingly serious.Aiming at volatile organic pollutants(VOCs),air sparging(AS)has been widely used in groundwater pollution remediation due to its advantages of simple operation,low cost and high efficiency.In addition,in the actual site remediation process,surfactants can often be added to further improve the removal rates of organic pollutants in AS process or to solve the "tailing effect" of AS remediating low-permeability formations,that is,surfactant-enhanced air sparging(SEAS).This technology can reduce the surface tension of groundwater,increase air saturation during the sparging process,and increase the area of zone of the influence(ZOI),thereby achieving the purpose of improving the removal efficiency of pollutants.Recently,scholars have discovered that alcohols with appropriate solubility and Henry's constant can accumulate organic pollutants at the water-air interface,thereby enhancing the volatilization rate of pollutants and increasing the removal rate of pollutants.That is alcohol-enhanced AS(AEAS).Compared with SEAS,AEAS technology has gradually received widespread attention as a new reinforcement method.The existing research on AEAS technology only observes the removal of pollutants in the water during sparging by adding alcohol to the water,and sparging in water is a two-phase system of water and air.While AS is often applied to aquifers,sparging in a aquifer is a multi-phase system in which water,gas and solid coexist.In addition,the current research also ignores that alcohol could also reduce the surface tension of groundwater.More importantly,the enhancing mechanism of alcohol in AEAS process,such as whether the alcohol volatilizes and whether the volatilization process is beneficial to the removal of pollutants,needs to be solved.In this study,medium carbon chain alcohol(1-heptanol)was used as the researchobject,and surfactant(sodium dodecylbenzene sulfonate,SDBS)commonly used in SEAS was selected as a comparison to explore the mechanism of gas migration and distribution and the remediation effects of benzene-contaminated aquifers in the enhancing process of different reagents.An improved two-dimensional airflow visualization device was selected to study the mechanism of gas migration and distribution during SEAS and AEAS in aquifers with different lithologies(medium sand,channelized flow;gravel,bubbly flow).In addition,one-dimensional simulation column experiments were conducted to investigate the effects of removing pollutants in benzene-contaminated aquifers in SEAS and AEAS and the concentration of alcohol under different media lithology conditions.And the theory of gas-liquid mass transfer explained why AEAS remediating benzene pollution is better than SEAS.The main research results of this study are as follows:1.Changes in main parameters in AS,SEAS and AEAS(1)The addition of 1-heptanol or SDBS increased the required sparging pressure in the same air injection rate and medium.(2)The addition of 1-heptanol or SDBS significantly reduced the surface tension of groundwater and greatly increased air saturation,but the mechanism of action was different in different media.It could reduce the surface tension of groundwater in medium sand,and play a role in stabilizing bubbles in gravel.2.Study on the zone of influence(ZOI)and airflow distribution in AS,SEAS and AEAS(1)When the air injection rate was 1000 L/h in three different systems,the addition of 1-heptanol and SDBS increased the area of ZOI in medium sand(channelized flow)by 25.7% and 21.8%,respectively,while the area of ZOI in gravel(bubbly flow)only increased by 8.1% and 5.2%,respectively.This showed that within a certain range of medium particle diameters,the finer the particle diameter of the medium,the more obvious the increase of the area of ZOI in AEAS and SEAS.(2)At the same air injection rate,the addition of 1-heptanol or SDBS made the airflow distribution range larger and the curve of airflow distribution relatively flat in medium sand,while the increase in the airflow distribution range was not obvious andthe curve of airflow distribution was relatively steep in gravel.(3)The distribution of airflow in ZOI under different air injection rates showed obvious similarities in three different systems.3.Study on the effects of AS,SEAS and AEAS remediation in a benzene-contaminated aquifer(1)The time required for AEAS to remediate benzene pollution was significantly less than that of SEAS.In the study of benzene pollution removal.It was found that when the average removal efficiency of benzene reached more than 95%,compared with the time required for AS,the time required for SEAS and AEAS was shortened by 28.6% and 52.4%,respectively,in the medium sand and by 16.7% and 58.3%,respectively,in the gravel.This showed that the addition of SDBS or 1-heptanol could significantly improve the removal rate of benzene pollutants,and the removal rate of AEAS was significantly better than that of SEAS.(2)Under the same surface tension condition,the addition of 1-heptanol was better than SDBS in removing benzene pollutants.This was because the flow field disturbance in AEAS process caused the alcohol existing on the gas-liquid interface to volatilize in the carrying of air,thereby inducing Marangoni convection at the interface,enhancing the gas-liquid mass transfer rate,and increasing the benzene removal rate at the interface.Therefore,this study further theoretically explained the mechanism of alcohol at the gas-liquid interface during AEAS process,providing a theoretical basis for the application of AEAS technology.