Determination Of Air-water Interfacial Area And PFOA Transport Characteristics In Vadose Zone

Groundwater is an important element in the environment composition,while the vadose zone is a necessary way for pollutants to enter the underground environment.In the vadose zone,the air-water interfacial area is considered to affect multiphase flow and control the retention and migration of chemical contaminants,pathogens,colloids,andvariousmasstransferprocessesatthe interface.Perfluorooctanoic acid is a new pollutant with surface activity,and it has inherently strong chemical stability and environmental sustainability,making it extremely difficult to be degraded.In the vadose zone,the adsorption of air-water interface is an important factor for the retention of a series of surface-active pollutants such as PFOA when they transport in porous media,but the methods that are currently applied to measure the air-water interfacial area of porous media are so limited.Therefore,a new method for measuring the air-water interfacial area of porous media is needed to be established.And based on the measurement of the air-water interfacial area of porous media,it is necessary to comprehensively conduct environmental monitoring and transport characteristics of perfluorinated compounds in the vadose zone for providing protection for the water resources.The objective of this study is to apply the gas absorption/chemical reaction method in chemical engineering to establish a new measurement method for measuring the air-water interfacial area of the porous media in the vadose zone.In this study,the influences of experimental conditions such as the gas flow rate,liquid flow rate,liquid concentration,and liquid temperature on the measurement results were investigated,and the results were verified and compared with the traditional X-ray microtomography method and interfacial partitioning tracer test method,to determine the correctness and feasibility of chemical reaction absorption method.Finally,based on the measurement of the air-water interfacial area of porous media,the effect of the air-water interfacial area of porous media and the influence factors such like flow rate,PFOA concentration,ionic strength and pH on the retention and adsorption characteristics of perfluorooctanoic acid were investigated,and the fraction of the measured total retention associated with adsorption at the air-water interface is also determined.This study is of great significance to deeply explore the transport characteristics of perfluorinated compound pollutants in the vadose zone,strengthen the environmental risk and health risk assessment of perfluorinated compounds and finally protect our ecological environment.Through this study,we got the following conclusions:（1）The air-water interfacial area of porous media decreased with the increase of saturation in the porous media,and they are in good inverse linear relationship.With the linear relationship above,the air-water interfacial area of porous media at various saturations can be calculated.The maximum air-water interfacial area of the glass beads measured by the X-ray microtomography method and direct measurement of the X-ray micro-scanning are basically the same with the value of the solid surface area of the glass beads,and are both statistically similar to the GSA and N₂/BET,which demonstrated the accuracy of X-ray microtomography in measuring the air-water interfacial area of porous media.The air-water interfacial area measured by the interfacial partitioning tracer method is close to the specific surface area of the porous media when the surface of the porous media is smooth.However,when measuring a porous medium with a rough surface,the measurement results are generally much larger than the specific surface area of the porous medium due to the influence of the roughness.（2）The gas absorption/chemical reaction method can perform robust and accurate measurement of the air-water interfacial area of the natural porous media.Compared with the two traditional methods:The interfacial partitioning tracer test method and X-ray microtomography method,the gas absorption/chemical reaction is simple,economic and easy to operate,and has a short experimental period.In a summary,it is a brand new method that can be used to measure the air-water interfacial area of porous media.The column diameter,gas flow rate,liquid flow rate,liquid concentration,and reaction temperature are important factors that affect the measurement of air-water interfacial area of porous media by gas absorption/chemical reaction method.（3）The air-water interfacial area of glass beads measured by the gas absorption/chemical reaction method is very consistent with the results of the other two standard method.However,the maximum air-water interface area of quartz sand measured by chemical reaction absorption method and X-ray microtomography method are significantly smaller than that measured by interfacial partition tracer method due to the significant microscopic surface roughness of the quartz sand.The difference indicates that the gas absorption/chemical reaction cannot characterize the interfacial area related to the microscopic surface roughness existing in quartz sand under the applied conditions.（4）When PFOA transports in the unsaturated sand porous media,the retention factor and adsorption capacity increase with the increase of interfacial area,and they are greater than the adsorption capacity of adsorption equilibrium and saturation state,which indicates that the effect of adsorption at the air-water interface attributed to the larger retention observed under the unsaturation condition of porous media.The relative contribution of adsorption at the air-water interface to the retention of PFOA depends on the nature medium and the system influencing conditions（saturation,flow rate,pollutant concentration,ionic strength,and p H）.In addition,the adsorption at the air-water interface is an important source of PFOA retention in most conditions,accounting for about 50%of the total retention.