| Tetrachloroethylene(PCE)is one of the commonly found organic pollutants in groundwater.After entering and accumulating in aquifer,the pollutant is difficult to dissolve,move and desorb from media.Although surfactant-enhanced flushing technology can improve the fluidity of pollutants by micellar solubilization and reducing interfacial tension,and it has achieved the effect of enhanced removal to a certain extent,however,the phenomenon of "channel flow" and "priority flow" during the migration of flushing fluid caused by the inherent spatial heterogeneity of the aquifer medium cannot be avoided,leading to problems such as long removal cycles,poor effects,and rebound.Colloidal gas aphrons(CGAs)are heterogeneous dispersion systems composed of a mixture of gas and liquid,and they are easy to deform,have fluidity,and have the properties of non-Newtonian fluid.CGAs can enter the pores of media with different sizes,and they are more evenly distributed in the media,which effectively avoids problems such as channel effect.In this study,CGAs flushing technology was used to increase the solubility and the mobility of PCE in aquifer,by which the transport flux of the pollutant could be increased and the remedial effect could be enhanced.First,a series of static experiments were conducted to investigate the surface activity of representative surfactants such as sodium dodecyl sulfate,tea saponin and sodium alpha-olefin sulfonate,and the stability of the CGAs produced by them,and the main factors affecting the stability of CGAs were analyzed.The results showed that sodium ?-alkenyl sulfonate exhibited the best comprehensive performance in terms of foam performance and underground environmental activity,and it is used to generate CGAs for flushing aquifers contaminated by NAPL.Secondly,the migration characteristics of two phases of gas and liquid during the migration of CGAs in aquifers,the temporal and spatial distribution and regularity of system pressure were explored through simulated column and simulated tank experiments,and the effects of CGAs injection rate,surfactant concentration and media condition on the migration anddistribution of CGAs were also explored.Finally,we comprehensively evaluated the remediation effect of CGAs on homogeneous and heterogeneous aquifers contaminated by PCE.The main conclusions of this study are as follows:(1)Study on the stability of CGAsCGAs can be prepared by stirring the surfactant solution at high speed.The size of the bubbles is micron level(10~100 ?m in diameter),the shape is regular spherical,the liquid film is thick,the bubbles are spaced apart from each other,and they are arranged uniformly.Taking AOS as an example,CGAs had good stability when the stirring speed was 4000~5000 r/min;when the surfactant concentration was greater than CMC,the stability of CGAs was better,and in the experimental concentration range(CMC~67 CMC),the stability enhanced with the increase of surfactant concentration;CGAs had better stability than regular foams,and gradually transformed to ordinary foams during the drainage process;PCE had an adverse effect on the stability of CGAs.(2)Study on the migration and distribution characteristics of CGAs in aquifers(1)CGAs bursted continuously during the migration in aquifers,causing the separation of the gas and the liquid,and forming the foam liquid and the foam gas.The foam liquid was distributed at the front of the CGAs migration front,and the foam gas accumulated above the foam liquid,and they migrated forward together under the push of CGAs.The system pressure in the area covered by CGAs migration would increase,and the system pressure increased with the increase of CGAs migration distance.The greater the injection rate,the greater the growth rate of the injection pressure of the CGAs migration system;the worse the permeability of the medium,the greater the injection pressure of the CGAs migration system.(2)Under single-point and single-side injection conditions,the shape of the sweeping area of CGAs was approximately semi-circular,and the migration front was relatively regular;due to the buoyancy effect during the migration process,CGAs appeared in the "drift up" phenomenon that the upward migration distance in the vertical direction was greater than the downward migration distance.Surfactant concentration had little effect on the migration and distribution of CGAs;when the injection velocity was low or the permeability of the medium was poor,CGAs appeared an obvious "drift up" phenomenon in the later stage of migration,at the same time,the shape of the sweeping area gradually changed from semi-circular to semi-elliptical,and the ratio of the migration distance of CGAs in the horizontal and vertical directions gradually increased;injected CGAs at a constant flow,there was a maximum sweeping area in the aquifer.Compared with traditional liquid flushing fluids,CGAs were less affected by the heterogeneity of the medium,and their migration and distribution were more uniform in the heterogeneous aquifer.(3)Study on the remediation effect of CGAs flushing PCE contaminated aquifersCompared with liquid flushing,the flushing of CGAs had better effects on enhancing the solubility and mobility of PCE,and they had a significant remediation effect on aquifers contaminated by PCE.CGAs flushed homogeneous aquifers with sizes of 0.5~1.0 mm,0.25~0.5 mm,and 0.1~0.25 mm,the corresponding removal rate of PCE was 98.2%,90.8%,and 83.7%,respectively.CGAs flushing could effectively remove PCE contamination in lens with low permeability in heterogeneous aquifers,and it was not found that the situations occured such as PCE sinked and the pollution plume expanded due to the mobilization effect.The removal rate of CGAs flushed heterogeneous aquifer contaminated by PCE reached 91.3%,of which the liquid phase removal rate was 68.7% and the gas phase removal rate was 22.6%.About 3/4 of the removal mechanism was the solubization effect and mobilization effect,and about 1/4 was the volatilization effect of the foam gas. |
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