Enhanced Flushing Remediation Of Tetrachloroethylene Contaminated Aquifers By In Situ Microemulsions

As important chemical raw materials,halogenated hydrocarbons are widely used in the process of industrial production,which are highly toxic and poorly water soluble.The occurrence of various pollution accidents has caused the release of such contaminants into the underground environment,resulting in serious pollution of groundwater.Surfactant-enhanced aquifer remediation technology is one of the most commonly used remediation technologies for aquifer halogenated hydrocarbon pollution.This technology uses the solubilization and flow-enhancing properties of surfactants to increase the solubility of contaminants in groundwater and enhance their mobility in the medium,thus accelerating the migration and removal of contaminants from the aquifer.However,the solubilization and flow-enhancing ability of single surfactant is limited,and the trailing and rebounding of pollutant concentration will occur in the late stage and after the end of flushing,making it difficult to achieve rapid and efficient removal of contaminants.In this paper,the effect of in situ microemulsion flushing technology on the remediation of contaminated aquifers was investigated with a typical halogenated hydrocarbon tetrachloroethylene(PCE)as the target contaminant.The microemulsion is a stable colloidal dispersion system formed spontaneously by oil,water,surfactant and co-surfactant of different identities,which has extremely strong solubilization ability and ultra-low interfacial tension,and its solubilization and flow-increasing performance is substantially improved compared with that of traditional surfactants.The mechanism of in-situ microemulsion flushing is to mix surfactant and co-surfactant in a set ratio,and then inject the mixture into the aquifer as flushing solution.When the mixture encounters the contaminant,the contaminant is removed by redistributing into the aqueous phase and forming microemulsion with the mixture.Compared with direct injection of microemulsions,the in-situ flushing method has the advantages of lower injection difficulty and more stable microemulsion formation.In this paper,we selected a variety of surfactants commonly used in environmental remediation field,compared the properties of biological toxicity and solubilization ability,and conducted a phase formation study of different surfactant-co-surfactant binary systems with PCE,and screened out the compound combination with good microemulsion phase formation ability;then we conducted solubilization and desorption experiments with this system,analyzed the influence of factors such as composition ratio and media type,and comprehensively evaluated the solubilization and media contamination desorption effect of this system on PCE.we continued to carry out the simulated contaminated aquifer flushing experiments of the in situ microemulsion system to characterize the formation of in situ microemulsions and clarify the influence of several parameters on the formation and phase change process of microemulsions.The contribution of in situ microemulsions to PCE removal was also calculated,and the remediation efficacy of microemulsion flushing on contaminated aquifers was evaluated in terms of pollution removal rate and solute residue.The specific conclusions are as follows.(1)Sodium dodecyl sulfate(SDS),Tween 80(Tween80)and lauryl polyoxyethylene ether(Brij35)all have good solubilization ability for PCE,among which Tween80 and Brij35 are more adaptable to the subsurface environment.The microemulsions were constructed with isopropanol and n-butanol,respectively.The Tween80 system could not form microemulsions due to severe emulsification during mixing with PCE;the formation of Brij35-isopropanol-PCE microemulsion system required a higher concentration of isopropanol,while microemulsions can be successfully constructed at 5%-30% volume of n-butanol.The density of the constructed microemulsions was similar to that of pure water,and the viscosity was in the range of 10.34-49.77 c P;the oil-water interfacial tension could be reduced to 1.00 m N/m,which was more than 4.22 m N/m lower than that of the single Brij35 solution.(2)The microemulsion-based Brij35-n-butanol system has better effect on PCE solubilization and desorption than single Brij35 or compound system.The system can dissolve up to 61.12 g/L of PCE and desorb 89.4% of contaminants in the medium.The system with high content of Brij35 has high viscosity,and the increase of n-butanol content can reduce the viscosity of the systems.Without medium,when the volume fraction of n-butanol is higher than 20%,the dissolved amount of PCE is proportional to the concentration of Brij35,and when the medium is present;the viscosity has a greater impact on the desorption of medium contamination,and the low viscosity system containing low concentration of Brij35 always has a better desorption effect,and it is also found that the smaller the particle size of the medium and the higher the organic matter content,the lower the desorption efficiency of PCE.(3)The process of PCE solubilization during the flushing of contaminated aquifers with Brij35-n-butanol mixed solution was micelle-microemulsion-micelle.Microemulsions contribute more to pollutant removal,W/O microemulsions enhanced the mobility of contaminants,while O/W microemulsions mainly promoted solubilization.The dissolved concentration of PCE in microemulsions can be more than30 g/L,which was more than 2.4 times of micelle structure.Microemulsions readily formed in situ in aquifers with high contaminant loadings;the increase of flushing flow rate enhanced the solubilization of contaminants and promoted the formation of W/O microemulsions;the smaller the particle size of the medium,the more difficult the flow of contaminants was and the less likely the W/O microemulsions were formed;the shorter the migration distance of the flushing fluid,the earlier the microemulsions appear and the longer maintaining period.(4)The microemulsion-based Brij35-n-butanol flushing system had a good effect on the remediation of PCE-contaminated aquifers,with removal rates of 94.42% and94.26% in the medium sand and fine sand aquifers,respectively,which were 53.48%and 66.86% higher than those of the single Brij35 solution,and could significantly reduce the trailing effect in the late flushing stage in the medium sand aquifer;during the flushing process,rates of Brij35 and n-butanol attrition was 18.71% and 4.59% in the medium sand aquifer and 26.11% and13.28% in the fine sand aquifer,respectively;after flushing,the injection of equal volume of water could reduce the concentration of Brij35 and n-butanol to 1.91 g/L,respectively.After the flushing,the concentration of Brij35 and n-butanol could be reduced to below 1.91 g/L and 1.21 g/L respectively,effectively avoiding the generation of secondary pollution.