Study On Desorption Performance Of In Situ Microemulsion For Mixed Chlorinated Hydrocarbons In Aquifer

As an important chemical raw material,chlorinated hydrocarbons are widely used in the production process.It has a"triple"effect,low water solubility and high oil-water（O/W）interfacial tension,which is difficult to be removed once released into the underground environment.Microemulsion has been applied in the field of environmental remediation by more and more researchers because of its super-strong solubability and ultra-low interfacial tension.Compared with direct injection of microemulsion,in situ microemulsion does not need to inject oil phase,and can achieve desorption and removal of pollutants by directly forming microemulsion with pollutants in aquifer,which has the advantages of low secondary pollution risk,low injection difficulty,and less influence by environmental conditions.At present,the research on in situ microemulsion solubilization of chlorinated hydrocarbons is mostly focused on a single contaminant,while in actual contaminated aquifers,there are usually complex systems with multiple components co-existing.Therefore,it is of great practical significance to study the desorption performance of in situ microemulsion on aquifers polluted by chlorinated hydrocarbon mixtures.In this paper,perchloroethylene（PCE）,trichloroethylene（TCE）,carbon tetrachloride（CT）,trichloromethane（CF）were selected as the target pollutants,sodium dodecyl sulfate（SDS）as the surfactant,n-butanol as the co-surfactant and potassium chloride（KCl）to construct in situ microemulsion.Firstly,the influence of SDS,KCl,n-butanol concentration and chlorinated hydrocarbon mixture on the phase state of microemulsion was compared and analyzed.The law of solution-enhancing formula of different chlorinated hydrocarbon mixtures was explored.The applicability of the precursor solution in underground environment was confirmed by the determination of its physicochemical properties.Then,by static solution-increasing experiments,the solution-increasing rule of in situ microemulsion on different chlorinated hydrocarbon mixtures and the influence of hydrogeo-conditions on solution-increasing ability were analyzed.Finally,a one-dimensional simulated column experiment was carried out to evaluate the desorption performance of in situ microemulsion on the chlorinated hydrocarbon mixture in the simulated column,and the influence of multiple parameters on the desorption efficiency of in situ microemulsion was determined.The desorption mechanism was analyzed according to the morphology of the effluent and the mass concentration of pollutants.Specific conclusions are as follows:（1）Under the condition of 3 wt.%SDS/2 wt.%KCl,the alcohol-scanning method was used to determine the concentration of n-butanol in the soluble formula of different chlorinated hydrocarbon mixtures.There was a quadratic function relationship between the required n-butanol concentration and the equivalent alkyl carbon number of chlorinated hydrocarbon mixtures,which could be used to determine the formula parameters.The viscosity of the microemulsion precursor liquid is not more than 2 cp,and it is easy to be injected into underground environment and migrate in underground environment.The Zeta potentials are all negative,which is not easy to be absorbed by the underground medium with negative charge,indicating that the Zeta potentials have good adsorption resistance.The precursor solution with higher n-butanol concentration does not precipitate at low temperature or in the presence of common inorganic salts,indicating that it has good ionic interference resistance and low temperature resistance.（2）The microemulsion has good solubability for different chlorinated hydrocarbon mixtures,and the total solubability concentration can reach more than 500 g/L,which has good solubability.The composition of chlorinated hydrocarbon mixtures in microemulsion was different from that before solubination,indicating that the microemulsion was selective to the solubination of chlorinated hydrocarbon mixtures,and CF with higher solubination polarity was selected.The solubility of chlorinated hydrocarbons was PCE>CT>TCE>CF;At low temperature,the solubability of the microemulsion for chlorinated hydrocarbon mixtures is enhanced,while the selective solubability of chlorinated hydrocarbon mixtures is weakened.Among the hydrochemical components,inorganic cations(K⁺,Na⁺,Ca²⁺,Mg²⁺)can improve the solubility of the microemulsion on the chlorinated hydrocarbon mixture,while inorganic anions(SO₄^2-,CO₃^2-,HCO₃^-,Cl^-)have no significant effect.（3）SDS and n-butanol could be migrated synchronously in the microemulsion precursor solution,and 96.2%SDS and 98.1%n-butanol were washed out in the dynamic experiment,with less adsorption loss.The chlorinated hydrocarbon mixtures in polluted aquifers can be removed by forming in-situ microemulsion.The total concentration of chlorinated hydrocarbons in the effluent can reach 237.9 g/L,which is19.2 times of that under the condition of SDS solution rinsing with the same mass concentration.The decolorization process can be divided into three processes:micellar,microemulsion and micellar.In the experimental range,the optimal conditions of in situ microemulsion technique are as follows:fine sand medium（particle size 0.1-0.25 mm）,injection flow rate of 0.5 m L/min,residual saturation of 15%.