Density Modification Displacement Using Colloidal Biliquidaphron For DNAPL Contaminated Aquifer Remediation

Dense nonaqueous liquid phases（DNAPLs）are one of the most common contaminants in groundwater.In the process of DNAPL migration in the groundwater,part of it is retained in the pores of the aquifer media to form a residual phase,the free phase part continues to migrate vertically downward under the action of gravity and aggregates in the low permeability layer or impermeable layer.These“residual”or“aggregated”DNAPL in the media have low water solubility and act as a secondary source of pollution to groundwater for a long time slowly releasing contaminants.Hence,the remediation of DNAPL-contaminated groundwater has received widespread attention.It has been shown that surfactant enhanced aquifer remediation is one of the effective methods for DNAPL-contaminated groundwater.However,uncontrolled vertical movement may occur after the decrease of interfacial force of DNAPL,this will expand the area of pollution for surfactant flushing process.The in situ density-modification displacement is to reduce the density of DNAPL by adding water-soluble light organic solvents to turn DNAPL into light non-aqueous phase liquid（LNAPL）,reducing the risk of downward migration and expansion of contamination.However,the density modification based on water-soluble light organic solvents depend on the concentration equilibrium in water and DNAPL,and once the equilibrium is disrupted,the light organic solvents will return to water,resulting in LNAPL becoming DNAPL again,and the density modulation is reversible.Colloidal gas aphrons have good stability,migration and deformability,and have been used in groundwater remediation in recent years.Based on the above status,this thesis proposes to prepare a colloidal biliquid aphron（CBLA）by replacing the gas in the colloidal gas aphron with light non-aqueous phase organic solvent,which is injected into groundwater and then demulsification to release the non-aqueous light organic solvent to miscible with DNAPL,so that DNAPL is directly transformed into LNAPL to achieve irreversible density modification remediation of DNAPL.In this thesis,a systematic study was conducted on the in situ density modification remediation for DNAPL-contaminated groundwater by CBLA.The thesis prepared CBLA with good stability,migration,and shear thinning characteristics;the demulsifier-polymeric aluminum chloride（PACl）was preferentially selected for this CBLA,and the density modification system of CBLA-PACl solution-DNAPL was constructed;the mechanism of in situ irreversible density modification remediation of DNAPL by CBLA was revealed;the effect of CBLA on the density-regulated remediation of DNAPL in homogeneous and heterogeneous aquifers was investigated.The study lays a theoretical foundation for the enhanced remediation of DNAPL-contaminated groundwater by CBLA in situ density modification.The main conclusions are as follows:（1）Preparation and properties of CBLAIn this thesis,micron-scale CBLA was successfully prepared using n-octane,water,sodium dodecyl sulfate（SDS）and dodecyl alcohol polyoxyethylene triether（AEO-3）,with an inner layer of n-octane dissolved with AEO-3 and an outer layer of aqueous phase soap film of SDS.The common anions and cations(Na⁺,Mg²⁺,Fe³⁺and Cl)in groundwater and the aquifer medium had little effect on the stability of CBLA.the horizontal distance of CBLA migration distribution was positively correlated with the injection pressure.（2）Mechanism of CBLA demulsification and influencing factorsIn this thesis,PACl was selected from six iron salts and aluminum salts（ferric chloride,aluminum chloride,aluminum nitrate,aluminum sulfate,ferric sulfate and polymeric aluminum chloride）as an efficient demulsifier for CBLA,0.5 g/L PACl can reach 99%within 10 min,and the demulsification process is in accordance with the pseudo-secondary kinetic model.The mechanism of demulsificaiton is that the positively charged Alb component in PACl adsorbed on the surface of negatively charged CBLA,which neutralizes its charge and reduces the electrostatic repulsion,causing CBLA to destabilize.The demulsification efficiency is influenced by the alkalinity,p H and aquifer material.When the alkalinity is 2 and the p H is less than 3.97,PACl has an efficient demulsification on CBLA.The presence of aquifer material accelerates the demulsification process of CBLA,and the final demulsification efficiency decreases with the decrease of media particle size.（3）Kinetics and influencing factors of irreversible density modificationThe density modification of DNAPL by CBLA conforms to pseudo-second-order reaction kinetics,and the higher the density of DNAPL,the lower the correlation coefficient.The common anions in groundwater have little effect on the efficiency of density modification,while cations have a promoting effect on the efficiency of density modification.CBLA can still maintain structural stability at 10°C,and is suitable for use in underground environments.In the presence of aquifer material,nitrobenzene（NB）,dichloromethane（DCM）,trichloroethylene（TCE）,carbon tetrachloride（CTC）and tetrachloroethylene（PCE）have good density regulation effects,and the regulated densities are 0.966,0.931,0.967,0.986 and 0.983 g/cm³ respectively,with good universality.（4）Mechanism of dynamic density modification of CBLA for remediation of DNAPL-contaminated aquifersWhen the mass ratio of CBLA to PCE is greater than or equal to 2.34,PCE can achieve a good irreversible density-modification remediation effect;the decrease of injection flow rate and the increase of PACl injection amount will enhance the density-modification remediation effect.The process and mechanism of density-modification remediation of DNAPL-contaminated aquifers by CBLA are as follows:PCE produces emulsified and dissolved phases after contact with CBLA,and its density decreases in these two phases in turn;LANPL phase appears after injection of PACl solution,and the density of PCE is further reduced to less than that of water,which completes the irreversible density-modification remediation.Furthermore,the viscosity ratio of CBLA to PCE controls its repellent effect on PCE during the remediation process,and the mass of contaminants remaining in the aquifer after remediation does not exceed（5）The effect of CBLA density modification to strengthen the remediation of DNAPL-contaminated aquifersCBLA can achieve good irreversible density-modification remediation of DNAPL-contaminated homogeneous and heterogeneous aquifers.In the process of density-modification remediation of homogeneous and heterogeneous aquifers,PCE showed a trend of upward migration,the density of PCE in the effluent was maintained at about 0.71 g/cm³,and the residual mass of PCE in the medium was only 0.42 and2.01%of the injected amount.The heterogeneity of the aquifer does not significantly hinder the remediation process and effect,but the lower the permeability coefficient of the aquifer,the longer the time to reach the maximum remediation efficiency and the larger the residual mass of PCE in the remediated medium.