Micro-mechanism of residual PCE removal from aquifers by in-situ microemulsion: behavior and proportion of mobilization and solubilization.

In-situ microemulsion has shown great potential for remediation of dense non-aqueous phase liquids (DNAPLs) contaminated aquifers due to the capacity to enhance DNAPL solubility and mobility. Understanding the pore-scale removal behavior of DNAPL and quantifying proportions of mobilization/solubilization in flushing process are essential to improve remediation efficiency. However, owing to the opacity of aquifer medium, the sand columns commonly used in flushing experiments are hard to reveal the dynamic behaviors and removal mechanism of DNAPLs in aquifer by in-situ microemulsion. In this work, emerging microfluidic visualization techniques were employed to investigate the tetrachloroethylene (PCE) interface dynamics and removal behaviors by microemulsion at pore-scale, quantify the proportions of mobilization and solubilization for PCE removal with various flushing conditions, and elucidate the working mechanisms of PCE-contaminated aquifer remediation by in-situ microemulsion. The results indicated that PCE removal was rapid mobilization initially followed by stable solubilization. With the exception of aquifers with low initial PCE saturation (<7 %), the removal of PCE by mobilization dominated in most conditions, which was positively correlated with flow rate and initial residual saturation, and negatively correlated with surfactant and co-solvent ratio in microemulsion. During mobilization, the long and large droplets were broken up into smaller patches with the reduced IFT, and migrated until the formation of a new stable distribution. In addition, PCE droplet breakup was also observed during solubilization, attributing to the differences in mass transfer. The new empirical model of mass transfer rate with respect to injection flow rate, PCE saturation and microemulsion fraction during solubilization has been developed. This study clarifies the pore-scale process and mechanism of DNAPLs removal, facilitating the predictive modeling and run optimization of DNAPLs remediation by in-situ microemulsion.