Foam-Enhanced Air Sparging Coupled with Soil Vapor Extraction for Remediation of Trichloroethylene DNAPL-Contaminated Soil.

Conventional remediation technologies, such as soil vapor extraction (SVE) and air sparging combined with SVE (AS-SVE), are often constrained by gas channeling and mass-transfer limitations when treating trichloroethylene (TCE) dense nonaqueous phase liquid (DNAPL) source zones. To overcome these challenges, this study developed an integrated oxidative foam-enhanced AS-SVE (Oxi-FEAS-SVE) system to improve contaminant accessibility, contaminant mass transfer, and degradation efficiency. The system employed oxidative foam composed of 32 mM α-olefin sulfonate (AOS) and 1500 mM sodium persulfate (SPS) to replace conventional air injection, with an AS/SVE flow ratio of 1:1 in a 5 L cylindrical soil reactor. Under these conditions, SVE alone and AS-SVE removed less than 2% of TCE through volatilization, whereas the Oxi-FEAS-SVE system achieved a total removal efficiency of 91% in this laboratory-scale study. This enhanced performance resulted from a two-stage process: an initial 3 h active operation that removed 54% of TCE primarily through DNAPL volatilization, followed by a 48 h static period that contributed an additional 37% removal through oxidative mineralization. Mechanistically, the AOS/SPS oxidative foam performs dual functions. It enhances interfacial contact area, promoting dissolution and volatilization for SVE capture, while simultaneously generating sulfate radicals via ambient-temperature activation of SPS to sustain oxidative degradation. Detected intermediates (C2H2Cl4, CHCl3, CH2Cl2, and CH3Cl) suggest degradation pathways involving radical addition, hydrogen abstraction/addition, and sequential dechlorination. Overall, the Oxi-FEAS-SVE system demonstrates promise as an innovative approach for treating TCE DNAPL source zones, while highlighting the need for additional studies to evaluate its viability under variable site conditions.