Sequential Air Sparging and Groundwater Circulation for Effective In Situ Chemical Oxidation of Trichloroethylene in Simulated Fe(II)-Bearing Aquifers

Air sparging (AS) normally removes volatile chlorinated solvents in aquifers by stripping. Here, we found that oxygen (O2) supplied by AS could be transformed to hydroxyl radicals (•OH) by aquifer Fe(II) and led to a significant degradation of contaminants. In a simulated two-dimensional (2D) sand-filling aquifer contaminated with 22 μM trichloroethylene (TCE), long-term groundwater circulation subsequent to short-term AS was applied to facilitate the dissolving and mass transport of O2, and dissolved Fe(II) in the form of TPP–Fe(II) complex was added as the source of Fe(II). With the addition of 1.6 mM TPP–Fe(II), TCE removal increased from 52.9 to 89.0% with the increase in air flow rate from 20 to 120 L/h, and degradation contributed to over 89% of the total removals. TCE degradation also increased significantly with the increase in TPP–Fe(II) concentration. Degradation played a much more significant role than stripping for TCE removal in the sequential process. Temporal and spatial distributions of dissolved O2, dissolved Fe(II), and TCE corroborated the predominant contribution of the O2–Fe(II) interaction for TCE degradation due to •OH production. This study demonstrates the important role of oxidation in air sparging and provides a new strategy for applying O2–Fe(II)-driven oxidation by O2 in aquifer treatment.