Evaluation of Persulfate Infiltration in Degrading Soil Residual Trichloroethylene in the Vadose Zone – A One-Dimensional Column Study

Abstract

Trichloroethylene (TCE) contamination commonly occurs in both soil and groundwater. Because regulatory standards differ between these media, soil may meet compliance levels, while residual TCE remains in the unsaturated zone. Such residues can later diffuse into groundwater during rainfall infiltration or water table fluctuations, resulting in secondary contamination. This study integrated porous pipe infiltration with in situ chemical oxidation to assess whether adjusting infiltration cycles and retention times can effectively deliver sodium persulfate (Na₂S₂O₈, SPS) into unsaturated soils via matrix flow. One-dimensional soil column experiments were conducted to observe SPS distribution over time and evaluate soil TCE residues removal. The research proceeded in two phases: (a) examining the SPS distribution under varying infiltration cycles and (b) assessing the TCE removal efficiency. A longer SPS residence time, achieved with a pulse/no-pulse interval (t_p/t_np) = 5/25 (min) over three cycles, produced more uniform oxidant distribution. Subsequent tests compared SPS retention times (24–120 h) to evaluate their effect on residual TCE degradation. Soil TCE removal ranged from 82 to 97% depending on SPS retention time. In the 120 h retention test, a 20 h water flushing of the treated soil column demonstrated TCE removal, achieving 99.9% efficiency, with effluent TCE concentrations of 0.05–0.45 mg/L, versus up to 11.8 mg/L in untreated controls. Overall, results demonstrated that under fixed t_p, increasing t_np enhances matrix potential, improving liquid transport and sustaining moisture at depth. This study shows that controlled oxidant infiltration is a promising remediation approach for addressing TCE contamination in unsaturated zones.