Pore-scale investigation of residual DNAPL remediation via in situ hydrogen peroxide oxidation in porous media

Hydrogen peroxide (H2O2) is widely used as an oxidant in the process of in situ chemical oxidation (ISCO) for remediating groundwater contaminated by dense non-aqueous phase liquids (DNAPLs). However, the remediation efficiency is often limited by excessive gas generation and H2O2 self-decomposition. A pore-scale understanding of the dynamic interactions among DNAPL, H2O2, and gas evolution during the ISCO is essential to optimize field-scale treatment applications. This study experimentally investigated the remediation process of residual trichloroethylene (TCE) in porous media using H2O2 as the oxidant. The pore-scale dynamics of the remediation were visualized through time-resolved three-dimensional micro-computed tomography (3D micro-CT). Results demonstrate that the TCE remediation involves a competition between TCE oxidation and H2O2 self-decomposition. At low H2O2 concentrations (<2.0 wt%), remediation improves with increasing H2O2 concentration. However, beyond the critical concentration (2.0 wt%), self-decomposition dominated, consuming H2O2 without degrading TCE. Additionally, gas generation induces pore blockage, restricting oxidant transport to the TCE phase. This study provides pore-scale insights into the mechanisms of H2O2-based TCE remediation, highlighting the multiphase interactions. It reveals the dual role of H2O2 as both an effective oxidant and a source of inefficiency in TCE remediation, leading to H2O2 consumption shift from diffusion-limited TCE degradation to H2O2 self-decomposition.