Mechanisms of Ultrasound‐Assisted In Situ Chemical Oxidation for Groundwater Remediation: A Laboratory‐Scale Column Study

Abstract Groundwater contamination has become an increasingly critical environmental concern worldwide. In situ chemical oxidation (ISCO) is considered a promising technology for groundwater remediation, but its performance in heterogeneous aquifers is often constrained by mass‐transfer limitations in low‐permeability zones, leading to suboptimal treatment efficiency. Theoretically, ultrasound (US) can enhance permeability in low‐permeability regions, accelerate oxidant transport, and activate oxidants to strengthen their degradation capacity; however, the remediation performance and underlying mechanisms of US‐ISCO coupling have not yet been systematically validated through experiments. To address this gap, this study conducted laboratory‐scale packed‐column degradation experiments under three ultrasonic modes (no US, ultrasonic pre‐treatment, and continuous US) and employed nuclear magnetic resonance to characterize the temporal evolution of OTC‐related hydrogen‐bearing signals and pore‐structure changes. The results show that ultrasonic pretreatment modifies the pore structure of the porous medium through cavitation‐ and vibration‐related mechanical effects, thereby enhancing permeability and oxidant mass transfer and consequently accelerating contaminant removal. When US is continuously applied during ISCO, it further intensifies oxidation through thermal and other US‐associated effects. The measured temperature rise promotes oxidation kinetics and mass transfer, while sustained US alters the transformation pathways of several key intermediates, indicating additional reaction routes. These coupled effects yield the highest degradation efficiency through permeability enhancement and in‐reaction US intensification. This study demonstrates the potential of US‐ISCO coupling to improve contaminant removal in low‐permeability porous media and provides a mechanistic basis for its further scale‐up and evaluation under complex hydrogeological conditions.