Kinetic mismatch drives tetrachloroethene (PCE) rebound in Fe(II)/persulfate in situ chemical oxidation.

Ferrous iron-activated persulfate (Fe(II)/PDS) is widely used for in situ chemical oxidation (ISCO) of chlorinated solvents, yet aqueous rebound complicates treatment evaluation. Phase-resolved mass balances were combined with electron paramagnetic resonance (EPR) and surface characterization to examine tetrachloroethene (PCE) redistribution and removal. In homogeneous aqueous systems, PCE removal was rapid within 2 h, with rapid dissolved Fe(II) consumption. Among evaluated Fe(II):PDS ratios, 1:1 yielded the highest removal, with > 30% PDS persisting beyond the initial stage. Following Fe(II)/PDS addition to pre-equilibrated soil-water systems, aqueous PCE increased while soil-associated PCE decreased and total PCE mass declined monotonically. SEM-EDS, FTIR, and XRD indicated Fe-rich surface deposition and altered organic functional groups, supporting interfacial changes that weaken PCE retention. Soil addition attenuated EPR spin-adduct responses. In pre-equilibrated systems, the fitted sulfate-adduct response in soil extracts exhibited negligible variation between 10 and 120 min, consistent with continued soil-associated activation as intensive aqueous oxidation subsided. These observations support a kinetic mismatch between declining effective oxidative capacity and continued soil-to-water PCE release, allowing aqueous accumulation despite net removal. The findings provide a mechanistic basis for coordinating reagent transport and local activation with delayed contaminant release through alternating PDS and Fe(II) injection in groundwater circulation well (GCW) systems.