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

Phase-resolved mass balances reveal aqueous PCE rebound during net PCE removal. • Rapid Fe(II) consumption weakens oxidation despite residual persulfate. • Fe-rich deposition and altered organic groups support changes in PCE retention. • Delayed PCE release outpaces effective oxidation, creating a kinetic mismatch. 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. Download: Download high-res image (273KB) Download: Download full-size image Mechanism of Radical‑pulse induced PCE rebound The aqueous PCE rebound observed in the investigated pre-equilibrated soil–water system is interpreted as the combined outcome of rapid initial oxidation, treatment-induced surface reconstruction, and delayed interphase mass transfer. Fe(II)/PDS activation generates an initial radical pulse involving SO 4 •− and ·OH, while dissolved Fe(II) is rapidly consumed. Reactive species can oxidize readily accessible PCE and competing soil constituents, including surface-associated organic moieties. Oxidative disturbance of SOM- and soft-carbon-associated sorption domains, together with the formation of Fe(III)-bearing surface deposits, may weaken PCE retention and reduce its immediate re-adsorption. Release from condensed SOM/hard-carbon domains and less-accessible intraparticle regions may persist as the initial aqueous oxidative activity declines. The pH evolution and relatively persistent sulfate-adduct response in soil extracts are consistent with continued soil-associated PDS activation, which may sustain slower oxidation within accessible domains. This activity, however, may not provide sufficient aqueous oxidative capacity to remove PCE at the rate of its release. Aqueous accumulation can therefore coexist with continued net PCE loss, as demonstrated by the phase-resolved mass measurements. The proposed mechanism links treatment-induced changes in contaminant retention to a kinetic mismatch between declining aqueous oxidative capacity and continued soil-to-water PCE release.