Source Zone Remediation of Toluene NAPL by Heat-Activated Peroxydisulfate in Columns: Oxidant Dose Based on the Second Damköhler Number and Identification of Reaction Products

Selection of an appropriate oxidant dose and minimization of byproduct formation are key to the successful implementation of in situ chemical oxidation (ISCO). Past ISCO column studies have mostly selected oxidant dose based on results from batch experiments, without considering mass transfer phenomena and have not thoroughly evaluated the byproducts. We selected peroxydisulfate (PDS) doses based on the second Damköhler (DaII) number and performed extensive monitoring of byproduct formation during heat-activated PDS treatment of toluene NAPL emplaced in sand columns. Toluene was removed by both oxidation as well as dissolution. The PDS utilization efficiency was lower for 50 mM PDS (DaII ≥ 1) columns than the 5 mM (DaII ≪1) columns. Toluene removal and formation/disappearance of its primary intermediate "benzaldehyde" were much higher in high PDS columns, which resulted in greater accumulation of secondary reaction products including precipitates that were observed visually and microscopically. Targeted/nontargeted GC–MS and LC-HR-MS analyses of effluents detected many CxHy, CxHyOz, and SwCxHyOz (organo-sulfur) compounds. GC–MS analysis of sand extracts detected SwCxHy (organo-sulfides) and elemental sulfur; XPS also indicated the presence of reduced sulfur species. No transformation products were detected in controls. Our results suggest that ISCO design could benefit from DaII calculations to determine the PDS dosage.