Dual-Element Compound-Specific Isotope Analysis of Diethyl Phthalate Degradation across the Vadose Zone and Saturated Zone: Divergent Isotope Behavior and Mechanisms

Dual-element compound-specific isotope analysis and associated isotope fractionation has been widely used to distinguish environmental transformation pathways and to resolve spatiotemporal distribution of contaminants. However, isotope fractionation during the degradation of organic pollutants across the soil-groundwater interface remains unclear. This study presents a model system for analyzing diethyl phthalate (DEP) degradation processes across the interface of the vadose zone and saturated zone with different simulated contamination scenarios. Across all scenarios, the isotopic fractionation of DEP markedly shifted as it migrated from the vadose zone into the saturated zone. εC ranged from -5.6 to -4.2‰ and εH ranged from -26 to -21‰ in the vadose zone, εC ranged from -2.8 to -2.4‰ and εH ranged from -15 to -11‰ in the saturated zone. The abundance of carboxylesterases suggests that hydrolysis dominated in both zones, whereas biological oxidation likely varied spatially. Cytochrome P450 predominated in vadose zone, while peroxidase contributed more in saturated zone. Differences in oxidative enzymes and the contributions to DEP degradation resulted in distinct carbon and hydrogen isotope fractionation between the two zones. Such substantial differences in isotope fractionation across the vadose zone and saturated zone revealed in this study help identify interfacial processes of pollutant fate.