Multi-element isotopic evidence for the microbial degradation of polybrominated diphenyl ethers and polychlorinated...

The in-situ microbial degradation of halogenated organic pollutants is a key natural attenuation process, but it remains poorly understood. In this study, compound-specific bromine and chlorine stable isotope analysis (Br-/Cl-CSIA) was first employed to elucidate the microbial degradation of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in field sediment cores. The results demonstrate that Br-/Cl-CSIA offers distinct advantages for tracing the fate of low-concentration and low‑halogenated congeners. For PBDEs, a clear trend of ⁸¹Br enrichment with depth was observed for BDEs 28 and 37, with the exception of BDE 28 in core 2, which displayed ⁸¹Br depletion. Given their differing abundances in technical mixtures, these isotopic fractionation patterns suggest that BDE 37 in all cores, along with BDE 28 in core 2, primarily serve as microbial debromination products, whereas BDE 28 in cores 1 and 3 mainly function as parent compounds. Regarding PCBs, pronounced chlorine isotopic fractionation was observed in a subset of congeners, which were identified mainly as either parent compounds or dechlorination products based on the direction of their isotopic fractionation (³⁷Cl enrichment or depletion) with depth. Critically, by coupling carbon isotopic fractionation patterns in dual-element isotope plots, this study provided the first evidence of potential inverse isotope effects and interval-/core-specific mechanistic variations in the in-situ degradation of specific congeners (e.g., PCB 99). These findings advance our understanding of the in-situ microbial degradation of these contaminants and provide guidance for investigating contaminant fate in complex environments.