Mechanistic Aspects Regarding the Ultraviolet Degradation of Polychlorinated Biphenyls in Different Media: Insights from Carbon and Chlorine Isotope Fractionation.

Abstract

In this study, the carbon and chlorine isotope fractionation during ultraviolet-photolysis of polychlorinated biphenyls (PCBs, including PCB18, PCB77, PCB110, and PCB138) in n-hexane (Hex), methanol/water (MeOH/H2O), and silica gel was first investigated to explore their mechanistic processes. We observed a significant variation in ΛCl-C (εCl/εC) for the same PCBs in different photochemical systems, implying that PCB degradation processes in various photoreaction systems could differ. Although all substrates showed normal apparent carbon/chlorine kinetic isotope effects (C-/Cl-AKIE >1), the putative inverse C-AKIE of nondechlorinated pathways was suggested by 13C depletion of the average carbon isotope composition of PCB138 and corresponding dechlorinated products in MeOH/H2O, which might originate from the magnetic isotope effect. Significant negative correlations were found between C-AKIE and relative disappearance quantum yields ("Φ") of ortho-dechlorinated substrates (PCB18, PCB110, and PCB138) in Hex and MeOH/H2O. However, the C-AKIE and "Φ" of PCB77 (meta/para-dechlorinated congener) obviously deviated from the above correlations. Furthermore, significantly different product-related carbon isotope enrichment factors of PCB77 in Hex were found. These results demonstrated the existence of dechlorination position-specific and masking effects in carbon isotope fractionations.