Halogenation of Polycyclic Aromatic Hydrocarbons by Photochemical Reaction under Simulated Tidal Flat Conditions

Selected six kinds of polycyclic aromatic hydrocarbons (PAHs) (naphthalene: NA, phenanthrene: (PHE), anthracene (ANT), fluoranthene (FLR), pyrene (PYR), and benzo[a]pyrene (B[a]P)) were exposed to natural sunlight under simulated tidal flat conditions, to check possibilities for photochemical productions of halogenated derivatives in the coastal environment. Moreover, 11 molecular descriptors were derived based on PM6 Hamiltonian for each PAH. Photochemical reaction induced productions of various 13 kinds of halogenated derivatives, which include 7 unknown products. In the determined 6 halogenated derivatives, the highest production amount was 6-chlorobenzo[a]pyrene. Moreover, PYR showed the behavior that produced halogenated derivatives significantly higher than its structural isomer of FLR. After the determination of production amounts, correlation between the production amounts of halogenated derivatives and the molecular descriptors of individual PAHs were examined. The energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (E-HOMOLUMO), which is a measure of chemical stability, showed the strongest significance (p < 0.01) and give enough explanation for differences in the production amounts among structural isomers. PAHs that have small E-HOMOLUMO showed high potential to produce halogenated derivatives. ANT, PYR and B[a]P would have potentially high capability to induce halogenated derivatives in the coastal environment. We further discussed the impact of photochemical reaction on environmental fate of PAHs and halogenated PAHs. Congener profiles observed in our previous field surveys seemed to suggest contributions of photochemical reaction to occurrences of Cl-PAHs.