Photochemical Behavior Of Polycyclic Aromatic Hydrogens And Polybrominated Diphenyl Ethers

Persistent toxic substances (PTS) are mainly produced by human activities and stable in theenvironment, which have impact on humans and biota due to their toxicological andecotoxicological characterization. Most of PTS could not be biodegraded easily so photolysisis one of the major abiotic transformation processes of these chemicals in the environment.Polycyclic aromatic hydrogens (PAHs) and polybrominated diphenyl ethers (PBDEs) aretypical PTS in the environment and their environmental transformation and fates are of greatconcern because of their significant toxic effect to the health of human being. In this study,we performed some basic researches on their photochemical behavior in environmentalmediums.Firstly, the solar photodegradation of 16 PAHs, sorbed on surfaces of pine [Pinuathunbergii] needles was investigated. The PAHs were produced by combustion of polystyreneand exposed onto the surfaces of pine needles. The disappearance of PAHs sorbed on the pineneedle surfaces is mainly caused by volatilization and photolysis. The volatilization andphotolysis of the 16 PAHs follow first-order kinetics.The volatilization half-lives are largerthan photolysis half-lives for PAHs on the pine needle surfaces, indicating photolysis plays amajor role. The volatilization rates correlate with PAH molecular weight significantly.Compared with water, the cuticular waxes of pine needles can stabilize photolysis of PAHsand facilitate accumulation of PAHs. The photolysis half-lives for selected PAHs correlatewith semi-empirically calculated energy of the highest occupied orbital (E_HOMO).Photochemical behaviors of PAHs are dependent not only on their molecular structures butalso the physical-chemical properties of the substrate on which they are adsorbed.Secondly, photolysis of selected PAHs, phenanthrene, pyrene and benzo(a)pyrene, wasconducted in nine organic solvents. A noticeable solvent effect can be observed for thephotolytic rates of PAHs. Photolytic rates of PAHs increased with the increase of electronaffinity, illustrating the key role of electron-accepting potential of solvents in the photolysisreaction. The electron-accepting potential of solvents plays a vital role in the photoinducedelectron transfer reaction. The implication of this study is that photochemical degradation ofPAHs will be accelerated by the environmental medium with high electron-accepting potential.Finally, in order to understand the relationship between photochemical behavior andmolecular structures for PBDEs, quantitative structure-property relationship (QSPR) approachwas adopted. Using semi-empirical quantum chemical descriptors, by partial least squares(PLS) regression, QSPRs were established for direct photolysis quantum yields (Φ) and rateconstants (k) of PBDE congeners, dissolved in water/methanol and methanol solutions,respectively, and irradiated by artificial UV-A light. Q²_cum, a parameter indicating robustnessand predictive abilities of PLS models, for the significant QSPR models are larger than 0.702.The gap of frontier molecular orbital energies (E_LUMO-E_HOMO and the most positiveMulliken atomic charges on a hydrogen atom (q_H⁺) are two main molecular structural factorsgoverning the logΦvalues, logΦincreases with increasing E_LUMO-E_HOMO and q_H⁺ values.logk is mainly related with bromination degree and pattern that can be characterized bymolecular weight (Mw), average molecular polarizability (α) and the average Mulliken atomiccharges on bromine atoms (q_Br). logk increases with bromination degree (Mw,α) and q_Br.These results show that photochemical behavior of PBDEs is correlated with their molecularstructures and the molecular structures of PTS can affect their photochemical behavior.