Investigation of the Photosensitized Mechanisms of PCBs in the Presence of Natural Organic Matter

摘要

The structures of 26 different congeners of polychlorinated biphenyls(PCBs, including monothrough deca-chlorinated) were optimized using density functional theory(DFT) calculations with the 6-31+G(d,p) basis set. The activation energies for the dechlorination of these systems were calculated for direct photodegradation and photosensitized degradation reaction pathways in the presence of natural organic matter(NOM). The dechlorination mechanism of these PCBs and the ring-opening reaction mechanisms(using QST3 method) of the photosensitive degradation products were analyzed. The results showed that(i) the activation energy for the photosensitized degradation of PCBs was much lower than that of direct photodegradation;(ii) the degradation activities(i.e., C–Cl bond cleavage energies) were the same for both degradation pathways and followed the order ortho > meta > para;(iii) the degradation activities of asymmetric PCBs were higher than those of the corresponding symmetrical PCBs for the direct photodegradation and it was completely opposite in the photosensitive degradation;(iv) there was no correlation between the dissociation energy and the number of C–Cl bonds for the direct photodegradation and dechlorination products were all biphenyl;(v) the degradation activity of PCBs decreased as the number of C–Cl bonds increased in the presence of NOM; and(vi) even when the dechlorination reaction was incomplete, it produced chlorophenol. Furthermore, the free radicals of NOM led to the ring-opening reactions of PCBs via an initial addition step. The main site of these ring-opening reactions was the ortho position. Notably, the likelihood of ring-opening reactions occurring involving the degradation products increased as the degradation degree of PCBs increased.