Photolysis of polychlorinated biphenyls on octadecylsilylated silica particles

PCBs were decomposed in a quartz column packed with octadecylsilylated silica particles (ODS-silica) by external UV irradiation. The photolysis of PCBs having two to eight chlorines, including those which are the major constituents of Aroclors (PCB-180, PCB-153, PCB-118, and PCB-105) as well as coplanar PCBs, allowed the examination of the factors that affect the reactivity and product distribution. Steric congestion and molecular symmetry were found to be the major factors affecting the reactivity and the site of the reaction. Some general rules can explain the seemingly complicated decomposition pathways of various congeners. The results indicate that the dechlorination at the ortho-position which is predominant leads to products having not only higher toxic equivalency factors but also much longer life under photolysis conditions. Highly chlorinated PCBs showed tendency to lose chlorines from meta- or para-positions. The reaction pathways were not much affected by the light source, a mercury lamp with or without a pyrex filter or sunlight, in spite of the reduction in decomposition rate with a weaker light source. Continuous photolysis of highly chlorinated PCBs trapped on ODS-silica from aqueous solution was also possible.