Dechlorination pathways of ortho-substituted PCBs by UV irradiation in n-hexane and their correlation to the charge distribution on carbon atom
Fang-Cheng Chang, Tzu-Chuan Chiu, Jui-Hung Yen and Yei-Shung Wang*
The photodegradations of twenty-two individual polychlorinated biphenyl (PCB) congeners (including twenty-one non-coplanar ortho substituted and one non-ortho substituted) by irradiation with ultraviolet lamp in n-hexane solution were studied. Photoproducts were identified by matching their retention times and mass spectra with those authentic standards. Substituted chlorine less than two, PCB congener was photodegraded within half an hour, if more than three chlorine on ring, the photodechlorination time for PCB needs one and half hours or more, even longer than 15 hours. The half-life of PCB degradation by photodechlorination was much shorter than that of by anaerobic biological dechlorination. Charge distribution on carbon atom combined with the monitoring products of individual PCB congeners were used to deduce the photodegradation pathways. The higher the charge distribution for carbon which chlorine attached, the easier photodechlorination occurred. A lot of chlorine atoms attached PCB, the dechlorination was found to occur prior to the carbon with higher charge distribution at the benzene ring with more chlorine atoms attached.
Keywords: Non-coplanar ortho substituted PCBs; Half-life; Electric charge distribution; Photodechlorination pathway