Dechlorination pathways of ortho-substituted PCBs by UV irradiation in n-hexane and their correlation to the charge distribution on carbon atom.

The photodegradations of 22 individual polychlorinated biphenyl (PCB) congeners (including 21 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 of authentic standards. PCB congener with less than two chlorides 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, sometimes even longer than 15 h. The half-life of PCB degradation by photodechlorination was much shorter than that 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 to which chlorine is attached, the easier for photodechlorination to occur. 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.