Aquatic Photodegradation of Polychlorinated Dibenzofurans:  Rates and Photoproduct Analysis

The aquatic photochemistry of 2,3,7,8-tetrachlorodibenzofuran (T4CDF) and 2,3,4,7,8-pentachlorodibenzofuran (P5CDF) was studied in the laboratory using a xenon light source and under sunlight conditions at 50° N latitude at the Experimental Lakes Area of northwestern Ontario, Canada. Under midsummer sunlight conditions 3H-T44CDF and 14C-P5CDF photolyzed slowly in 25:10 (v/v) distilled water/acetonitrile solutions with rate constants of 0.11 ± 0.01 and 0.015 ± 0.007 d-1, respectively. Both congeners experienced enhanced degradation rates in lake water, with rate constants of 0.58 ± 0.05 and 3.6 ± 0.3 d-1. Photoproduct analysis confirmed reductive dechlorination as a degradative pathway for both PCDFs in lake water. Three T4CDFs were detected as photoproducts of 23478-P5CDF, with two of these products identified by GC/MS as 2368- and 2348-T4CDF. Similarly, one T3CDF was detected in the photolysis of 2378-T4CDF, although the specific congener was not identified. One polar photoproduct was detected in the photolysis of 23478-P5CDF in lake water. Although the identity of this product could not be confirmed, GC/MS analysis of the TMS derivative suggested a hydroxylated, completely dechlorinated compound with an apparent M+ = 330 amu. Photolysis of nonchlorinated dibenzofuran in lake water yielded 2,2‘-dihydroxybiphenyl as photoproduct, with this product producing a trihydroxybiphenyl, tentatively identified as 2,2‘,3-trihydroxybiphenyl, under similar conditions. The results show that reductive dechlorination, C−O cleavage, and hydroxylation all play a role in the photolytic transformation of PCDFs and the dibenzofuran nucleus in natural water.