Aquatic Photodegradation of Polychlorinated Dibenzofurans: Rates and Photoproduct Analysis

The aquatic photochemistry of 2,3,7,8-tetrachlorodibenzofuran (T 4 CDF) and 2,3,4,7,8-pentachlorodibenzofuran (P 5 CDF) 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 3 H-T 4 4CDF and 14 C-P 5 CDF 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 T 4 CDFs were detected as photoproducts of 23478-P 5 CDF, with two of these products identified by GC/MS as 2368- and 2348-T 4 CDF. Similarly, one T 3 CDF was detected in the photolysis of 2378-T 4 CDF, although the specific congener was not identified. One polar photoproduct was detected in the photolysis of 23478-P 5 CDF 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.