Photolysis of highly chlorinated dibenzo-p-dioxins by sunlight

Abstract

CHLORINATED dibenzo-p-dioxins (CDDs) are now widely recognised as contaminants of commercial chlorinated phenols and related products. These compounds possess a remarkable range of toxicity; the LD₅₀ for 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) in female rats is 45 µg per kg, that of the octachloro compound (OCDD) is > l g per kg (ref. 1). Although the degree of toxicity is related to the extent of chlorination, positional isomerisation also Plays a critical and perhaps dominant part; 1,2,3,4-TCDD is at least 800 times less toxic than 2,3,7,8-TCDD (ref. 2). The extreme toxicity of some of these compounds, coupled with the large variations with extent of chlorination and positional isomerisation, make accurate identification of CDDs essential in any assessment of environmental or safety implications. The study reported here is concerned with the photolysis of octa-, hepta (Hp)- and some hexa (Hx)-chlorodibenzo-p-dioxins and shows that the structural isomers which are expected to be the most toxic are most rapidly photodecomposed. We also report that despite photo-lytic reductive dechlorination which gives rise to less highly chlorinated dioxins, the reaction mechanism favours the formation of the less toxic isomers.