EVIDENCE OF SENSITIZED PHOTOLYSIS OF POLYCHLORINATED DIBENZO-PARA-DIOXINS IN NATURAL-WATERS UNDER SUNLIGHT CONDITIONS

The sunlight photolysis of carbon-14 labeled 1,2,3,4,7-pentachlorodibenzo-p-dioxin (P5CDD) and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (H7CDD) was studied at 50° N latitude in filter-sterilized natural water and in distilled water-acetonitrile (2:3, v/v) solutions to determine the extent of direct versus indirect photolysis under environmental conditions. Under midsummer sunlight conditions, pseudo-first-order sunlight photolysis rate constants (kp) of 0.74 and 0.28 day−1 were measured for P5CDD and H7CDD, respectively, in natural water. However, considerably slower direct aqueous photolysis rates were observed for the two PCDD congeners in distilled water–acetonitrile, with rate constants of 0.058 and 0.019 day−1 for P5CDD and H7CDD, respectively, in relatively good agreement with predicted rates of direct aqueous photolysis. The results indicate that an indirect or sensitized photolytic mechanism may dominate the photodegradation of PCDDs in natural waters. The major degradation product(s) appeared as nonextractable 14C activity, with increases in this fraction paralleling the disappearance of the parent PCDD. © 1990, American Chemical Society. All rights reserved.