Environmental photochemistry of PCDDs. 2. Quantum yields of the direct phototransformation of 1,2,3,7-tetra-, 1,3,6,8-tetra-, 1,2,3,4,6,7,8-hepta-, and 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin in aqueous acetonitrile and their sunlight half-lives

Photochemistry of four polychlorodibenzo-p-dioxins (PCDDs), namely 1,2,3,7-tetrachlorodibenzo-p-dioxin (1,2,3,7-T{sub 4}CDD), 1,3,6,8-T{sub 4}CDD, 1,2,3,4,6,7,8-H{sub 7}CDD, and 1,2,3,4,6,7,8,9-O{sub 8}CDD, in water-acetonitrile (2:3, v/v) was investigated at 313 nm; the quantum yields for the direct phototransformation of the candidate PCDD isomers in these solvent systems were (5.42 {plus minus} 0.42) {times} 10{sup {minus}4}, (2.17 {plus minus} 0.14) {times} 10{sup {minus}3}, (1.53 {plus minus} 0.17) {times} 10{sup {minus}5}, and (2.26 {plus minus} 0.33) {times} 10{sup {minus}5}, respectively. These quantum yields and the measured absorption spectra together with solar intensity data available in the literature were utilized to estimate the direct sunlight (environmental) phototransformation first-order rate constants of these PCDD congeners in water under conditions of variable sunlight intensity during various seasons; the corresponding half-lives were also determined. In summer, typical midday, midseason half-lives for the direct phototransformation of 1,2,3,7-T{sub 4}CDD, 1,3,6,8-T{sub 4}CDD, 1,2,3,4,6,7,8-H{sub 7}CDD, and O{sub 8}CDD near the surface of water bodies at 40{degree} north latitude would be 1.8 {plus minus} 0.1, 0.31 {plus minus} 0.02, 47 {plus minus} 5, and 18 {plus minus} 3 days, respectively, time being expressed in terms of 24-h day. Furthermore, the experimentally determined sunlight photolysis half-life of {sup 14}C-labeled 1,3,6,8-T{sub 4}CDD in pond water was 3.5 days.