Environmental photochemistry of PCDDs. Part I. Kinetics and quantum yields of the photodegration of 1,2,3,4,7-penta- and 1,2,3,4,7,8-hexachlorodibenzo-p-dixin in aqueous acetontrile

For the determination of the quantum yields of 1,2,3,4,7-pentachlorodibenzo-$\text{p}$-dioxin ($\text{1}$) and 1,2,3,4,7,8-hexachlorodibenzo-$\text{p}$-dioxin ($\text{2}$), ten Pyrex glass photochemical cells were constructed and their correction factors determined. Quantum yields at 313 nm for $\text{1}$ and $\text{2}$ in aqueous acetonitrile (4:6 v/v) were (9.781 ± 2.376) 10^?5 and (1.096 ± 0.020) × 10^?4, respectively. These quantum yields and the measured absorption spectra were used to estimate the environmental phototransformation first-order rate constants and photochemical half-lives of $\text{1}$ and $\text{2}$ in water under conditions of variable sunlight intensity during various seasons from solar intensity data available in the literature. In summer, typical half-lives for the phototransformation of $\text{1}$ and $\text{2}$, respectively, near the surface of water bodies at 40° north latitude would be 364 ± 88 and 151 ± 3 h; whilst such values in winter would be 1257 ± 309 and 518 ± 10 h.