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The photolysis at 222 and 308 nm of 2-chlorodibenzo- p -dioxin and 1,2,4-trichlorodibenzo- p -dioxin dissolved in acetonitrile—water and methanol—water solutions was studied, using excimer incoherent -UV lamps (KrCl excimer, 222 nm: XeCl excimer, 308 nm) and an HgXe lamp (wavelength of 308 nm was selected with a monochromator). Numerous products are formed: dibenzodioxin, hydroxydibenzodioxin, trihydroxybiphenyl (two isomers), dihydroxychlorobiphenyl (two isomers) and trihydroxychlorobiphenyl (two isomers) were detected by gas chromatography—mass spectroscopy (GC—MS) and high performance liquid chromatography (HPLC), and chloride ion by high performance ion chromatography (HPIC). 2,2'-Dihydroxybiphenyl is formed as a secondary product. The relative yields of the products depend on the nature of the organic cosolvent and on whether or not the solutions are oxygented. Quantum yields of substrate destruction and of dibenzodioxin and chroloride ion formation were determined. For example, at 308 nm, in deoxygenated and oxygenated acetonitrile—water (50:50, v/v) solution with a solute concentration of 7.5×10 −4 mol dm −4 , Φ(2-chlorodibenzodioxin destruction) was 0.08 and 0.10, Φ(dibenzodioxin) was 0.014 and 0.004 and Φ(Cl − ) was 0.04 and 0.05 respectively. In methanol—water (50:50, v/v/), Φ(dibenzodioxin) is about 30 higher and is not decreased in the presence of oxygen. The dependence of the nature of the products and the dependence of their relative importance on the nature of the solution indicates that several primary processes play a part: (i) homolytic scission of the CCl bond; (ii) heterolytic scission of the CCl bond; (iii) homolytic cleavage of a CO bond. The latter reaction may lead to reactive quinone-type compounds. Photodegradation experiments involving pulp sludge were performed with 1,2,4-trichlorodibenzodioxin. Higher fluences are required to produce equal effects when the compound is adsorbed on pulp sludge rather than homogenously dissolved, but the destruction of the dioxin follows the same (first-order) rate low. |
