APPLICATION OF EXCIMER INCOHERENT-UV SOURCES AS A NEW TOOL IN PHOTOCHEMISTRY - PHOTODEGRADATION OF CHLORINATED DIBENZODIOXINS IN SOLUTION AND ADSORBED ON AQUEOUS PULP SLUDGE

The photolysis at 222 and 308 nm of 2-chlorodibenzo-p-dioxin and 1,2,4 -trichlorodibenzo-p-dioxin dissolved in acetonitrile-water and methano l-water solutions was studied, using excimer incoherent-UV lamps (KrCl excimer, 222 nm; XeCl excimer, 308 nm) and an Hg-Xe lamp (wavelength of 308 nm was selected with a monochromator). Numerous products are fo rmed: dibenzodioxin, hydroxydibenzodioxin, trihydroxybiphenyl (two iso mers), dihydroxychlorobiphenyl (two isomers) and trihydroxychlorobiphe nyl (two isomers) were detected by gas chromatography-mass spectrometr y (GC-MS) and high performance liquid chromatography (HPLC), and chlor ide ion by high performance ion chromatography (HPIC). 2,2'-Dihydroxyb iphenyl is formed as a secondary product. The relative yields of the p roducts depend on the nature of the organic cosolvent and on whether o r not the solutions are oxygenated. Quantum yields of substrate destru ction and of dibenzodioxin and chloride ion formation were determined. For example, at 308 nm, in deoxygenated and oxygenated acetonitrile-w ater (50:50, v/v) solution with a solute concentration of 7.5 x 10(-4) mol dm-3, PHI(2-chlorodibenzodioxin destruction) was 0.08 and 0.10, P HI(dibenzodioxin) was 0.014 and 0.004 and PHI(Cl-) was 0.04 and 0.05 r espectively. In methanol-water (50:50, v/v), PHI(dibenzodioxin) is abo ut 30% higher and is not decreased in the presence of oxygen. The depe ndence of the nature of the products and the dependence of their relat ive importance on the nature of the solution indicates that several di fferent primary processes play a part: (i) homolytic scission of the C -Cl bond; (ii) heterolytic scission of the C-Cl bond; (iii) homolytic cleavage of a C-O bond. The latter reaction may lead to reactive quino ne-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 pul p sludge rather than homogeneously dissolved, but the destruction of t he dioxin follows the same (first-order) rate law.