γ-radiolysis and pulse radiolysis of aqueous 4-chloroanisole |
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Authors: | R.M. Quint H.R. Park P. Krajnik S. Solar N. Getoff K. Sehested |
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Affiliation: | a Institut für Theoretische Chemie und Strahlenchemie der Universität Wien, Althanstraße 14, A-1090, Wien, Austria b Department of Chemistry, Chonnan National University, Kwang-Ju, 500-757, South Korea c Environmental Science and Technology Department, Risø National Laboratory, DK-4000, Roskilde, Denmark |
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Abstract: | Using pulse radiolysis and steady state γ-radiolysis in combination with product analysis by HPLC the radiolytic degradation mechanism of 4-chloroanisole (4-ClAn) has been elucidated. Pulse radiolysis experiments show that OH radicals react in neutral aqueous N2O saturated solutions with 4-ClAn by addition to all aromatic ring positions to yield hydroxycyclohexadienyl radicals (OH-adducts), k(OH + 4-ClAn) = 6.5 × 109dm3mol−1s−1. Those OH-adducts formed on ipso positions of the molecule subsequently undergo HCl or CH3OH elimination forming methoxyphenoxyl- and chlorophenoxyl radicals. Their yield corresponds to 20% of the OH-radicals, whereby the distribution is roughly 3:1 in favor of the methoxyphenoxyl radicals, which reflects the stronger ortho-, para-directing activity of the methoxy group. The OH-adducts decay second order, 2k = 1 × 109dm3mol−1s−1. The presence of oxygen leads to its addition on the hydroxycyclohexadienyl radicals, k(OH-adduct + O2) = 3.2 × 108dm3mol−1s−1. In airfree solution the reaction of H-atom with the substrate, k(H + 4-ClAn) = 1.2 × 109dm3mol−1s−1, results in H-adducts which decay in bimolecular reactions, 2k = 8.2 × 108dm3mol−1s−1. The rate constant for the reaction of the solvated electrons has been determined to k(eaq− + 4-ClAn) = 2 × 109dm3mol−1s−1. The absorption spectra of H- and OH-adducts were measured in the range of 280–450 nm. The products analysed by HPLC after γ-radiolysis in dependence of dose (100–600 Gy) are given for N2O-, air-, oxygen- and argon saturated neutral aqueous solutions. In conditions favoring the OH radical oxidation 4-chlorophenol, 4-methoxyphenol, 5-chloro-2-methoxyphenol and 2-chloro-5-methoxyphenol were determined as final products. In the presence of Ar, where about equal amounts of OH and eaq− are present, additionally anisole could be detected. Under both reaction conditions the amount of identified products is about 20% of decomposed 4-ClAn. The reaction of eaq− leads to reductive dechlorination which corresponds quantitatively to the degradation of the substrate. In the presence of air or solutions saturated with pure oxygen predominantly hydroquinone, 4-chlorophenol and muconic acids are formed and the material balance is 50%. The efficient dechlorination (60% of the decomposed 4-ClAn) as well as ring fragmentation products as intermediates en route to complete mineralization in oxygenated solution indicate that high energy radiation is a promising method for degradation of halogenated aromatic compounds in water. Variation of dose rates from 79 Gy min−1 to 266 Gy min−1 did not show any influence on the product distribution. |
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