Photoinduced adduct formation between Orange II and [Fe3+(aq)] or Fe(ox)33-–H2O2 Photocatalytic degradation and laser spectroscopy

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Abstract
The mechanism of the photo-Fenton catalytic degradation of an azo-dye has been studied by a laser photolysis technique. The effect of added oxalate on the activity of the Fenton reagent (Fe 2+/3+ –H 2 O 2 ) is reported for dark and light reactions during the degradation of Orange II via steady-state and pulsed laser photolysis. Different LMCT of the excited states of the Fe(ox) 3 3- or the (Fe III ···Orange II) complexes are the active species during dye decoloration and/or degradation. The kinetics of the LMCT states reacting with H 2 O 2 on a timescale of ns to ms were studied. Based on a radical theory of Fenton catalysis and direct evidence obtained from laser spectroscopy, it was possible to demonstrate the formation of ·OH adducts for Orange II and other organic compounds. The formation of these adducts in fast reactions was followed by optical methods and appears to be the initial degradation step leading to the observed oxidation of the three organic compounds considered. An interpretation of the fast kinetics in relation to the parameters affecting the degradation and insight into the reactions taking place in the short-time domain are provided.

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