Iodine-Sensitized Degradation of 2,4,6-Trichlorophenol under Visible Light

Molecular iodine has been studied, for the first time, as a sensitizer for the degradation of 2,4,6-trichlorophenol (TCP) in aqueous solution under visible light (lambda >= 450 nm). TCP was degraded in the presence of commercial I-2, but the reaction rate decreased significantly after 2 h. When a solution of NaI and H2O2 was used as an iodine source with phosphotungstic acid (PW) as a catalyst, TCP degradation was not only fast but also followed zero-order kinetics. Importantly, the I-2 concentration remained unchanged with time, indicative of I-2 recycling as a kind of photocatalyst. During TCP degradation, 2,6-dichloro-1,4-benzoquinone was produced as the main intermediate (76%), which slowly degraded in the irradiated solution. For every equivalent of TCP consumed at the 2 h time point, approximately 1.7 equivalents of chloride ions were produced. Further study of the effect of variables including the type of polyoxometalates (POM) and the initial concentration of each component revealed that the rate of TCP degradation under visible light was determined by the rate of I-2 production in the dark. The optimum pH and apparent activation energy for TCP disappearance were 4.5 and 42.8 kJ/mol, respectively. It is proposed that TCP degradation is initiated by iodine radicals produced from I-2 photolysis, followed by I-2 regeneration through a POM-catalyzed oxidation of I-3(-) by H2O2.