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

Abstract

Molecular iodine has been studied, for the first time,\nas a sensitizer for the degradation of 2,4,6-trichlorophenol (TCP)\nin aqueous solution under visible light (λ ≥ 450 nm).\nTCP was degraded in the presence of commercial I₂, but\nthe reaction rate decreased significantly after 2 h. When a solution\nof NaI and H₂O₂ was used as an iodine source\nwith phosphotungstic acid (PW) as a catalyst, TCP degradation was\nnot only fast but also followed zero-order kinetics. Importantly,\nthe I₂ concentration remained unchanged with time, indicative\nof I₂ recycling as a kind of photocatalyst. During TCP\ndegradation, 2,6-dichloro-1,4-benzoquinone was produced as the main\nintermediate (76%), which slowly degraded in the irradiated solution.\nFor every equivalent of TCP consumed at the 2 h time point, approximately\n1.7 equivalents of chloride ions were produced. Further study of the\neffect of variables including the type of polyoxometalates (POM) and\nthe initial concentration of each component revealed that the rate\nof TCP degradation under visible light was determined by the rate\nof I₂ production in the dark. The optimum pH and apparent\nactivation energy for TCP disappearance were 4.5 and 42.8 kJ/mol,\nrespectively. It is proposed that TCP degradation is initiated by\niodine radicals produced from I₂ photolysis, followed by\nI₂ regeneration through a POM-catalyzed oxidation of I₃^– by H₂O₂.