Visible light induced degradation of chlorophenols in the presence of H2O2 and iron substituted polyoxotungstate

A water-soluble and pale-yellow Keggin-type iron (III) substituted polyoxotungstate (PW11Fe) has been examined as a photocatalyst for chlorophenol degradation in water under visible light at wavelengths longer than 420 nm. In an aerated aqueous solution, PW11Fe was not active for the photodegradation of phenol, 4-chlorphenol (4-CP), 2,4-dichlophenol and 2,4,6-trichlorophenol, except that H2O2 was also added. However, all the observed reactions of organic degradation were slow in the beginning, followed by a fast, first-order process. As the initial concentration of PW11Fe or H2O2 increased, this induction period for 4-CP degradation declined, together with increase in the overall reaction rate. During 4-CP degradation, chloride ions were produced, the amount of which was about 94% of the 4-CP degraded, while 1,4-benzo-quinone, 2-hydroxy-BQ catechol, and 4-chlorocatechol were identified as the main intermediates, all of which were degradable in situ. Moreover, 4-CP could degrade at initial pH 2.2-7.2, whereas PW11Fe was stable. Through a spin-trapping electron paramagnetic resonance spectroscopy, hydroxyl radicals were found in the irradiated aqueous solution of PW11Fe and H2O2. Finally, a plausible mechanism responsible for the observed slow and fast phases of 4-CP degradation is proposed, involving photolysis of a PW11Fe complex with H2O2, the Fenton-type production of hydroxyl radicals, and BQ photolysis. (C) 2014 Elsevier B.V. All rights reserved.