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

Abstract

Abstract A water-soluble and pale-yellow Keggin-type iron (III) substituted polyoxotungstate (PW 11 Fe) 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, PW 11 Fe was not active for the photodegradation of phenol, 4-chlorphenol (4-CP), 2,4-dichlophenol and 2,4,6-trichlorophenol, except that H 2 O 2 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 PW 11 Fe or H 2 O 2 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-benzoquinone, 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 PW 11 Fe was stable. Through a spin-trapping electron paramagnetic resonance spectroscopy, hydroxyl radicals were found in the irradiated aqueous solution of PW 11 Fe and H 2 O 2 . Finally, a plausible mechanism responsible for the observed slow and fast phases of 4-CP degradation is proposed, involving photolysis of a PW 11 Fe complex with H 2 O 2 , the Fenton-type production of hydroxyl radicals, and BQ photolysis.