Photosensitized Degradation of Azo Dyes on Fe, Ti, and Al Oxides. Mechanism of Charge Transfer during the Degradation

Azo dyes disappear in photocatalyzed processes mediated by either hematite or geothite and TiO 2 (anatase and rutile) under visible light irradiation. The details of the charge injection from the excited state from Orange II into the catalyst are reported by laser-pulsed spectroscopy. The electron injection from the Orange II to α-Fe 2 O 3 occurs within the duration of the laser pulse (<10 -8 s) and is followed by two processes: charge recombination with a rate of 1.1 × 10 7 s -1, followed by a slower process with a rate of 2.1 × 10 5 s -1 between the dye and the HO • radical at the iron oxide surface. At pH 3 the electrostatic attraction between FeOH 2 + and HOR - leads to the formation of a strong complex between Orange II and α-Fe 2 O 3 necessary for an efficient charge-transfer process. This interaction is seen to be much weaker at neutral pH values. The complex [Orange II···α-Fe 2 O 3 ] was identified at acid pH. At a concentration of Orange II of about 2.3 mM, monolayer coverage of this dye on α-Fe 2 O 3 was observed. Photocorrosion of α-Fe 2 O 3 with mononolayer dye coverage is much higher than that with multilayer dye coverage under visible light irradiation. The photogenerated intermediates generated on the oxide surface by the adsorbed azo dye during irradiation were identified by FTIR and in solution by HPLC. Experimental observations with COD and BOD techniques confirmed the improved biodegradability of the intermediates generated in the presence of α-Fe 2 O 3 .