Photocatalytic degradation of Reactive Black 5 using Ag3PO4 under visible light

Photocatalytic degradation of Reactive Black 5 (RB5) solution using Ag3PO4 particles was studied under visible light irradiation as a function of the initial pH of the reaction medium. At pH 5 and 7, RB5 decolourization was limited to 43% and 27%, respectively, at the end of 2 h. Under identical experimental conditions, at pH 9 and 11, more than 90% of RB5 was decolourized. The typical blue colour of RB5 turned pinkish orange when RB5 was subjected to Ag3PO4 photocatalysis at pH 9 or greater, with a readily observable shift of the characteristic wavelength from 595 nm initially to approximately 500 nm at the end of 30 min irradiation. Liquid chromatography-mass spectrometry analysis revealed the accumulation of intermediates with imine (=NH) and hydmazo (-HN-NH-) groups, indicating partial oxidation of amines or partial reduction of azo links. Theoretical calculations of band potentials besides the experimental evidence indicated that oxidation of RB5 by hydroxyl radicals and photogenerated holes in the valence band occurred concomitantly with reduction by electrons in the conduction band. The effect of the presence of some common anions on RB5 degradation was studied. The presence of carbonate ions did not have an adverse effect on RB5 decolourization; however, sulfate ions significantly inhibited the decolourization. The extent of RB5 decolourization remained almost unchanged but the rate of reaction was slower in the presence of chloride ions.