Photocatalytic degradation of the alimentary azo dye amaranth: Mineralization of the azo group to nitrogen

Abstract

The photocatalytic degradation of amaranth, an alimentary dye, was investigated in an irradiated titanium dioxide aqueous suspension. FT-IR spectroscopy of amaranth adsorbed on TiO2 powder provided an insight to the mode of its adsorption on TiO2 via the sulfonate group located in the ortho-position with respect to the OH group. The photodisappearance of amaranth was followed by monitoring the degradation of the dye either by UV-Vis spectroscopy or by HPLC/UV-Vis analysis while the total mineralization was monitored by DCO, TOC, GC/catharometer and HPLC ion-chromatography. A prompt removal of the colors was observed which was, however, slightly less rapid than the disappearance of amaranth itself; this indicates the presence of other colored molecules, such as hydroxylated or desulfonated amaranth as suggested by the initial changes in DCO, TOC and sulfate concentrations. In addition to the decolorization, TiO2/UV-based photocatalysis was simultaneously able to fully oxidize the dye, with a complete mineralization of organic carbon into CO2. Sulfur heteroatoms were converted into SO42− ions, which remain partially adsorbed on TiO2, while the central –N=N– azo group was 100% selectively transformed into gaseous dinitrogen this represents. An ideal issue for the elimination of a nitrogen-containing pollutants. A TiO2-mediated photodegradation mechanism for the evolution of aliphatic organic acids during the amaranth photocatalytic degradation was proposed.The formation of lactic and formic acids at the beginning of irradiation-time is indicative of a the fast and easy naphtalenic ring breaking.