Photoassisted degradation of dye pollutants. 8. Irreversible degradation of alizarin red under visible light radiation in air-equilibrated aqueous TiO2 dispersions

Alizarin red undergoes rapid photoassisted decomposition in air-equilibrate d aqueous TiO2 dispersions under visible light radiation (lambda > 420 nm). Proton NMR, chemical oxygen demand (CODCr), UV-visible, IR, CC-MS, and spi n-trapping ESR spectroscopic techniques were employed to obtain details of the photodegradation of alizarin red. Peroxides and carbonyl species are th e first intermediates observed during the process. The major component of t he peroxides produced is H2O2, and no organoperoxides were detected. We hav e also confirmed that in addition to CO2 and to smaller carbonyl species th e principal intermediate produced is phthalic acid, which is unable to degr ade further because it does not absorb the actinic visible light radiation (lambda > 420 nm). The photodegradation kinetics are discussed in terms of the Langmuir-Hinshelwood model, a case of saturation type kinetics. A mecha nism for the photoassisted degradation of alizarin red mediated by TiO2 par ticles under visible light radiation is proposed.