Photochemical decoloration of Remazol Brilliant Blue and Uniblue A in the presence of Fe3+ and H2O2

The primary fast kinetic steps of the decoloration of the nonbiodegradable textile dyes Remazol Brilliant Blue R and its close analogue, Uniblue A, were studied in the presence of the Fe3+/H2O2 under light irradiation. Special attention was devoted to the visible excitation at wavelengths >400 nm during dye decoloration. Stopped-flow experiments under light irradiation were carried out at different wavelengths. Laser flash photolysis suggested that electron transfer between the excited dye and Fe3+ is the initiating step, either as a bimolecular process (D* + Fe3+ --> D.+ + Fe2+) or through a dye/iron complex (D* + Fe3+ --> (D ... Fe3+) --> D.+ + Fe2+). A direct correlation was found between the absorption spectrum of the dye and the rate observed for dye decoloration as a function of the wavelength of light used. The latter rate was kinetically modeled using a radical-chain sequence of reactions, obtaining a good agreement between the modeling and the experimental data. A meaningful acceleration during the initiation step in the radical-chain reaction by Fe3+/H2O2 is observed involving the photochemical reduction of Fe(III) to Fe(II). The photodissociation reaction of the [D ... Fe3+] complex is seen to be more important in the initiation of the chain reaction than the bimolecular quenching between D* and Fe3+. The latter observation is also related to the enhancement of the decoloration rate observed under light irradiation. The usefulness of the reactions described above is related to the beneficial effect of the visible light activation of the decoloration of nonbiodegradable reactive textile dyes by Advanced Oxidation Technologies (AOTs).