Kinetic modelling of colour changes in aqueous solutions of phenol oxidized by sono-Fenton technology

Colour formation in oxidized phenolic waters treated with (ultrasonic, US) sono-Fenton is favoured by operating at pH = 3.0. In these conditions, FeOH2+ species is predominant and its presence would generate an additional supply of hydroxyl radicals, which would increase the degradation yield of phenol. The maximum intensity of colour originated in the water (0.74 AU) is obtained when conducting the treatment with 4.0 mol H2O2/mol phenol and ratios about 20.0 mg Fe2+/kW. At these conditions, a notable presence of catechol is detected. The formation of Fe(OOH)(2+) with US waves could establish interactions with the organic matter, as catechol or certain carboxylic acids, like 2,5-dioxo-3-hexenedioic, generating (hydro) peroxo iron (II) complexes that would induce colour. A kinetic modelling is proposed of the catalytic effect exerted by the dosage of iron and the irradiation of US waves in colour kinetics. The induction of colour is adjusted to zero-order kinetics, while the discolouration evolves in a kinetics of third order. Iron exerts a catalytic effect greater than that of US waves in the colour induction stage. Thus, the colour formation increases with iron concentration according to 0.0131 (AU L/mg/min), while the decolouration rate increases according to 0.0262 (AU(2) L/mg/min). Working at US = 0.50 kW, the rate of water colouring shows an important increase. This effect may be due to the fact that the ratio between iron and ultrasound waves 20.0 mg Fe2+/kW may lead to the formation of (hydro) peroxo-iron(II) complexes.