Photochemical Kinetics of Organic Dye Oxidation in Water

Abstract

Photochemical kinetics of dye colour removal and destruction with the use of two types of UV sources has been studied. The mathematical model of the oxidation kinetics with the use of a Xe-eximer lamp (λ= 172 nm) has been developed. This model considers a quasi-heterogeneous stage of OH radical formation, their diffusion into the bulk water and the chemical reaction between radicals and dye molecules. The model is valid if the depth of light penetration is less then the diffusion layer thickness. Diffusion, chemical and mixed control are possible. The model can be used for VUV-photoreactor modelling. At chemical control the process rate should be independent of the stirring intensity while at mixed and diffusion control the process rate depends on hydrodynamic conditions. Chemical reaction rate constants for different dyes, as well as the steady-state concentrations of OH adicals in the photolayer were evaluated. The homogeneous reaction between OH radicals and dye molecules controls the process rate when an Hg low-pressure lamp (λ1 185 and λ2 254 nm) was utilized. Colloidal particles of TiO2 do not affect the rate of colour removal, however crystalline powders of TiO2 vary the rate. The dependencies of colour change on time and quantity of crystalline TiO2 were found to be complex functions. Neither the Hg lamp nor the Xe-eximer light source permitted us to obtain essential mineralization of dyes in the conditions used in our experiments. In all cases only a minute decrease in the chemical oxygen demand was observed.