Mathematical modelling of photomineralization of phenols in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide

Photomineralization of phenol, 2,6-dimethyl phenol, 1,2,3-benzenetriol, 4-chloro phenol, and 2,4-dichloro phenol, in 9.9 x 10^-3-5.6 x 10^-5 M aqueous solutions, in the presence of stoichiometric hydrogen peroxide, was studied, using PHOTOPERM® CPP/313 membranes containing immobilized 30 ± 3 wt.% TiO₂, by analysis of total organic carbon (TOC) content. With phenol and 2,6-dimethyl phenol also the rate of disappearance of the substrate molecules was measured fluorimetrically. Polychromatic or monochromatic irradiation was carried out (radiant power in the absorption range 145 and 8 W respectively). The initial rate of photodegradation was studied as a function of the initial concentration of substrate using the linearized form of the Langmuir-Hinshelwood equation, from which the rate constants k and apparent adsorption constants K were evaluated. These parameters, which are unable to fit the whole photomineralization kinetic curves, were employed to optimize, by numerical integration, a kinetic model which considers appearance and disappearance of all intermediates, as if they were represented by a hypothetical single molecule, mediating all of them. By this way, two couples of parameters, k₁ and K₁, k₂ and K₂ were obtained, relative to the two successive steps of the model (substrate disappearance and mineralization), able to reproduce satisfactorily well the whole kinetics. Experimental data, as compared to modelling, show evidence of a main rate determining path, and of another, minor but not unsignificant, parallel path leading to a faster mineralization. Mean quantum yields of organic carbon mineralization, calculated by k₂ parameters, reach 10-20% of the maximum allowable efficiencies.