Mathematical modelling of pilot-plant photomineralization of chlorophenols in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide

Photomineralization of 4-chlorophenol, 2,4-dichlorophenol, and pentachlorophenol, in 9.9×10−4−3.8×10−5 M aqueous solutions, in the presence of stoichiometric hydrogen peroxide, was studied using PHOTOPERM® CPP/313 membranes containing immobilized 30±3 wt.% TiO2, by following analytically both disappearance of absorbable chloro-organics (AOX), and mineralization of total organic carbon (TOC) content. Experiments were carried out in a PHOTOPERM®WP pilot plant, using monochromatic irradiation (254 nm) in the absorption range of semiconductor, with an absorbed power of 22±1 W. The initial rate of photodegradation, both regarding AOX and TOC analysis, 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 κ and apparent adsorption contants were evaluated. These parameters, which are unable to fit the whole photomineralization kinetic curves, because they account only for initial rates, were employed, as starting values, 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. In this way, two couples of parameters, κ1 and 1, κ2 and 2 were obtained, relative to the two successive steps of the model (substrate disappearance and mineralization), able to reproduce the whole kinetics satisfactorily. Mean quantum yields of organic carbon mineralization, calculated by κ2 parameters, reach 75, 71, and 31 % for the chlorinated phenols in the order reported above, with respect to their maximum allowable quantum yields. These values are from 3 to 5 times greater than those measured in laboratory-scale experiments thus showing the excellent performance of the pilot-plant employed.