Nonlinear Modelling of Kinetic Data Obtained from Photocatalytic Mineralisation of 2,4‐Dichlorophenol on a Titanium Dioxide Membrane

Ignazio Renato Bellobono,Franca Morazzoni,Cristina Costache,Roberto Scotti,Mauro Rossi,Liliana Bobirica,Mauro Luigi Bonardi,Paola Maria Tozzi,Massimiliano D'Arienzo,Luigi Gini,Gabriela Cobzaru,Rodica Stanescu,Flavia Groppi,Riccardo Bianchi

doi:10.1155/2009/631768

Abstract

Photomineralisation of 2,4‐dichlorophenol (DCP) in aqueous solutions (10.0–100.0 mg/L of C) was systematically studied at 318 ± 3 K, in an annular laboratory‐scale reactor, by photocatalytic membranes immobilizing titanium dioxide, as a function of substrate concentration, and absorbed power per unit length of membrane. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed (first series of experiments). At a fixed value of irradiance (1.50 W⋅cm−1), other series of mineralization experiments were repeated (second series of experiments) by carrying out only analyses of chemical oxygen demand (COD), in order to compare modelling results of the two sets of experiments. In both sets of experiments, stoichiometric hydrogen peroxide was used as oxygen donor. For the first series of experiments, a kinetic model was employed, already validated in previous work, from which, by a set of differential equations, four final optimised parameters, k1 and K1, k2 and K2, were calculated. By these parameters, the whole kinetic profile could be fitted adequately. The influence of irradiance on k1 and k2 could be rationalised very well by this four‐parameter kinetic model. Modelling of quantum yields, as a function of irradiance, could also be carried out satisfactorily. As has been found previously for other kinds of substrates, modelling of quantum yields for DCP mineralization is consistent with kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading finally to carbon dioxide, paralleled by a second competition kinetics involving superoxide radical anion. For the second series of experiments, on the contrary, the Langmuir‐Hinshelwood model was employed. Uncertainties of COD analyses, coupled with discrepancies of this model and with its inability to reproduce kinetics up to complete mineralization, are underlined.

Highlights

Among the advanced oxidation processes, which were developed during the last twenty five-thirty years, photocatalysis [1,2,3] in the presence of semiconductors, such as titanium dioxide, with UV radiation, is certainly one of the most studied
Modelling kinetic data for most of experiments, that is those for which the rate of DCP disappearance was measured together with the rate of total organic carbon (TOC) disappearance, the same procedure described in preceding papers ([23, 24] and [11]) was closely followed, by using the four-parameter model illustrated with all details in these works
For the series of experiments in which only chemical oxygen demand (COD) was determined, COD data were first transformed into TOC data, by supposing that the ratio between total organic compound, as expressed by COD, and carbon itself was the same as that which was present in DCP

Summary

Introduction

Among the advanced oxidation processes, which were developed during the last twenty five-thirty years, photocatalysis [1,2,3] in the presence of semiconductors, such as titanium dioxide, with UV radiation, is certainly one of the most studied. In many cases described in literature, no regard is made to total organic carbon (TOC) mineralisation, but merely to transformation of substrate into some intermediate compound, for example, bleaching of a dye, as the first step of the complicated reaction sequence leading to carbon dioxide It is a common practice, even when quantitative evaluations are made, to consider the photocatalytic process as a first-order reaction. In a wide range of concentrations, and over all the kinetic concentration profile of TOC, leading to full mineralisation, has been approached systematically in previous papers of this series [11, 12], by using a fourparameter kinetic model This has been done from the stand-point of quantum yields and energy efficiencies, a very important aspect, which is often neglected in the literature pertinent to photocatalysis. We deemed useful to evaluate treatment of data obtained by these two analytical methods

Experimental

Results and Discussion

Conclusions