Photocatalytic degradation rate of oxalic acid on the semiconductive layerof n-TiO2 particles in the batch mode plate reactor - Part I: Mass transfer limits

Organic compounds dissolved in water can be decomposed on a layer of n-TiO2 particles irradiated by u.v. light, which generates holes and electrons in the TiO2 material. Dissolved oxygen was used as electron scavenger and hol es reacted with water to give OH radicals. The rate of degradation of the d issolved organic compounds by OH radicals is limited by the transfer of eit her oxygen or of the organic compounds to the surface of n-TiO2 particles. The consequence of these limits is that, in the batch mode reactor with rec irculation of the liquid, the dependence of the concentration of an organic compound on time has either a linear or an exponential form. Experiments w ith decomposition of oxalic acid in aqueous solutions using a plate reactor (60 cm x 120 cm) confirmed the analysis. Equations for evaluation of the m ass transfer coefficient of the dissolved species to the surface of the pla te reactor with a moving liquid film were developed for the case of the thi ckness of the Nernst diffusion layer being thinner than the thickness of th e liquid. The experimentally obtained decomposition rate of oxalic acid was about 60 to 80% of the theoretical decomposition rate limited by oxygen fl ux through the film of a moving liquid. The present theory neglects the dif fusion of oxygen into the porous layer of n-TiO2.