Photocatalytic degradation of model organic pollutants on an immobilized particulate TiO2 layer: Roles of adsorption processes and mechanistic complexity

The kinetics of photocatalytic degradation of four different model organic compounds, formic acid (FA), oxalic acid (OA), 4-chlorophenol (4-CP) and the herbicide monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in a self-constructed batch-mode plate photoreactor with a thin flow of contaminated aqueous solution circulating over an illuminated particulate layer of TiO2 P25 (Degussa) was compared. Both OA and FA were adsorbed on TiO2 surface; their mineralization, induced by direct transfer of photogenerated holes, proceeded in a single step, without observable intermediates, following approximately zero order kinetics. Numerical simulations were performed using a newly proposed kinetic model based on the photostationary state assumption. The model allowed an explanation of the observed reaction order as well as the comparison of independent with competitive adsorption of organic compound and oxygen on the photocatalyst surface, yielding a better fit for the case of competition. 4-CP and monuron, which were not adsorbed under the conditions used, were degraded through the action of photogenerated hydroxyl radicals. Their degradation proceeded with lower photoefficiency than for the adsorbed compounds (FA and OA). While the mineralization of both 4-CP and monuron followed zero order kinetics, their degradation was close to first order. The different reaction orders were consistently explained using the photostationary state approach.