Kinetic studies of photocatalytic degradation in a TiO2 slurry system:: Distinguishing working regimes and determining rate dependences

A systematic analysis has been carried out to distinguish "kinetic" and "transport" limited regimes for photocatalytic degradation in a TiO2 slurry system using benzoic acid as a model component.. Experiments were performed to investigate the effect of both monoparametric (e.g., catalyst loading, circulation rate, and initial concentration) and biparametric (e.g., catalyst loading and circulation rate, initial concentration of solute, and light intensity) on the degradation rate to differentiate the kinetic regime from the transport regime. It is shown that there is no external mass-transfer limitation at any catalyst loading, but at high catalyst loading, there exist both internal mass-transfer limitation and light limitation. The domain of kinetic limitation, the domain of internal mass-transfer limitation (particle agglomeration effect), and the domain of light transport limitation (shielding effect) were distinguished. Experimental results demonstrate a significant difference in the rate values between the "optimal catalyst loading" regime and the kinetic regime. The true kinetic dependences of the photocatalytic degradation rate on light intensity and the substance initial concentration were determined.