Oxidation of organic contaminants in a rotating disk photocatalytic reactor: reaction kinetics in the liquid phase and the role of mass transfer based on the dimensionless Damkِhler number

The photocatalytic degradation of a model non-volatile chlorinated aromatic compound 4-chlorobenzoic acid (4-CBA) was investigated as a function of disk angular velocity, contaminant concentration, and incident light intensity using a rotating disk photocatalytic reactor (RDPR). The study was designed to investigate the effect of all three parameters on the degradation rate as well as their importance on unveiling the existence of mass transfer limitations in the liquid phase under specific conditions. The results showed that the reaction rate increased with disk angular velocity in accordance with a saturation-type dependency. In the range of 2–6 rpm the degradation rate increased almost linearly with disk angular velocity. Above 6 rpm, however, the influence of disk angular velocity was not significant. The initial increase in the reaction rate with disk angular velocity was attributed to the longer time available per rotation resulting in higher down flow of liquid carried by the disk and to the increase in the overall mass transfer coefficient. The rates of 4-CBA acid decomposition and Cl− mineralization at 6 rpm as a function of initial 4-CBA concentration followed Langmuir–Hinshelwood kinetics. At 6 rpm, the rates of 4-CBA degradation followed a linear dependency with incident light intensity. This was attributed to the existence of low local values of incident light intensity on the illuminated disk. With respect to the effect of these three parameters on the degradation rate, the obtained results suggested the absence of significant mass transfer limitations at disk angular velocities higher than 6 rpm. The latter was verified by additional calculations of the Damköhler (Da) number based on dimensionless analysis. The Da number was found to decrease significantly with disk angular velocity and at high disk angular velocities (ω>15 rpm), Da was much lower than 0.1, even when the concentration of the contaminant in the bulk was extremely small (i.e. 1 μmol/l).