Kinetic modeling of pilot plant photomineralization of aqueous Cibacron CR as model molecule of industrial azo dyes on titanium dioxide-immobilizing membranes

The kinetics of photobleaching by spectrophotometric analysis and of photomineralization by total organic carbon (TOC) analysis of Cibacron CR in aqueous solutions were studied using titanium dioxide-immobilizing membranes. The experiments were carried out in a pilot plant under monochromatic irradiation. From the rate data as a function of initial concentration, the kinetic parameter k and the pseudo-thermodynamic parameter K for photobleaching were evaluated by complying with the Langmuir-Hinshelwood equation. These parameters were employed to optimize a kinetic model, which considered the mineralization of substrate S to CO2 through one single intermediate I, as being able to represent and mediate the behaviour of all the intermediates effectively formed, both S and I acting as competitive apparent adsorption agents on the immobilized semiconductor surface. By this way, the two couples of constants -k1 and K1 as well as k 2 and K2 - were obtained. With these two couples of parameters the calculated curves fitted very well to the experimental ones, both for photobleaching and photomineralization, whereas the formal Langmuir model was unable to predict rates other than those at the beginning of the bleaching process. Therefore, the Langmuir model alone is unsuitable to infer temporal disappearance of TOC during the photomineralization process. The influence of the membrane's position within the annular photoreactor at several distances from the irradiating lamp was systematically studied with optical paths varying from 2.5 to 12.8 cm. The optimum pH value in order to reach the maximum degradation was around 6, whereas at lower and higher values, a monotonous decrease in rate occurred.