Photocatalytic Degradation of Azo Dyes. Pilot Plant Investigation

The UVB-induced photocatalytic degradation of Methyl Red and Methyl Orange (azo dyes used in the textile industry) containing solutions was carried out by the use of a laboratory-scale pilot plant where the catalyst, TiO2 (anatase), was immobilized at the bottom of a channel through which the liquid was recirculated under UVB irradiation. The plant was preliminarily characterized hydrodynamically, i.e., flow-rate, hydraulic gradients, and residence time. Photodegradation kinetics were followed by UV−vis absorption measurements of the residual dye concentration in the liquid-phase, and the synergistic effects of the catalyst and radiation in promoting the abatement of dyes was demonstrated in the concentration range 0.3−5.0 mg/L. Kinetic data were correlated by the use of first-order (or pseudo-first-order) models up to the concentration range 0.7 mg/ L; at higher concentrations, zero-order models (pure catalytic control) better correlated the experimental data. Photocatalytic degradation of Methyl Red was faster than Methyl Orange, possibly due to the Coulomb repulsion of the negatively charged sulfonate functionalities present on this latter compound. A better hydrodynamic of the liquid recirculating in the channel, i.e., higher flow rate (lower contact time), associated with an improved surface catalyst renovation and a higher frequency of exposition of the substrate to the UVB radiation, together with an improved oxygen dissolution in the liquid-phase, played a positive role in the overall kinetic performance.