Application of photocatalysis to the treatment of complex industrial aqueous effluent in a pilot-scale bubble column reactor

Abstract

In this study, the photocatalytic mineralization of the industrial dump-site leachate was evaluated using an internally-irradiated 18-Litre pilot-scale aerated annular bubble column photoreactor. The study includes evaluating the effect of catalyst loading, leachate initial concentration, initial solution pH, light intensity and oxygen partial pressure. The reaction runs were performed over a 48-hours period at room temperature and atmospheric pressure.

Titanium catalyst loading was optimized to be 3 gL-1 where the reaction rate constant 20x10-6 mol L-1 min-1.Beyond this dosage, the effect of light scattering by the catalyst particles were noticed on dropping the degradation rate. Moreover, at high catalyst loading, particles aggregates reduce the interfacial area between the reaction solution and the photocatalyst resulting in significant reduction in the number of active sites on the catalyst surface. It is also noticed that when the initial leachate concentration is high, the number of the active sites are decreased because of their competitive adsorption on the TiO2 particles; while on the other hand, during the light intensity illumination period, the OH radicals formed on the catalyst surface are remaining constant as evidenced by constant hydroxyl production rate. Thus, the reactive O2 attacking the contaminants molecules decrease and simultaneously the overall photodegradation efficiency also decrease dramatically. The plot of the apparent reaction rate constant versus the initial leachate concentration exhibits almost a quadratic behaviour which has an optimum value at concentration of 50 mM. Finally, it was found that the degradation rate constant increased with O2 partial pressure until a maximum was obtained around 50% O2/N2 of gas feed composition. The drop in the rate beyond 50% can be explained by the fact that the dissolved oxygen molecular oxygen is strongly electrophilic and thus increasing the dissolved oxygen content probably reduced electron-hole recombination rate and hence the system was able to maintain favourable charge balance necessary for the photocatalytic-redox process. Moreover, in the presence of excess O2, the photocatalyst surface may become highly hydroxylated to the point of inhibiting the adsorption of organic species causing decrease in the degradation rate.

Effect of upflow co-current and counter current continuous operation mode were performed in the 18-litre bubble column photoreactor for the photo-oxidation degradation treatment of the dump-site landfill leachate. The best situation is liquid flow rate at 800 mL min-1 and total gas flow rate at 5 Lmin-1 for the counter current operation, while for the up-flow co-current operation, the best situation is liquid flow rate at 600 mL min-1 and total gas flow rate at 5 Lmin-1