Effect of radical scavengers on the performance of advanced oxidation processes

Abstract

Advanced Oxidation Processes (AOPs) rely on the generation of short-lived, powerful and non-selective hydroxyl radicals (·OH) that are responsible for the destruction of a wide range of contaminants in wastewaters. The efficiency of AOPs depends on the production of ·OH and utilization of it effectively by the target contaminants. Non-target substances, or radical scavengers that exist in wastewaters reduce the efficiency of AOPs by consuming significant amounts of ·OH.The purpose of this study was to investigate the kinetics of reactive dyestuff degradation by an advanced oxidation system using ultraviolet light (UV) and hydrogen peroxide (H2O2), and the effect of ·OH scavenging species such as bicarbonate, carbonate and chloride on the efficiency of the process.The method involved preparation of synthetic dye solutions using a reactive azo dye, Procion Red HE-7B, which was selected as a model compound for its wide use in the industry. The effectiveness of the employed system was tested by monitoring the degradation of color and organic carbon in the absence and presence of scavengers.It was found that in the absence of scavengers, 15 minutes was sufficient for complete color removal, whereas 1 hour contact time was necessary for 92 per cent organic carbon degradation. In the presence of scavenging species, the rate of color removal was inhibited and the degree of inhibition increased with increasing concentrations of bicarbonate and carbonate species. It was further found that chloride inhibited the degradation of color only at 100-1250 mM chloride, and did not affect the rate at higher concentrations.The rate of color and organic carbon degradation was found to follow first order kinetics with respect to concentration, and second order kinetics with respect to ·OH. The bimolecular color and organic carbon rate constants with ·OH was estimated by using a simplified kinetic model describing the reaction mechanism. The estimated constants were in good agreement with the published data for azo dyes.