Role of HO and SO4- radicals on the photodegradation of remazol red in aqueous solution

The aim of this study was to determine the effectiveness of oxidation processes (AOPs) based on the use of UV radiation (UV, UV/H2O2, and UV/K2S2O8) in the degradation and mineralization of dye remazol red (RR) in aqueous solution. Results showed that ultraviolet (UV) radiation was unable to degrade RR and that addition of H2O2 or K2S2O8 during the photodegradation markedly accelerated oxidation of this dye. The effectiveness of the UV/H2O2 system to remove RR was favored at low H2O2 concentrations, low dye concentrations, and high temperatures. Thus, the results obtained show that the decolorization process becomes slower with higher dye concentrations; thus, 99.9%, 81.0%, and 47.7% decolorization was obtained after 60 min when using initial concentrations of 20, 50, and 100 mg/L. Results obtained show that the degradation kinetics at 288 and 298 K were very similar, achieving 66% decolorization at both temperatures after 60 min of treatment. Decolorization kinetics were not accelerated at high H2O2 concentrations, but there was an increase in organic matter mineralization. Apparent reaction rate constants were around 2.5-fold higher with the UV/K2S2O8 system than with the UV/H2O2 system. The results obtained indicate that RR molecules are more selectively degraded by SO4- radicals than by HO radicals. The kAP values and TOC reduction were higher with the UV/S2O82- system than with the UV/H2O2 system in ultrapure water but the inverse was observed in natural waters, when higher values were obtained with the UV/H2O2 than with the UV/S2O82- system. Results obtained show that the toxicity of degradation by-products largely depends on the process used. With the UV/H2O2 system, the toxicity of degradation by-products was much higher than that of the original compound, reaching 100% bacteria inhibition after 30 min of treatment. In contrast, the degradation by-products were less toxic than the original compound with the UV/S2O82- process.