Decolorization of CI reactive red 198 in UV/oxidant and UV/TiO2/oxidant systems

In this study, UV/oxidant and UV/TiO2/oxidant systems were employed to treat textile wastewater. The parent compound was C.I. reactive red 198 (RR198). The selected oxidants were H2O2, Na2S2O8, NaBrO3, and NaIO4. The effects of oxidant dosage (1-24 mM), wavelength of UV (254 and 365 nm) and radical scavenger addition (C2H5OH) were determined in UV/oxidant systems. The experimental results revealed that all oxidants effectively decolorized RR198 under 254 nm irradiation; however, only Na2S2O8 and NaIO4 can decolorize RR198 under 365 nm irradiation. The decolorization rates fit a pseudo-first order reaction model. Under 254 nm irradiation and 6 mM oxidant addition, the decolorization rate constants (k) of H2O2, Na2S2O8, NaBrO3, and NaIO4 for RR198 were 10.24, 17.93, 13.37, and 11.90 h(-1). Under 365 nm irradiation, 1 g/L TiO2 and 1 mM NaIO4 addition, the k values of the UV/TiO2, UV/NaIO4, and UV/TiO2/NaIO4 systems were 0.50, 0.52, and 11.67 h(-1). The inhibition of RR198 decolorization by the addition ethanol indicates that the primary decolorization pathway involves hydroxyl radicals in UV/H2O2 and UV/Na2S2O8 systems, and that oxidation by other radicals is probably important in UV/NaBrO3 and UV/NaIO4 systems.