Degradation Of Azo Dyes By The Persulfate Catalytic Oxidation

The printing and dyeing industry produces a large number of wastewater, whichcontains high strength organic compounds and salt. Due to the high concentration of COD,dark colour and high variability in composition, the printing and dyeing wastewater presentsa great challenge for its treatment. The azo dyes-containing effluent from the printing anddyeing industry is a more challenging type of wastewater to deal with, because of the poorbiodegradability and high toxicity. Moreover, during the biodegradation treatment, the azodyes are capable to produce intermediate called anilines, which are carcinogenic,terarotogenic and mutagenic, imposing a great menace to the environment. Currently, thedecolourization and mineralization of azo dyes have become the two difficult problemsduring the treatment of dyeing wastewater. Therefore, in this study, methyl orange andorange G, which are categorized into azo dyes, were selected as the target pollutants andtreated with advanced oxidation of persulfate by oxidative degradation of methyl orange andorange G.Firstly, oxidative degradation of methyl orange （MO） using zero-valent iron-activatedsodium peroxydisulfate （PDS） as catalyst was investigated, the kinetics of Fe⁰/PDS systemoxidation of MO was also studied. The results demonstrated that the degradation processwas based on generation of sulfate radicals as powerful oxidizing species. The effects ofdosage of PDS and Fe⁰, initial pH, temperature and initial concentration of MO on MOdegradation were examined in batch experiments. Under the conditions of initial MOconcentration of100mg·L－1, PDS dosage of0.912mmol·L－1, Fe⁰dosage of0.16g·L－1, pHof4.0and temperature of298K, MO degradation efficiency in aqueous solution reached80.2%within2.5h, and the degradation of MO occurred in two stages, and the second stageof the reaction followed first-order kinetics behavior.Secondly，the kinetic of oxidation degradation of orange G （OG） in aqueous solutionsby the activated carbon catalyzed peroxydisulfate （PDS） was investigated. This process isalso based on the generation of sulfate radicals. The results demonstrated that OG can bedegraded by GAC/PDS reagent effectively. Moreover, the dosage of PDS and GAC, initial pH had an impact on OG oxidation. In addition, the dosage of radical quencher was alsoexamined, indicated that the degradation of Gold Orange G by the sulfate radicals formedin the GAC/PDS system. Finally, the catalytic performance of GAC was studied after reusedseveral times, and the reason of the decline in the catalytic properties was also analyzed.In addition, kinetic model of the oxidation of the orange G with persulfate wasestablished. The results showed that higher temperature and larger GAC dosage resulted inhigher OG degrading rates. In addition, the empirical kinetic equation for OG oxidation byGAC/PDS combined system under the conditions of0.050mmol·L^-10.125mmol·L^-1of OG,5.0of pH,10/1¹60/1of PDS/OG molar ratio,0.1g·L^-11.6g·L^-1of GAC,298K³38K oftemperature, could be reasonably represented by a first order kinetics, which were fitted verywell with the experimental data.