| Industrial wastewater usually contains large amount of chloride ions with high non-biodegradable organic concentration, which can seriously inhibit the effectiveness of aerobic and anaerobic biological treatment of wastewater. Therefore, it is urgent to develop higher efficiency, low cost and more suitable treatment methods for saline wastewater, in order to protect the water quality safety and achieve the sustainable development of the industry.Recently, advanced oxidation processes (AOPs) are gaining significant importance in detoxification of many organic contaminants, due to their high treatment efficiency, energy saving, wide application scope and convenience of management. However, most of the previous investigations have ignored the influence of halide ions (Cl-, Br-) on the AOPs treatment of saline wastewaters. For example, hydroxyl radical (·OH,2.8V) can oxidize Cl-to less reactive chlorine species, such as Cl2/2Cl-(1.36V) and HOCl/Cl-(1.48V). Consequently, the involvement of Cl-not only influences the degradation efficiency of the organic pollutants, but also possibly leads to the formation of higher toxic or even carcinogenic halogenated organic compounds. Therefore, it is urgent to study the effect of halide ions on AOPs treatment processes systematically.In this study, an azo dye Acid Orange7(AO7) was selected as a model nonbiodegradable substrate. The homogeneous UV/H2O2system based on hydroxyl radicals, the heterogeneous UV/TiO2system and the emerging cobalt/peroxymonosulfate (Co/PMS) advanced oxidation process were chosen to treat the simulated saline dye wastewater. The effect of halide ions (Cl-, Br-) on dye decoloration, mineralization and AOX (halogenated organic compounds adsorbable on activated carbon) formation in aqueous solution were investigated, considering various solution pH. Our attention has also been focused on the dye degradation mechanisms in high saline solutions based on the identification of the transformation products. The results are presented as following:(1) UV/H2O2systemHydroxyl radicals, generated by the photolysis of H2O2, can degrade the organic contaminants effectively. The degradation kinetics can be affected by the strong competition of halide ions (Cl-, Br-) in the saline wastewater with dye molecules for·OH. The experimental results showed that the decoloration and mineralization rate of AO7was significantly reduced by the addition of Cl-or Br-in UV/H2O2system, especially in acidic conditions. Kinetic modeling based on radical reactions was implemented to examine the impact of both chloride ion concentrations and solution pH on the degradation efficiency of organic contaminant. The simulation results were as follows:①In the neutral conditions, the yield of chlorine radicals (C12·-\Cl· and ClOH·-) increased with the increasing content of Cl-present. When the initial concentration of Cl-exceeded about200mM, Cl2·-instead of·OH became the predominant species during the overall oxidation process. The inhibitory effect of Cl-on AO7degradation observed in the laboratory experiments could be attributed to both scavenging effect of Cl-on OH and the much lower reactivity of chlorine radicals formed.②Under acidic conditions, the C1OH·-can be easily converted to Cl· rapidly through the protonation reaction. Both C1OH·-and Cl’can also react with Cl-generating Cl2·-, which can be the predominant radical species especially in acid medium. Due to the lower reactivity of Cl2·-with AO7, the inhibitory effect of Cl-is more significant under lower pH conditions.③The conversion of ClOH·-to·OH and Cl-was favored in alkaline conditions, leading to less inhibition on AO7degradation.④The reaction of·OH with dye molecule is the rate-governing step and is critical during the dye degradation process. Changing this rate constant by an order of magnitude has the largest effect. The GC-MS results confirmed that the halide iolis were involved in the dye degradation process, leading to the formation of several high toxic organic halogenated byproducts, including2,4-dichloro-l-naphthalenol,5-chlorobenzofuran,3-chloropropiophenone,3-chloropropiophenone,1-bromo-2-naphthalenol and1,1-dibromo-propanone. AOX was found to increase with the increasing content of Cl-. Based on the intermediate products and especially the toxic halogenated byproducts, the possible degradation pathways of saline dyeing wastewater were proposed.(2) UV/TiO2systemConduction band electrons (e-) and valence band holes (h+) can be produced when aqueous TiO2suspension is irradiated with UV light. The photogenerated holes can react with OH-, H2O or O2, leading to the formation of·OH and O2·-. Such highly oxidants species have been reported to be responsible for the heterogeneous TiO2photodecomposition of organic substrates as dyes. In this UV/TiO2system, the effects of chloride salt on dye degradation may affect the degradation of organic substrates based on the following hypothesis:a) Cl-may competitively reduce the adsorption of dye on the catalyst surface; b) Cl-may be directly oxidized by the valence band holes; and c) a large amount of Cl-adsorbed on the catalyst surface may form considerable inner-sphere=Ti-Cl surface complexes. Accordingly, the accumulation of Cl-on the TiO2surface promoted the photodegradation of AO7with Cl’radical-initiated reactions. The average extent of AO7(anionic dye) adsorbed on the catalyst was observed to decrease dramatically with the increasing Cl-content. The chloride ion was found to have a dual effect on both the dye decoloration and mineralization in UV/TiO2system due to different mechanisms involved. Cl-ion (0-50mM) showed an obvious enhancement for both decolorization and mineralization of AO7. The observed decreasing acceleration activity or even inhibitory effect with much higher chloride content may be caused by the deactivation of the photocatalyst and reduction in photon receiving efficiency. In the presence of chloride, the chlorine radicals may react with dye and its intermediates by one-electron oxidation, H-abstraction or addition to unsaturated C-C bonds. Several toxic chlorinated byproducts, such as7-chloro-2-naphthalenol,4-chloro-l-naphthalenol,2,4-dichloro-l-naphthalenol,1,3,5-trichlorobenzene and2,4,6-trichloroaniline, were identified during the dye degradation in UV/TiO2process using GC-MS method.(3) Co/PMS systemCo2+can efficiently activate peroxymonosulfate (PMS) to produce sulfate radicals (SO4·-,2.5～3.1V), which can destroy the pollutants present in wastewater by one-electron oxidation, H-abstraction or addition reactions. This emerging AOP has attracted great scientific and technological interest in environmental application. The degradation efficiency of AO7was found to be positively correlated with the concentration of cobalt ion and PMS. Consumption of SO4·-radicals by Cl-and formation of less reactive chlorine radicals should be responsible for the adverse effect of chloride on Co/PMS performance. However, the formation of HOCl and Cl2via the reaction between Cl-and PMS would make the dye bleaching rate higher with the addition of large amount of chloride ions. A dual effect of chloride or bromide (i.e. inhibitory and accelerating effect) on AO7degradation in Co/PMS system was observed. High concentrations of chloride (>5mM) can significantly enhance the dye decoloration independent of the presence of the Co2+catalyst, but did greatly inhibit the dye mineralization to an extent which was closely dependent upon the chloride content. Such dual effect also appeared when Br-was added. The degradation rate of AO7was enhanced with the increasing pH. Both UV-Vis absorbance spectra and AOX determination indicated the formation of some refractory degradation intermediates. Several chlorinated compounds, including chlorobenzene, chlorophenols, chlorination aromatic hydrocarbon and low molecule chlorinated products, were indentified by GC-MS measurement. The formation of such byproducts can be attributed to two possible branched reaction pathways (SO4·-radical-based and non-radical).It should be point out that many industries such as meat canning, olive oil mills, petroleum, petrochemical, agro-food, seafood processing, vegetable canning, pickling and cheese processing also generate effluents with high salt content, ranging from1.5to80g/L. The present studies suggested that the appreciable level of salts in practical wastewater possibly reduces the level of pollutants treatment efficiency and even leads to the formation of more toxic halogenated compounds during AOPs treatment. Therefore, it is very necessary to monitor the concentration of halide ions and develop effective strategies before the large scale application of the advanced processes. In this work insights into the reaction mechanisms based on the chain radical reactions involving halide ions, hydroxyl radicals, sulfate radicals and organic molecules may have significant technical implications for the application of more complex systems of relevance to oxidation of organic contaminants in the high salinity wastewater. |
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