Degradation Of Typical Organic Pollutants In Aqueous Solution With Advanced Oxidation Technology Based On Sulfate Radicals

Advanced Oxidation Technologies based on the generation of sulfate radicals (50₄^?) isone of the most effective ways to remove the nonbiodegradable organic pollutants. Amongthem, homogeneous Co²⁺ catalyzed peroxymonosulfate (Co/PMS) and CO₃O₄ catalyzed PMS(Co₃O₄/PMS) are very promising technique and can generate SO₄^? as the main species due tothe simpleness of methods and mildness of reaction conditions. As good electron acceptor,PMS, which has high oxidation-reduction potential and electrophilic transfer ability, canenhance generation rate of radicals and degradation rate of dye with TiO₂ photocatalysisunder visible light irradiation. In this study, azo-dye Orangeâ…¡(AO7) and pesticideImidacloprid were chosed as model compounds to study the degradation kinetics, influentialfactors and mechanism by homogeneous Co/PMS, heterogeneous Co₃O₄/PMS andVis/TiO₂/PMS system. The following studies were carried out:(1) The degradation kinetics of AO7 in aqueous solution induced by homogeneousCo/PMS system was investigated. A kinetics model was proposed to describe theexperimental data. The pseudo first-order decolorization rate constants (k_obs) are related to[Co²⁺], [PMS] and reciprocal of [AO7]₀. Activation energy of the AO7 decolorization byCo/PMS process was determined to be 75.7 kJ/mol. UV and visible light can accelerate thedecolorization and mineralization process due to different mechanisms. In the combinedUV/Co/PMS system, UV light can decompose PMS to generate ^·OH and SO₄^?, while in theVis/Co/PMS system, excited AO7 molecules can transfer electrons to PMS or Co³⁺ and thusaccelerate the decomposition of PMS and catalytic cycle of Co³⁺/Co²⁺.(2) Effects of [PMS], [Co²⁺] and various inorganic anions (H₂PO₄^-, HCO₃^-, NO₃^-, Cl⁾on the degradation of Imidacloprid, a kind of insecticide as a model compound, induced byCo/PMS system were investigated. The degradation kinetics of Imidacloprid followed pseudofirst-order kinetics and the reaction rates increased with increasing [PMS] and [Co²⁺] whenthe other parameters were constant. However, the decrease of degradation rates was foundwhen the molar ratio of PMS versus Imidacloprid was over 20. The effects of H₂PO₄^- hadpositive effect on the degradation of Imidacloprid, low concentration HCO₃^- had positiveeffect and high concentration HCO₃^- has negative effect, Cl^-had negative effect, while NO₃^-had little effect. Based on the results of GC/fMS, two main intermediates were identified: 6-chloronicotinic acid and 6-chloronicotinamide, and the reaction pathway for SO₄^? inducedby homogeneous Co/PMS was proposed accordingly.(3) Nano-Co₃O₄ was prepared by precipitation method and was successfully applied asheterogeneous catalyst to activate peroxymonosulfate (PMS) and degrade AO7. The catalystexhibits spherical morphologies with minor particle agglomeration, small particle averagesize (20 nm) and high specific surface area (18 m²/g). The degradation kinetics of AO7induced by nano-Co₃O₄/PMS system was investigated both at acidic and neutral pH. Theheterogeneous character of PMS activation with nano-Co₃O₄ is more pronounced at neutralpH as indicated by fast degradation rate of AO7 and low dissolved Co ion. There is an optimalcatalyst dosage and oxidant molar concentration in the degradation of AO7 induced bynano-Co₃O₄/PMS system. The catalyst presented a long-term stability through using thecatalyst for multiple runs in the degradation of AO7. The main degradation intermediates ofAO7 identified by GC/MS and LC/MS were 4-hydroxybenzenesulfonic acid,1, 2-naphthalenedione, coumarin, phthalic anhydride, phthalimide and 2-formyl-benzoic acid.Proposed degradation pathways were elucidated in light of the analyzed degradation productsand frontier electron density theory.(4) According to the low degradation efficiency of AO7 by TiO₂ under visible lightirradiation, the addition of PMS can greatly improve the degradation rate of AO7 and thedegradation rate was better than K₂S₂O₈ and H₂O₂. The effect of humic acid has little effecton the Vis/TiO₂/PMS degradation effiency. In Vis/TiO₂ system, the observed inhibition effectsof inorganic anions, such as H₂PO₄^-,HCO₃^- and Cl^-, can be interpreted by competitiveadsorption. While in Vis/TiO₂/PMS system, the inhibition phenomenon was not obviousexcept Cl^- probable due to the different mechanism. In Vis/TiO₂/PMS system, the mainoxidative species for the degradation of AO7 were ^·OH and SO₄^?. The possible pathway wasproposed based on the analysis of degradation intermediates of AO7.