Radical And Non-radical Mechanisms For Degradation Of Organic Pollutants

Advanced Oxidation Processes?AOPs?are one of the common methods for removing refractory organic contaminants from water.AOPs mainly include radical based and on non-radical based pathways,but there is still a lack of comprehensive understanding of the mechanism of radicals and non-radicals for contaminants degradation.Therefore,this paper selected oxcarbazepine?OXC?and neonicotinoid insecticide thiacloprid?THIA?as the research object,comprehensive evaluation the mechanism of radicals and non-radical degradation of pollutants under UV/H₂O₂ AOPs and unactivated monopersulfate?PMS?treatment conditions.The details are summarized as follows:?1?OXC is ubiquitous in the aqueous environment.And due to its ecotoxicological effects and potential risks to human,an effective way to eliminate OXC from aqueous environment has aroused public concerns in recent years.Radical-based reactions have been shown to be an efficient way for OXC destruction,but the reactions of OXC with reactive oxygen species?ROS?and carbonate radical(CO₃^·-)are still unclear.In this study,we focused the degradation of OXC and ROS,CO₃^·-generation mechanism,and their roles in OXC degradation via UV and UV/H₂O₂.The triplet state of oxcarbazepine?³OXC^*?was found to play an important role in OXC degradation via UV.And hydroxyl radicals?^·OH?and singlet oxygen?¹O₂?were found to be the dominant ROS in OXC degradation.Superoxide radical(O₂^·-)did not react with OXC directly,but it may react with intermediate byproducts.Generation of CO₃^·-played a positive role on OXC degradation for both UV and UV/H₂O₂.In addition to^·OH,³OXC*also contribute to CO₃^·-production.The second-order rate constants of OXC with^·OH and CO₃^·-were 1.7×10¹⁰ M^–1 s^–1 and 8.6×10⁷ M^–1 s^–1,respectively.Potential OXC degradation mechanisms by^·OH were proposed and included hydroxylation,?-ketol rearrangement,and benzylic acid rearrangement.Compared with non-selective^·OH,the reactions involving CO₃^·-are mainly electron transfer and hydrogen abstraction.And the acute toxicity of OXC was lower after UV/H₂O₂ and UV/H₂O₂/HCO₃^-treatments,which was confirmed by luminescent bacterial assay?Vibrio fischeri bacterium??Chapter 2?.?3?PMS is a promising oxidant that has been widely used in advanced oxidation technology to eliminate the organic contaminants from water.Very limited studies have focused on using unactivated PMS to treat toxic insecticide,however,this aspect of knowledge is urgently needed to fulfill the overall understanding on the advanced oxidation technology.In this study,the reaction mechanisms,byproducts and toxicity evaluation of the neonicotinoid insecticide THIA by unactivated PMS treatment in aqueous solutions were comprehensively investigated.According to electron spin resonance?ESR?spectra and quenching experiments,the non-radical mechanism?i.e.,direct PMS oxidation and ¹O₂ oxidation?was responsible for the THIA degradation.¹O₂ can be favorably produced during the process of PMS self-decomposition and inner electron transfer between PMS and THIA.According to kinetic solvent isotopic effect?KSIE?analysis,the overall THIA degradation can ascribe to 36.5%¹O₂oxidation and 63.5%direct PMS oxidation.Structure-activity assessment and frontier electron densities?FEDs?calculation confirmed that thioether sulfur,amidine nitrogen and cyanoimino group on the thiazole ring were the main reaction sites.Moreover,three major THIA degradation pathways were proposed,including???heterolytic cleavage of peroxo bond and oxygen atom transfer,???electron transfer and???oxidation of cyanoimino group to generate nitroso and nitro-THIA.The water matrixes(HA,HCO₃^?,Cl^?,SO₄^2?)showed negligible effect of on THIA degradation.In addition,the degradation of THIA by PMS-induced oxidation was also effective in natural water matrices.These results suggest that PMS is a promising oxidant for the treatment of THIA in water,because PMS exhibits specific high reactivity and suffers less interference from the water matrix?Chapter 3?.