Development And Degradation Efficiency Of Advanced Oxidation Processes Based On Dithionite

Toxic refractory organic substances have posed a serious threat to the ecosystem and influenced the quality and safety of the ecosystem seriously,which was difficult to be effectively degraded by traditional treatment processes.Advanced oxidation process based on sulfate radical(SO₄^·--AOP),a novel technology for most refractory organic matter,had great potential to control the organic pollution of the environment.Generally,the traditional free radical generating sources of SO₄^·-,involving persulfate,permonosulfate and sulfite,could form sulfate radicals with the stronger oxidizing capacity to degrade organic contaminations activated by transition metals,light,and sound etc.In this study,we have developed two novel processes based on dithionite(DTN)of PS/DTN and Fe²⁺/O₂/DTN in which DTN were served as activator of persulfate or sulfate radical generation source,respectively,with the advantages of simple operation and low risk of secondary pollution.In addition,we have conducted the identification of the possible reactive species and explored the formation process of these radicals.Moreover,we have studied the degradation rules of organic contaminations and investigated the degradation mechanism.As to the traditional SO₄^·--AOP with the sulfate radical resource of persulfate,we have investigated the novel AOP system based on PS activated by DTN.Compared with the system of PS alone,atrazine(ATZ)degradation rate could be increased by about 10 times with the addition of DTN.There was the highest ATZ degradation rate of 3.42×10^-2 min^-1 with the C_PS/C_DTN of 500:1 and the activation energy of ATZ degradation also could significantly be reduced to 97.98 k J/mol in PS/DTN system.Then we have identified the reactive species of SO₄^·-and·OH based on free radical scavenging tests and electromagnetic spin resonance spectroscopy(ESR).In addition,ATZ degradation would be inhibited by the ubiquitous substances(Cl^-,HCO₃^-,NOM,etc.)in the environment in PS/DTN system and the main degradation pathways have been proposed including dechlorination,hydroxylation,dealkylation and deamination.Organic pollutants without halides,such as bisphenol A(BPA),could also effectively degraded and mineralized in PS/DTN system and the concentrations of BPA and TOC were reduced by about 85%and 30%,respectively.Based on the response surface methodology(RSM),we have obtained the optimal parameters for the degradation of BPA in the PS/DTN system with the of PS and DTN concentrations of 131.77 and 93.64?mol/L,respectively,and the initial p H of 3.62.Moreover,the co-existing substances of Cl^-,HCO₃^-,NOM,etc.also have the inhibition on BPA degradation and the primary degradation pathways of BPA included the active groups attacking the benzene ring structure,?cleavage of C-C bond and radical binding processes.In addition,we have developed another SO₄^·--AOP of Fe²⁺/O₂/DTN system with the generation resource of DTN without persulfate or permonosulfate.When the dissolved oxygen concentration was about 7.0 mg/L,Fe²⁺could activate DTN to degrade ATZ effectively with the degradation rate of about 0.1620 min^-1,which was significantly higher than the systems of PS/DTN and Fe²⁺/PS.Furthermore,the Fe²⁺/O₂/DTN system could effectively degrade ATZ under neutral and acidic solutions.The intermediates of ATZ degradation were identified based on gas-mass spectrometry(GC-MS)and the main degradation mechanisms of ATZ in Fe²⁺/O₂/DTN system have been concluded including dechlorination,dealkylation,carbonylation and hydroxylation.Based on the scavenging tests and ESR detection,we also confirmed the formation of several reactive species including SO₃^·-,SO₄^·-and·OH derived from the main intermediates of Fe HSO₃⁺and Fe SO₃⁺,whose formation included two parallel pathways:pathway I was the reaction between Fe²⁺and HSO₃^-generated from DTN decomposing;pathway II was the reaction between DTN and Fe³⁺derived from Fe²⁺.