Identification And Chemical Transformation Mechanism Of Typical Spin Adducts In Persulfate-Based Oxidation Processes

Relying on electron paramagnetic resonance（EPR）combined with spin trapping technique,free radicals produced in persulfate-based advanced oxidation processes can be detected by forming stable and long-life spin adducts with spin trapping agent such as 5,5-dimethyl-1-pyrrolin-N-oxide（DMPO）or N-tert-butyl-α-phenylnitrone（PBN）.However,spin trapping agents or their spin adducts may undergo chemical transformation under highly oxidizing environment,generating the false EPR signal via non-radical mechanism,thus causing the inaccurate interpretation on the reaction mechanism.Therefore,this thesis investigates the trapping of free radicals by DMPO or PBN and chemical transformation mechanism in currently controversial systems of peroxygens/spin trapping agents and Cu（Ⅱ）（or phosphate）/peroxygens/spin trapping agents via a series of methods,such as isotope labeling,fluorescence spectra,electrochemistry,and theoretical calculation.The main conclusions are as follows:1.For identification of free radicals,it is the prerequisite that persulfate does not react with spin trapping agent in persulfate-based advanced oxidation processes.However,in this study,it is found that persulfate（PDS）could directly react with spin trapping agent（i.e.,DMPO or PBN）to produce time-dependent DMPO-OH or PBN-OH signal.Radical quenching experiments show that neither of these two adducts is derived from the trapping of hydroxyl radical（^·OH）.However,the generation pathways of DMPO-OH and PBN-OH are greatly different.In PDS/DMPO system,PDS directly oxidizes DMPO to produce DMPO-SO₄,then producing DMPO-OH through the substitution reaction with H₂O.Isotope labeling experiment confirms that the hydroxyl oxygen in DMPO-OH originates from H₂O.In PDS/PBN system,PBN cationic radical(PBN^·+)is firstly generated through one-electron oxidation of PBN by PDS,then PBN-OH is formed by nucleophilic addition of PBN^·+with H₂O.2.^·OH or Cu（Ⅲ）is likely to be the reactive species of Cu（Ⅱ）-based advanced oxidation systems.In this study,EPR combined with spin trapping method is used to investigate the multiple generation pathways of DMPO-OH adduct in Cu（Ⅱ）-based oxidation systems.In Cu（Ⅱ）/DMPO system,DMPO is oxidized by Cu（Ⅱ）to produce DMPO-OH signal.Based on the diverse radical adducts generated by methanol with^·OH or Cu（Ⅲ）,it is proved that DMPO-OH in Cu（Ⅱ）/H₂O₂ system is mainly generated from the capture of^·OH by DMPO.In Cu（Ⅱ）/peroxymonosulfate（PMS）system,DMPO-OH is mainly derived from the oxidation of DMPO by Cu（Ⅲ）.While in Cu（Ⅱ）/PDS system,the trapping of SO₄^·-by DMPO or the hydrolysis of DMPO-SO₄,and the oxidation of DMPO by Cu（Ⅲ）may lead to the generation of DMPO-OH.Isotope labeling experiments demonstrate that the hydroxyl oxygen in DMPO-OH signal which generated from Cu（Ⅱ）/DMPO system is entirely derived from H₂O,while hydroxyl oxygen produced in Cu（Ⅱ）/H₂O₂/DMPO,Cu（Ⅱ）/PMS/DMPO,or Cu（Ⅱ）/PDS/DMPO system mainly originate from the oxidant.3.It is reported that some organic pollutants can be effectively degraded by phosphate/PMS systems,but the reaction mechanism is still controversial.In this study,it is confirmed that different types of phosphates（i.e.,Na₂HPO₄,NaH₂PO₄,Na₂HPO₃,and NaH₂PO₂）/PMS systems could oxidize acid yellow 17,an azo dye.However,no significant amount of^·OH is found in such reaction systems in fluorescence probe experiments.Based on the data of electrochemical half-wave potential and density functional theory（DFT）calculation,this thesis proposes that phosphates might react with PMS to form reactive intermediates,which could degrade organic pollutant,but not oxidize coumarin to produce 7-hydroxycoumarin.Thus,DMPO-OH or DMPOX signal detected by spin-trapping EPR method should mainly result from the oxidation of DMPO by the reactive intermediates.Overall,the phosphate/PMS system mainly follows the non-radical mechanism.