Mechanism Of Peroxymonosulfate/Peracetic Acid Activation By Co(Ⅱ) For The Degradation Of Organic Contaminants

In recent years,advanced oxidation processes based on peroxymonosulfate（PMS）and peracetic acid（PAA）have been extensively investigated for degradation of various organic compounds.Transition metal ions have been widely used as catalysts to activate PMS and PAA because of their simplicity and lower cost.Among them,Co（Ⅱ）has shown excellent catalyst activity.At present,free radicals have been generally considered to be the primary reactive species in the above processes.However,the exact mechanism still exists controversial.Therefore,we specifically investigate the reaction mechanism of Co（Ⅱ）/PMS and Co（Ⅱ）/PAA processes in this study.The specific study contents and conclusions are as follows:（1）Firstly,the electron-transfer mechanism and electrophilic/nucleophilic nature of reactive species involved in Co（Ⅱ）/PMS process were investigated.Using methyl phenyl sulfoxide（MPSO）as a chemical probe,we find that the yield of MPSO₂ is 18.4%～89.9%at p H=3.0～8.0,indicating that the Co（Ⅱ）/PMS process exists two types of electron-transfer mechanisms（both one-electron transfer and oxygen-atom transfer pathway）.Moreover,the solution p H plays a crucial role in determining their contribution.Furthermore,we find that the reactive species generated in the Co（Ⅱ）/PMS process have electrophilic/nucleophilic nature by using 2-phenylpropionaldehyde（2-PPA）as the probe compound.Based on the above results,we speculate that the main reactive species is considered to be the Co（Ⅱ）-PMS complex in the Co（Ⅱ）/PMS process.Meanwhile,the Co（Ⅱ）-PMS complex decomposes to form a few free radicals and Co（III）as the secondary reactive species for degradation of contaminants.（2）The exact mechanism of Co（Ⅱ）/PAA process was further investigated,and the degradation efficiency of BPA in Co（Ⅱ）/PAA process was systematically evaluated.We find that the Co（Ⅱ）/PAA process achieves removal of 99.3%for sulfamethoxazole,93.6%for bisphenol A,66.2%for phenol,and 18.8%for nitrobenzene within 20 min,indicating that the Co（Ⅱ）/PAA process could be employed for removal of various contaminants from water.Using MPSO as a chemical probe,we find that there are two types of electron-transfer mechanisms in the Co（Ⅱ）/PAA process.Moreover,the proportion of oxygen-atom-transfer pathway is up to77.2%.Combined with the degradation results of carbamazepine,R-O^·radicals are excluded as the main reactive species in the Co（Ⅱ）/PAA process.We demonstrate that Co（Ⅱ）-PAA complex is the main reactive species,while R-O^·and Co（III）act as the secondary reactive species that play a minor role in the Co（Ⅱ）/PAA process by using electron paramagnetic resonance analysis,radical quenching experiments and other methods.In addition,neutral conditions were more favorable for degradation of BPA.The degradation of BPA is significantly inhibited in the presence of HCO₃^-and natural organic matter,while other inorganic ions(Cl^-,NO₃^-,SO₄^2-)have no significant effect on BPA degradation.This study provides novel insights into exploring the mechanism of transition metal-catalyzed peroxide activation process.