Strengthening Efficiency And Mechanism Of Nitrilotriacetic Acid Assisted Iron/Peroxymonosulfate System For Degradation Of Orange G

Water pollution caused by nitrogen-containing organic substances has attracted considerable attention in recent years.Azo compounds,as a typical nitrogen-containing organic substance that has strong carcinogenesis and environmental persistence,are posing a threat to water environment safety.The iron activated peroxymonosulfate system can produce active species such as high-valence iron（Fe（IV））,sulfate radical（SO₄^-·）and hydroxyl radical（·OH）,which is an advanced oxidation technology to reach an efficient removal of azo compounds in water.However,the formation of iron precipitation and the passivation of the surface of zero-valent iron limit the application of ferrous activated peroxymonosulfate（Fe（Ⅱ）/PMS）and zero-valent iron activated peroxymonosulfate（Fe（0）/PMS）systems under neutral p H conditions.In this study,orange G（OG）was used as the target pollutant,and nitrilotriacetic acid（NTA）was added to improve the oxidation efficiency of Fe（Ⅱ）/PMS and Fe（0）/PMS systems under neutral conditions.Firstly,the effects of the dosing mode of Fe（Ⅱ）,the particle size of Fe（0）,p H value,and different complexing agents were investigated on the performance of OG oxidation degradation in Fe（Ⅱ）/PMS and Fe（0）/PMS systems.The results indicated that in the Fe（Ⅱ）/PMS system,batch dosing of Fe（Ⅱ）could achieve a higher OG removal effect than one dosing.And,the smaller particle size of Fe（0）would lead to better removal of OG.Besides,the two systems had poor OG removal efficiency under neutral p H conditions.But the addition of NTA could significantly improve the oxidation efficiency of these two systems.Secondly,a comparative study was carried out to explore the degradation efficiency of OG in Fe（Ⅱ）/PMS and NTA/Fe（II）/PMS systems with neutral p H conditions.The results demonstrated that the introduction of NTA could enhance the degradation of OG in the Fe（Ⅱ）/PMS system.And,increasing the concentration of NTA（0～1.0m M）,Fe（II）（0.1～1.5m M）and PMS（0.2～2.0m M）within a certain range is conducive to improving the removal efficiency of OG in the NTA/Fe（II）/PMS system.Meanwhile,the system has a good OG removal effect in the range of p H=4～6.5.Through the establishment and analysis of the empirical kinetic model of the reaction,the order of the factors’ability to affecting the oxidation efficiency of the NTA/Fe（II）/PMS system was found to be Fe（II）>p H>NTA>PMS.Quenching experiments of methanol,tert-butanol and benzoic acid,and oxidation experiment of methyl phenyl sulfoxide（PMSO）indicated that the main active species in the NTA/Fe（II）/PMS system is Fe（IV）and SO₄^-·,which account for approximately 72%and 28%respectively.And as the concentration of NTA increased,the active species gradually shifted from SO₄^-·to Fe（IV）.The results also showed that the acidic condition is favourable to SO₄^-·domination and Fe（IV）can occupy a dominant position under neutral conditions.On these bases,the formation mechanism of active species in the NTA/Fe（II）/PMS system was analyzed,8 degradation products were identified by using the UPLC-Q Exactive.Further,the degradation path of OG was proposed.Meanwhile,the effect of coexisting substances in water on the degradation of OG in the NTA/Fe（II）/PMS system proved that the system has a certain ability to resist the impact of water quality background.Finally,the mechanism and efficiency of NTA enhancing OG degradation in the Fe（0）/PMS system were studied.According to the results,adding NTA to the Fe（0）/PMS system facilitated the oxidative removal of OG under neutral p H conditions.Increasing the concentration of NTA（0～8.0m M）,Fe（0）（0～10.0m M）and PMS（0～1.0m M）within a certain range is also beneficial to promote OG degradation in the NTA/Fe（0）/PMS system.And the system has a good OG removal effect in the range of p H=3～7.Observations indicated that adding NTA could affect the morphology of Fe（0）and also inhibit the formation and deposition of the iron sulphate layer on Fe（0）surface.Meanwhile,the formation of soluble iron was enhanced with NTA existed.The experiments to explore the inhibition of methanol,tert-butanol and phenol on OG,and the oxidation experiment of PMSO show that free radicals（70%）including SO₄^-·and·OH,and Fe（IV）（30%）are the main active species in the NTA/Fe（0）/PMS system.Similar to the results of the NTA/Fe（II）/PMS system,the active species gradually changed from free radicals（SO₄^-·,·OH）to Fe（IV）with the increase of NTA concentration.The initial p H value of the solution also affected free radicals and Fe（IV）in the system.However,compared to the results of the NTA/Fe（II）/PMS system,free radicals still dominated while Fe（IV）has increased under neutral conditions.On these bases,the formation mechanism of active species in the NTA/Fe（0）/PMS system was analyzed.10 degradation products were identified by using the UPLC-Q Exactive.Likewise,then the degradation path of OG was proposed.Among the common water coexisting substances,HCO₃^-,H₂PO₄^-,EDTA,citric acid and sodium acetate have an inhibitory effect on the removal of OG,while Cl^-has a certain promoting effect.It can be seen that the introduction of NTA in Fe（Ⅱ）/PMS and Fe（0）/PMS systems can facilitate the generation of active species and the degradation of organic pollutants in the system under neutral conditions,which has good practical application significance.