Catechol Degradation On The Surface Of Hematite And Goethite And The Formation And Stabilization Mechanism Of EPFRs

Environmentally persistent free radicals(EPFRs)had a longer half-life and a wider distribution in the environment than the traditional short-lived free radicals.As a new class of pollutants,it has been detected in natural environments such as tar balls,soil in coking plants and suspended particulate matter in the atmosphere.EPFRs can increase the production of reactive oxygen species,which can be used to remove organic pollutants from the environment.On the other hand,its strong oxidative activity can also reduce the antioxidant capacity of cells and inhibit the metabolism of the body and clearance of exogenous substances,which in turn impair cardiopulmonary function and induce cancer,pose a potential risk to human health.Previous studies have shown that stable phenoxy or semiquinone radical can be produced during the interaction between chlorine-substituted or hydroxy-substituted benzene ring with transition metal oxides.These studies focused on EPFRs produced by the interaction between parent organic and transition metals,while ignoring the contribution of by-products produced during the degradation of parent organics to EPFRs.Iron was one of the most abundant elements on earth,and almost all rocks and soil contained at least traces of iron.Hematite and goethite mainly present in the soil in the form and were the most stable forms of iron oxides and hydroxides.However,the degradation products of organics on hematite and goethite may be different for the different crystal structure.In this study,hematite and goethite were loaded on silica to simulate soil particles.Catechol was selected as a model compound which was widely in the environment.The degradation of catechol on simulated soil particles and the formation of EPFRs were studied.The effect of the iron oxide crystal structure on the formation of EPFRs was investigated by analysing the degradation products.The main conclusions of this research were as follows:(1)The experimental results shown that the degradation tendencies of catechol on silica,hematite-silica and goethite-silica were different in dark and under ultraviolet light.EPFRs on hematite-silica were the most stable both in dark and under ultraviolet light irradiation,while the EPFRs on silica were the most unstable.In the dark,hematite and goethite inhibited the catechol degradation.But the inhibition extent of goethite was more significant.But the EPFRs signal strengths on hematite-silica and goethite-silica were weak and similar.The half-lives of EPFRs on hematite-silica,goethite-silica and silica in dark were 69.4 days,23.1 days and 14.9 days,respectively.However,hematite and goethite facilitated catechol degradation under continuous ultraviolet light irradiation.The strengths of EPFRs signal on hematite-silica and goethite-silica were strong.The EPFRs signal on goethite-silica was stronger than that on hematite-silica.The half-lives of EPFRs on hematite-silica,goethite-silica and silica were 11.0 days,8 days and 6.9 days,respectively.The EPFRs signal strengths in the particles were not in correspondence with the catechol degradations,which contradicted the previous theory that the altered organics degradation was attributed to the stabilized free radicals on particle surface.(2)According to the results of GC-MS,goethite may only stabilize phenoxy radical,but hematite may stabilize dimer-type free radicals produced by further degradation of phenoxy radical.Under ultraviolet light irradiation,the dimer converted from the dimer free radical would be further degraded to produce 2,4-hexadienediic acid.The catechol degradation pathways in the three degradation systems explained the difference in the catechol degradation and the EPFRs stabilities on hematite and goethite.The results of this research indicated that the formation of EPFRs was not only related to the parent organics,but also related to the byproducts of the parent organics.The byproducts were an important contribution to the formation of EPFRs.The results of this research indicated that in the research of the geochemical behavior of organic pollutants,in order to more comprehensively evaluate the environmental behavior of organic pollutants,it was necessary to consider not only the interaction of metal oxides with parent organics to form EPFRs,but also the interaction of different metal oxides with degradation byproducts.This research provided one important information for EPFRs formation and stabilization mechanism on hematite and goethite.It also provided risk assessment data guidance for the soils containing different iron oxides contaminated by organic compounds.