Persistent Free Radicals In Humic Substances:characteristics And Reactivity

Compared to transient free radicals,persistent free radicals(PFRs)generally have a half-life of several hours to months,which are organic groups or compounds with unpaired electrons and are widely found in soil environment.Previous studies have shown that humic substances(HS)are the important medium of PFRs in soil.Due to the various sources of HS,the type and activity of PFRs in HS may be significantly different,which in turn affect their potential environmental behavior.Therefore,the characteristics and stability of PFRs in HS were investigated by electron paramagnetic resonance(EPR)spectrometer,fourier transform infrared spectroscopy(FTIR)and ultraviolet spectroscopy.And the objectives of this study were to(i)investigate the characteristics of PFRs and reactive oxygen species(ROS)in the different components of HS(humic acid(HA),fulvic acid(FA)and humin(HM),different molecular weight(MW)HA,different sources of HM),(ii)evaluate the evolution and stability of PFRs in HM under various irradiation or redox conditions,and(iii)clarify the role of PFRs in the transformation of bisphenol A(BPA),polycyclicaromatichydrocarbons(PAHs),and iron oxides in HS.In addition,the relationship between the stability and reactivity of PFRs in HS was revealed by mathematical statistical analysis methods,such as analysis of principal components and correlation analysis.This study provides a scientific basis for evaluating the environmental behavior of PFRs and offers new ideas for soil remediation.The main results were as follows:(1)The composition,MW and source of HS all affected the type and content of PFRs.HA,FA and HM were extracted from peat soil by classical acid-base extraction methods,and the distribution characteristics of PFRs in these three components were analyzed by EPR.Results showed that the g-factor of HM was?2.0026,which indicated that the PFRs in HM were carbon-centered radicals,while the g-factor of PFRs in HA and FA were between 2.0030 and 2.0040,indicating that two dissolved organic matter contain not only“carbon-centered”but also“oxygen-centered”radicals.The concentration of PFRs in HM was 8.6×10¹⁶spins/g,which was much higher than that of the other two components.This was attributed to the relatively high degree of aromatization of HM.Further research found that the PFRs in low molecular weight fractions(MWF)HA(MW<3500 Da,MW<7000Da,MW<14000Da)were oxygen-centered free radicals,while that in large MWF HA(MW>14000 Da)was carbon-centered free radicals.The concentration of PFRs in the low MWF HA was in the range of 0.20×10¹⁶ to 0.45×10¹⁶ spins/g,which was much less than that in the large MWF HA(1.30×10¹⁶ spins/g).Furthermore,the distribution characteristics of PFRs in HM from 10 types of soils were studied.It was found that PFRs in HM from different soils were assigned to carbon-centered“aromatic”radicals and their concentration ranged from 0.11×10¹⁶ to 5.79×10¹⁶ spins/g.There was a positive correlation between the concentration of PFRs and the humification index in HM,namely,the humification process was conducive to the formation and accumulation of PFRs.(2)Environmental conditions(light,redox)affected the type and concentration of PFRs in HS.The concentration and g-factor of PFRs in HA,FA and HM increased rapidly first and then reached an equilibrium under the irradiation conditions.After irradiation for 2 h,the varation range of PFRs in FA was significantly higher than that in HA and HM,which indicated that FA had the higher photoelectron transfer ability.This was related to more C=O and C-O groups in FA.However,PFRs in HA,FA and HM decreased rapidly to the initial level under the dark condition(cut off the light irradiation).Likewise,redox conditions also had an important effect on the formation of PFRs in HM.When the concentration of H₂O₂/VC was less than 0.08 M,the concentration of PFRs in HM increased with the increase of the concentration of H₂O₂/VC,that is,oxidation/reduction treatment significantly increased the concentration of PFRs in HM.The PFRs produced by the redox treatment were more reactive than that in the original HM,and were quickly decay to the original level under natural environment.It was found that the concentration of PFRs in HM was positively correlated with the content of ROS.These results indicated that irradiation/redox could promote the formation of PFRs,and the newly formed PFRs can easily transfer electrons to O₂,promoting the formation of ROS.(3)PFRs in HS were reactive and could promote the degradation of organic pollutants.The redox capacity of HM from different soils was different,and all of them could promote the degradation of BPA.The removal rate of BPA was positively correlated with the oxidative capacity of HM,which in turn was positively correlated with ROS,confirming that the ROS induced by PFRs playd an important role in the removal of BPA.Taking PAHs as model pollutants,it was found that PFRs in HM from different soils could promote the degradation of PAHs.And the degradation rate was related to the molecular structure of PAHs,the abundance of PFRs and ROS in HM.The degradition rate of PAHs was in the order:benzo[a]pyrene>anthracene>phenomenon>naphthalene.Compared with PAHs with lower molecular rings,the compounds with higher molecular weight were more likely to be degraded.At the same time,it was found that the degradition rate of ANT increased with the increasing of PFRs concentration in HM,which proved the promotion effect of PFRs in the transformation of PAHs.The inhibition experiment proved that ROS induced by PFRs could promote the degradation of PAHs.(4)HA/goethite was used as a model system to study the effects of PFRs on the environmental geochemical process of soil mineral.Various MWF of HA could promote the transformation of goethite,and the transformation rate was related to MW,phenolic hydroxyl content,types and concentrations of PFRs and ROS in HA.Among them,the large MWF HA(MW>14000 Da)and bulk HA had the higher adsorption and reduction capacity for goethite than the low MWF HA.The amount of Fe(II)production was positively correlated with the concentration of PFRs and phenolic hydroxyl group,while negatively correlated with the concentration of ROS.That is,PFRs in HA promoted the reduction and dissolution of goethite,while ROS inhibited the reduction of goethite.