| Environmentally persistent free radicals(EPFRs),considered as a new class of emerging pollutants,have paramagnetic stability,oxidative stress on the biological system.EPFRs can induce oxidative stress on the biological system,causing damage to cells and organisms.Previous reports on EPFRs mainly focused on industrial incineration of fly ash,atmospheric particles,and low-temperature pyrolysis carbon-based materials,etc.However,there are few studies on the formation process and stabilization mechanism of EPFRs in soil environment of contaminated sites.It has been reported that the formation of EPFRs in soil was closely related to the transformation process of organic pollutants.Among many organic pollutants,polycyclic aromatic hydrocarbons(PAHs),as a kind of persistent organic pollutants(POPs),are widely present in soil.The migration and transformation processes of PAHs in soil have become the focus in the fields of soil science and environmental science.Previous studies mainly focused on the microbial transformation of PAHs in soil,while ignoring the abiotic transformation.This processe was related to the active components of clay minerals such as metal oxides and organic matter in soil.Clay minerals have attracted extensive attention due to their unique nanometer layered structure and surface physical and chemical properties.Previous studies have confirmed that clay minerals were important sites for the transformation of a variety of organic pollutants.However,there was insufficient understanding of the abiotic transformation process and mechanism of PAHs on clay mineral surface,and whether PAHs can promote the formation of toxic intermediates-EPFRs during the transformation process.Therefore,this paper system studied the electron transfer and pollutants conversion in the PAHs-clay minerals,analyzed the types of PAHs,clay types,the physical and chemical properties of clays,and the influence of environmental conditions on the conversion process of PAHs to EPFRs,and further discussed the formation mechanism of EPFRs at the PAHs/clay mineral and the environmental stability of EPFRs.On the one hand,it can provide new knowledge for the environmental behavior of PAHs in contaminated soil and pollution control technology.On the other hand,it can provide a theoretical basis for comprehensively assessing the environmental risks of PAHs in the soil environment.The main research contents and results of this study are as following:(1)Abiotic transformation of PAHs on the surface of clay minerals.PAHs(anthracene(ANT),phenanthrene,pyrene,benzo[a]anthracene and benzo[a]pyrene(B[a]P))with different molecular structures were selected as model compounds,and the conversion rate of the surface properties of clay minerals(such as the types of metal ions and clays)to PAHs was investigated.The results show that the transformation rate of PAHs on the surface of montmorillonite was higher than that of kaolinite and illite.PAHs with lower oxidation potential and ionization potential(IP)can easily transformed on the surface of montmorillonite,and the conversion rate of B[a]P can reach?98%after 2 d.The type of cations adsorbed on the clays has a greater impact on the conversion effciency of PAHs,which was expressed as:Fe(III)>Cu(II)>Ni(II)>Co(II)>Zn(II)>Na(I).At the same time,the effects of environmental conditions such as with/without light and moisture content on PAHs conversion were analyzed.The results showed that light and relative humidity?33%were favorable for the conversion of PAHs.Fourier transform infrared spectroscopy(FTIR),diffuse reflectance spectrum,and gas chromatography mass spectrometry(GC-MS)analysis found that the conversion process of PAHs was related to the interface electron transfer,and its degradation products mainly include quinone-PAHs derivatives(2)Established an extraction method of PAHs-EPFRs on the surface of clay minerals.The establishment of EPR measurement method was the basis for effective acquisition of EPFRs information in PAHs-clay mineral.In order to prevent other paramagnetic components(elements)from greatly interfering with the solid phase EPR measurement results,the organic solvent extraction method was proposed in this paper.However,the type of solvent and extraction method will directly affect the extraction efficiency of EPFRs.There are problems such as incomplete extraction of EPFRs or the quenching of free radicals caused by the extraction process,resulting in the measured value is lower than the actual level.Therefore,we simulated PAHs contaminated clays and screened and optimized the solvent types,extraction methods,reaction time and other conditions.The results showed that acetone or dichloromethane/acetone mixed solvents can effectively extract PAHs-EPFRs and the extraction efficiency of EPFRs decreases with the increase of ultrasound intensity and ultrasound time.The environmental stability of EPFRs formed by different PAHs extracted in solvent showed two processes,namely rapid decay(0-5 h)and slow decay(5-50 h),and the decay rate<5%after extraction 5 min.Based on the above results,for the determination of EPFRs in soil or clay minerals,a method of"low frequency ultrasound+solvent selection+fast EPR liquid analysis"was proposed,which improves the accuracy of EPFRs determination data in solid samples.(3)The formation and mechanism of EPFRs on the surface of PAHs contaminated clay minerals was revealedThe influence of PAHs molecular structure,types and interface characteristics of clays on the formation of EPFRs was explored.The formation mechanism of EPFRs by PAHs contaminated clay minerals was analyzed by combining the evolution trend of EPFRs and transformation products of PAHs with density functional theory(DFT).The results show that the smaller the delocalization energy of PAHs,the easier it is to transform into EPFRs on the surface of clays.The content of PAHs-EPFRs on Fe(III)-montmorillonite was much higher than that on Fe(III)-illite and Fe(III)-kaolinite.B[a]P/ANT can form EPFRs on the surface of montmorillonite containing metal ions with high oxidation potential(such as Fe3+,Cu2+).By combining the results of DFT and PAHs transformation products,the formation mechanism of EPFRs at room temperature was proposed.The?Cation-??formed by the exchangeable cations on the surface of clays and PAHs first forms a bond with clay minerals.With strong electron-acquiring ability(E0?-0.280 V(versus NHE))and PAHs with a certain electron-donating ability(IP<7.5 e V),electrons are transferred from PAHs to surface cations,and stabile PAHs-EPFRs are formed on the surface of clays.The radicals can further react with small molecular substances(such as H2O/O2)to form new free radical intermediates or products.(4)Effects of environmental conditions on the formation and stability of EPFRs by PAHs contaminated clay mineral.Environmental conditions such as moisture content and light were the key influencing factors for the abiotic transformation of PAHs in soil.The result showed that light was a key factor affecting the accumulation of EPFRs.On the one hand,light promotes the transfer of interface electrons from PAHs to active sites and accelerates the formation of EPFRs.On the other hand,the active oxygen generated by photocatalysis on the surface of inorganic minerals(or soil organic matter)can be oxidized.The formation of EPFRs at the interface of clay minerals was inhibited when the moisture content was>97%.In general,environmental factors affect the formation rate,cumulative abundance,and attenuation trend of EPFRs by acting on the electron transfer process at the soil particle interface.(5)The formation of ROS on the surface of PAHs/clay minerals and its toxicological analysisEPFRs formed in PAHs/clay mineral can induce the generation of ROS with the effect of oxidative stress,and then produce toxic effects on organisms or human cells.The formation of ROS in the system was analyzed by using spin capture technique.It was found that hydroxyl radical(·OH),H2O2 and superoxide radical(O2·-)were generated,and the content of O2·-was greater than that of·OH.The content of ROS first increased and then decreased with the increase of PAHs aging time,which was more consistent with the evolution trend of EPFRs.This result indicated that EPFRs in organic contaminated soil can also induce the formation of ROS.Through toxicological,it was found that Microtox?and human gastric epithelial GES-1 cells showed obvious toxic effects after exposure to PAHs/clay minerals,and the trend of toxicity was consistent with the evolution of ROS and EPFRs.These results indicated that the generation of EPFRs and ROS were the main cause of toxicity of organisms.In addition,it also shown that mineral particles carrying EPFRs was more toxic to biological cells than their parent compounds(ie,PAHs). |
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