| With the rapid development of the economy and changes in land use patterns,the concentration of polycyclic aromatic hydrocarbons(PAHs)and micro-nano plastics(M-NPs)is increasing,and the combined pollution of PAHs and M-NPs in soils is not optimistic.M-NPs can cause damage to respiratory system,reproductive system,immune system and liver function.PAHs have cytotoxicity,genotoxicity,immunotoxicity,carcinogenesis,teratogenicity,mutagenicity and so on.With small particle size,large surface area and strong fluidity,M-NPs can adsorb organic pollutants by affecting the physical and chemical properties and structure of soil,and affect the trend of soil pollutants,which has potential ecological pollution and human health risks.On the basis of extensive and in-depth literature research,this study focuses on the common PE M-NPs and PS M-NPs in soil.Based on the Py-GC-MS method,a simultaneous qualitative and quantitative analysis method for soil microplastics(MPs)and nanoplastics(NPs)was established by optimizing density flotation and alkaline digestion processes.Using batch experiment,column leaching experiment and pot experiment,combined with adsorption kinetics model fitting and isothermal adsorption model fitting,as well as scanning electron microscopy(SEM),X-ray diffraction(XRD),specific surface area analysis(BET)and potentiometric titration(Zeta)characterization methods,Effects and mechanisms of the types and particle sizes of M-NPs on the migration and transformation behavior of soil PAHs under different environmental conditions were studied.Specific research results were as follows:(1)Based on literature research,a simultaneous qualitative and quantitative analysis method for soil MPs and NPs was established by density flotation combined with thermal cracking and gas chromatography-mass spectrometry(DF-Py-GC-MS)experiment.The detection limits of qualitative and quantitative soil MPs methods was 0.27-0.81 μg.The detection limit of qualitative and quantitative soil NPs methods was 0.082 μg·g-1.The method was used to detect the MPs and NPs of Shenyang farmland soil.The detection rate of MPs was 100%.The detection rate of PE MPs was 60%with a concentration of 685.55μg·g-1.The detection rate of PP MPs was 20%with a concentration of 1069.98 μg·g-1.The detection rate of PS MPs was 20%with a concentration of 864.23 μg·g-1.The detection rate of PS NPs in soil was 66.7%,and the concentration was 3.45 μg·g-1-10.26 μg·g-1.(2)The influence of the type and particle size of M-NPs on the adsorption behavior of soil PAHs under different pH and DOM concentration was studied by isothermal adsorption experiment and kinetic adsorption experiment.Through fitting adsorption kinetics model and isothermal adsorption model,combined with SEM,XRD,BET and Zeta characterization,the mechanism of different types and sizes of M-NPs Adsorbing Soil PAHs under different pH and DOM concentration was clarified.The results showed that the R2 value of Freundlich fitting curve was the largest(0.9954-0.998),which was more suitable for describing the isothermal adsorption behavior of PAHs by M-NPs in soil.The quasi-second-order kinetic model had the best fitting effect(R2 ranging from 0.9947 to 0.9969),and was more suitable for describing the chemical kinetic adsorption behavior of PAHs by PE M-NPs soils and PS M-NPs soils.At different initial concentrations of PAHs,the adsorption capacity of PAHs in the three soils increased with the increase of temperature.Under different pH values,the adsorption capacity of PAHs in the three soils firstly increased and then decreased with the increase of pH.The maximum adsorption capacity of PAHs in PE M-NPs,PS M-NPs and CK soil was about 5,7 and 3,respectively.When the pH value was greater than zero charge point,the adsorption capacity of PAHs on the three soils gradually decreased with the increase of pH value,which was mainly affected by electrostatic interaction.For PS M-NPs soil,the adsorption capacity of PAHs increased with the increase of DOM concentration,which mainly through π-π mechanism,but for PE M-NPs soil and CK soil,the adsorption capacity of PAHs decreased.The adsorption capacity of PAHs in M-NPs soil of different particle sizes was significantly higher than that in CK soil,and the maximum adsorption capacity of PAHs in PS M-NPs soil and PE M-NPs soil appeared at particle sizes of 50 μm and 0.8 μm,respectively.Hydrophobic interaction,electrostatic interaction,Van der Waals force interaction,polarity interaction and π-π interaction are the main adsorption mechanisms of PAHs by M-NPs soil.(3)Column leaching experiments were conducted to study the effects of MPs type and particle size on the vertical leaching behavior of soil PAHs under different pH and DOM concentrations,and the related mechanism was also analyzed.The results showed that both acidic and alkaline environments could significantly improve the leachability of HMW-,MMW-,and LMW-PAHs in soils,and promote their migration to the depth of soil.DOM can obviously increase the leachability of MMW-and LMW-PAHs in soils,because the benzene ring in the DOM structure can adsorb PAHs through π-π mechanism;in addition,DOM can increase the solubility of soil PAHs,and thus promoting their migration in the soil.The higher the DOM concentration in the leaching solution,the deeper the downward migration of soil PAHs and MMW-and LMW-PAHs.However,the effect of DOM concentration on HMW-PAHs was limited as it has a higher octanol-water partition coefficient and it is easyly adsorbed by the soils with microplastics and organic matter,which weakened the leachability of it.Under different pH and DOM concentrations,the presence of PE MPs reduced the leachability of soil PAHs and the possibility of soil PAHs migration to deeper soil layers,thereby reducing the risk of soil PAHs polluting groundwater.(4)Pot experiment was conducted to study the effects of MPs type and particle size on the migration and transformation behavior of soil PAHs at the root-soil interface under different pH and DOM concentration conditions,and to analyze the ecological risks of plant absorption of soil PAHs.The results showed that the PAHs and their composition concentrations in the roots and leaves of cabbage was the lowest when the particle size of PS MPs and PE MPs was 50 μm,but as the particle size increases or decreases,it increases to varying degrees.In CK and PS MPs soils,the PAHs and their components of was the lowest when pH was 7,and in PE MPs soils,it was he lowest when pH was 5.But as the pH increases or decreases,the PAHs and their composition concentrations in the roots and leaves of cabbage increases to varying degrees.Compared with LMW-PAHs,pH value showed better inhibition effect on HMW-PAHs of Chinese cabbage.The PAHs and their components of Chinese cabbage was significantly lower in PE MPs soils and PS MPs soils than that in CK soils under different DOM concentrations.As the concentration of DOM increases,the PAHs content of Chinese cabbage significantly decreases in PS MPs soil,while in PE MPs soil is exactly the opposite.DOM has a more significant effect on inhibiting the absorption of HMW-PAHs by Chinese cabbage.Compared with the PAHs threshold of the national food standard(GB 2762-2005)(5 μg/kg),the content of PAHs in CK soil was significantly higher than that in PS MPs soil under different PH,DOM and particle size conditions,while the content of PAHs in PS MPS soil was significantly lower.The content of PAHs in PE MPs soil was also significantly higher under different DOM concentration and pH(except when pH was 5)conditions. |
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