The Interfacial Behavior Of DEHP And Cd/Cu Composite Pollutants In Molisol Was Studied With Computational Chemistry

Agricultural soil is contaminated by both phthalates（PAEs）and heavy metals due to agricultural film mulching,industrial production and irrational use of pesticides and fertilizers.More and more attention has been paid to the combined pollution of heavy metals and PAEs in agricultural land.In particular,in molisol with high organic matter content,due to the limitations of uneven distribution of heavy metal-PAEs composite pollutants and complex environmental factors,the relevant mechanism of heavy metal-PAEs composite pollution behavior in molisol is rarely studied.In this study,typical PAES organic pollutants--DEHP and heavy metal Cd/Cu were selected as the research objects.Our aim was to investigate the interfacial behavior of DEHP-Cd/Cu combined contamination in facility agricultural soils in a complex system of molisol.On this basis,computational chemistry was combined with experiments to provide practical proof and theoretical support for elucidating the interaction mechanism between pollutants and different components of soil at the micro scale.Explore the adsorption,retention,dissipation,morphology and interaction of PAEs and heavy metals in agricultural soil when they coexist,which is greatly helpful to clarify the accumulation,migration and transformation of PAEs and heavy metals in the soil environment.This study has important scientific value and significance for the development of soil science and environmental pollution control.Specific research contents are as follows:（1）The interaction of heavy metals and DEHP on the adsorption behavior in molisol was studied.The presence of Cd/Cu increased the capacity of DEHP adsorption（>23%）and increased the nonlinearity of the adsorption.However,the effect of DEHP on the adsorption of Cd/Cu in molisol is not obvious.Fourier transform infrared spectrum analysis showed that the expansion vibration of soil functional groups changed under different pollution combinations.The results showed that the presence of heavy metals increased the capacity of soil to adsorb DEHP,and there was a binding relationship between heavy metals and DEHP.（2）The effect of DEHP on the forms of Cd/Cu in soil from 0 to 90 days was investigated.DEHP delayed the aging and fixation process of Cd/Cu in soil and reduced the proportion of heavy metal residue.Due to the differences in the interaction ability between different heavy metals and DEHP and soil components,DEHP increased the availability and toxicity of Cd in soil,while Cu did not.With the degradation of DEHP and the aging of heavy metals,this effect did not show a reversible trend.（3）The effect of Cd/Cu on natural degradation of DEHP in soil during 0-90 days was studied.The degradation of DEHP in soil was delayed by Cd/Cu.In the presence of Cd/Cu,the half-life of DEHP was extended from 30.81d^-1 to 40.53 d^-1 and 35.40 d^-1,respectively.The inhibition effect of Cd on DEHP reduction in soil was stronger than that of Cu.（4）Through quantum chemical simulation,including independent gradient model（IGM）and localized orbital functions（LOL）,we found that the external orbital complex can be formed by electrostatic interaction between Cd/Cu ions and DEHP.The electron transfer process is analyzed in detail by charge decomposition analysis（CDA）and extended CDA（ECDA）method.Bond critical point analysis（BCP）was used to reflect the change of bond strength in DEHP,and it was found that Cd/Cu ions can reduce the stability of DEHP.（5）In order to analyze the mechanisms affecting the adsorption of pollutants,the key organic matter（SOM）in soil was analyzed by Pyrolysis Chromatography/Mass Spectrometry（PY-GC/MC）,and the binding energy and binding free energy of different contaminant-SOM composite systems were calculated and analyzed.The results showed that the presence of Cd/Cu ions enhanced the binding capacity of DEHP and SOM,which effectively explained that the presence of Cd/Cu ions improved the DEHP adsorption capacity of soil.In addition,SOM,which contains polar functional groups,plays an important role in the process of DEHP adsorption by heavy metals.（6）In order to further explore the interfacial behavior of these composite pollutants in soil clay minerals,molecular dynamics simulation was used to simulate the adsorption behavior of DEHP and Ca/Cd/Cu divalent ions in typical clay mineral-montmorillonite（MMT）.The results show that DEHP increases the density distribution of three metal ions on the MMT surface.Compared with the common metal cation Ca²⁺,Cd²⁺significantly increased the density distribution of DEHP on the MMT surface.According to the diffusion coefficients of pollutants calculated by mean azimuth shift（MSD）,it is found that DEHP and metal ions inhibit each other’s migration and diffusion compared with single pollution.In this study,the interaction between heavy metals and PAEs in typical agricultural soils was revealed for the first time.These results provide additional evidence for the interaction,accumulation,migration and transformation of hydrophobic organic-heavy metal complex pollution in soils.