Study On The Migration Mechanism And Numerical Simulation Of Heavy Metals Cu And Cd In Soils In Different Functional Areas

With the rapid increase of social population,the continuous expansion of industrial production scale,the acceleration of urbanization,sewage irrigation and the use of large quantities of chemical fertilizers and pesticides in agriculture,substances with high heavy metal content enter the soil through various ways,causing soil pollution,which has become one of the most significant environmental pollution problems worldwide.Study on the chemical form transformation,adsorption mechanism and migration law of heavy metals in the soils of different functional areas is conducive to comprehensively clarifying the migration characteristics and release mechanisms of heavy metals under different pollution conditions,and can provide a theoretical basis for the environmental risk assessment and pollution prevention of heavy metal contaminated soils.The research content can predict and evaluate groundwater pollution.In this study,SA agricultural soil,SI industrial soil and SU urban soil were used as test soils,and the adsorption characteristics of soils on pH,temperature and salinity under single-factor and multi-factor coupling conditions were studied through adsorption experiments.Through the isothermal adsorption experiment of Cu and Cd in the three soils,the conservative ion dispersion experiment and the Cu and Cd penetration experiment,the water and solute transport parameters were obtained and combined with the actual situation,an thus the Cu and Cd migration model was established by using the HYDRUS-2D software.The intra-laboratory soil column experiment was conducted to investigate the migration and transformation process of Cu and Cd in the soil under rainfall,sewage irrigation and leakage conditions,followed by numerically simulating the migration process of Cu and Cd.On this basis,this study generalized the study area and combined the parameters obtained in the experiment to predict the migration of Cu and Cd in the soil under the conditions of rainfall evaporation,sewage irrigation and leakage.Finally,the Morrios global sensitivity analysis method was used to analyze the sensitivity of the main parameters of the Cu and Cd migration model.The main conclusions are as follows:(1)Adsorption experiments show that the adsorption capacity of soil to Cu and Cd is as follows:SU soil>SA soil>SI soil,and the adsorption capacity of Cd is lower than that of Cu.Temperature and pH are generally positively related to the soil’s ability to adsorb Cu and Cd.The adsorption capacity of soil for Cu increases with the increase of salinity level,and the adsorption capacity for Cd with the increase of salinity level shows a trend of first weakening and then gradually steadying.Under the coupling effect of multiple factors,the interactions between salinity and pH and bwtween temperature and pH are obvious,while the interaction between salinity and temperature is relatively weak.The main factors that affect the soil’s adsorption of Cu are pH,temperature and specific surface area.The direct contribution rate to Cu adsorption is:pH>temperature>specific surface area.The main factors affecting the adsorption of soil on Cd are pH,temperature,specific surface area,clay content and salinity.The direct contribution rate to the Cd adsorption is:pH>specific surface area>clay content>temperature>salinity.(2)The results of laboratory soil column experiments HYDRUS-2D numerical simulationshow that the migration capacity of Cu and Cd in the three soils under rainfall conditions is:SA soil>SI soil>SU soil,and heavy metal migration capacity:Cd>Cu.The greater the proportion of exchangeable ion states,the stronger the migration ability of Cu and Cd.Under the conditions of leakage and sewage irrigation,the measured values of Cu and Cd concentrations are relatively consistent with the simulated values,indicating that the model after parameter inversion optimization can well describe the migration process of Cu and Cd under different pollution conditions.(3)The simulation predicts the migration of Cu and Cd in the soil under the conditions of rainfall evaporation,sewage irrigation and leakage.The research results show that under the conditions of rainfall evaporation and sewage irrigation,Cu and Cd do not penetrate the 1.5m soil layer.Under the leakage condition,Cu and Cd penetrate the soil layer,but the peak concentration of Cu and Cd penetration is low,which has little risk for groundwater pollution.(4)In the Morrios element effect global sensitivity analysis method,the parameters in the Cu and Cd migration model include the residual moisture content θr,saturated moisture content θs,VG model empirical coefficients a and n,saturated hydraulic conductivity Ks,the proportion of equilibrium adsorption sites f,the first order Kinetic rate constants α,and Langmuir equilibrium constants Kd and η.These parameters are subjected to sensitivity analysis,and the analysis results show that the parameters that have a greater impact on the output of the model are Kd,η,Ks and f,and the other parameters have less impact.Among them,θr and n are the smallest that can be ignored.Furthermore,the influence of parameters a,n,Ks and η on the simulated concentration value is positively correlated,and the influence of parameters θr,θs,f,α and Kd on the simulated concentration value is negatively correlated.