Desorption/Migration And Numerical Simulation Of Cu/Cd Composite Polluted Soil Under The Influence Of Different Chemical Factors

With the development of economic society,the problem of soil heavy metal pollution is becoming more and more serious,and has become one of the research focuses in the field of environment.Heavy metals entering the soil and exceeding a certain threshold can cause harm to the soil and further affect environmental substrates such as groundwater and organisms.Indoor experiments with heavy metals Cu and Cd as the research objects were used to investigate the effects of different chemical factors（ionic strength,pH,and ion types）on the desorption and migration of Cu and Cd in contaminated soil.In order to provide a theoretical basis for soil remediation and the re-release of residual heavy metals after soil remediation.This paper mainly studied two aspects.First,the effects of different ionic strength,pH and ion types on the desorption and migration behavior of heavy metals Cu and Cd in contaminated soil were investigated by soil column experiments and desorption kinetic experiments.Then,the soil was stratified and sampled after the soil column experiments were completed to determine the exchangeable state of heavy metals at various depths.Second,the Elovich equation,Parabolic equation and Two-constant equation were used to fit the dynamic desorption process of Cu and Cd,and the Hydrus-1D software with coupled chemical reaction process was used to simulate the migration characteristics of Cu and Cd in soil.The following conclusions were mainly drawn.（1）In the dynamic desorption experiment,the desorption process of Cu and Cd was divided into two stages,the fast desorption stage and the slow desorption stage.The fast desorption was completed in about 2 h,and the desorbed heavy metals were mainly in the electrostatic adsorption state;while the slow desorption took more time,the amount of heavy metal desorption basically did not change anymore,and the desorbed heavy metals were mainly in the special adsorption state.In the dynamic desorption experiments of soils,the effects of different chemical factors on the desorption of heavy metals Cu and Cd are different.In general,the lower pH,the higher ionic strength and the higher valence state,the greater the desorption of heavy metals Cu and Cd.At different ionic strengths,the desorption of Cd²⁺in single and composite contaminated soils increased significantly when the concentration of CaCl₂increased from 0.005 mol/L to 0.05 mol/L.The amount of Cd²⁺desorbed in single-contaminated soil increased by 10 times,and the amount of Cd²⁺desorbed in compound-contaminated soil increased by 3 times.For Cu²⁺in composite contaminated soils,the increase of ionic strength promoted the desorption of Cu²⁺.In both contaminated soil types,the desorption of Cd²⁺at pH 3-6 decreased with the increase of pH;when the pH was 7,the desorption of Cd²⁺increased in both contaminated soil types,probably because the presence of Ca²⁺played a dominant role at pH 7 and the radius of Ca²⁺and Cd²⁺were similar,which facilitated the release of Cd²⁺.For Cu²⁺in multi-contaminated soils,the desorption amount decreased with the increase of pH.Under different ion types,Ca²⁺was more favorable to the desorption of Cu and Cd in soil compared to Na⁺.（2）At different ionic strengths,the Elovich equation was a better fit for the desorption kinetic process of Cd in single and composite contaminated soils,followed by the Two-constant equation,and the Parabolic equation was the worst fit.For the desorption kinetic process of Cu,the Parabolic equation was the best fit for it.At different pH,the Parabolic equation was the best fit for the desorption kinetics of Cd in the single contaminated soil;in the composite contaminated soil,all three equations were better fits for the desorption kinetics of Cd,with R²mostly above 0.8.The Parabolic equation was the best fit for the desorption kinetic process of Cu in the composite contaminated soil.In general,the fitting of the three equations for the kinetics of heavy metal desorption at different NaCl concentrations was not satisfactory.（3）In the soil column experiment,the effects of different ionic strength,pH and ion types on the migration of heavy metals Cu and Cd varied.pH decreased,the desorption of Cu²⁺and Cd²⁺increased.The acidic environment was favorable to the desorption of Cu²⁺and Cd²⁺.However,in terms of the peak concentrations of Cu²⁺and Cd²⁺,the peak concentrations of Cd²⁺at pH 7 were greater than those at pH 5 and 6 in the single-contaminated soil;in the composite contaminated soil,the peak concentrations of Cu²⁺at pH 3 were smaller than those at pH 4 and 5.The peak concentrations of Cu²⁺at pH 3 were smaller than those at pH 4 and 5.The peak concentrations of Cu²⁺and Cd²⁺leaching at different ionic strengths increased with the increase of ionic strength.In addition,the desorption of Cd²⁺was inhibited at 0.5 mol/L CaCl₂in the composite contaminated soil,mainly because the increased amount of Cl^-formed a protective layer on the soil surface,which prevented the dissolution of heavy metals.Ca²⁺was more favorable to the desorption of heavy metals Cu and Cd at different cation valence states.In the leaching stage of deionized water,the outflow of soil particles occurred under three NaCl concentrations,mainly because the increase of Na⁺led to the dispersion and expansion of soil,the decrease of soil pores and the decrease of soil permeability.The effects of different ionic strength,pH and ion types on the residual exchangeable state Cu,Cd in contaminated soil were different.The ionic strength increased,the content of residual exchangeable state Cu and Cd was smaller,and more heavy metals were desorbed.In the composite contaminated soil,the effect of ionic strength changes on Cd was more significant compared with Cu.The effect of different pH on the content of residual exchangeable state heavy metals in single and composite polluted soils was not significant.In general,Ca²⁺was more conducive to the desorption of exchangeable heavy metals than Na⁺.（4）The HP1 model was used to simulate the transport of heavy metals at different pH,ionic strength and NaCl concentration.The fitting results for Cd²⁺under different conditions were satisfactory for both single and composite contaminated soils,while for Cu²⁺in composite contaminated soils,the fitting results were not satisfactory.It may be due to the fact that soils exhibit different adsorption affinities for Cu and Cd,with Cu preferentially adsorbed on the surface of soil particles than Cd and undergoes specific adsorption and surface precipitation,while Cd mainly undergoes electrostatic adsorption and is prone to migration.