Study On Adsorption Characteristics And Vertical Migration Of Cu And Nd In Sandy Loam Soil

In recent years,more and more heavy metal pollutants are discharged into the soil with industrial waste gas and sewage,resulting in serious environmental pollution problems.Studying the adsorption mechanism and migration law of the two multi-element systems in soil can provide theoretical support for the control,treatment and remediation of soil rare earth heavy metal composite pollution.This study selects the sandy loam produced in Nanjing,Jiangsu Province,and takes copper and neodymium as the research object to analyze the adsorption characteristics of copper and neodymium in sandy loam under single and multiple systems,and explore the vertical migration law of copper and neodymium in sandy loam.In this study,the effects of initial solution concentration,adsorption reaction time,adsorption reaction temperature and other factors on the adsorption of Cu or Nd by sandy loam were investigated under a single system,and the adsorption kinetics fitting,adsorption isotherm fitting and adsorption thermodynamics characterization were carried out respectively.In order to explore the adsorption characteristics of copper and neodymium adsorbed by red sand under multi-element system,adsorption kinetic test and adsorption thermodynamic test were carried out.In order to explore the vertical migration law of copper and neodymium in red sand,an indoor dynamic leaching soil column was built.In order to explore the influence of environmental conditions on the vertical migration law of copper and neodymium in red sand,the soil column was treated with p H reduction and freeze-thaw cycle.The solute transport model was established by using hydraus-1d software to simulate and predict the migration of Cu and Nd in sandy loam.The main conclusions are as follows:（1）The experimental results of single system adsorption kinetics of Cu and Nd in sandy loam show that the correlation coefficients R²of the pseudo-first-order kinetic equations for the three concentrations of Cu²⁺are all larger than the correlation coefficients of the pseudo-second-order kinetic equations,so The adsorption process is mainly physical adsorption;the correlation coefficient R²fitted by the pseudo-second-order kinetic equation of the three concentrations of Nd³⁺is larger,so the adsorption process mainly involves chemical adsorption;the multi-system test results show that when the solution concentration is 20mg/L.For example,when the sandy loam soil adsorbs Cu²⁺and Nd³⁺ions,the equilibrium adsorption capacity is smaller than the adsorption capacity of a single system of Cu²⁺and Nd³⁺in the sandy loam soil.Among them,the adsorption capacity of Cu²⁺is reduced by 50.25%,and the adsorption capacity of Nd³⁺is reduced by 42.02%.It can be seen that The two heavy metals are inhibiting each other,and the inhibition effect of Cu element is greater than that of Nd element.The adsorption capacity of the medium sandy loam soil for the two heavy metal ions is less than that of the single system sandy loam soil for the two heavy metal ions.The unit and multivariate adsorption thermodynamic test results of Cu and Nd in sandy loam show that the equilibrium adsorption of Cu and Nd in sandy loam increases with the increase of temperature,indicating that the adsorption belongs to an endothermic reaction;the adsorption enthalpy changeΔH^θ>0,indicating that the adsorption is an endothermic process;the adsorption free energy changeΔG^θis negative,indicating that the adsorption is a spontaneous process;ΔS^θ>0,indicating that the entropy of the adsorption process increases,and the degree of freedom of the liquid-solid interface increases.（2）The results of the mechanically dispersed soil column test showed that the content of Cu and Nd in each soil layer increased with the increase of sampling times（time）and decreased with the increase of depth.The migration in sandy loam soil can reach the bottom layer（35 cm）,and the migration ability of Nd element in soil is lower than that of Cu element.The results of the Cu and Nd contents in each soil layer of the soil column after p H reduction showed that the p H reduction could promote the vertical migration of the two heavy metal elements in the sandy loam soil.The results of the freeze-thaw cycle soil column test showed that the freeze-thaw cycle could inhibit the vertical migration of the two heavy metal elements in the sandy loam soil.The correlation analysis results of the migration process of the mechanically dispersed soil column showed that the migration rate of the solution had a very significant negative correlation with time（P<0.01）,showed a very significant positive correlation with the solution migration rate（P<0.01）;the regression equations of the two heavy metal migration rates showed a very significant positive correlation（P<0.01）,and the two showed a significant positive correlation with the soil column depth（P<0.01）.Negative correlation（P>0.05）.（3）The HYDRUS-1D software was used to establish a model to simulate and predict the vertical migration of Cu and Nd in sandy loam soil.The results show that with the increase of the mechanical dispersion time,the heavy metals Cu and Nd are mainly adsorbed in the upper layer of the soil profile,and the heavy metals Cu and Nd are continuously migrated downward in the soil profile;the longer the mechanical dispersion time is,the deeper the migration depth of the two heavy metals is.The deeper it is,when the mechanical dispersion time is 90 d,the migration depth of Cu and Nd reaches 10 cm and 11 cm.The migration of heavy metals Cd and Nd in the soil column is simulated and predicted by HYDRUS-1D software using the deterministic model.The results show that,using the model prediction results show that long-term sewage irrigation and other behaviors will still cause heavy metals Cu and Nd to migrate and accumulate downwards in the soil.