Effect Of Layered Double Hydroxide On Adsorption And Migration Of Cadmium In Soil And Its Numerical Simulation

In recent years,the pollution of heavy metals in soils has always been a hot issue concerned by scientists in environmental and related disciplines,and cadmium pollution in soils has affected human health.Heavy metals in soils migrate horizontally and vertically to varying degrees due to factors such as tillage and leaching,which gradually enlarges the scope of heavy metal pollution.When heavy metals are transported to groundwater environment with water flow,the hazards are serious and the pollution control is more difficult.The adsorption and transport characteristics of heavy metals in soils are closely related to the soils environmental conditions,and the complex soils components make the solute transport behavior more complex.Under certain conditions,metal ions in soils can form layered double hydroxides in situ.Layered double hydroxides（LDH）in soils and layered double hydroxides entering soils through various channels have certain effects on pH,surface charge of soils,and then affect the adsorption and migration behavior of heavy metals in soils.Therefore,by adding Mg-Al-CO₃ LDH,which is common in nature,into the soils and comparing with the original soils,a large number of laboratory experiments have been carried out to explore the effects of Mg-Al-CO₃ LDH on the change of soils environment and the adsorption and migration of cadmium in soils,and to clarify the adsorption mechanism of Mg-Al-CO₃ LDH on heavy metal cadmium,so as to provide a basis for the prevention and control of heavy metal pollution in soils and groundwater.The main contents and conclusions of the study were as follows:I.Adsorption characteristics of cadmium in soils in the presence of Mg-Al-CO₃LDH.The adsorption characteristics of cadmium on soils with different LDH contents（0,300,500,1000,1200 mg/kg）,cation types(Na⁺,K⁺,Ca²⁺,Mg²⁺),pH（4.5,5.5,6.5）and ionic strengths（0.005,0.01,0.05 mol/L）were studied by batch static adsorption experiments.The adsorption and diffusion process of cadmium was described by adsorption kinetics experiments.Fourier transform infrared（FTIR）and X-ray diffraction（XRD）were used to analyze the samples before and after the adsorption of cadmium by Mg-Al-CO₃ LDH,to further explore the adsorption characteristics of cadmium by Mg-Al-CO₃ LDH.Freundlich thermodynamic model,Langmuir thermodynamic model and linear thermodynamic model were used to fit the experimental data and obtain the relevant parameters.With the increased of LDH content in soils,the adsorptive capacity of cadmium in soils increased,that was,the presence of LDH can promote cadmium adsorption.Highly valent cations were more likely to compete with cadmium for adsorption sites and then inhibit the adsorption.Soils adsorptive capacity for cadmium is enhanced under alkaline conditions,and the presence of Mg-Al-CO₃ LDH increased soil pH and increased the adsorption sites of cadmium.The existence of background ions tends to compete with Cd²⁺for adsorption sites and inhibit adsorption.Spectral analysis showed that the adsorption mechanism of Mg-Al-CO₃ LDH on Cd²⁺was mainly the reaction of Cd²⁺and CO₃^2-to form CdCO₃ precipitation.In natural soils and L soils（the soils added LDH,referred to as L soils）,the fitting effect of Freundlich and Langmuir adsorption isotherms was better than that of linear adsorption isotherms.The results of adsorption kinetics showed that the existence of Mg-Al-CO₃ LDH prolonged the equilibrium time of adsorption reaction and increased the specific adsorption sites.The adsorption rate of soil to Cd²⁺was mainly controlled by particle diffusion and chemical reaction.II.Migration characteristics of cadmium in soil in the presence of Mg-Al-CO₃ LDH.Though soil column leaching experiments,the effects of Mg-Al-CO₃ LDH on the migration of heavy metal cadmium in soils under different cation types and ionic strengths were investigated.The characteristics of cadmium transport in homogeneous soils,layered soils and changing the order in which the soils layers were revealed.Under different cation types,the Cd²⁺outflow time in L soils was later than that in natural soils,that was,the presence of Mg-Al-CO₃ LDH can inhibit the migration of Cd²⁺and delay the outflow time.However,Cd²⁺adsorbed by Mg-Al-CO₃ LDH can be easily replaced by background ions,which plays a sustained release role.The existence of high valence cations can compete with Cd²⁺for adsorption sites and promote migration.In natural soils,with the increased of ionic strength,the adsorptive capacity of cadmium decreased and promoted the migration of cadmium.Cd²⁺and the surface of soils particles formed an outer surface complex due to electrostatic interaction.Under high ionic strength,Cd²⁺formed outer surface complexes with soils surface,while under low ionic strength,Cd²⁺formed inner surface complexes with soils.At high ionic strength,soils colloids are easy to flocculate and precipitate due to the compression effect of background ions on soils colloidal electric double layer,which blocks soils pore and reduces soils hydraulic conductivity.As an anionic clay mineral,Mg-Al-CO₃ LDH expands easily with water,which reduces soil pore size and reduces soil water conductivity.The outflow time of cadmium in homogeneous soils and different soils layers was as follows:the sequence of layered soils in natural soils had little effect on the migration of cadmium,which was between single layer sandy soils and single layer loam soils;in L soils,the outflow time of cadmium in the sequence of loam-sand layers is earlier than that in the sequence of sand-loam layers.When Mg-Al-CO₃ LDH exists,the interception ability of soils is weaker when the soils structure is loam on the top and sand at the bottom,and the contaminants are easier to migrate to the groundwater.III.Numerical simulation of cadmium transport in soils in the presence of Mg-Al-CO₃ LDH.One-site nonequilibrium model（OSM）was used to simulate the migration behavior of cadmium in soils under different conditions.The fitting results showed that R²>0.8and RMSE<0.08,which indicated that one site nonequilibrium model could describe the migration characteristics of cadmium in soils.However,the phenomenon of steady outflow or small protuberance in the backwash stage cannot be well described.The f value fitted by non-equilibrium two-point adsorption model was smaller,which indicated that the proportion of cadmium adsorption in soils is very small,and the kinetic adsorption was dominant.The empirical constantβobtained by simulation was small,which indicated that the heterogeneity of soil surface is stronger.And the empirical constantβobtained by fitting in L soils was smaller than that in natural soils,indicating that the existence of Mg-Al-CO₃ LDH enhances the heterogeneity of soil surface.By simulating and predicting the transport process of pollutants at different depths of soil profiles in layered soils,it can be seen that both natural soils and L soils presented similar transport processes at different depths of lower sandy soils,and the interception of pollutants is weak.There are differences in solute concentration between natural soils and L soils at the same depth profile.There was a discontinuous jump in the adsorption capacity at the boundary of layered soils,which reflected the difference of cadmium transport process between loam and sandy soils.