Stabilization Effect Of Heavy Metals In Soil Using Palygorskite Modified With Ca（OH）₂

A large amount of heavy metals enter the soil with the rapid development of industry and agriculture,which lead to serious pollution to the soil,and eventually endangers food production and human health.Therefore,it is urgent to control pollution of heavy metal in soil.As a clay mineral with abundant reserves,palygorskite can be applied to remediate heavy metal-contaminated soil after modified,which not only provide materials for the current treatment of heavy metal contaminated soil,but also promote the development and utilization of palygorskite in Gansu,further enhance its application value.In this study,the palygorskite from Linze,Gansu was mixed with Ca（OH）₂in different proportions（5%,10%,15%,20%,25%）,and the first type of palygorskite modified with Ca（OH）₂applied to the simulated Cd contaminated soil was prepared by immersion.In order to improve further the adsorption performance of palygorskite for heavy metals in soil,The second type of stabilizer applied in the actual heavy metals contaminated soil was prepared optimizing the preparation method and adding calcinition at high temperature.Using characterization experiments,stabilization cultivation experiments,pot experiments and seed germination experiments to analyze the modification mechanism of palygorskite modified with Ca（OH）₂,and the effect on physical and chemical properties of soil,the bioavailability contents of heavy metals in soil,chemical speciations of heavy metals in soil,plant growth status and heavy metal enrichment,seed germination and root elongation.The stabilization effect of palygorskite modified with Ca（OH）₂on heavy metals in soil were studied.The results show that:（1）Palygorskite mostly exists in the form of crystal bundle aggregates,and the structure is dense and not loose.After modification by Ca（OH）₂,the aggregates of palygorskite crystal bundles were dispersed,more rod crystals were exposed,and crystals with spherical particle distribution were formed at the same time,and the adsorption capacity of palygorskite were improved.In addition,the Si-O-Si bonds were broken,the active sites of palygorskite were increased,and the adsorption capacity was further improved.High-temperature calcination could remove adsorbed water,zeolite water and part of crystallization water in palygorskite.（2）The application of palygorskite modified with Ca（OH）₂could significantly improve soil pH,electric conductivity and cation exchange capacity.The soil pH,electric conductivity and cation exchange capacity reached the maximum when the ratio of Ca（OH）₂was 25%.（3）The DTPA and TCLP extracted contents of heavy metals in the simulated Cd-contaminated soil and actual heavy metals contaminated soil were significantly reduced after applying palygorskite modified with Ca（OH）₂.The DTPA and TCLP extracted contents of Cd,Cu and Zn in the soil reached the minimum value when the ratio of Ca（OH）₂was 25%.Compared with the control treatment,the DTPA extracted contents of Cd,Cu and Zn in soil coud be decreased by up to 31.28%,16.00%and 23.81%,and the TCLP extractive contents up to 33.29%,35.90%and 33.50%.When the ratio of Ca（OH）₂was 20%.The DTPA and TCLP extracted contents of Pb and Ni in the soil reached the minimum value.The DTPA extracted contents of Pb and Ni could be reduced by 40.19%and 31.06%at the highest,and the TCLP extractble content could be reduced by 31.12%and 25.93%,respectively.（4）The palygorskite modified with Ca（OH）₂could promote the transformation of heavy metals in simulated Cd-contaminated soil and actual heavy metal-contaminated soil from active acid-soluble speciation to residual speciation and reduce the migration ability of heavy metals.Among them,the speciation transformation of Cd and Cu was the most obvious when the ratio of Ca（OH）₂was 25%.The speciation transformation of Pb,Ni and Zn was more obvious when the ratio of Ca（OH）₂was 25%and 20%.The acid-soluble speciation contents of Pb,Ni and Zn in soil reached the minimum value when the ratio of Ca（OH）₂was 25%,which decreased by 46.67%,29.20%and 57.64%,respectively,compared with the control treatment.The residual content of Pb,Ni and Zn in the soil reached the maximum when the ratio of Ca（OH）₂was 20%,with an increase of 49.52%,30.28%,and 51.77%,respectively.（5）The addition of the palygorskite modified with Ca（OH）₂could reduce the toxic effect of heavy metals on corn,and promote seed germination,root elongation and the growth of corn,reduced the accumulation and transport of heavy metal Cd in corn.The germination rate,relative root elongation rate and germination index of seeds were the highest and the root length,stem length,fresh weight and dry weight of corn plants were the largest,and the content of Cd in the stems and roots of corn plants was the smallest when the ratio of Ca（OH）₂was 25%.（6）Different palygorskite modified with Ca（OH）₂improved the stabilization effect of heavy metals in simulated Cd-contaminated soil and actual heavy metal-contaminated soil,and reduced the ecological risk and potential ecological risk of heavy metals.The stabilization effect of soil heavy metals Cd,Cu and Zn was the highest when the ratio of Ca（OH）₂was25%and Pb and Ni was the highest when the ratio of Ca（OH）₂was 20%.The application of palygorskite modified with Ca（OH）₂reduced the ecological risk and potential ecological risk of heavy metals in soil.The ecological risk of heavy metals Cd,Cu and Zn is the lowest when the ratio of Ca（OH）₂was 25%and Pb and Ni is the lowest when the ratio of Ca（OH）₂was20%.The potential ecological risk of soil Pb,Ni and Zn was the lowest when the ratio of Ca（OH）₂was 20%,Cd was the lowest when the ratio of Ca（OH）₂was 25%and Cu was the lowest when the ratio of Ca（OH）₂was 20%and 25%.The comprehensive potential risk of heavy metals in the soil was the lowest when the ratio of Ca（OH）₂was 25%.The palygorskite modified with Ca（OH）₂has a good stabilization effect on heavy metals in soil in this study,and has application value in the remediation of heavy metal-contaminated soil.