Study On Heavy Metal Stabilization Of Acid-activated Attapulgite Modified By Hydrothermal Crystallization

The pollution of heavy metals in soil caused by the rapid development of industrialization and urbanization should not be underestimated.Heavy metals accumulate in the soil and enter crops,affecting the normal growth and development of crops and threatening human health through the food chain.Attapulgite can stabilize heavy metals in the soil and reduce their mobility and bioavailability through surface adsorption,ion exchange,adsorption complexes and hydrolytic precipitation.However,the natural attapulgite clay in Gansu province has many impurities,which seriously affect its adsorption properties.Acid modification can dissolve carbonate impurities in attapulgite clay.Hydrothermal crystallization technology can realize mineral crystal rearrangement or produce new mineral crystals in a suitable reaction environment,and improve mineral adsorption performance.At present,there are few studies on the modification of attapulgite clay by acid treatment combined with hydrothermal crystallization technology,and the related studies on the use of this stabilization material for the remediation of actual heavy-metal contaminated soil are also rare.Therefore,this study can provide a new theoretical basis and data support for the current treatment of heavy metal-contaminated soil and the performance improvement of attapulgite clay.In this study,attapulgite clay from Linze County,Gansu Province was used as raw material after wet sieving.The raw material was treated with different mass contents of sulfuric acid（0%,2%,4%,6%,8%,10%and 12%）first,and hydrothermal crystallization was carried out with high brine to prepare hydrothermal crystallization acid-treated attapulgite clay.Scanning electron microscopy（SEM）,Fourier infrared spectrometer（FTIR）and X-ray diffraction（XRD）were used to characterize the microstructures,functional group characteristics and crystal phase composition of the modified attapulgite clay.The heavy metal complex contaminated soil of the actual mining field was collected,and the stabilization experiment was further verified by applying acid-activated attapulgite modified by hydrothermal crystallization to the target soil with a mass fraction of 4%.The changes in soil’s basic physical and chemical properties and the content of heavy metals from the single-stage extraction state of calcium chloride（CaCl₂）,diethylenetriamine pentaacetic acid（DTPA）,and toxic characteristic leaching（TCLP）were measured to evaluate the stabilization effect of acid-activated attapulgite modified by hydrothermal crystallization when the stabilization was complete.The chemical speciation distribution of heavy metals in soil was measured through continuous extraction of BCR to evaluate the environmental risk index of heavy metals.The results showed that:（1）The SEM,FTIR and XRD patterns of the materials showed that the impurities such as montmorillonite,gypsum,sepiolite,chlorite and calcite in the raw ore were dissolved in acid-activated attapulgite modified by hydrothermal crystallization.The attapulgite rod crystals became shorter,part of the rod crystal bundles was dissociated,forming loose pore structures,and the characteristic peaks of the silicon oxygen/silicone hydrogel group were enhanced.The stabilized material crystallizes to form calcite and a small amount of potassium nepheline with the increase of acid mass fraction.（2）The physical and chemical properties of soil was significantly improved by acid-activated attapulgite modified by hydrothermal crystallization.The p H value of soil decreased significantly,and alkaline soil became neutral soil.The conductivity and cation exchange capacity of soil increased significantly.The electrical conductance increased by 0.15 to 2.91ms·cm^-1,and the cation exchange capacity increased by 6.02%to 30.44%compared to the control group.（3）Acid-activated attapulgite modified by hydrothermal crystallization significantly reduced the bioavailable content and leaching toxicity of heavy metals in soil,and the stabilization ability of the same stabilization material to different heavy metals was different.The attapulgite clay treated with hydrothermal crystallization after adding 4%sulfuric acid had the best stabilization effect on Cd,and the content of Cd in the extracted state of TCLP decreased by 36.67%compared with the control group.Hydrothermal crystallization-treated attapulgite clay with 6%sulfuric acid has the best stabilization effect on Cr,Cu and Pb.Compared with the control group,the content of Cr extracted from CaCl₂,Cu extracted from TCLP and Pb extracted from DTPA are reduced by 42.58%,44.32%,and 60.52%,respectively.The attapulgite clay treated with hydrothermal crystallization containing 12%sulfuric acid had the best stabilization effect on Ni and Zn.The content of Ni extracted by TCLP and Zn extracted by DTPA decreased by 56.31%and 55.80%compared with the control group.The total ecological risk index of complex heavy metals was the lowest when the mass fraction of sulfuric acid was 4%or 6%.（4）Both wet-sieved attapulgite clay and acid-activated attapulgite modified by hydrothermal crystallization can transform heavy metals in soil from exchangeable and reducible speciation in high activity to residual speciation in low activity.The speciation transformation of Cd in soil was the most obvious after the stabilization treatment of attapulgite clay with hydrothermal crystallization after adding 4%sulfuric acid.The content of exchangeable Cd decreased by 66.53%compared with the control group,and the percentage of residual Cd increased from 15%to 39%.,The form transformation of Cr,Cu,Ni,Pb and Zn in soil was the most obvious prepared after stabilization treatment with 6%sulfuric acid-activated attapulgite modified by hydrothermal crystallization,and the residue content increased by13.60%,24.96%,20.59%,130.31%and 29.02%,respectively.Stabilization materials significantly reduced the environmental risk index and potential environmental risk index of different heavy metals.stabilization materials can effectively reduce the potential comprehensive environmental risk index of composite heavy metals when the mass fraction of sulfuric acid was 4%or 6%.（5）The growth and development of maize were promoted by wet-sieved attapulgite clay and acid-activated attapulgite modified by hydrothermal crystallization.The ecotoxicity of soil and heavy metals concentration in above-ground and underground parts of maize were reduced.The bioconcentration factors of heavy metals were the lowest in maize planted in stabilized soil with 6%sulfuric acid-activated attapulgite modified by hydrothermal crystallization.The bioconcentration factor of Cd,Cr,Cu,Ni,Pb and Zn were reduced by 51.95%,13.66%,60.09%,63.86%,65.60%and 21.66%,respectively,compared with the control group.