Effect Of Temperature Controlled Hydrothermal Crystallization Of Attapulgite Clay On Heavy Metal Stabilization Performance

Due to the rapid development of China’s economy and industrial technology,the"three wastes"discharged by enterprises during mining and smelting in mining areas,as well as long-term sewage irrigation,have led to increasing soil heavy metal pollution issues.In situ chemical stabilization technology is a common method for remediation of heavy metal contaminated soil due to its low cost,high efficiency,simple operation,and effective reduction of the bioavailable content of heavy metals.For in situ chemical stabilization technology,the selection of passivating agents is particularly critical.Attapulgite is widely used as a stabilization material in soil remediation due to its special chain layered crystal structure and fibrous morphology,developed pore structure,large specific surface area,and good adsorption,catalysis,and ion exchange properties.In this study,attapulgite was used as raw material,and after acid activation pretreatment,hydrothermal crystallization reactions were conducted at different temperatures of 25℃,30℃,40℃,60℃,80℃,and 100℃to prepare six stabilization materials.Six hydrothermal crystallization attapulgite stabilization materials were added to heavy metal contaminated soil at a ratio of 4%.After stabilization for50 days,the physical and chemical properties such as pH,electrical conductivity（EC）,cation exchange capacity（CEC）,and the chemical speciation distribution of heavy metals Cd,Cr,Cu,Ni,Pb,Zn in the CaCl₂extracted state,TCLP extracted state,DTPA extracted state,and BCR.Planting corn in stabilized soil,analyzing the root length,stem length,fresh and dry weight of the aboveground and underground parts of the corn,as well as the content of heavy metals Cd,Cr,Cu,Ni,Pb,and Zn in the corn plant.The stabilization effect of hydrothermal crystallization attapulgite stabilization agent on Cd,Cr,Cu,Ni,Pb,Zn in heavy metal contaminated soil was analyzed through ecological risk assessment,environmental risk assessment,and corn transport enrichment coefficient.The results indicate that:（1）Adding hydrothermal crystallization attapulgite to stabilize the soil,the pH,EC,and CEC of the soil showed a trend of first increasing and then decreasing with the increase of temperature.Compared with the control group without adding stabilization materials,when the hydrothermal crystallization temperature was 30℃,the pH,EC,and CEC in the soil were the maximum values.（2）Adding hydrothermal crystallized attapulgite to stabilize the soil,the contents of CaCl₂,TCLP,and DTPA extracted states of Cd,Cr,Cu,Ni,Pb,and Zn in the soil first decreased and then increased with the increase of temperature.When the hydrothermal crystallization temperature is 30℃,the bioavailable content of Cd,Cr,Cu,Pb,and Zn in the stabilized soil is the lowest.The extracted CaCl₂content of Cd,Cr,Cu,Pb,and Zn decreased by 20.38%～35.14%compared to the control group without stabilization materials;Compared with the control,the extracted content of TCLP decreased by 16.50%～38.80%;Compared with the control,the extracted content of DTPA decreased by 19.28%～39.06%.When the hydrothermal crystallization temperature was 60℃,the contents of CaCl₂,TCLP,and DTPA extracts of Ni in stabilized soil were the lowest,which decreased by 22.00%,28.78%,and 23.95%,respectively,compared to the control group without stabilization materials.（3）After adding hydrothermal crystallized attapulgite to stabilize the soil,the chemical forms of heavy metals in the contaminated soil change,the content of acid soluble and reduced states of Cd,Cr,Cu,Ni,Pb,and Zn decreases,and the content of oxidized and residual states increases.When the hydrothermal crystallization temperature is 30℃,the acid soluble state and residual state of Cd and Cr vary greatly.Compared with the control group without stabilization material,the acid soluble state of Cd and Cr decreases by 9.04%and8.22%,respectively,and the residual state increases by 10.06%and 8.30%,respectively.The reduced state and residual state of Cu,Pb,and Zn vary greatly,and the reduced state decreases by 7.12%,4.51%,and 7.26%,respectively,compared with the control group without stabilization material,and the residual state increases by 12.15%,respectively 7.19%and 9.30%.When the hydrothermal crystallization temperature is 60℃,the acid soluble state and residual state of Ni vary greatly,decreasing by 9.15%compared to the control group without stabilization materials,and increasing by 7.62%（4）After adding hydrothermal crystallization attapulgite to stabilize the soil,the stem length,root length,fresh weight,and dry weight of corn were significantly improved.When the hydrothermal crystallization temperature was 30℃,the stem length,root length,aboveground fresh weight,underground fresh weight,aboveground dry weight,and underground dry weight of corn increased by 40.86%,87.22%,35.79%,51.63%,31.61%,and46.95%,respectively,compared with the control group without stabilization materials.（5）After adding hydrothermal crystallized attapulgite to stabilize the soil,the concentration of heavy metals in the aboveground and underground parts of corn plants significantly decreased,inhibiting the accumulation and transportation of heavy metals in corn.When the hydrothermal crystallization temperature is 30℃,the content of Cd,Cr,Cu,Pb,Zn in corn in stabilized soil is the lowest,and the enrichment coefficient and transport coefficient are the lowest;When the hydrothermal crystallization temperature is 60℃,the Ni content of corn in stabilized soil is the lowest,and the enrichment and transport coefficients of Ni are the lowest.（6）After adding hydrothermal crystallized attapulgite to stabilize soil,the stabilization efficiency of heavy metals was significantly improved,and the ecological risk index and potential risk index of heavy metals were significantly reduced.When the hydrothermal crystallization temperature is 30℃,the stabilization efficiency of Cd,Cr,Cu,Pb,and Zn in the stabilized soil is the highest,while the ecological risk index and potential risk index are the lowest.When the hydrothermal crystallization temperature is 60℃,the stabilization efficiency of Ni in stabilized soil is the highest,and the ecological risk index and potential risk index are the lowest.When the hydrothermal crystallization temperature is 30℃,the comprehensive potential risk index is the lowest,from 109.59 to 72.31.Hydrothermal crystallization of attapulgite stabilization materials effectively reduces the bioavailability of heavy metals in contaminated soil,changes the chemical forms of heavy metals,improves stabilization efficiency,reduces ecological risk index,and effectively reduces the absorption and enrichment of heavy metals by corn in contaminated soil.