Stabilization Of Heavy Metal In Soil Using Stabilizers Prepared With Attapulgite And Phosphate

Soil is an indispensable resource in agricultural production.In recent years,with the rapid development of global economy and industrialization,the intensity of soil development by human activities is increasing,which leads to the serious excessive heavy metals in most areas of China,and the soil environmental quality is not optimistic.In order to reduce the content of heavy metals in soil more economically and effectively,seven kinds of heavy metal stabilization materials were prepared by mixing attapulgite and phosphate in the mass ratio of0:1,8:1,3:1,1:1,1:3,1:8 and 1:0.The stabilization material was added to the contaminated soil with 4%addition for stabilization test and pot experiment.After 30 days of stabilization,the physical and chemical properties of soil,the contents of TCLP,DTPA,CaCl₂,H₂O extracted heavy metals（Cr,Cd,Cu,Ni,Pb,Zn）and four chemical forms of heavy metals were determined.The root length,stem length,fresh weight,dry weight and heavy metal content in plants were determined in pot experiment.The results show that:（1）Seven kinds of stabilization materials were added to the soil polluted by heavy metals.The results showed that different stabilization materials had obvious effects on the physical and chemical properties of the soil.The p H value of the soil was the highest when attapulgite was applied alone,and the conductivity and cation exchange capacity of the soil reached the maximum when the ratio of attapulgite and phosphate was 3:1.（2）The results showed that the content of TCLP,DTPA,CaCl₂and H₂O extracted heavy metals was significantly affected by the combination of phosphate and attapulgite stabilization materials.With the increase of attapulgite dosage,the content of six heavy metals decreased first and then increased.The reduction efficiencies of TCLP,DTPA,CaCl₂ and H₂O were Pb>Cu>Cd>Cr>Zn>Ni,Zn>Cu>Ni>Cr>Cd>Pb,Cr>Pb>Cu>Cd>Zn>Ni,Pb>Ni>Zn>Cu>Cd>Cr,respectively.When the ratio of attapulgite to phosphate was 3:1,the extraction amounts of Cr,Cd,Cu and Zn in TCLP,DTPA,CaCl₂and H₂O were the lowest.The lowest bioavailable extractable content of Ni and Pb appeared when the ratio of attapulgite to phosphate was 1:1.Compared with the control,the contents of TCLP,DTPA,CaCl₂and H₂O in Cr decreased by 30.2%,44.7%,48.8%and 35.2%,respectively,the four forms of Cu decreased by 32.9%,46.1%,40.7%and 44.6%,the four forms of Cd decreased by 31.1%,43.8%,38.4%and 40.3%,the four forms of Cr decreased by 26.7%,54.3%,36.4%and 47.8%,and the four forms of Ni decreased by 25.5%and 47.8%,respectively 4%,38.9%,41.2%,53.4%respectively.（3）The combination of stabilization materials promoted the transformation of heavy metals from acid soluble state to residual state in soil.With the decrease of acid soluble content of six heavy metals,the oxidation and reduction states of heavy metals also gradually changed,but most of them were finally transformed into residual state and fixed.When the ratio of attapulgite to phosphate was 3:1,the acid soluble and residual states of Cr,Cd,Cu and Zn changed most significantly,which decreased by 14.7%,19.6%,14.2%and 11.4%respectively compared with the control.Ni and Pb also changed from acid soluble state to residual state,especially when the ratio of attapulgite to phosphate was 1:1.Compared with the control,the acid soluble Ni and Pb decreased by 21.6%and 18.9%respectively.（4）Phosphate combined with attapulgite stabilization material can significantly increase the biomass of plants and reduce the enrichment,enrichment factor and transport coefficient of heavy metals in plants.When the ratio of attapulgite to phosphate was 3:1,the root length,stem length,dry weight and fresh weight of maize were significantly increased.Compared with the control,the root length,stem length,aboveground fresh weight,underground fresh weight,aboveground dry weight and underground dry weight of maize increased by 81.6%,34.7%,50.9%,79.8%,55.1%and 41.0%respectively when the stabilization ratio was 3:1.The content of heavy metals in plants decreased by 62.3%,56.5%,56.3%,47.5%,44.2%and41.1%,respectively.When the ratio of attapulgite to phosphate was 3:1,the enrichment factors of Cr,Cd,Cu and Zn in the aboveground and underground parts of the plant decreased most obviously,and the transport coefficient also decreased most obviously under this ratio,which decreased by 35.2%,39.9%,32.1%and 28.8%respectively compared with the control.When the ratio of attapulgite to phosphate was 1:1,the enrichment factor and transport factor of Ni and Pb in aboveground and underground parts of plants decreased most.（5）The addition of phosphate attapulgite stabilization material can improve the stabilization efficiency of heavy metals and reduce the ecological risk index of heavy metals.Compared with the control treatment,the stabilization efficiency of DTPA extracted,TCLP extracted and CaCl₂ extracted heavy metals was increased by 0.81-17.06%,0.046%-9.45%and 1.88%-17.23%respectively.Compared with the control,the ecological risk index of DTPA,CaCl₂and TCLP extracts decreased in varying degrees.The ecological risk index of Ni,Pb,Cr,Cu,Cd and Zn extracted by DTPA,TCLP and CaCl₂decreased from medium risk to low risk or from high risk to medium risk.The results show that the application of phosphate and attapulgite stabilization materials can effectively reduce the bioavailability of heavy metals and prevent the further migration of heavy metals to organisms.It provides a new method for the development and utilization of attapulgite clay and the stabilization of heavy metals in soil.