Biogeochemical Characteristics And Environmental Influence Of Main Heavy Metals In Oasis Soils

Accumulation and transfer for Cu,Zn,Ni,Pb,Cd in soil-wheat system,the fraction distribution for heavy metals and their bioavailability were carriede to study the chemical behavior of heavy metal in soil.These studies will provid a scientific basis for the management of environment and remediation of soils contaminated by heavy metals.Tessier sequential extraction procedure were carried to study the fraction distribution for Cu,Zn,Ni,Pb,Cd.Total content of heavy metals in soil,the Tessier sequential extraction procedure and single extraction procedure which included DTPA extraction procedure,EDTA extraction procedure,NH₄OAC extraction procedure were carded to assess the bioavailability for heavy metals.The main results show as follows:(1) The pollution index based theâ…¡grade standard of soil environmental quality indicated 78%of the samples were contaminated by heavy metals and the major pollutant were copper and nickel.Middle pollution point and serious pollution point respectively was 32%and 24%of all sample points.(2) The accumulated capacity of heavy metals in different apparatus of wheat were different, with the order of Cu in wheat being root＞leaves＞tip shell＞straw＞grain,with the order of Ni in wheat being root＞＞leaves＞tip shell＞straw and grain.Roots enriched Zn easily,whereas roots, leaves and tip shell enriched Cd easily.The transfer capacity of heavy metals in wheat was＞Cd＞Cu＞Ni.Cu content in grains is reaching the warning level,the Zn,Cd content in wheat is below food safety level,while the Ni content in wheat of all samples exceeds food safety level.(3) Copper was distributed mainly in organic fraction,the next were residual fraction and Fe-Mn oxides fraction,the content of Cu in exchangeable fraction was least.Zinc was distributed as follow:residual fraction＞Fe-Mn oxide fraction＞organic fraction＞carbonate fraction＞exchangeable fraction.Nickel was distributed mainly in Fe-Mn oxide fraction,the next were organic fraction and the residual fraction,content of Ni in exchangeable fraction was least.Lead was distributed mainly the residual fraction and organic fraction,the next was Fe-Mn oxide fraction,content of lead in carbonate fraction was least.Soil pH and organic matter content influenced the distribution of Cu,Ni in soil.The distribution of Zn in soil was influenced by soil organic matter content,as well as the distribution of Pb in soil was influenced by soil organic matter content and electrical conductivity.(4) Total content of heavy metal in soil,DTPA extraction procedure,EDTA extraction procedure and NH₄OAC extraction procedure all are suitability to assess the bioavailability for Cu in the studied region soil.Total content of heavy metal in soil and NH₄OAC extraction procedure can not be used to assess the bioavailability for Zn in the studied region,while Tessier sequential extraction procedure,DTPA extraction procedure,EDTA procedure can provide some indication of bioavailability for Zn.DTPA extraction procedure is a good method to evaluating the bioavailability for Ni in the studied region soil.Total content of heavy metal in soil,EDTA extraction procedure,NH₄OAC extraction procedure can provide some indication of Ni content in roots and wheat.But Tessier sequential extraction procedure was unsuitability to evaluate the bioavailability for Ni in the studied region.Total content of heavy metal in soil,DTPA extraction procedure,EDTA extraction procedure,NH₄OAC extraction procedure can be used to evaluate the bioavailability for Cd in the studied region,but Tessier sequential extraction procedure is not a good method to assess the bioavailability for Cd.However the NH₄OAC extraction procedure provide less indication of Ni content in roots.