Analytical Methods Research On Exchangeable Form Of Heavy Metals In Soil

Heavy metal pollution in soil has become an important global concern environmental issue. Because of accumulation and migration, heavy metals in soil either directly or indirectly harm the environment and human health. The total amount of heavy metals in soil contains heavy metal background value and the level of heavy metals entered from the external environment. The total amount of heavy metals in soil is a prerequisite for evaluating the extent of heavy metal contamination and heavy metal bioavailability of soil, but a large number of domestic and foreign research results showed that only using the total amount of heavy metals in soil could not be a good assessment of the forecast of heavy metal contamination and bioavailability of heavy metals in soil. The heavy metal be potentially harmful on the environment or used by bio-absorption is only the physical and chemical properties of water-soluble and a more lively part of heavy metal, and this part is generally believed very close to exchangeable heavy metal. In this paper, the exchangeable form of heavy metals in soil was studied by different analytical methods, the characteristics of various types of extraction methods were discussed, its advantages and limitations in the application were evaluated, and this information could be used as reference in the research field of heavy metal contamination in soil.In this article, Panjiazhuang Cinnamon and North Kangzhuang Brown soil of Jinan were selected as soil samples for study, the total amount of heavy metals in soil was studied by using microwave digestion, and chemical speciation of heavy metals in soil was studied by BCR extraction method; At the same time, the influencing factors of exchangeable form of heavy metals were analysed systemically.1. Through the determination of total heavy metals in the soil, the distribution of total heavy metals in the two kinds of soil was close, the order of heavy metal content of Panjiazhuang Cinnamon was: Fe> As> Mn> Zn> Pb> Cr> Ni> Cu> Co> Cd, the order of heavy metal content of North Kangzhuang Brown soil was: Fe> Mn> As> Zn> Pb> Cr> Ni> Cu> Co> Cd.2. Through heavy metal speciation analysis of Cinnamon and Brown soil, experimental results showed that the distribution of heavy metal form of the two types of soil was very different: Cu,Mn and Pb were mainly in the potential available state in Cinnamon, but in unavailable state in Brown soil; In addition, the form of heavy metal of the two types of soil had similar distribution: Zn was mainly in the potential available state in the Cinnamon and Brown soil; elements Fe,Cd,Co,Ni,As and Cr in the Cinnamon and Brown soil were mainly in unavailable state.3. In accordance with analysis results that the extraction rate of heavy metals in soil affacted by extraction time, under normal circumstances, 2h could be choosed as extraction time for extacting the exchangeable forms of heavy metals in soil.4. According to experimental results of extraction under different pH, pH = 2 could be choosed as critical pH for exchangeable form of heavy metals in soil, that is, pH of extraction solution needs to meet pH≥2.5. By comparing the experiments of extracting Exchangeable form of heavy metals in the soil by different types of extractants, the optimum extractants for heavy metals are as follows: Extraction for Cu element: MgCl₂ solution of 2.0mol/L is suitable for Cinnamon, and NaAc solution of 1 .0mol/L is suitable for Brown soil;Extraction for Zn element: 1.0mol/L NH₄NO₃ is suitable for Cinnamon, and 0.1mol/L MgCl₂ is suitable for Brown soil;Extraction for Fe element: deionized H₂O is suitable for both Cinnamon and Brown soil;Extraction for Mn element: 0.5mol/L MgCl₂ is suitable for both Cinnamon and Brown soil;Extraction for Cd element: 0.01mol/L HCl is suitable for Cinnamon, and 0.01mol/L EDTA is suitable for Brown soil;Extraction for Co element: 0.01mol/L EDTAis suitable for both Cinnamon and Brown soil;Extraction for Ni element: 0.01mol/L EDTA is suitable for Cinnamon, and buffer solution of. 0.1mol/L(NH₄)₂C₂O₄ is suitable for Brown soil.6. As a result of the heavy metal type difference: the adsorption situation in the soil surface's is different with the type of heavy metal ion, and their desorption situation is also not correspondingly same.7. Competition function of metal positive ion: for element Cd, the positive metal ion's competition order is: Ca²⁺>Mg²⁺>Na⁺; for element Co, the positive metal ion's competition order is: Mg²⁺>Ca²⁺>Na⁺; for element Ni, the positive metal ion's competition order is: Ca²⁺>Mg²⁺>Na⁺.8. Influence of anion: Regarding element Cd, the anion competition order is: Ac>NO₃>C₂O₄^2-; Regarding element Co, the anion competition order is : C₂O₄^2->NO₃^->Ac^-; Regarding element Ni , the anion competition order is : C₂O₄²>Ac^->NO^3-;9. Influence of onic strength: In the ordinary circumstances, the extraction rate of the exchangeable form of heavy metals in soil is in proportional relations with the ionic strength, but possibly could also be in reverse proportion relation because of the heavy metal type extracted;10. Influence of soil type: In ordinary circumstances, ifsoil adsorptive capacity was strong, then its desorption ability would be weak; The adsorptive capacity of Cd,Co,Ni in the Brown soil was slightly higher than the Cinnamon; The desorption ability of Cd and Ni in the Cinnamon was slightly higher than the Brown soil; The desorption ability of Co in the Cinnamon was slightly higher than the Brown soil by using the metal salt and the nonmetallic salt as extrants, but the desorption ability in the Brown soil was slightly higher than the Cinnamon by using the acid,the buffer solution and complex organic as extrants.