Reliability Assessment And Geochemical Significance Of Heavy Metal Sequential Extraction In Soils And Sediments

Heavy metal pollution has been one of the main research fields in soil contamination. It brings great damage to human beings and the environment. It is increasingly realised that the distribution, mobility and biological availability of heavy metal depend not simply on their concentrations but, critically, on the forms in which they occur in natural systems. Hence, including chemists, biochemists, physicists, geochemists, and water and soil scientists, they have more interest in speciation of heavy metals. Considerable recent research has focused on the topic of chemical speciation of heavy meatal in the environment.However, it is difficult to carry through this investigation due to the complexity of environment. Up to now, chemically sequential extraction technique has been wildly adopted, especially amended Tessier Sequential Extraction (Seven-step method) being employed in the eco-geochemical survey and evaluation in our country.In the paper, the certified reference material GBW07436 (sediment) and GBW07437 (soil) was utilized to estimate the reliability of the modified Tessier Sequential Extraction, so as to overcome the difficulty in speciation content confirmation and poor comparability of inter-laboratory results. Seven fractions of 13 kinds of heavy metals (Cu, Pb, Co, Ni, Cr, Cd, Mo, Zn, As, Mn, Sb, Hg and Se) were sequentially extracted and calculated statistically in accordance to the principle of mass conservation. The reasons for the measurement errors were discussed including the selection of extractant, ion reabsorption and so on. The major results achieved are described as follows:1. Modified Tessier Sequential Extraction scheme is suitable for 11 kinds of elements such as Cd, Co, Ni, Cu, Pb, Zn, Cr, Mo, Mn, As and Sb. But it is unsuitable for the Hg, Se in soils and sediments.2. As the exchangeable fraction extractant, Magnesium chloride exerts evident influences on the carbonate phase of some heavy metal such as Co, Cr, Ni, Cu Pb. and Zn. However, it has little effect on carbonate fractions of As, Sb, Hg, Se, Cd, and Mo which content of exchangeable, acid and water-soluble fractions are low. Magnesium chloride makes no impact on the extraction of other fractions such as the easily oxidisable, the hydrous oxides of iron and manganese, the oxidisable and the residual.3. After the modified Tessier sequential extraction scheme, the proportion of the mineral facies of the certified reference materials shows little change such as quartz, albite, K-feldspar, and muscovite. The clay-mineral facies altered pratly: Palygorskite disappeared in the residual of the exchangeable fraction and clinochlore vanished in the residual of the oxidisable fraction.