Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils
Various extraction procedures were employed for measuring extractable concentrations of potential toxic elements in soil. The extractability of Cd, Cu, Pb and Zn in four contaminated and four non-contaminated soils of Japan, was compared by single extraction (CaCl2, DTPA, NH4Cl, 0.1 M HCl and 1 M HCl ) and sequential extraction procedures [(six operationally defined chemical phases, viz. water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5) and residual (F6) fractions)]. Extractability of metals from soils samples varied depending on metals and/or extractants used. Among the extractants, 1 M HCl extracted the largest proportion of Cd (79 to 96% of total), Cu (61 to 83%), Pb (51 to 99%) and Zn (23 to 52%) from soils followed by 0.1 M HCl, NH4Cl, DTPA and CaCl2. In all the extractants, the proportion of extractability of metals was higher in the contaminated soils than the non-contaminated soils. Regardless of soils and extractants, relative extractability was higher for Cd as compared to other three metals. The use of 1 M HCl may be recommended for first-level screening of soil contamination with heavy metals. The other four weak extractants are believed to provide a better assessment of bioavailable/mobile metals content in soils than 1 M HCl extractant. However, 0.1 M HCl mobilized all four metals irrespective of soil types, therefore, might be the best choice if only one extractant is to be used. The sequential extraction procedures showed 22 to 64% of total Cd was in the mobile fraction (sum of F1 to F3), while the corresponding values for Cu, Pb and Zn in this fractions were 2 to 23% suggesting higher mobility of Cd than other three metals. The single extraction procedures are simple and easy to perform and obtained results are comparable with sequential extraction procedure.