METAL FRACTIONATION IN SEDIMENTS: A COMPARATIVE ASSESSMENT OF FOUR SEQUENTIAL EXTRACTION SCHEMES

Four sequential chemical extraction schemes for metal analysis (a 4-step modified protocol originally proposed by the Standards, Measurements and Testing Programme (SM&T - formerly BCR) of the European Union, Tessier and Hall 5-step procedures and 6-step Grimalt's scheme) were compared. Each procedure was applied to four sediment samples collected from two rivers, Nakagawa (NR) and Tamagawa (TR), Tokyo, Japan. Additionally, a certified marine sediment reference material (JMS-1) was also extracted for the comparison. To evaluate the partitioning of metals among different geochemical forms, the concentration of chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) were measured by Zeeman-corrected atomic absorption spectrometry. Other major elements and mineralogy of sediments were also determined on bulk prior to extracting by X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis, respectively, which allowed qualitative correlation between the fractionation results obtained and the presence of defined geochemical phases. The total concentrations of metals were determined after strong acid attack and the analytical precision was verified by using JMS-1. Significant proportions of elements were found in non-residual fractions among the schemes (average 83, 91, 76, 93 and 59% for Cr, Cu, Pb, Zn and Ni, respectively) in the more anthropogenically impacted NR sediments. The highest mobility was observed for Pb and Zn among the four schemes while the lowest was for Cr and Ni. Results of the comparison among the total metal levels for the four schemes based upon the sum of sequential extraction steps for each element showed significant differences for Cr, Ni and Zn. On the other hand, the sum of non-residual phases of the four schemes showed no significant differences for Cr, Cu and Zn but the SM&T and Hall schemes showed significant differences from others for Pb and Ni, respectively. Generally, good recoveries were achieved with the Hall scheme (range 92-109%) and the Fe-Mn oxides occlued phase of this scheme was the most effective owing to the high tendency to extract metals. In addition, Grimalt scheme produced the lowest recovery of Cr and the concentrations of residual fraction of other elements were also lower.