Sequential fractionation of chromium and nickel from some serpentinite‐derived soils from the eastern Transvaal

Abstract

Soils derived from ultramafic serpentinitic rocks are inherently infertile. These soils support plant species that are able to hyperaccumulate both chromium (Cr) and nickel (Ni). This study was conducted to determine the efficacy of a sequential extraction technique in explaining sources of Cr and Ni that are taken up by plant species growing on these soils. The sequential extraction of soil samples obtained from the eastern Transvaal involved the following reagents: H₂O and 0.5M KNO₃, 0.5M NaOH, 0.05M Na₂EDTA, and 4M HNO₃. More than 95% of the total Cr was extracted by HNO₃ while the remaining extractants fell into the order NaOH > EDTA ≫ KNO₃ + H₂O. There would appear to be a loose correlation between easily soluble Cr (KNO₃ + H₂O) and the uptake of Cr by the plant. A somewhat higher proportion of Ni was extracted prior to the HNO₃ treatment although amounts removed by KNO₃ + H₂O were all less than 1% of the total. It would appear that plant species growing on these soils are able to accumulate these elements from sources other than those considered easily available. A highly significant coefficient of determination was obtained between Ni extracted by oxalate and EDTA extractable. The fraction extracted by the steps in the sequential procedure can be related to exchangeable and sorbed (KNO₃ and H₂O) and an easily acid soluble inorganic fraction (HNO₃). The NaOH and EDTA fractions are probably related to the Cr and Ni bound in the form of organic complexes and associated with iron oxides.

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Department of Agronomy, University of Natal, P. O. Box 375, Pietermaritzburg, 3200, South Africa