Chemical Fractionation of Cadmium, Copper, Nickel, and Zinc in Contaminated Soils

TLDR

Heavy metals pose significant environmental and health risks, and their toxicity is governed by their chemical associations in soils. The study aimed to determine the chemical forms of Cd, Cu, Ni, and Zn in contaminated soils to assess their mobility and bioavailability. Sequential extraction partitioned the metals into six operational groups—water soluble, exchangeable, carbonate, Fe‑Mn oxide, organic, and residual—across nine soils. The residual fraction dominated for all metals, yet Zn (2.4–44% in the potentially available pool) and Cu (40–74% in organic, Fe‑Mn oxide, and carbonate fractions) were most mobile, with Cd and Ni less affected, yielding a mobility hierarchy of Zn > Cu > Cd > Ni, and metal distribution across fractions correlated with total concentrations except for Zn.

Abstract

Abstract Heavy metals are potentially toxic to human life and the environment. Metal toxicity depends on chemical associations in soils. For this reason, determining the chemical form of a metal in soils is important to evaluate its mobility and bioavailability. Sequential extraction was used to fractionate four heavy metals (Cd, Cu, Ni, and Zn) from nine contaminated soils into six operationally defined groups: water soluble, exchangeable, carbonate, Fe‐Mn oxide, organic, and residual. The residual fraction was the most abundant pool for all four metals examined. A significant amount (2.4–44%) of Zn was present in the potentially available fraction: nonresidual fraction. A major portion (40–74%) of Cu was associated with the organic, Fe‐Mn oxide, and carbonate fractions in most of the soils. Contamination of Cd and Ni in these soils was not as severe as Zn and Cu. Assuming that mobility and bioavailability of these metals are related to their solubility and geochemical forms, and that they decrease in the order of extraction sequence, the apparent mobility and potential bioavailability for these four metals in the soils were: Zn > Cu > Cd > Ni. Metal distributions in different chemical fractions in these soils depended on respective total metal concentrations, except for Zn.