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

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 hioavailability . 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, FeMn 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. ONTAMINATION of heavy metals in the environment C is of major concern because of their toxicity and threat to human life and the environment (Purves, 1985). Much research has been conducted on heavy metal contamination in soils from various anthropogenic sources such as industrial wastes (Haines and Pocock, 1980; Parry et al., 1981; Culbard et al., 1983; Gibson and Farmer, 1983), automobile emissions (Lagerwerff and Specht, 1970; Fergusson et al., 1980; Garcia-Miragaya, 1984), mining activity (Davies and Ginnever, 1979; Culbard and Johnson, 1984), and agricultural practice (Colbourn and Thornton, 1978). Based on primary accumulation mechanisms in sediments, heavy metals can be classified into five categories: (i) adsorptive and exchangeable, (ii) bound to carbonate phases, (iii) bound to reducible phases (Fe and Mn oxides), (iv) bound to organic matter and sulfides, and (v) detrital or lattice metals (Salomons and Forstner, 1980). Heavy metals present in these categories have different remobilization behaviors under changing environmental conditions (Forstner, 1985). Geochemical forms of heavy metals in soil affect their solubilities, which directly influence their bioavailability (Xian, 1987). Therefore, determining total content of heavy metals is insufficient to assess the environmental impact of contaminated soils and sediments (Salomons and Forstner, 1980), because it is the chemical form that deterSoil and Water Science Dep., Univ. of Florida, Gainesville, FL 3261 10290. Received 17 Nov. 1995. *Corresponding author ([email protected].