Kinetic Extractions of Nickel and Lead from Some Contaminated Calcareous Soils

The mobility of heavy metals in contaminated soils is dependent on the kinetics release from soils. Metal extraction over time is commonly used to distinguish two or more fractions of metal based on differences of extraction or release rates. Kinetic studies using 0.01 M CaCl₂, 0.01 M malic acid, and 0.01 M EDTA extractions were performed to characterize nickel (Ni) and lead (Pb) kinetic release in 10 contaminated calcareous soils. Proportions of Ni and Pb extracted with EDTA were higher than when using malic acid and CaCl₂, respectively. The release of Ni and Pb was characterized by an initial fast rate followed by a slower rate and could best be described by a two first-order reactions model with rate constants k₁ and k₂ and two metal pools: readily labile (Q ₁) and less labile (Q ₂). In an EDTA extractant, different Q₁ /Q₂ ratios for Ni and Pb were observed, indicating binding energies to soil constituents is not comparable. The k₁ of the model for Ni (average of 10 soils: 0.2204 h^?1 and 0.2359 h^?1 for 0.01 M CaCl₂ and 0.01 EDTA, respectively) was higher than Pb (0.1044 h^?1 and 0.1631 h^?1 for 0.01 M CaCl₂ and 0.01 M EDTA, respectively), indicating a higher potential of Ni for leaching and groundwater contamination in contaminated calcareous soils. Relationships between the fraction of Ni and Pb determined with the two first-order reactions model and the soil composition and Pb fractions were established. The results indicated that the efficiency of the extractions Ni and Pb from soils depends both on the Ni and Pb content and soil composition. Overall, the results indicated that the use of a 0.01 M malic acid washing solution would be preferred in the field condition.