Adsorption-solubility equilibria and speciation of Pb, Cd, and Zn in a savanna soil
Emission of contaminants into the environment is a common feature of industrialization. The danger of these substances has generated interest in their solubility and speciation. In this study, the nature of adsorption, solubility and speciation of Pb, Cd and Zn under high levels of accumulation were evaluated through a model sorption-solubility experiment. Equilibrium concentrations of metals and other soil characteristics were used as input parameters in a metal speciation model. The solubility controls of metal activities were evaluated through the use of activity/stability diagrams. The solubility of Pb, Cd and Zn was enhanced between pH 4.8 and 6.6. Increased solubility at this pH range indicates higher bioavailability and toxicity. Speciation results showed a predominance of free metal species in the soil solution. The activity diagrams used to test the stability of possible mineral bearing compounds in the soils did not show any particular solid phase as possible control of the activities of Pb, Cd and Zn in solution. This suggests that solubility controls might be due to the formation of metastable minerals which are not well crystallized.
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