Metal ions speciation in a soil and its solution:: experimental data and model results

The bioavailability and the potential toxicity of heavy metals in the environment depend on their speciation in the soil and the soil solution. The analytical task is rendered difficult because the individual chemical species are often present at nano- and picomolar concentration in natural systems. To determine the free metal ion (FMI) concentration, both analytical and computational approaches were used. The recently developed Donnan Membrane Technique (DMT) succeeded in measuring the free metal ion concentration. In a first analytical step, some Zn smelter impacted topsoil (0-5 cm) columns were linked to the DMT cell and metal concentrations were measured at different pHs. The measured free metal concentrations in the soil solution were compared to those obtained from model calculations. In the second part, metal concentrations in the soil solution are predicted with the soil major constituents characteristics. These combined approaches allow a good description of the soil solution chemistry. Cd and Zn are mainly in solution as free aquo-ion that will migrate easily. The speciation of Pb and Cu is regulated by the dissolved organic matter. Their transport will be controlled by organic matter mobility. Cd, Zn and Pb speciation in the soil is controlled by organic matter, metallic oxides (Fe, Mn) and newly formed Zn-bearing minerals. All these calculated speciation and distribution are in reasonable agreement with EXAFS data previously published for the same soil sample. (C) 2002 Elsevier Science B.V All rights reserved.