Chemical speciation of heavy metals in sandy soils in relation to availability and mobility

The environmental risk of heavy metals which are present in soil at a certain total content is highly dependent on soil properties. Chemical speciation is a comprehensive term for the distribution of heavy metals over all possible chemical forms (species) in soil solution and in the solid phase. The chemical behaviour of heavy metals depends among others on the quantity and type of sorption sites at the solid surface (e.g. organic matter) and environmental conditions (e.g. pH, competitors, complexes).In this thesis, emphasis is given to the effects of pH, solid and dissolved organic matter, inorganic complexation, and calcium competition on the speciation of the heavy metals cadmium and copper in sandy soils. For cadmium, inorganic complexation (e.g. by chloride) is of great importance whereas for copper organic complexation (dissolved organic matter) (DOC) is more important with respect to availability and mobility. Copper binding by dissolved organic matter (DOC) and by (soil) solid organic matter could be described well with the Non-Ideal Competitive Adsorption (NICA) model and with the Two Species Freundlich (TSF) model using parameters derived from purified humic acid.Both models accurately predicted the copper concentration at different depths in a field. Leaching experiments revealed that copper mobility depends on DOC concentration. DOC coagulation was described as a function of complexed cations such as aluminium, calcium, copper and protons. The long term effect of changes in soil pH, and solid and dissolved organic matter on copper mobility are illustrated using a dynamic soil (solution) composition balance approach. Understanding of the chemical behaviour of cadmium and copper makes it possible to estimate the availabile and mobile contaminant fraction from routine accessible parameters such as total content, pH and organic matter.