Determination of labile Cu in soils and isotopic exchangeability of colloidal Cu complexes

Summary Measurement of labile (isotopically exchangeable) pools of metals (E values) in soil is required to assess the size of metal pools potentially available to soil organisms, from both a micronutrient deficiency and metal toxicity viewpoint. In this paper, E values of soil Cu were measured by an isotope dilution technique using different solution extracts – water with and without resin purification, water coupled with Donnan dialysis (free ion determination) and 0.01 m CaCl2. Using these techniques, the isotopic exchangeability of Cu species in water extracts was investigated. The results showed that the specific activity of 64Cu in the water-soluble fraction was less than in the free metal ion fraction or in the fraction that adsorbed to resin. The isotopically non-exchangeable Cu in water extracts ranged between 4% and 40% of water-soluble Cu (16% on average), and appeared to be associated with dispersed colloids. The existence of isotopically non-exchangeable Cu in water extracts led to overestimation (17.7% on average) of isotopically exchangeable Cu in soils when based on the specific activity in water extracts. The method of isotope dilution coupled with resin extraction is recommended for the determination of isotopically exchangeable Cu in soils when water extracts are used.