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Trace Metal Speciation in Three Unbuffered Salt Solutions Used to Assess their Bioavailability in Soil

INRA, Laboratoire d'Analyses des Sols, 273 rue de Cambrai, F-62000 Arras, France.

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ABSTRACT

Three unbuffered salt solutions used to estimate soil trace metal bioavailability were compared. The objective was to evaluate chemical speciation of Cd, Cu, Pb, and Zn in 0.01 M CaCl₂, 0.1 M NaNO₃, and 1 M NH₄NO₃ extracts. The metal in each extract were speciated with a model that was optimized experimentally using an exchange resin. In NH₄NO₃, Cd, Cu, and Zn were completely complexed by NH₃. In extracts of pH lower than 6.5, Pb was essentially present as free Pb²⁺ and metal-organic complexes. In extracts of higher pH, PbCO₃ and free Pb²⁺ were dominant. In CaCl₂, Cu and Zn were mainly present as organic complexes and free Zn²⁺, respectively. In all extracts, 50% of Cd was free Cd²⁺ and 50% CdCl⁺. As pH increased, the proportions of free Pb²⁺ and organic complexes decreased while those of inorganic complexes increased. In NaNO₃, Cd, Cu, and Zn were, respectively, mainly present as free Cd²⁺, organic complexes and free Zn²⁺. Speciation of Pb changed with pH as in NH₄NO₃. Similarities were expected between the qualitative composition of the extracts and the soil solution, justifying the use of unbuffered salt solutions, particularly NaNO₃ and CaCl₂, in the evaluation of soil trace metal availability.

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