Trace metal speciation in three unbuffered salt solutions used to assess their bioavailability in soil

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 mere 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.