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

TLDR: In this article, the authors evaluated chemical speciation of Cd, Cu, Pb, and Zn in 0.01 M CaC l2 0.1 M NaNO 3, and 1 M NH 4 NO 3 extracts.

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 CaC l2 0.1 M NaNO 3 , and 1 M NH 4 NO 3 extracts. The metal in each extract were speciated with a model that was optimized experimentally using an exchange resin. In NH 4 NO 3 , Cd, Cu, and Zn were completely complexed by NH 3 . In extracts of pH lower than 6.5, Pb was essentially present as free Pb 2+ and metal-organic complexes. In extracts of higher pH, PbCO 3 and free Pb 2+ were dominant. In CaCl 2 , Cu and Zn were mainly present as organic complexes and free Zn 2+ , respectively. In all extracts, 50% of Cd was free Cd 2+ and 50% CdCl + . As pH increased, the proportions of free Pb 2+ and organic complexes decreased while those of inorganic complexes increased. In NaNO 3 , Cd, Cu, and Zn were, respectively, mainly present as free Cd 2+ , organic complexes and free Zn 2+ . Speciation of Pb changed with pH as in NH 4 NO 3 , Similarities were expected between the qualitative composition of the extracts and the soil solution, justifying the use of unbuffered salt solutions, particularly NaNO 3 and CaCl 2 , in the evaluation of soil trace metal availability.