Extended X-ray Absorption Fine Structure Spectroscopy Evidence for the Complexation of Cadmium by Reduced Sulfur Groups in Natural Organic Matter

It is widely accepted that the bioavailability, toxicity, and mobility of trace metals are highly dependent on complexation reactions with functional groups in natural organic matter (NOM). In this study, the coordination chemistry of Cd in NOM was investigated by extended X-ray absorption fine structure spectroscopy. Soil organic matter (SOM) from different types of organic soils and dissolved organic matter (DOM) from an organic and a mineral soil horizon of a Spodosol and aquatic DOM from Suwannee River were investigated. In SOM samples (1000−25000 μg of Cd g^-1, pH 4.6−6.6), Cd was coordinated by 1.0−2.5 S atoms at a distance of 2.49−2.55 Å and by 3.0−4.5 O/N atoms at a distance of 2.22−2.25 Å. In DOM samples (1750−4250 μg of Cd g^-1, pH 5.4−6.3), Cd was coordinated by 0.3−1.8 S atoms at a distance of 2.51−2.56 Å and 3.6−4.5 O/N atoms at a distance of 2.23−2.26 Å. In both SOM and DOM samples a second coordination shell of 1.7−6.0 carbon atoms was found at an average distance of 3.12 Å. This is direct evidence for inner-sphere complexation of Cd by functional groups in NOM. Furthermore, ion activity measurements showed that less than 1% of total Cd was in the form of free Cd²⁺ in our samples. Bond distances and coordination numbers suggest that Cd complexed in SOM and DOM is a mixture of a 4-coordination with S (thiols) and 4- and 6-coordinations with O/N ligands. Given that Cd−S associations on average are stronger than Cd−O/N associations, our results strongly indicate that reduced S ligands are involved in the complexation of Cd by NOM also at native concentrations of metal in oxidized organic-rich soils and in humic streams.