| Abstract
| - It is widely accepted that the bioavailability, toxicity, andmobility of trace metals are highly dependent on complexationreactions with functional groups in natural organicmatter (NOM). In this study, the coordination chemistry ofCd in NOM was investigated by extended X-ray absorptionfine structure spectroscopy. Soil organic matter (SOM) fromdifferent types of organic soils and dissolved organicmatter (DOM) from an organic and a mineral soil horizonof a Spodosol and aquatic DOM from Suwannee River wereinvestigated. 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 adistance of 2.49−2.55 Å and by 3.0−4.5 O/N atoms at adistance of 2.22−2.25 Å. In DOM samples (1750−4250 μgof Cd g-1, pH 5.4−6.3), Cd was coordinated by 0.3−1.8S atoms at a distance of 2.51−2.56 Å and 3.6−4.5 O/N atomsat a distance of 2.23−2.26 Å. In both SOM and DOMsamples a second coordination shell of 1.7−6.0 carbonatoms was found at an average distance of 3.12 Å. Thisis direct evidence for inner-sphere complexation of Cd byfunctional groups in NOM. Furthermore, ion activitymeasurements showed that less than 1% of total Cd wasin the form of free Cd2+ in our samples. Bond distancesand coordination numbers suggest that Cd complexed inSOM and DOM is a mixture of a 4-coordination with S (thiols)and 4- and 6-coordinations with O/N ligands. Given thatCd−S associations on average are stronger than Cd−O/Nassociations, our results strongly indicate that reducedS ligands are involved in the complexation of Cd by NOMalso at native concentrations of metal in oxidized organic-rich soils and in humic streams.
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