Complexation of zinc in organic soils--EXAFS evidence for sulfur associations

Even if it is generally accepted that associations with natural organic matter (NOM) to a great extent determine the bioavailability and mobility of trace metals in soils and waters, the knowledge about the identity of NOM functional groups involved is still limited. In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry of zinc (Zn) in two organic soils (500-10,000 microg Zn g(-), pH 5.6-7.3). In both soils Zn was coordinated by a mixture of oxygen/nitrogen (O/N) and sulfur (S) ligands in the first coordination shell. In average, 0.4-0.9 S atoms were located at a distance of 2.29-2.33 angstom, well in agreement with a 4-fold coordination with thiolates (RS-) in proteins. In addition 2.7-3.7 O/N atoms were located at 1.99-2.04 angstrom. The improved merit of fit by inclusion of S atoms was shown to be significant after adjusting for the improvement caused merely by increasing the number of fitting parameters. Two second shell Zn-C distances were used in our model: 3.0-4.2 carbon (C) atoms, associated to first shell O/N, were encountered at an average distance of 2.84 amgstrom, and 0.4-0.9 C atoms, associated to first shell S, were encountered at an average distance of 3.32 angstrom. These Zn-C distances are well in agreement with distances determined in well-defined organic molecules. It is concluded that Zn forms mainly inner-sphere complexes with a mixture of 4-fold coordination with S and O/N ligands and 6-fold coordination with O ligands in organic soils.