Complexation of copper(II) in organic soils and in dissolved organic matter - EXAFS evidence for chelate ring structures

Associations with functional groups of natural organic matter (NOM) are of great importance for bioavailability, toxicity, and mobility of trace metals in soils and waters. In this study, the coordination chemistry of copper, Cu(II), in organic soils and dissolved organic matter (DOM) from soils and streams was investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. In both soil organic matter (SOM) and DOM (990-11 000 mu g Cu g(-1) dry weight, pH 2.8-6.3), Cu(II)was coordinated by 4 oxygen/nitrogen (O/N) atoms at a distance of 1.92-1.95 angstrom in the first coordination shell. These four atoms are positioned in the equatorial plane of a Jahn-Teller distorted octahedron. In samples with a pH of 4.8-6.3, a second coordination shell with 2.0-3.8 C atoms was located at a distance of 2.76-2.86 angstrom. A significant improvement (19-39%) of the fit was obtained by including a third coordination shell with 2.0-3.8 O/C atoms involved in single scattering at an average distance of 3.69 angstrom and multiple scattering at an average distance of 4.19 angstrom. Our results provide evidence for inner-sphere complexation of Cu(II) in NOM and suggest that Cu(II) is complexed by either one or two five-membered chelate rings involving possible combinations of amino, carboxyl, or carbonyl functional groups. Ion activity measurements showed that less than 0.2% of total Cu was in the form of free Cu2+ in our samples at pH 4.8-6.3.