EXAFS Study of the Inner Shell Structure in Copper(II) Complexes with Humic Substances

The structure of humic substances (HS) and their complexes with metal cations is a matter of ongoing debate. In this paper, the structure of Cu 2+ −HS complexes was studied by extended X-ray absorption fine structure spectroscopy (EXAFS). This method is highly sensitive to the local structure around the target (e.g., Cu 2+ or other cations). The CuO 6 octahedron was used to model the inner complexation shell of the Cu 2+ −HS complexes. The quality of the fit of the EXAFS data was tested using model systems (copper(II)−aqua and −salicylate complexes). On the basis of the dissimilarity of the EXAFS spectra for the Cu 2+ −tetrahydrofurantetracarboxylic acid complex from those of the Cu 2+ −HS complexes, it is concluded that complexation sites of HS are not likely to contain carboxylic groups attached to a furan ring. The Cu−O distances in all Cu 2+ −HS complexes were shorter than those in aqua or salicylate complexes. The contraction of the Cu−O distance was especially prominent for the axial direction, and the distortion of the inner shell was much reduced. The mean-square disorder values for the axial Cu−O pairs in Cu 2+ −HS complexes indicate that the axial oxygens are more tightly bound with the central cation than those in either aqua or salicylic complexes, possibly indicating six-dentate coordination of Cu 2+ by HS.