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 Cu²⁺-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., Cu²⁺ or other cations). The CuO₆ octahedron was used to model the inner complexation shell of the Cu²⁺-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 Cu²⁺- tetrahydrofurantetracarboxylic acid complex from those of the Cu²⁺-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 all Cu²⁺-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 Cu²⁺-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 Cu²⁺ by HS.