EXAFS STUDY OF THE INNER-SHELL STRUCTURE IN COPPER(II) COMPLEXES WITHHUMIC 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 struc ture spectroscopy (EXAFS). This method is highly sensitive to the loca l 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 usin g model systems (copper(ll)-aqua and -salicylate complexes). On the ba sis of the dissimilarity of the EXAFS spectra for the Cu2+-tetrahydrof urantetracarboxylic acid complex from those of the Cu2+-HS complexes, it is concluded that complexation sites of HS are not likely to contai n carboxylic groups attached to a furan ring. The Cu-O distances in ai l Cu2+-HS complexes were shorter than those in aqua or salicylate comp lexes. The contraction of the Cu-O distance was especially prominent f or 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 bou nd with the central cation than those in either aqua or salicylic comp lexes, possibly indicating six-dentate coordination of Cu2+ by HS.