XANES study of Cu2+-binding sites in aquatic humic substances

The structure and composition of the inner shell df copper (II) complexes with aquatic humic substances (HS) were studied by X-ray absorption near-edge structure (XANES) spectroscopy. The interpretation of the XANES data was based on FEFF8 code real-space multiple-scattering calculations for the prototype [CU(H2O)(6)](2+) complex. These calculations reproduced the tetragonal distortion of the CuO6 octahedron representing the inner shell. They also demonstrated the polarization nature of this effect's manifestations ("splitting" of the derivative) in XANES. The extension of this interpretation to the Cu-HS system confirmed suppression of the tetragonal distortion of the inner shell in these species established by independent EXAFS experiments. XANES measurements for Cu/C ratios varying from 0.0005 to 0.03 demonstrated the nonuniform nature of copper binding sites. Experiments with Cu-ethylenediamine and Cu-glycine model systems and FEFF8 calculations indicated that for low copper concentrations (Cu/C < 0.005) nitrogen-containing functional groups dissimilar to those of amino acids are likely to be engaged in the complexation.