XANES Study of Cu 2+ -Binding Sites in Aquatic Humic Substances

The structure and composition of the inner shell of 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(H 2 O) 6 ] 2+ complex. These calculations reproduced the tetragonal distortion of the CuO 6 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.