Studies of the nature of Cu2+ and Pb2+ binding sites in soil humic substances using X-ray absorption spectroscopy

We have investigated the binding environments of Cu2+ and Pb2+ complexed by soil humic substances using synchrotron-based X-ray absorption spectroscopy. With the assistance of bond network analysis, analysis of X-ray absorption near edge structure (XANES) and radial structure functions derived from extended X-ray absorption fine structure (EXAFS) spectra of Cu-humate at pH 4, 5, and 6 yielded a tetragonally-distorted octahedral binding environment for Cu with 4 O atoms at an average distance of 1.94 (A) over circle, 2 O atoms at an average distance of 2.02 (A) over circle, and 4 C atoms at an average distance of 3.13 (A) over circle. Analysis of Pb-humate samples at pH 4, 5, and 6 yielded 4 O atoms at average distances between 2.46 (A) over circle to 2.32 (A) over circle and 2 C atoms at an average distance of 3.26 (A) over circle for Pb. We interpret the presence of C atoms in the second atomic shell of the metal binding site as evidence that both Cu(2+)and Pb2+ form innersphere complexes with soil humic substances. Within the pH range 4-6,there is no significant change in the structure of the binding sites for either Cu or Pb. Copyright (C) 1997 Elsevier Science Ltd