STUDIES OF THE NATURE OF CU2+ AND PB2+ BINDING SITES IN SOIL HUMIC SUBSTANCES USING X-RAY ABSORPTION SPECTROSCOPY

Abstract

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 #9, 2 O atoms at an average distance of 2.02 #9, and 4 C atoms at an average distance of 3.13#9 . Analysis of Pb-humate samples at pH 4, 5, and 6 yielded 4 O atoms at average distances between 2.46 #9 to 2.32 #9 and 2 C atoms at an average distance of 3.26 #9 for Pb. We interpret the presence of C atoms in the second atomic shell of the metal binding site as evidence that both Cu2+ 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.