Studies of the nature of binding sites of first row transition elements bound to aquatic and soil humic substances using X-ray absorption spectroscopy

Abstract

The coordination environments of first row transition elements (Co, Ni, Cu, and Zn) complexed by aquatic and soil humic substances were studied using X-ray absorption spectroscopy (XAS). With the assistance of bond network analysis, analysis of X-ray absorption near edge structure (XANES) spectra and radial structure functions (RSF) derived from extended X-ray absorption fine structure (EXAFS) spectra of Co, Ni, Cu, and Zn in humic substances indicate an octahedral binding environment for Co, Ni, and Zn and a tetragonally-distorted octahedral binding environment for Cu. Analysis of EXAFS spectra of each element provides detailed information on the internuclear distances, bond angles, and Debye-Waller factors within the first two atomic shells surrounding Co, Ni, Cu, and Zn bound to humic substances. We interpret the presence of C atoms in the second atomic shell of the metal binding site as further evidence that Co, Ni, Cu, and Zn form innersphere complexes with humic substances. The number and type of ligands involved in the binding are different for different elements.