Molecular scale characteristics of Cu(II) bonding in goethite–humate complexes

Abstract

—Interactions between oxide minerals and natural organic matter affect metal adsorption properties of mineral surfaces, but the mechanisms of metal bonding are not well understood. Extended X-ray absorption fine structure (EXAFS) spectroscopy analyses were performed on aqueous pastes containing Cu(II) and goethite (α-FeOOH) with humic acid adsorbed at 0, 14, 28, 57, 88, 216, and 236 g kg −1 goethite, and with aqueous suspensions of humic acid only. Analyses were conducted at the Cu K edge with 40 mmol Cu(II) kg −1 goethite or 2 mmol Cu(II) kg −1 suspension for the humic acid system. Samples were equilibrated at pH 5.6 in a 0.1 M NaNO 3 background electrolyte. For all systems, analysis of EXAFS results suggests that Cu(II) is present in a distorted octahedral configuration containing four short equatorial (1.94–1.97 Å) and two longer axial bonds with oxygen. When the concentration of adsorbed humic acid on goethite was increased from 0 to 28 g kg −1, the axial Cu-O bond length decreased to 2.24 ± 0.03 Å, which was less than for individual humic acid (2.32 ± 0.02 Å) or goethite (2.29 ± 0.03 Å) samples. The apparent decrease in the axial Cu-O bond length was attributed to a decrease in the ligand field splitting energy. When humic acid ligands replace equatorial water molecules in the Cu(II) coordination sphere, a weaker ligand field strength occurs. For absorbed humic acid up to 88 g kg −1 goethite, second-shell iron neighbors were observed between 3.17 and 3.20 Å, suggesting that Cu(II) was bonded on average to both inorganic (goethite) and organic (humic acid) functional groups. In addition, derivative X-ray absorption near edge structure (XANES) spectra for Cu(II) on goethite–humate complexes (<88 g kg −1) were poorly fit by use of a linear combination of spectra for Cu(II) on goethite or humate alone. At humate concentrations between 216 and 236 g kg −1 goethite, second-shell iron neighbors could not be identified, and the Cu-O axial distance (2.32 ± 0.02 Å) and derivative XANES spectra strongly resembled that of Cu(II) adsorbed to humate only. Analysis of the XANES and EXAFS data suggested that Cu(II) was bonded on average to both inorganic and organic functional groups as a type A ternary complex at lower levels of adsorbed humate, and to organic groups as a type B complex at higher levels.