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

Interactions between oxide minerals and natural organic matter affect metal adsorption properties of mineral surfaces, but the mechanisms of metal bon ding are not well understood. Extended X-ray absorption fine structure (EXA FS) spectroscopy analysts were performed on aqueous pastes containing Cu(II ) and goethite (alpha -FeOOH) with humic acid adsorbed at 0, 14, 78, 57, 88 , 216, and 136 g kg(-1) goethite, and with aqueous suspensions of humic aci d 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. Sam ples were equilibrated at pH 5.6 in a 0.1 M NaNO3 background electrolyte. F or all systems, analysis of EXAFS results suggests that Cu(II) is present i n a distorted octahedral configuration containing four short equatorial (1. 94-1.97 Angstrom) and two longer axial bonds with oxygen. When the concentr ation 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 Angstrom, which wa s less than for individual humic acid (2.32 +/- 0.02 Angstrom) or goethite (2.29 +/- 0.03 Angstrom) samples. The apparent decrease in the axial Cu-O b ond length was attributed to a decrease in the ligand field splitting energ y. When humic acid ligands replace equatorial water molecules in the Cu(II) coordination sphere, a weaker ligand field strength occurs. For absorbed h umic acid up to 88 g kg(-1) goethite, second-shell iron neighbors were obse rved between 3.17 and 3.20 Angstrom, suggesting that Cu(II) was bonded on a verage to both inorganic (goethite) and organic (humic acid) functional gro ups. In addition. derivative X-ray absorption near edge structure (XANES) s pectra for Cu(II) on goethite-humate complexes (<88 g kg(-1)) were poorly f it by use of a linear combination of spectra for Cu(II) on goethite or huma te alone. At humate concentrations between 216 and 236 g kg(-1) goethite, s econd-shell iron neighbors could not be identified, and the Cu-O-axial dist ance (2.32 <plus/minus> 0.02 Angstrom) and derivative XANES spectra strongl y resembled that of Cu(II) adsorbed to humate only. Analysis of the XANES a nd 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 lev els. Copyright (C) 2001 Elsevier Science Ltd.