SPECTROSCOPIC INVESTIGATION OF PB(II) COMPLEXES AT THE GAMMA-AL2O3 WATER INTERFACE

We have characterized the structure and composition of aqueous Pb(II) complexes sorbed onto γ-Al2O3, in situ, using X-ray absorption spectroscopy. Qualitative comparison of X-ray absorption near edge structure (XANES) spectra from Pb(II) sorbed on γ-Al2O3, aqueous Pb(II) nitrate solution and crystalline Pb-O model compounds suggests that Pb is not sorbed as hexaaquo Pb(II) in an outersphere complex and does not form a three-dimensional solid precipitate or large multinuclear complex. Analysis of radial structure functions derived from extended X-ray absorption fine structure (EXAFS) spectra of sorbed Pb(II) (5 and 15 mM initial Pb concentrations or 0.4 and 1.3 μmmoles Pb/m2) yielded one O atom at 2.23 Å and two O at 2.46 Å. Further analysis of the 15 mM sorption sample yielded approximately one second-neighbor Pb atom at 3.45 Å and one second-neighbor Al atom at 3.72 Å, in addition to the three first-neighbor O ligands. These data suggest that Pb(II) bonds directly to the γ-A12O3 surface as an inner-sphere complex and rule out the possibility of outer-sphere complexes, surface precipitates, or diffusion into the adsorbent. The presence of Pb atoms in the second coordination shell suggests that at least some Pb is sorbed as small multinuclear complexes; the number or size of these complexes apparently increases with increasing surface coverage. Based on the presence of second-neighbor Al and the observed interatomic distances, we infer that the adsorption site is monodentate. © 1990.