AN X-RAY ABSORPTION FINE STRUCTURE (XAFS) STUDY OF LEAD COMPLEXES AT THE GIBBSITE AND PYROPHYLLITE SOLID/LIQUID INTERFACES

Alcacio, Tim E.

MS

University Of Delaware

1997

X-ray absorption fine structure spectroscopy was combined with macroscopic batch studies to investigate the molecular environment of Pb(II) at the solid/liquid interface of gibbsite and pyrophyllite. The XAFS spectra demonstrated that Pb(II) surface complexes on gibbsite differ significantly from those found on pyrophyllite. Pb(II) surface complexes (RPb-O=2.25-2.31 Å, NPb-O=2.18-2.21) on pyrophyllite were found coordinated in a bidentate fashion. Based on steric hindrances of Pb(II) corner sharing with silica tetrahedrons, and on the Pb-Si/Al coordination shell determined (RPb-Si/Al=3.29-3.32 Å, NPb-Si/Al=1.01-1.03), Pb(II) adsorption sites on pyrophyllite are expected to be comprised of aluminol functional groups, rather than silanol groups.

Gibbsite displayed both short and long Pb-O distances. Short Pb-O bond distances (RPb-O=2.25-2.31 Å, NPb-O=1.00-1.60) were attributed to monodentate Pb(II) surface complexes while long Pb-O (RPb-O=2.58-2.60 Å, NPb-O=6.25-6.27) distances were attributed to outer-sphere complexes. Second Pb neighboring atoms (RPb-O=3.77-3.78 Å, NPb-O=2.73-2.76) were also found on the gibbsite surface suggesting the presence of multinuclear complexes. The Pb-Pb coordination distances are comparable to Pb(II) hydroxy polymers and basic Pb(II) nitrate and thus cannot be discriminated. These results demonstrate that Pb(II) complexes found on gibbsite differ significantly from Pb(II) surface complexes on pyrophyllite, as well as other aluminum oxides found in the literature, e.g., corundum and alumina.