Structures and compositions of lead(II) sorption products and surface sites on aluminum and iron oxides

Pb(II) sorption on {dollar}alpha{dollar}- and {dollar}gamma{dollar}-Al{dollar}rmsb2Osb3,{dollar} hematite ({dollar}alpha{dollar}-Fe{dollar}rmsb2Osb3){dollar} and goethite ({dollar}alpha{dollar}-Fe{dollar}cdot{dollar}OH) was studied as a function of pH (5-8), adsorption density and (Pb) {dollar}rmsb{lcub}T{rcub}{dollar} in 0.1 M NaNO{dollar}sb3{dollar} and 0.1 M NaCl using XAFS spectroscopy. Pb(II) adions on Al and Fe-oxides are hydrolyzed, having hydroxide ions and surface oxygens as ligands, which are arranged to form the bases of trigonal pyramids. On Fe-oxides, Pb(II) adsorbs preferentially as mononuclear bidentate complexes to edges of FeO{dollar}sb6{dollar} octahedra. At these sites, Pb(II) is complexed by (Fe{dollar}rmsbsp{lcub}Fe{rcub}{lcub}Fe{rcub}rightarrow{lcub}rm O{rcub}sp{lcub}-1/2{rcub}{dollar}) surface functional groups. On Al-oxides, Pb(II) adsorbs as bidentate edge-sharing and monodentate corner-sharing complexes. Both (Al{dollar}rmsbsp{lcub}Al{rcub}{lcub}Al{rcub}rightarrow{lcub}rm O{rcub}sp{lcub}-1/2{rcub}{dollar}) and (Al-OH{dollar}sp{lcub}-1/2{rcub}{dollar}) groups complex Pb(II). Multinuclear Pb(II) surface complexes were also observed on Al-oxides and are believed to have the composition Pb{dollar}rmsb2(OH)sb4,{dollar} but with three of the hydroxide ion ligands replaced by surface oxygens. On single-crystal {dollar}alpha{dollar}-Al{dollar}rmsb2Osb3{dollar} (0001), Pb(II) forms outer-sphere sorption complexes, which are hydrogen-bonded to the surface. Hence, ({dollar}rmsbsp{lcub}Al{rcub}{lcub}Al{rcub}>{dollar} OH) groups, which terminate {dollar}alpha{dollar}-Al{dollar}rmsb2Osb3{dollar} (0001), do not bond to Pb(II) under the experimental conditions. The differences in Pb(II) adsorption on Al- and Fe-oxides can be ascribed to the size differences of FeO{dollar}sb6{dollar} and AlO{dollar}sb6{dollar} octahedra.; A bond-valence description of the relative stabilities of Al- and Fe-oxide surface functional groups is proposed. It explains the reactivities of the different surface functional groups, and leads to constraints on the stoichiometries of Pb(II) adsorption.; The presence of Cl{dollar}sp-{dollar} has little detectable effect on Pb(II) adsorption complexes on goethite and Al{dollar}rmsb2Osb3{dollar} at pH 7. However, at pH {dollar}leq{dollar} 6, Pb(II)-chloro ternary complexes occur on goethite, which are attached to the surfaces through Pb(II)-O{dollar}rmsb{lcub}sfc{rcub}{dollar} and Cl{dollar}sp-{dollar}-Fe{dollar}rmsb{lcub}sfc{rcub}{dollar} bonds. No such complexes were observed on Al{dollar}rmsb2Osb3.{dollar} It was demonstrated that EXAFS but not XANES spectra can distinguish between inner-sphere Pb(II) and PbClOH precipitates present simultaneously in the Al{dollar}rmsb2Osb3{dollar} samples.