ABSTRACT
The products of aqueous Zn(II) sorption on high-surface-area alumina powders (Linde-A) have been studied using XAFS spectroscopy as a function of Zn(II) sorption density (Gamma=0.2 to 3.3 µmol/m(2)) at pH values of 7.0 to 8.2. Over equilibration times of 15-111 h, we find that at low sorption densities (Gamma=0.2-1.1 µmol/m(2)) Zn(II) forms predominantly inner-sphere bidentate surface complexes with AlO(6) polyhedra, whereas at higher sorption densities (Gamma=1.5 to 3.5 µmol/m(2)), we find evidence for the formation of a mixed-metal Zn(II)-Al(III) hydroxide coprecipitate with a hydrotalcite-type local structure. These conclusions are based on an analysis of first- and second-neighbor interatomic distances derived from EXAFS spectra collected under ambient conditions on wet samples. At low sorption densities the sorption mechanism involves a transformation from six-coordinated Zn-hexaaquo solution complexes (with an average Zn-O distance of 2.07 Å) to four-coordinated surface complexes (with an average Zn-O distance of 1.97 Å) as described by the reaction identical withAl(OH(a))(OH(b))+Zn (H(2)O)(6)(2+)--> identical withAl(OH(a)') (OH(b)')Zn(OH(c)')(OH(d)'+4H(2)O+zH(+), where identical withAl(OH(a))(OH(b)) represents edge-sharing sites of Al(O,OH,OH(2))(6) octahedra to which Zn(O,OH,OH(2))(4) bonds in a bidentate fashion. The proton release consistent with this reaction (z=a-a'+b-b'+4-c'-d'), and with bond valence analysis falls in the range of 0 to 2 H(+)/Zn(II) when hydrolysis of the adsorbed Zn(II) complex is neglected. This interpretation suggests that proton release is likely a strong function of the coordination chemistry of the surface hydroxyl groups. At higher sorption densities (1.5 to 3.5 µmol/m(2)), a high-amplitude, second-shell feature in the Fourier transform of the EXAFS spectra indicates the formation of a three-dimensional mixed-metal coprecipitate, with a hydrotalcite-like local structure. Nitrate anions presumably satisfy the positive layer charge of the Al(III)-Zn(II) hydroxide layers in which the Zn/Al ratio falls in the range of 1 : 1 to 2 : 1. Our results for the higher Gamma-value sorption samples suggest that Zn-hydrotalcite-like phases may be a significant sink for Zn(II) in natural or catalytic systems containing soluble alumina compounds. Copyright 2000 Academic Press.
ABSTRACT
The uptake and release behavior of cobalt(II) was studied over thousands of hours in CO(2)-free aqueous suspensions of kaolinite under three pairs of total cobalt concentration (Co(T)) and near-neutral pH (7.5-7.8) conditions. Dissolved cobalt, aluminum, and silicon concentrations were monitored by ICPMS, and cobalt-containing products were identified by EXAFS spectroscopy. In each uptake experiment, cobalt sorbed to kaolinite as a mixture of surface-adsorbed monomers or polymers and hydrotalcite-like precipitates of the approximate composition Co(x)Al(OH)(2x+2)(A(n-))(1/n), where 2
ABSTRACT
Thecomposition and mode of attachment of Cu(II) complexes at the gamma-Al(2)O(3)-water interface in suspensions containing a simple amino acid (glutamate) were characterized with EXAFS and FTIR spectroscopies. The spectroscopic results indicate that two types of Cu(II)-glutamate-alumina interactions are primarily responsible for Cu(II) and glutamate uptake between pH 4 and 9. In acidic suspensions of alumina, glutamate forms a bridge between Cu(II) ions and the (hydr)oxide surface (Type B complex). In this Type B surface complex, Cu(II) is bonded to amino acid headgroups (i.e., (+)H(3)NCHRCOO(-)) of two glutamate molecules. Spectroscopic and ionic strength dependent uptake results are combined to propose that the nonbonded side chain carboxylate groups of this complex are attracted to the oxide surface through long-range forces, leading to enhanced Cu(II) uptake relative to the glutamate-free system. In alkaline suspensions the relative amount of surface-bound Cu(II) complexed by glutamate decreases, and a direct Cu(II)-surface bond becomes the dominant mode of attachment (Type A complex). These surface complexes differ markedly from the species found in the alumina-free Cu(II)-glutamate aqueous system under similar solution conditions, where Cu(H(2)O)(2+)(6) and Cu(glutamate)(2-)(2) are the dominant species in acidic and alkaline solutions, respectively. Based on these spectroscopic results, surface complexation reactions are proposed for the Cu(II) and glutamate ternary interactions with the alumina surface in this system. Similarities between the results of this study and Cu(II) uptake behavior and complexation in the presence of natural organic material (NOM) indicate that Cu(II)-glutamate interactions mimic those in more complex Cu(II)-NOM-mineral-water systems. Copyright 1999 Academic Press.
ABSTRACT
We have examined the molecular-scale details of aqueous Co(II) surface complexes and the types of surface sites to which these complexes bind on the (110) and (001) surfaces of single-crystal TiO(2) (rutile) using polarization-dependent grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy under ambient conditions in a humid atmosphere. On both surfaces, Co(II) adsorbs at sites corresponding to Ti-equivalent positions in an extension of the rutile structure. This result suggests that even if different crystallographic surfaces of metal oxides have strongly differing adsorption properties for gaseous species in ultra-high vacuum, they can have similar properties for adsorption of metal ions in aqueous solution, probably due to the tendency of liquid water to heal defects and satisfy the bonding requirements of coordinatively unsaturated surface atoms. Using a bond valence approach in combination with the XAFS results, we have proposed specific surface reactions for sorption of Co(II) on the (110) and (001) rutile surfaces as a function of pH and Co surface concentration. No evidence was found for well-ordered Co(II)-hydroxide-like precipitates that would show Co-Co pair correlations, or for Co-Ti pair correlations similar to those in anatase, as have been observed in other studies. These results demonstrate the utility of GI-XAFS spectroscopy on adsorbed metal ions at submonolayer surface coverages for determining the types of reactive sorption sites on metal oxide surfaces. Copyright 1999 Academic Press.
ABSTRACT
We have characterized the adsorption of Co(II) on the (0001) and (1102) surfaces of alpha-Al(2)O(3) single crystals under ambient conditions using polarization-dependent grazing-incidence X-ray absorption fine structure spectroscopy, in combination with bond valence modeling. Co(II) ions were found to be adsorbed on both surfaces in an inner-sphere fashion. Adions were found to adsorb dominantly in a tridentate fashion (i.e., bonded to three surface oxygens) on the (0001) surface and dominantly in a tetradentate fashion on the (1102) surface. Based on EXAFS results and bond valence analysis, plausible surface complexation reactions for Co(II) sorption on these two surfaces can be written as represent surface water molecules, hydroxyl groups, and oxygens bonded to one, two, and three Al cations, respectively. Copyright 1999 Academic Press.
ABSTRACT
We have used X-ray absorption fine structure (XAFS) spectroscopy to investigate Cu(II) sorption complexes on high surface area amorphous silica (am-SiO2) and gamma-Al2O3. For Cu(II) on gamma-Al2O3, analysis of XAFS data collected after a solution-solid total contact time of 80-170 h showed that monomeric Cu(II) species predominate at surface coverages of 0.007 and 0.05 µmol m-2. Cu(II) on the gamma-Al2O3 surface has aluminum second neighbors at about 2.8 Å. Geometrical considerations indicate that this distance is consistent with models of Cu(II) binding to the gamma-Al2O3 surface in inner-sphere bidentate or monodentate modes on Al(O,OH)6 octahedra. For Cu(II) sorbed on am-SiO2, analysis of XAFS data collected after a solution-solid total contact time of 80-110 h showed that a dimeric Cu(II)-surface complex predominates, along with a minority monomeric Cu(II) surface species, at 0.03 and 0.05 µmol m-2. The XAFS-derived Cu-Si distance is in the range of 2.98 to 3.05 Å, suggesting that Cu(II) binds to am-SiO2 in an inner-sphere, monodentate fashion. XAFS spectra of Cu(II) sorbed on am-SiO2 collected after 20-30 h total contact time are quantitatively different from those collected after 80-90 h contact time. Analyses of these spectra indicate that the ratio of dimeric to monomeric Cu(II) surface complexes has increased with contact time. A discussion of the processes responsible for the different Cu(II) complexes on am-SiO2 and gamma-Al2O3 is presented. Copyright 1998 Academic Press.
ABSTRACT
Surface precipitation is an important process in many areas of science and technology, including modeling contaminant segregation from groundwater to solid phases and dispersion of active phases on catalyst supports. XAFS, TEM, and XPS measurements of Co(II) sorbed on Al2O3 demonstrate that surface precipitates have formed from solutions that are undersaturated with respect to any known bulk solid phase. The precipitates have a structure similar to that of Co(OH)2(s), but are disordered and have a high concentration of Co vacancies. The data plus thermodynamic reasoning have been used to analyze the plausibility of various models for surface precipitation and to show that for Co(II)/Al2O3 it occurs by forming a double-hydroxide phase containing substrate-derived Al(III) ions. This idea was corroborated by mixing aqueous solutions of Al(III) and Co(II) at the pH and concentration of the sorption samples, forming a stable colloidal precipitate that is less soluble than either Al(OH)3 or Co(OH)2. The Co XAFS of this material was similar to that of the sorption samples. Successful quantitative models of metal ion transport in groundwater need to include the possibility of forming ternary and higher order precipitates that include ions derived from sparingly soluble solids. For catalyst impregnation, surface coprecipitation can prevent production of a well-dispersed precursor material.
ABSTRACT
The structures and compositions of Pb(II) adsorption complexes and surface binding sites on alpha-Al2O3 (0001) and (1&1macr;02) surfaces were investigated in the presence of water using grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy. Pb(II) ions were found to adsorb in an inner-sphere mode on alpha-Al2O3 (1&1macr;02) but as outer-sphere complexes on alpha-Al2O3 (0001). The distance between the outer-sphere complexes and the surface places useful constraints on double-layer properties of water. A bond-valence model is described that relates the reactivities of surface functional groups and adsorption complexes to their molecular structures and compositions, and places constraints on the stoichiometries of adsorption reactions, including proton release. The EXAFS and modeling results suggest that Pb(II) and Co(II) ions bond to [AlAlAl-->O-1/2--> and [Al-OH+1/22] surface functional groups. In contrast, [AlAl > OH] groups complex Co(II) but not Pb(II). The results indicate the importance of using structurally defined surface sites to describe reactions at oxide-water interfaces.
