Outer-sphere Adsorption Research Articles

X-ray Absorption Spectroscopic Investigation of Arsenite and Arsenate Adsorption at the Aluminum Oxide–Water Interface

We investigated the As(III) and As(V) adsorption complexes forming at the γ-Al2O3/water interface as a function of pH and ionic strength (I), using a combination of adsorption envelopes, electrophoretic mobility (EM) measurements, and X-ray absorption spectroscopy (XAS). The As adsorption envelopes show that (1) As(III) adsorption increases with increasing pH and is insensitive to I changes (0.01 and 0.8 M NaNO3) at pH 3–4.5, while adsorption decreases with increasing I between pH 4.5 and 9.0, and (2) As(V) adsorption decreases with increasing pH and is insensitive to I changes at pH 3.5–10. The EM measurements show that As(III) adsorption does not significantly change the EM values of γ-Al2O3 suspension in 0.1 M NaNO3 at pH 4–8, whereas As(V) adsorption lowered the EM values at pH 4–10. The EXAFS data indicate that both As(III) and As(V) form inner-sphere complexes with a bidentate binuclear configuration, as evidenced by a As(III)–Al bond distance of ≅3.22 Å and a As(V)–Al bond distance of ≅3.11 Å. The As(III) XANES spectra, however, show that outer-sphere complexes are formed in addition to inner-sphere complexes and that the importance of outer-sphere As(III) complexes increases with increasing pH (5.5 to 8) and with decreasing I. In short, the data indicate for As(III) that inner- and outer-sphere adsorption coexist whereas for As(V) inner-sphere complexes are predominant under our experimental conditions.

Influence of Organic Solvents on the Kinetics of Electron Transfer and the Adsorption at Highly Oriented Pyrolytic Graphite

Acetonitrile and several other common organic solvents have been found to dramatically influence the kinetics of electron transfer and the adsorption at freshly cleaved highly oriented pyrolytic graphite (HOPG). Ferricyanide and anthraquinone-2,6-disulfonate were used to investigate the effect of several pretreatment procedures including cleaving, electrochemical pretreatment, and thermal etching on the activity of basal plane HOPG. The results of this work suggest that although active sites produced by cleaving are relatively stable in air and in aqueous solution, exposure to organic solvents quickly and irreversibly passivates them toward simple outer-sphere electron transfer and adsorption. In contrast, electrochemically pretreated and thermally etched HOPG is much less sensitive to organic solvents, suggesting that the natures of the active sites produced by these methods are significantly different. As a result, it is important to take into account this behavior when using freshly cleaved HOPG in surface and electrochemical studies.

Outer-sphere adsorption of Pb(II)EDTA on goethite

Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAPS) spectroscopic measurements were performed on Pb(II)ethylenediaminetetraacetic (EDTA) adsorbed on goethite as a function of pH (4–6), Pb(II)EDTA concentration (0.11–72 μM), and ionic strength (16 μM–0.5 M). FTIR measurements show no evidence for carboxylate-Fe(III) bonding or protonation of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS spectroscopic measurements suggest that EDTA acts as a hexadentate ligand, with all four of its carboxylate and both of its amine groups bonded to Pb(II). No evidence was observed for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA2−. Because substantial uptake of PbEDTA(II)2− occurred in the samples, we interpret that Pb(II)EDTA2− adsorbed as outer-sphere complexes and/or as complexes that lose part of their solvation shells and hydrogen bond directly to goethite surface sites. We propose the term “hydration-sphere” for the latter type of complexes because they should occupy space in the primary hydration spheres of goethite surface functional groups and to distinguish this mode of sorption from common structural definitions of inner- and outer-sphere complexes. The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal/ligand-promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precursor species.

The Use of XAFS to Distinguish between Inner- and Outer-Sphere Lead Adsorption Complexes on Montmorillonite

Adsorption mechanisms of Pb on montmorillonite were investigated by conducting equilibrium and X-ray absorption fine structure (XAFS) spectroscopy studies. Data from the batch equilibrium studies indicate that Pb could be adsorbing via two mechanisms, depending on ionic strength. At low ionic strength (I = 0.006 M) Pb adsorption is pH-independent: 97% of the available Pb was removed from solution at pH 4.42 and 100% at pH 8.0. This behavior is consistent with an outer-sphere complexation mechanism. At high ionic strength (I = 0.1 M) Pb adsorption is pH-dependent, suggesting inner-sphere complexation as the adsorption mechanism: 43% of the available Pb was removed from the solution at pH 4.11 and 98.9% at pH 7.83. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy results reveal that in the sample equilibrated at I = 0.006 M, pH 4.48–6.40 the local atomic structure (LAS) surrounding the adsorbed Pb is similar to the LAS surrounding Pb2+ (aq), confirming that the adsorption mechanism is outer-sphere complexation. In the system equilibrated at I = 0.1 M, pH 6.77 the XANES and EXAFS results show that the LAS surrounding the adsorbed Pb atom is similar to the LAS surrounding reference compounds in which Pb is forming covalent bonds (Pb4(OH)4+4 (aq) and a sample of γ-Al2O3 with Pb adsorbed via inner-sphere complexation). These similarities indicate that Pb is forming inner-sphere complexes on the montmorillonite at this ionic strength and pH. In samples equilibrated at I = 0.006 M, pH 6.77 and I = 0.1 M, pH 6.31 the XAFS results suggest that Pb is forming both inner- and outer-sphere adsorption complexes. This observation could not be distinguished by making macroscopic observations only. Thus, the results of this study reveal important information on Pb sorption behavior on clays and also provides insights into the use of XAFS to determine sorption mechanisms.

Molecular orbital calculations for modeling acetate-aluminosilicate adsorption and dissolution reactions

Possible molecular configurations of acetic acid and acetate adsorbed onto aluminosilicate minerals are examined. Molecular orbital calculations were performed on molecules and dimers that are intended to mimic inner sphere and outer sphere adsorption complexes on mineral surfaces. The results predict the structure, energetics, and vibrational spectra of the acetic acid and acetate bonded to aluminosilicate groups. The most likely surface complexes are determined by reaction energetics and comparison of theoretical to experimental vibrational spectra. In addition, a reaction pathway of Si0AI cleavage by acetic acid and chemisorption of acetate with tetrahedral Al 3+ is predicted. An activation energy for this reaction is estimated from constrained energy minimizations of the reactants along a reaction pathway.

Boron Adsorption Mechanisms on Oxides, Clay Minerals, and Soils Inferred from Ionic Strength Effects

Boron Adsorption Mechanisms on Oxides, Clay Minerals, and Soils Inferred from Ionic Strength Effects

Sensitivity of surface complexation modeling to the surface site density parameter

Previous research has shown that adsorption of many inorganic anions on soil mineral surfaces can be described equally well by chemical surface complexation models using either inner- or outer-sphere surface complexes. At the same time, goodness of fit of these models to adsorption data has been used to distinguish between inner- and outer-sphere adsorption mechanisms. In this study the ability of chemical surface complexation models to describe anion adsorption on goethite using both inner- and outer-sphere surface complexes was evaluated and found to be sensitively dependent on the value of the surface site density. Application of a goodness of fit criterion leads to the choice of an inner-sphere adsorption mechanism for small surface site densities and an outer-sphere adsorption mechanism for large values of this parameter. Experimentally determined values of surface site density vary by an order of magnitude depending upon the method used. It is suggested that uncertainty in the value of the surface site density parameter currently invalidates use of surface complexation models to predict anion adsorption mechanisms on soil mineral surfaces.