Outer-sphere sorption

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

One approach commonly used to study how strongly an aqueous ion binds to a sohd surface is measurement of adsorption isotherms as a function of pH, ionic strength, and total metal-ion concentration, in the presence or absence of other ions or organic coadsorbents, ft is often assumed that significant inhibition of adsorption with increasing ionic strength at a given pH indicates that the sorption complexes are dominantly of the weakly bound, outer-sphere type [123]. In contrast, when there is... 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Aqueous radionuclide species and other solutes can sorb to mineral surfaces by forming chemical bonds directly with the amphoteric sites or may be separated from the surface by a layer of water molecules and be bound through longer-range electrostatic interactions. In the TLM, complexes of the former type are often called inner-sphere complexes those of the latter type are called outer-sphere complexes (Davis and Kent, 1990). The TLM includes an inner plane (o-plane), an outer plane (/8-plane), and a diffuse layer that extends from the /8-plane to the bulk solution. Sorption via formation of inner-sphere complexes is often referred to chemisorption or specific... 

Mineral or particle surfaces are enriched with As due to several processes that are collectively referred to as sorption (Parks, 1990), but the chemical properties of surface-associated As have been difficult to study directly. Outer-sphere, or physisorption, describes weak, long range, attractive forces between the surface and sorbing As inner-sphere, or chemisorption, refers to the formation of chemical bonds between the surface and adsorbing As. Stronger adsorption is expected by the formation of a bidentate (two bond) adsorbed complex rather than a monodentate (1 bond) complex. Selective chemical extraction methods have been useful for empirical determination of the dominant chemical/mineralogical compartments retaining As in aquifer... 

Figure I. Schematic representation of possibie arsenic sorption complexes on mineral surfaces (Modifiedfrom Brown, 1990). Outer-sphere (physisorbed) complexes, in which As is fully coordinated by water molecules, are bound to the mineral surface by weak electrostatic forces. Inner-sphere (chemisorbed) complexes are characterized by the formation of one or more chemical bonds between the sorbing As oxoanion and the mineral surface. Surface precipitation refers to the formation of a new phase on the mineral surface. Reprinted with permission.

Figure 3. Schematic representation of mechanisms responsible for ion sorption on charged mineral surfaces. Key O.S., outer sphere I.S., inner sphere.

Current surface complexation models were developed with a focus on minor and trace ions and hence do not consider sorption in the diffuse layer. Even the triple-layer model (34), which can include electrolyte sorption as outer-sphere complexes, does not consider sorption in the diffuse layer. To... 

The sorption mechanism of chromate is unclear. Zachara et al. (1989) suggested that chromate forms an outer-sphere complex on the surfaces of Fe and Al oxides. However, spectroscopic studies have shown that chromate forms inner-sphere complexes (both bidentate and monodentate) on goethite (Fendorf et al., 1997). This anion has a smaller shared charge than do arsenite and arsenate. 

In contrast to the transition metals, divalent alkaline earth metal ions typically sorb at pH values above the pHp/c. For these cations, sorption is believed to occur primarily through relatively weak ion pair formation (outer sphere complexes). The relative affinity to form complexes with surface hydroxyls can be correlated with... 

The above sequence has been observed in studies of alkaline earth adsorption on y-Al203 (Huang Stumm, 1973). The trend is also consistent with the expectation based on the expected preference of harder Lewis acids for hard Lewis bases like surface hydroxyls. Limited spectroscopic evidence is available for sorption of alkaline earth metals because many of these metals do not exhibit sufficiently high K-shell fluorescence energies to be studied in the presence of corundum and water using current EXAFS methods. Chen and Hayes (1999) have shown that Sr(II) sorbs to montmorillonite, illite, and hectorite primarily as a weakly associated outer-sphere complex. Similar findings have been reported for sorption of Sr(II) to clay minerals (Parkman et al., 1998 O Day et al., 2000 Sahai et ah, 2000). 

Thus, current versions of the model allow sorption of inner-sphere complexes directly to surface hydroxyls whereas outer-sphere complexes are located at the [3-plane. Recent modifications have included an extension to allow parameter estimation of surface site densities, surface acidity constants and site densities using Bom solvation and crystal chemical theory (Sverjensky, 1993, 1994 Sverjensky Sahai, 1996 Sahai Sverjensky, 1997a,b) and to treat electrolyte ions as nonspecific adsorbing species that screen charge in the p-plane (Robertson Leckie, 1997). 

Modeling divalent metal ion sorption requires estimation of the proton stoichiometry (the number of protons released per metal ion sorbed), the type of surface complex (inner or outer sphere) formed, and the formation constants for each reaction selected. Table 7-2 presents a list of various reactions that may be incorporated into the TLM. Because a variety of combinations of different sorption reactions and constants may fit various aspects of the sorption data equally well (see, e.g., Westall Hohl, 1980 Hayes et al., 1991 Katz Hayes, 1995a), protocols are needed to insure the best choice of reactions and a more universally accepted set of guidelines to allow reproducibility from one laboratory to another. The strategy used in modeling Co(II) sorption to a-Al203 involved ... 

The final task in the evaluation of SCM for predicting sorption behavior was to assess its ability to predict sorption for a weakly sorbing (outer-sphere) divalent metal ion. Because previous studies reporting EXAFS results for Sr(II) sorption to aluminum oxides were not available in the literature prior to our modeling efforts, Sr(II) XAS data were collected as part of this work. The Sr(II) XAS data were collected for a strontium nitrate solution and the samples shown in Table 7-7. 

Fig. 7-14. TLM model calibration and predictions for outer sphere Sr(ll) to ot-ALOp (A) model calibration (optimization using FITEQL), (B) predictions of Sr(ll) sorption ionic strength dependency.

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