Surfaces inner-sphere complexes

If isomorphic substitution of Si(IV) by AI(III) occurs in the tetrahedral sheet, the resulting negative charge can distribute itself over the three oxygen atoms of the tetrahedron (in which the Si has been substituted) the charge is localized and relatively strong inner-sphere surface complexes (Fig. 3.10a) can be formed. 

We have argued that (inner-sphere) surface complex formation of a metal ion to the oxygen donor atoms of the functional groups of a hydrous oxide is in principle similar to complex formation in homogeneous solution, and we have used the same type of equilibrium constants. How far can we apply similar concepts in kinetics ... 

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

Surface spectroscopic techniques must be separated carefully into those which require dehydration for sample presentation and those which do not. Among the former are electron microscopy and microprobe analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy. These methods have been applied fruitfully to show the existence of either inner-sphere surface complexes or surface precipitates on minerals found in soils and sediments (13b,30,31-37), but the applicability of the results to natural systems is not without some ambiguity because of the dessication pretreatment involved. If independent experimental evidence for inner-sphere complexation or surface precipitation exists, these methods provide a powerful means of corroboration. 

A general formulation of the equilibrium, involving a monodendate, inner-sphere surface complex, and of the associated constants, is as follows ... 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

The constant-capacitance model (Goldberg, 1992) assigns all adsorbed ions to inner-sphere surface complexes. Since this model also employs the constant ionic medium reference state for activity coefficients, the background electrolyte is not considered and, therefore, no diffuse-ion swarm appears in the model structure. Activity coefficients of surface species are assumed to sub-divide, as in the triplelayer model, but the charge-dependent part is a function of the overall valence of the surface complex (Zk in Table 9.8) and an inner potential at the colloid surface exp(Z F l,s// 7). Physical closure in the model is achieved with the surface charge-potential relation ... 

An example of a speciation calculation involving metals and ligands that adsorb to form only inner-sphere surface complexes is shown in Table 9.10 for a soil solution at pH 7.5. The adsorption reactions for these metals and ligands are exemplified by the first and eighth rows in Table 9.7 ... 

A reaction sequence somewhat in parallel with the Eigen-Wilkins-Werner mechanism can also be expressed for the inner-sphere surface complexation of bivalent metal cations by an ionised surface hydroxyl group (cf equation (9.9a) ... 

Simulations of three representative Cs-smectites revealed interlayer Cs+ to be strongly bound as inner sphere surface complexes, in agreement with published bulk diffusion coefficients [78]. Spectroscopic and surface chemistry methods have provided data suggesting that in stable 12.4 A Cs-smectite hydrates the interlayer water content is less than one-half monolayer. However, Smith [81] showed using molecular simulations of dry and hydrated Cs-montmorillonite that a 12.4 A simulation layer spacing was predicted at about one full water monolayer. The results of MD computer simulations of Na-, Cs-and Sr-substituted montmorillonites also provide evidence for a constant water content swelling transition between one-layer and two-layer spacings [82]. 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

Table 4.2 Results of a Chemical Speciation Calculation at pH 7.5 Involving Specific Adsorption (Inner-Sphere Surface Complexation)3...

In the present example, the metals Cu, Cd, and Pb, and the ligands, F, P04, and B(OH)4 were permitted to undergo specific adsorption to form an inner-sphere surface complex with the species SO or SOHj, respectively. All but F were found to be primarily adsorbed species at equilibrium. 

Given that the concentration deviation for species E, H20, will be negligible, the second term on the right side of Eq. 4.32b will reduce to -k bAcD, making the corresponding term in Eq. 4.34b equal to - (k b + k f)AcD, and a equal to (k b + k f) in Eq. 4.36e. From Eq. 4.39 it then follows that the second term on the right in Eq. 4.41b will be simply k b, without the equilibrium concentrations of species D and E, in this case. Thus, if outer-sphere surface complexation is much faster than inner-sphere surface complexation and if the effect of any perturbation of the reactions in Eq. 4.40 on the concentration of water is negligible, the linear relationships... 

The adsorption of selenate (SeO ) by goethite (a-FeOOH) is thought to result primarily in outer-sphere surface complexes, whereas the adsorption of selenite (Se02 ) is thought to result primarily in inner-sphere surface complexes. Develop equations for the time constants rx and r2 in the adsorption kinetics of these two species and compare the resulting equations for t2. (Hint Consider Eqs. 4.40-4.45.)... 

Upon reaction with an adsorptive in aqueous solution (which then becomes an adsorbate), surface functional groups can engage in adsorption complexes, which are immobilized molecular entities comprising the adsorbate and the surface functional group to which it is bound closely [18]. A further classification of adsorption complexes can be made into inner-sphere and outer-sphere surface complexes [19]. An inner-sphere surface complex has no water molecule interposed between the surface functional group and the small ion or molecule it binds, whereas an outer-sphere surface complex has at least one such interposed water molecule. Outer-sphere surface complexes always contain solvated adsorbate ions or molecules. Ions adsorbed in surface complexes are to be distinguished from those adsorbed in the diffuse layer [18] because the former species remain immobilized on a clay mineral surface over time scales that are long when compared, e.g., with the 4-10 ps required for a diffusive step by a solvated free ion in aqueous solution [20]. Outer-sphere surface complexes formed in the interlayers of montmorillonite by Ca2+ or Mg2+ are immobile on the molecular time scale... 

These speciation concepts are illustrated in Fig. 3 for the idealized basal-plane surface of a smectite, such as montmorillonite. Also shown are the characteristic residence-time scales for a water molecule diffusing in the bulk liquid (L) for an ion in the diffuse swarm (DI) for an outer-sphere surface complex (OSQ and for an inner-sphere surface complex (ISC). These time scales, ranging from picosecond to nanosecond [20,21], can be compared with the molecular time scales that are probed by conventional optical, magnetic resonance, and neutron scattering spectroscopies (Fig. 3). For example, all three surface species remain immobile while being probed by optical spectroscopy, whereas only the surface complexes may remain immobile while being probed by electron spin resonance (ESR) spectroscopy [21-23]. 

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