Inner-sphere complex surface charge

The surface behavior of Na is similar to that of Cs, except that inner sphere complexes are not observed. Although Na has the same charge as Cs, it has a smaller ionic radius and thus a larger hydration energy. Conseguently, Na retains its shell of hydration waters. For illite (Figure 6), outer sphere complexes resonate between -7.7 and -1.1 ppm and NaCl... 

Figure 11.1. Schematic views of various ways in which an organic chemical, i, may sorb to natural inorganic solids (a) adsorption from air to surfaces with limited water presence, (b) partitioning from aqueous solutions to the layer of vicinal water adjacent to surfaces that serves as an absorbent liquid, (c) adsorption from aqueous solution to specific surface sites due to electron donor-acceptor interactions, (d) adsorption of charged molecules from aqueous solution to complementarily charged surfaces due to electrostatic attractions, and (e) chemisorption due to surface bonding or inner sphere complex formation.

Although each SCM shares certain common features the formulation of the adsorption planes is different for each SCM. In the DDLM the relationship between surface charge, diffuse-layer potential, d, is calculated via the Gouy-Chapman equation (Table 5.1), while in the CCM a linear relationship between surface potential, s, is assumed by assigning a constant value for the inner-layer capacitance, kBoth models assume that the adsorbed species form inner-sphere complexes with surface hydroxyls. The TLM in its original... 

Similar to solution complexation, surface complexation can be distinguished between inner-spherical complexes (e.g. phosphate, fluoride, copper), where the ion is directly bound to the surface, and outer-spherical (e g. sodium, chloride) complexes where the ion is covered by a hydration sleeve with the attraction working only electrostatically. The inner-sphere complex is much stronger and not dependent on electrostatic attraction, i.e. a cation can also be sorbed on a positively charged surface (Drever 1997). 

The adsorption reaction that occurs between metallic ions and the charged surfaces of clay-organics may involve formation of either relatively weak outer-sphere complexes, or strong inner-sphere complexes. 

Equation 4.26, as noted above, has two terms—a chemical complexation term, [-SM(m 1)[H+]/[-SHj[Mm+], and an electrical potential term, exp[(m-l)A /// T]. The electrical potential term is directly dependent on pH (see Eq. 4.27b). Therefore, by regulating the magnitude of the physical attraction between a metal and a surface, the electrical potential indirectly affects the magnitude of surface complexes that could form. Recall that before a chemical complex or inner-sphere complex between two reactants is formed, they must physically collide successfully at a given frequency. When a metal-ion (e.g., contaminant) and a surface are oppositely charged, physical collisions maximize, and, thus, inner-sphere complex formation maximizes. When a metal-ion (e.g., contaminant) and a surface are similarly charged, physical collisions minimize, and inner-sphere complex formation also minimizes. 

The solvent molecules form an oriented parallel, producing an electric potential that is added to the surface potential. This layer of solvent molecules can be protruded by the specifically adsorbed ions, or inner-sphere complexed ions. In this model, the solvent molecules together with the specifically adsorbed, inner-sphere complexed ions form the inner Helmholtz layer. Some authors divide the inner Helmholtz layer into two additional layers. For example, Grahame (1950) and Conway et al. (1951) assume that the relative permittivity of water varies along the double layer. In addition, the Stern variable surface charge-variable surface potential model (Bowden et al. 1977, 1980 Barrow et al. 1980, 1981) states that hydrogen and hydroxide ions, specifically adsorbed and inner-sphere... 

The edge charges can also bond ions with opposite charges. This process, however, is not directed clearly by electrostatic forces chemical properties play an important role. The ions are sorbed with no hydrate shell, that is, inner-sphere complexation occurs. These reactions and the surface complexation models for their quantitative treatment are shown in general in Chapter 1, Table 1.7. 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

There are two widely accepted mechanisms for adsorption of solutes by a solid surface. Outer-sphere surface complexation, or non-specific adsorption, involves the electrostatic attraction between a charged surface and an oppositely charged ion in solution (Fig. 3). The adsorbed ion resides at a certain distance from the mineral surface. Inner-sphere complexation, also termed specific adsorption, involves the formation of a coordinative complex with the mineral surface (Kingston et al., 1972 Fig. 3). Inner-sphere complex bonds are more difficult to break than outer-sphere complex bonds and result in stronger adsorption of ions. 

Macroscopic chemical evidence indicates that sulfate [S(V1)] adsorbs via electrostatic attraction as an outer-sphere complex and should therefore have much less effect on As adsorption than inner-sphere complex formers such as P(V). Sulfate would only be expected to adsorb when the net surface charge is positive, i.e., at pH values below the lEP of the solid (Hingston et al, 1972). This is consistent with experimental data for S(V1) adsorption by synthetic ferrihydrite (Dzombak and Morel, 1990) and natural aquifer solids (Stollenwerk, 1995). Experimental data from Jain and Loeppert (2000), replotted in Fig. 6, show the influence of S(V1) on adsorption of As(V) and As(lll) by ferrihydrite as a function of pH. The S(Vl)/As ratios for Fig. 6, 10 1 and 50 1 reflect the fact that S(VI) concentrations in groundwater are generally much greater than As. Sulfate had essentially no effect on As(V) adsorption over the entire pH range. At the highest S(V1) concentration. 

Inner- and Outer-Sphere Complexes. As illustrated in Figure 1, a cation can associate with a surface as an inner-sphere or an outer-sphere complex, depending on whether a chemical bond is formed (i.e., a largely covalent bond between the metal and the electron-donating oxygen ions, as in an inner-sphere complex) or whether a cation of opposite charge approaches the surface groups within a critical distance. As with solute ion pairs, the cation and the base are separated by one or more water molecules (1, 2). Furthermore, ions may exist in the diffuse swarm of the double layer. 

In the presence of s.a. ions one has to decide at which plane these ions adsorb. The most simple choice is that the s.a. ions are located at the Stern plane. This choice is appropriate for ions that form outer sphere complexes with the surface sites or for ions that have no affinity for the proton sites. Specifically adsorbing counterions that are forming inner sphere complexes with the surface groups screen the primary surface charge very effectively and the difference between primary and secondary surface charge becomes vague. In this case it is appropriate to place the s.a. charge at the surface plane, or partly at the surface plane and partly at the Stern plane. 

For inner sphere complexes with the entire charge at the surface plane Uxi x = Zx and the Boltzmann factor becomes simply exp(—ZxFV s/RT). For outer sphere complexes nx = 0 and both activities are determined by ipd only. Assuming that, except for the electrostatic interactions, the surface phase behaves ideal, the ratios of the surface group activities in Eqs. (46), (51) and (52) can be replaced by ratios of site fractions, 6x = SX/Nj where is the total density of surface sites ... 

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. 

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