Adsorption mechanisms inner sphere complexation

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 Table 4 there are presented data, concerning the specific adsorption of the ions at the silica/aqueous electrolyte interface. Metal ions may adsorb with formation of inner-sphere complexes, hydrocarbon clusters (for example Cu2+) or outer-spherical complexes as for example Mn2+ [105]. The determination of the adequate adsorption mechanism is possible with in situ spectroscopy method (Table 4). 

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. 

As shown by Tunesi and Anderson (48), the efficiency of the photoredox nrocess for organic compounds depends on their adsorption behavior. When direct charge transfer (inner-sphere complexes) occurs, this mechanism is more efficient than free radical attack. These authors interpret their results nth salicylate at low pH as a direct electron transfer from the adsorbed organic molecule—assumed to be an orbital configuration of the chelate ring— the semiconductor. 

Randall et al. (1999) studied the structure and composition of Cd-+ complexes sorbed on several iron oxyhydroxide minerals goethite, lepidocrocite, akagenite, and schwertmannite using EXAFS. In all cases, adsorbed Cd-+ formed inner-sphere complexes over a wide range of solution pH and Cd-+ concentration. However, the bonding mechanism differed between minerals and depended on the availability of different types of adsorption sites at the mineral surface. For example, sorbed to goethite by the formation of bidentate surface com-... 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

In an effort to understand adsorption mechanisms, Waite et al. (1994) (see also Chisholm-Brause and Morris 1992) examined the character of U(V1) adsorption sites on the HFO surface with uranium EXAFS spectroscopy. They concluded that a single inner-sphere, mononuclear, bidentate complex, (sFe02)U02, could explain their low pH-adsorption results and that U(VI) desorption at alkaline pH s could be modeled assuming a (sFe02)U02C03 surface species. Waite et al. (1994) used the DL model in their study and assumed the existence of both weak and strong adsorption sites (see Fig. 13.14 and Chap. 10). 

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