Montmorillonite outer-sphere complex

Unit layers of smectites, notably montmorillonite, can associate in stacked, roughly parallel alignment to form a quasicrystal [52]. This particle structure is stabilized by attractive interactions between the basal planes of unit layers, as mediated by adsorbed cations and water molecules. The prototypical example of a montmorillonite quasicrystal is that comprising stacks of four to seven unit layers, with Ca2+ adsorbed in outer-sphere complexes on the siloxane surfaces to serve as molecular cross-links binding the unit layers together through electrostatic forces. This kind of quasicrystal appears to form with any bivalent cation and for any smectite [21,54]. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

Bostick et al. (2002) studied Cs+ adsorption onto vermiculite and montmorillonite with EXAFS and found that Cs+ formed both inner-and outer-sphere complexes on both aluminosihcates. The inner-sphere complexes bound to the siloxane groups in the clay structure. Combes et al. (1992) found that NpOj adsorbed onto goethite as a mononuclear surface complex. Waite et al. (1994) were successful in describing uranyl adsorption to ferrihydrite with the diffuse layer model using the inner-sphere, mononuclear, bidentate surface complex observed with EXAFS. 

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). 

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]. 

Strawn, D. G., and D. L. Sparks. 1999. The use of XAFS to distinguish between inner- and outer-sphere lead adsorption complexes on Montmorillonite. J. Coll. Interface Sci. 216 257-269. 

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. 

Stragier H, Cross JO, Rehr JJ, Sorensen LB, Bouldin CE, Woicik JC (1992) Diffraction anomalous fine structure A new X-ray structural technique. Phys Rev Lett 69 3064-3067 Strawn DG, Scheidegger AM, Sparks DL (1998) Kinetics and mechanisms of Pb(II) sorption and desorption at the aluminum oxide-water interface. Environ Sci Technol 32 2596-2601 Strawn DG, Sparks DL (1999) The use of XAFS to distinguish between inner- and outer-sphere lead adsorption complexes on montmorillonite. J Colloid Interface Sci 216 257-269 Strawn DG, Sparks DL (2000) Effects of soil organic matter on the kinetics and mechanisms of ( ) sorption and desorption in soil. Soil Sci Soc Am J 64 144-156 Stumm W (1992) Chemistry of the Sohd-Water Interface. John Wiley Sons, Inc, New York... 

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