Inner-sphere bidentate surface complexes

Collins et al. (1999a) found that Hg2+ sorbed to goethite as an iimer-sphere bidentate complex. Cheah et al. (1998) found that Cu " " sorbed to amorphous silica and Y-AI2O3 as monomeric and monodentate iimer-sphere surface complexes. However, bidentate complexes may also form on Y-AI2O3. Using polarized EXAFS, Dahn et al. (2003) determined that Ni " " sorbed to montmorillonite edge sites as an inner-sphere mononuclear surface complex. Inner-sphere surface complexes were observed with XAS for Cr " " adsorption on manganese (Manceau and Charlet, 1992) and iron oxides (Charlet and Manceau, 1992). 

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. 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

EXAFS data showed that cations and oxyanions (e.g. selenite and arsenite) can form two kinds of bidentate, inner sphere complexes on iron oxides depending upon the surface site at which the adsorbate adsorbs (Manceau, 1995 Randall et al.. 

The pHPZC of ferric hydroxide surfaces is about 8 [127], so aqueous Pb2+ should be electrostatically repelled from these surfaces at pH values less than 8. However, as seen in Figure 7.6(a), the Pb2+ present in this aqueous solution is sorbed essentially completely to ferric hydroxide surfaces at pH 6. This behavior suggests that Pb2+ forms direct chemical bonds to these surfaces in order to overcome the repulsive electrostatic forces below the pHpzc of ferric hydroxide. This conclusion based on macroscopic uptake data has been confirmed by direct spectroscopic observation using X-ray absorption fine structure (XAFS) spectroscopy under in situ conditions (i.e., with aqueous solution in contact with a-FeOOH surfaces at ambient temperature and pressure) [133,134]. These studies showed that the aquated Pb(II) ion forms dominantly inner-sphere, bidentate complexes on a-FeOOH surfaces. 

Inner-sphere complex A type of Stern layer, where a chemical species bonds directly onto the surface of a solid material (adsorbent) in an aqueous solution. The formation of inner-sphere complexes is called chemisorption. Stern inner-sphere adsorption complexes are further divided into monodentate, bidentate-mononuclear, and bidentate-binuclear types (compare with outer-sphere complex). 

Figure 4.2. Schematic of inner-sphere monodentate and bidentate metal complexes between metal ions and hydroxyl groups of an oxide surface (adapted from Hayes, 1987).

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... 

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). 

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. 

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-... 

Figure 5. Cu(II)-humic complex on the goethite surface, showing a Type A complex (Cu(II) forms bridge between humic substance and the goethite surface) and a Type complex (humic substance forms a bridge between Cu(II) and the goethite surface). Also shown is an inner-sphere bidentate Cu(II) complex on the goethite surface. XAFS data consistent with this model were collected on NSLS beam line X-l 1 A. [Reprinted from Alcacio et al. (2001), Fig. 1, with permission from Elsevier Science]...

Figure 2. Schematic representation of principal surface sorption processes, (a) Projection of the CoOOH structure in the ab plane. MSC, multinuclear surface complexation represented by an epitaxy of a-FeOOH (left) ISC, mononuclear monodentate (middle right), mononuclear bidentate (top right) and binuclear bidendate (lower right) inner-sphere complexation OSC, outer-sphere surface complexation (top) LD, lattice diffusion (center), (b) Example of epitaxy without sharing of oxygens (Van der Waals forces). The. ..AB-AB... close-packed anionic layer sequence of Co(OH)2(s) is coherently stacked on the. ..AB-BC-CA... layer sequence of CoOOH. Co(OH)2(s) has a 1H polytypic structure, and CoOOH a 3R. Small circles are...

This interpretation has been supported by a study of oxalate sorption on corundum modelled by the CD-MUSIC model involving ATR-IR spedroscopy (Johnson et al., 2004). A mononuclear bidentate complex was found up to 14 pmol/m, whereupon oxalate additionally adsorbed as an outer-sphere complex. Sorption of oxalate has also been studied on boehmite and corundum by Yoon et al. (2004) The peaks assigned to the inner-sphere complex in previous works (near 1286,1418,1700 and 1720 cmi) were claimed to arise from the presence of several species. Evidence for this phenomenon comes from the observation that peaks at 1286 and 1418 cm-i are shifted to 1297 and 1408 cm-i as the oxalate surface coverage increases. The authors finally postulated the existence of two species species "A" at 1286 and 1418 cmi, and species "B" at 1297 and 1408 cm-i, respectively, which were... 

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