Bidentate surface complexes

There is also the possibility that bidentate surface complexes are formed ... 

Metal ion binding to hydrous oxides can occur as monodentate or bidentate surface complexes (Eqs. 2.9a and 2.9b) where, respectively, one or two protons are released per mol of metal ion bound. Develop a simple graphical method to distinguish between monodentate and bidentate metal binding. 

In principle, it appears possible to distinguish between the formation of monodentate and bidentate surface complexes if the stoichiometry of the H+ release is know. A mean surface complex stoichiometry can be formulated... 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

The photocatalytic oxidation of organic and inorganic compounds and the photo-catalytic production of H202 occurs also at the surface of iron(III)(hydr)oxides. It has been proposed (e.g., Hoffmann, 1990 Faust and Hoffmann, 1986) that the oxidation of S(IV) by 02 in atmospheric water is catalyzed by iron(III)(hydr)oxide particles. It is assumed that the reductant (HSO3) is specifically adsorbed at the surface of an iron(III)(hydr)oxide, forming either a monodentate or a bidentate surface complex ... 

As shown in Figure 1, the adsorption of Mn(II) on y-FeOOH can be successfully described using a constant capacitance model. In these calculations the hydrolysed surface complex =FeO-Mn-OH was not considered. The reason for not considering both the bidentate (sS0)2Mn and hydrolysed surface species is that both have virtually the same pH dependence, so it is impossible using the available data to make anything other than an arbitrary choice about the relative proportions of these two species. Based on the model calculations, in the pH range 8-9, the predominant Mn(II) species on the y-FeOOH surface is the bidentate surface complex or the hydrolysed surface complex. 

Previous workers concluded that cadmium ions adsorb onto the goethite surface by the formation of bidentate surface complexes.11,12 However, the cadmium adsorption data and our calculations (see Figure 3) shows that cadmium adsorbs on goethite mainly on one surface hydroxyl group, forming monodentate complexes, according the following reaction ... 

As shown by Moser et al. (47), surface complexation of colloidal Ti02 accelerates electron transfer from the conduction band to methyl viologen. The enhancement of interfacial electron transfer is much more pronounced with the bidentate benzene derivates (Figures 12b and 12c) (1700 times faster with salicylate than in its absence). Similar results have been obtained (47) on the acceleration of electron transfer to oxygen by bidentate surface complexation. 

The stability constant of the bidentate surface complex in reaction (5.69) is often defined as ... 

A ( react ion (5.69)) = [bidentate surface complex] x II ions released into solution X n ions adsorbed from solution x [surface site 1)] x [surface site (2)] x Boltzmann factor (5.70)... 

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. 

In the last step the U(VI) desorbs from the surface as a uranyl ion, U02 +. Two oxygen ions from the Pb02 surface remain coordinated to the high valent uranyl. Recently Combes (1989) has shown by EXAFS (extended x-ray absorption fine-structure spectroscopy) that uranyl indeed forms bidentate surface complexes on goethite. The local coordination sites on a-FeOOH are structurally very similar as the Pb02 sites with a rutile structure. 

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. 

ATR-FTIR combined with quantum chemical calculations (Figure 4.8), considering different crystal surfaces. Differences between surface complexes on anatase and rutile lie mainly on the denticity type. On rutile, the most stable species consist of two bidentate surface complexes, followed by a monodentate form in the third place. On the contrary, on anatase, the most stable forms are four species. 

Adsorption of 5-SSA to related minerals (i.e. alumina) has been previously studied previously by Jiang et aL (2002) by both a set of electrokinetic data and batch adsorption. Furthermore they repwrted IR data which they interpreted in terms of 5-SSA forming a bidentate surface complex involving the carboxylate group and the phenol group. 

Chromate adsorption onto ferrihydrite was studied by attenuated total reflection IR spectroscopy (ATR-IR) and theoretical frequency calculations (Johnston and Chrysochoou 2012). The calculations were done for several model arranganents, shown in Figure 9.21. It was found that the results are consistent with the formation of monodentate and bidentate surface complexes. Monodentate complexes (Figure 9.21e) are dominaut at low surface coverage aud pH > 6.5, and bidentate complexes (Figure 9.21f) form at high surface coverage aud pH < 6. 

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