Phosphate bidentate adsorption

Considering that -O-H may be a weaker complex than -O-M, formation of the latter would be relatively independent of pH. The latter complex would involve a strong bond (e.g., chemisorption). The same explanation applies to anion adsorption. For example, phosphate (P04) adsorption by oxides may take place in an outer- or inner-sphere mode of the monodentate or bidentate type (Fig. 4.7). 

The chemical part AG° corresponds in most cases to specific adsorption on reactive surface hydroxyl groups, whereas the electrical part AG is due to the cou-lombic interaction. In the first part, it arises quite naturally the concept of binding site as the place on the surface where the adsorbate binds these sites can be associated with individual hydroxyl groups, but not necessarily, because when bidentate adsorption takes place, as in the case of phosphate or arsenate adsorption onto iron oxides, two hydroxyls may be considered as forming a single site. 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

Figure 4.7. Schematic of adsorption of phosphate by an iron oxide in an inner-sphere bidentate or monodentate mode.

Use of surface speciation models for prediction of adsorption and transport requires specification of the mode of bonding and speciation of oxyanions on oxide surfaces. FTIR spectroscopy (especially ATR and DRIFT) offers the potential to establish symmetry of surface species, protonation, and determination of monodentate or bidentate bonding. Determination of surface speciation is greatly enhanced when the spectroscopic information is combined with measurements of electrophoretic mobility (EM), calculation of point of zero charge and proton balance measurements before and after adsorption. We review adsorption of phosphate, carbonate, boron, selenate and selenite on Fe and A1 oxides. New preliminary spectra and EM and proton balance information for arsenate and arsenite adsorption on amorphous Fe and A1 oxide suggest that HASO4 and H2ASO3 are the dominant surface species. 

As in the other surface complexation models, the chemical part consists in several adsorption reactions. In the application of the CD-MUSIC model, the choice of these reactions has been carefully based on spectroscopic evidence of course, that can be done also in other models such as TLM, BSM, and so on, but here it is rather essential for proper elucidation of the charge distribution. The original paper (Hiemstra and Van Riemsdijk 1996) applied the model to phosphate adsorption on goethite. Based on spectroscopic studies, they proposed the existence of monodentate-bound species (Reaction 12.44), an unprotonated bidentate-bound species (Reaction 12.45), and a protonated bidentate species, following ... 

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