Hydrous oxide surface

Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (ion pair) or be in the diffuse swarm of the electric double layer. (From Sposito, 1989)... 

Fig. b shows a schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ("s"), inner-sphere complexes ("a"), outer-sphere complexes ("P") and the diffuse ion swarm ("d"). (Modified from Sposito, 1984)... 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). 

We consider here first, the kinetics of the adsorption of metal ions on a hydrous oxide surface. 

Adsorption of Metai Ions to a Hydrous Oxide Surface... 

How could one distinguish between phosphate binding (ligand exchange) to a hydrous oxide surface and the precipitation of iron phosphate ... 

How could one distinguish experimentally in the interaction of a hydrous oxide surface with a fatty acid, whether the interaction is due to hydrophobic bonding or to coordinative interaction (ligand exchange of the carboxyl group with the surface functional groups of the hydrous oxide) ... 

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

The hydrous oxide surface, as a first approximation, is treated like a cross-linked polyhydroxo-oxo acid ... 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

The oxidation of Fe(II), V02+, Mn2+, Cu+ by oxygen is favored thermodynamically and kinetically by hydrolysis and by specific adsorption to hydrous oxide surfaces. As suggested in formula (IV) of Fig. 9.1 Fe(II) and the other transition elements Mn(II), VO(II), Cu(I) may more readily associate (probably outer-spherically) with... 

Wehrli, B., B. Sulzberger, and W. Stumm (1989), "Redox Processes Catalyzed by Hydrous Oxide Surfaces," Chemical Geology 78,167-179. 

Stumm, W., H. Hohl, and F. Dalang (1976), "Interaction of Metal Ions with Hydrous Oxide Surface", Croat. Chem. Acta 48, 491. 

As stated previously, it is difficult to assign ions to a few discrete "mean planes of adsorption" and have these mean planes correspond in every case to the location of ions expected from hypothetical structure and bonding at the hydrous oxide surface. 

Anion adsorption by goethite and gibbsite. II. Desorption of anions from hydrous oxide surfaces. J. Soil Sci. 25 16-26 Hiradate, S. Inoue, K. (1998) Interaction of mugineic acid with iron (hydr)oxides Sulfate and phosphate influences. Soil Sci. Soc. 

Dtsch. Keram. Ges. 43 677-684 Wehrli, B. Sulzberger, B. Stumm,W. (1989) Redox processes catalysed by hydrous oxide surfaces. Chem. Geol. 78 167-179. 

Stumm, W., Hohl, H. and Dalang, F. (1976) Interaction of metal ions with hydrous oxide surfaces. Croatica Chim. Acta, 48, 491M99. 

Hydrous oxide surfaces of sand immobilize As by adsorption processes ( 3). The results of our studies show that the extent of adsorption varies with the oxidation state of the As, the redox environment and/or the pH of the eluting water. The influence of these parameters on the mobility of As was studied by eluting As through sand columns waters of different redox... 

Westall, J. and Hohl, H. A general method for the computation of equilibria and determination of equilibrium constants for adsorption at hydrous oxide surfaces, iji "Abstracts of Papers," Amer. Chem. Soc. Meeting, Miami Beach, FL., 1978. 

Figure 9.10. (a) Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ( chemical bond ), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer. (The inner-sphere complex may still retain some aquo groups toward the solution side.) (From Sposito, 1989.) (b) A schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ( s ), inner-sphere complexes ( a ), outer-sphere complexes ( /3 ), and the diffuse ion swarm ( d ). (Adapted from Sposito, 1984.)... 

Hydrous Oxide Surfaces 547 Tableau 9.1. Surface Complex Formation of S—OH... 

High-Affinity and "Low-Affinity Surface Sites Often, in cation adsorption on hydrous ferric oxides two site types (high-affinity and low-affinity) are required for modeling the equilibrium. This is a result of heterogeneity of the hydrous oxide surfaces (see Dzombak and Morel, 1990). 

Sorption Depends on Sorption Sites Three-layer silicates contain on the crystal edges (broken bonds) end-standing OH groups, which, similar to the OH groups on hydrous oxide surfaces, can interact with OH", and metal ions (surface complex formation). 

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