Surface protonation ligand adsorption

Example 5.1 Change in Surface Protonation as a Consequence of Metal Ion or Ligand Adsorption... 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

F ure 9.21. The net charge at the hydrous oxide surface is established by the proton balance (adsorption of or OH and their complexes) at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidi-metric titration curve and from a measurement of the extent of adsorption of specifically adsort)ed 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). Addition of a ligand, at constant pH, increases surface protonation while the addition of a metal ion (i.e., specifically adsorbed) lowers surface protonation. (Adapted from Hohl et al., 1980.)... 

Figure 7. The effect of ligands and metal ions on surface protonation of a hydrous oxide is illustrated by two examples (1). Part a Binding of a ligand (pH 7) to hematite, which increases surface protonation. Part h Adsorption of Pb2+ to hematite (pH 4.4), which reduces surface protonation. Part c Surface protonation of hematite alone as a function of pH (for comparison). All data were calculated with the following surface complex formation equilibria (1 = 5 X 10"3 M >. Electrostatic correction was made by diffuse double layer model.

Longer times described by the dashed portion of the adsorption-desorption line represent the slower rates of metal and ligand adsorption found in some laboratory experiments and observed in most soil and groundwater systems (cf. Jenne 1995). Adsorption of metal ions generally involves concurrent desorption of protons or other metals. When adsorbed metals are strongly surface-bound (see Chap. 10) both adsorption and desorption reactions may be slow and desorption partially irreversible, requiring adsorption or desorption equilibration times that may approach 10 to 100 hours. Based on an analysis of published laboratory and field studies, Jenne (1995) concludes that metal adsorption and desorption reactions have initial fast and then slower reaction steps and are usually diffusion-rate limited. 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Any surface reaction that involves chemical species in aqueous solution must also involve a precursory step in which these species move toward a reactive site in the interfacial region. For example, the aqueous metal, ligand, proton, or hydroxide species that appear in the overall adsorption-desorption reaction in Eq. 4.3 cannot react with the surface moiety, SR, until they leave the bulk aqueous solution phase to come into contact with SR. The same can be said for the aqueous selenite and proton species in the surface redox reaction in Eq. 4.50, as another example. The kinetics of surface reactions such as these cannot be described wholly in terms of chemically based rate laws, like those in Eq. 4.17 or 4.52, unless the transport steps that precede them are innocuous by virtue of their rapidity. If, on the contrary, the time scale for the transport step is either comparable to or much longer than that for chemical reaction, the kinetics of adsorption will reflect transport control, not reaction control (cf. Section 3.1). Rate laws must then be formulated whose parameters represent physical, not chemical, processes. 

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