Binuclear surface complexes, effect

Our present information on the effect of surface speciation on the reactivity of the surface (i.e., its tendency to dissolve) is summarized in Figure 4. Evidence for the formation of binuclear surface complexes is often circumstantial. Most researchers who modeled surface complex formation with oxy-anions could fit the adsorption data only by assuming the formation of binuclear complexes, usually in addition to mononuclear ones. 

As suggested in Figure 13.7, binuclear or multinuclear surface complexes tend to block surface sites. A much higher activation energy is involved in detaching simultaneously two metal centers from the surface hence dissolution is retarded by binuclear surface species. This retardation is especially pronounced when the effect of surface coordination with an inhibitor upon surface protonation is not unfavorable. For example, in the reaction of an anionic inhibitor, L, ... 

Many of these oxoanions can form, depending on concentration and pH, various surface complexes. This ability may explain the different effects observed under different solution conditions. For example, Bondietti et al. (33) found that phosphate at low pH (where mononuclear complexes are probably formed) accelerated EDTA-promoted dissolution of lepidocrocite, whereas at near-neutral pH conditions (where binuclear complexes are presumably formed), phosphate was an efficient inhibitor. Furthermore, because of the several geometries involved, the extent of comer sharing or edge sharing by adsorbed oxoanions may differ with the type of oxide and with allotropic modifications of the same metal oxide. 

Case Examples. The effects of various oxoanions on EDTA-pro-moted dissolution of lepidocrocite (y-FeOOH) have been studied by Bondietti et al. (33). EDTA was chosen as a reference system because it is dissolution-active over a relatively wide pH range. Phosphate, arsenate, and selenite markedly inhibit the dissolution at near-neutral pH values. At pH <5 phosphate, arsenate, and selenite accelerate the dissolution. It is presumed that the bi-nuclear surface complexes formed at near-neutral pH values by these oxoanions (Table II) inhibit the dissolution. Figure 8a displays data on the effect of selenite on EDTA-promoted dissolution, and Figure 8b shows that calculations on surface speciation by Sposito et al. (35) support the preponderance of binuclear selenite surface complexes in the neutral-pH range. Mononuclear surface species prevail at lower pH values. 

Figure 8. The effect of selenite on the EDTA-promoted dissolution of y-FeOOH 0.5 gIL). Part a At low pH the dissolution rate is increased by selenite at pH 7 it is strongly inhibited. Concentration of the ligands is given in inol/L. Part b Surface speciation on lepidocrocite as a function of pH according to Sposito et al. (35). These data suggest that binuclear selenite surface complexes are formed in the neutral pH range (from reference 33).

The possibility of using surface modification of cheap metals to make them effective electrode materials has been mentioned (Section 57.3.2.3(1)). A further example employs cyanoferrates and cyanoruthenates as the redox centres.76 Complexes such as [M(CN)5L]" (M = Fe, Ru L = CN, H20, NO, L-histidine) may be immobilized on a partially corroded nickel surface. The surfaces have good stability and diffuse reflectance IR spectroscopy shows the presence of bridging cyano groups, implying the presence of a binuclear (Ni, M) species in the surface. A general equation for the redox reaction is ... 

The effect of pH on heavy metal ion adsorption capacity was studied by previous researchers using the shake flask experiments. Eric and Roux used the shake flask experiment to study the influence of pH on the heavy metal ion binding onto a fimgus-derived bio-sorbent in the year, 1992. Also the evaluation of the effect of the hydrochloric acid concentration on the adsorption of platinum group metal ions onto chemically modified chitosan was done by Inoue et al., using the shake flask experiment [85]. Depending upon the type of P complexation with the surface such as monodentate, bidentate mononuclear, and bidentate binuclear the phosphorus desorption is potentially controlled. These complexes can be either non protonated or protonated depending on the suspension pH [184]. 

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