Constant capacitance model anion adsorption

The intent of this chapter is not to survey noninvasive surface spectroscopy but to illustrate briefly how it is applied to resolve the Stummian issue of whether inner-sphere surface complexes form. For this purpose, the application of electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) spectroscopies to elucidate metal cation speciation and the use of extended X-ray absorption fine structure (EXAFS) spectroscopy to detect surface anion species will be described. Emphasis will be on the interpretation of spectra. Sample preparation and instrumentation details were reviewed in recent volumes edited by Hawthorne (55) and Perry (27). Because the constant capacitance model was developed in the context of adsorption by hydrous oxides, these... 

Most studies of metal ion complexation rely on the two-pKH model. Schindler and Stumm [59, 69, 78, 82, 87] combine the two-pKH constant capacitance model with stoichiometric reactions of the metal ions with surface hydroxyls. Huang et al. [62] and Dzombak and Morel [63] tabulate ion affinity constants on the basis of the two-pKH GC model. Leckie and co-workers [88-90] combine the model cation and anion adsorption with the two-pKH TL model. Hayes makes a distinction between strongly and weakly adsorbing ions [89, 90]. A series of reviews on s.a. using the two-pKH model can be found in [91]. 

A conceptual and mechanistic model of particle interactions in silica-iron binary oxide suspensions is described. The model is consistent with a process involving partial Si02 dissolution and sorption of silicate onto Fe(OH)3. The constant capacitance model is used to test the mechanistic model and estimate the effect of particle interactions on adsorbate distribution. The model results, in agreement with experimental results, indicate that the presence of soluble silica interferes with the adsorption of anionic adsorbates but has little effect on cationic adsorbates. 

ANION ADSORPTION. The Constant capacitance model postulates that anions react with surface hydroxyl groups through the ligand exchange mechanism embodied in Eq. 5.37b.In typical applications, the value of b in Eq. 5.37b is either 1 or 2 and the corresponding conditional equilibrium constants have the form... 

The constant capacitance model has been applied successfully to describe the adsorption of both inorganic and organic anions by hydrous oxides.Both adsorption isotherms and adsorption envelopes like those illustrated in Fig. 4.8 are accounted for quantitatively, except in the case of sulfate adsorption. It is possible, however, that this anion adsorbs through outer-sphere instead of inner-sphere complex formation, as discussed in Sec. 4.4. Aside from sulfate, anion adsorption envelopes are predicted in the constant capacitance model on the basis of an increase in [OH ], which causes the concentrations of the surface complexes in Eq. 5.53 to decrease, and a decrease in o-p. 

Other modeling efforts include soil acidification models of the macroscopic type that account for the process of S04 sorption in different ways. These approaches, which assume equilibrium conditions to prevail, include the adsorption isotherm, solubility product, and anion exchange. Prenzel (1994) discussed the various limitations of the above approaches in their capability to account for changes in pH. Recently, Fumoto and Sverdrup (2000) used a constant capacitance approach to describe the pH dependency of S04 sorption isotherms in an andisol. Other modeling efforts of S04 isotherms were reported by Gustafsson (1995) in a spodosol. Such isotherm models are of the equilibrium type and include linear and Temkin types of models. 

VIBRATIONAL SPECTROSCOPY Infrared and Raman spectroscopies have proven to be useful techniques for studying the interactions of ions with surfaces. Direct evidence for inner-sphere surface complex formation of metal and metalloid anions has come from vibrational spectroscopic characterization. Both Raman and Fourier transform infrared (FTIR) spectroscopies are capable of examining ion adsorption in wet systems. Chromate (Hsia et al., 1993) and arsenate (Hsia et al., 1994) were found to adsorb specifically on hydrous iron oxide using FTIR spectroscopy. Raman and FTIR spectroscopic studies of arsenic adsorption indicated inner-sphere surface complexes for arsenate and arsenite on amorphous iron oxide, inner-sphere and outer-sphere surface complexes for arsenite on amorphous iron oxide, and outer-sphere surface complexes for arsenite on amorphous aluminum oxide (Goldberg and Johnston, 2001). These surface configurations were used to constrain the surface complexes in application of the constant capacitance and triple layer models (Goldberg and Johnston, 2001). 

The model used to evaluate surface chemistry in these systems is the constant capacitance surface complexation model. This model has been used to describe the adsorption of cations (41) and anions (4,8) onto oxides similar to those used in our experiments. A significant difference between those studies and the present study is that we have adapted the model to simulate some of the interactions that might occur between particles in a binary suspension. 

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