Surface complex formation model

In the surface complex formation model the amount of surface charge that can be developed on an oxide surface is restricted by the number of surface sites. (This limitation is inherently not a part of the Gouy-Chapman theory.)... 

Stipp and Hochella (1991), on the basis X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED), have shown that CaC03 exposed to water, contains at the surface =C03H and =CaOH functional groups and van Capellen (1991) has proposed a surface complex formation model for carbonates. Similarly, Ronngren et al. (1991) have proposed =SH and =ZnOH functional groups for the surface of hydrous ZnS s). 

It is surprising that data on natural particles can be fitted over a range of concentrations (representative of those encountered in natural waters) on the basis of a "single-site" surface complex formation model. Apparently similar types of binding groups are predominant and of importance in these particles. 

Horst, J., W. H. Holl, and S. H. Eberle. 1990. Application of the surface complex formation model to exchange equilibria on ion exchange resins. Part I. Weak-acid resins. React. Polym. 13 209-231. 

Whether the knowledge of surface charge and its variations with pH is a sufficient condition for predicting uptakes requires closer scrutiny. In this regard, the conclusion by Corapcioglu and Huang [171] that [e]lectrostatic interaction plays an insignificant role in adsorption reaction certainly needs to be reexamined. The.se authors preferred the surface complex formation model and thus postulated... 

In the surface complex formation model, the amount of surface charge that... 

Surface complex formation models for carbonates and sulfides, respectively, have been proposed by van Capellen et al. (1993) and by Ronngren et al. (1991). 

There are many models that describe the interaction between solute ions and surfaces. These have been reviewed by several authors (1,2,3) and include general ion exchange (4,5,6), surface complex formation (7,8) (the Swiss model), and various electrostatic models (Gouy-Chapman-Stern (9), Grahame (10,11), and James and Healy (12)). For hydrolyzable species sorbing onto hydrous oxide surfaces, the surface complex formation model and the solvent-ion interaction model of James and Healy have been shown to be in good agreement with observations. In this chapter, data are analyzed via the James and Healy model. 

With a chapter on particle-particle interaction (coagulation) the characteristics of particles and colloids as chemical reactants are discussed. Since charge, and in turn the surface potential of the colloids is important in coagulation, it is illustrated how in simple cases the modelling of surface complex formation permits the calculation of surface charge and potential. The role of particle-particle interaction in natural water and soil systems and in water technology (coagulation, filtration, flotation) is exemplified. 

The central ion of a mineral surface (in this case we take for example the surface of a Fe(lll) oxide and S-OH corresponds to =Fe-OH) acts as Lewis acid and exchanges its stuctural OH against other ligands (ligand exchange). Table 2.1 lists the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic for all surface complexation models (Dzombak and Morel, 1990.)... 

The "classical" theory of nucleation concentrates primarily on calculating the nucleation free energy barrier, AG. Chemical interactions are included under the form of thermodynamic quantities, such as the surface tension. A link with chemistry is made by relating the surface tension to the solubility which provides a kinetic explanation of the Ostwald Step Rule and the often observed disequilibrium conditions in natural systems. Can the chemical model be complemented and expanded by considering specific chemical interactions (surface complex formation) of the components of the cluster with the surface ... 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

This scheme disregards mass transfer limitations and represents only a simplified model. Formation of A S may involve specific interactions, such as hydrogen bonds, coordination, or ir-complex formation, or non-specific interactions, such as van der Waals or hydrophobic bonds. Non-specific interactions are insignificant for small polar molecules, but may contribute significantly to the surface complex formation if the hydrophobic moiety is large ( 5, 6) ... 

Using this model, one cannot forecast the adsorption of the background electrolyte ions because this model do not consider the reactions responsible for such a process. Zeta potential values, calculated on the basis of this model, are usually too high, nevertheless, because of its simplicity the model is applied very often. In a more complicated model of edl, the three plate model (see Fig. 3), besides the mentioned surface plate and the diffusion layer, in Stern layer there are some specifically adsorbed ions. The surface charge is formed by = SOHJ and = SO- groups, also by other groups formed by complexation or pair formation with background electrolyte ions = SOHj An- and = SO Ct+. It is assumed that both, cation (Ct+) and anion (A-), are located in the same distance from the surface of the oxide and form the inner Helmholtz plane (IHP). In this case, beside mentioned parameters for two layer model, the additional parameters should be added, i.e., surface complex formation constants (with cation pKct or anion pKAn) and compact and diffuse layer capacities. 

Noh and Schwarz proposed a modified Huang s and Stumm method for the calculation of surface hydroxyl group ionization constants, based on Gouy-Chapman model [118]. In this method the reactions of the surface complex formations are neglected ... 

Schindler (29, 30) has proposed a similar model in that ions are adsorbed, yet adsorption is understood in terms of surface complex formation with deprotonated surface OH-groups as ligands. His schematic example using Si as a typical oxide surface is ... 

In a more recent study. Nelson and Yang [494] pre.sented a surface complex-ation model to describe the effect of pH on adsorption equilibria of chlorophe-nols, i.e., the electrostatic effect they also discussed the potential importance of 7t-7t interactions and donor-acceptor complex formation but could not distinguish between the two and concluded, somewhat vaguely, that [t]hese proposed mechanisms provide plausible explanations for the surface complexation reactions between chlorophenols (neutral or anionic forms) and the surface of activated carbon (acidic or basic sites). ... 

Table 9.1 presents the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic of all surface com-plexation models (Dzombak and Morel, 1990) ... 

Figure 10.8. Schematic model of metal ion uptake through a membrane of a phytoplankton cell, (a) The metal ion is bound to the outside surface of the cell either by biologically released ligands or by surface functional ligand groups subsequently to the surface complex formation. The metals are carried—usually by porter molecules—to the inside of the cell. If the transport into the cell is slow in comparison to the pre-equilibration process on the solution side, then the uptake of the metal ion of the cell depends on the free metal ion activity, (b) Solution variables outside and inside the cell.

In surface precipitation, cations (or anions), which adsorb to the surface of a mineral, may form a precipitate of the cation (anion) with the constituent ions of the mineral at high surface coverage. Figure 13.28 shows schematically the surface precipitation of a cation Me " to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion that is. 

Figure 13.28. Schematic representation of surface precipitation on hydrous ferric oxide, Fe(OH)3(s). (a) At low surface coverage with Me, surface complex formation dominates. Instead of the usual short-hand notation (=Fe—OH + Me " 5= FeOMe" + H ), we use one that shows the presence of Fe(0H)3(s). (b) With progressive surface coverage, surface precipitation may occur. The surface precipitate is looked at as a solid solution of Fe(OH)3(s) and Me(0H)2(s) some isomorphic substitution of Me(II) for Fe(III) occurs. This model has been proposed by Farley et al. (1985).

Since the complex formation model can estimate the net surface charge, op, and thus in turn the surface potential, (equation v. Table 14.3), the colloid stability and pHp c can, at least in principle, be predicted (see Figure 14.8b). 

J. P. Chen and M. S. Lin, Surface charge and metal ions adsorption on an H-type activated carbon experimental observation and modeling simulation by the surface complex formation approach, Carbon 39, 1491-1504 (2001). 

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. 

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.

The preceding discussion relies on an analogy between complexation in solution and complexation at the mineral surface, a fundamental tenet of the surface complexation model (27). Strong complexation of metals in solution by humic substances is well-documented (16, 42-44). Thus surface complex formation is a likely mechanism for the adsorption of humic substances on oxide surfaces. 

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). 

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