Surface complexation models site concentration

FIGURE 2.3 Potentiometric titration curve of copper-montmorillonite in 0.1 mol dm-3 NaC104 solution, m = 50 mg, V = 20 cm3 (upper left). Vs are the experimental points, line is the plotted curve by the surface complexation model. The concentration of surface sites—lower left interlayer cations upper right silanol sites lower right aluminol sites (Nagy and Konya 2004). 

It is useful to compare the capacity for each metal to be sorbed (the amount of each that could sorb if it occupied every surface site) with the metal concentrations in solution. To calculate the capacities, we take into account the amount of ferric precipitate formed in the calculation (0.89 mmol), the number of moles of strongly and weakly binding surface sites per mole of precipitate (0.005 and 0.2, respectively, according to the surface complexation model), and the site types that accept each metal [As(OH)4 and ASO4 sorb on weak sites only, whereas Pb++, Cu++, and Zn++ sorb on both strong and weak]. 

The Concentration of Edge Sites and Intrinsic Stability Constants of Protonation and Deprotonation of Silanol and Aluminol Sites of Montmorillonite Samples Calculated by the Surface Complexation Model... 

Methods for measurement of parameters used in SCM s have been described in the literature. Only a brief summary is presented here. Surface complexation model parameters that can be measured directly include, (1) the solid concentration, (2) surface site density, (3) surface area, and (4) equilibrium constants for the mass action equations describing all relevant adsorption reactions. The relation between surface charge and potential is calculated in geochemical equilibrium models. 

A diffuse layer surface-complexation model was used to model adsorption of As. Two different types of sites were used Surfr OH represents the total concentration of surface complexation sites, which were used to model adsorption/desorption of H ... 

Surface complexation models of the solid-solution interface share at least six common assumptions (1) surfaces can be described as planes of constant electrical potential with a specific surface site density (2) equations can be written to describe reactions between solution species and the surface sites (3) the reactants and products in these equations are at local equilibrium and their relative concentrations can be described using mass law equations (4) variable charge at the mineral surface is a direct result of chemical reactions at the surface (5) the effect of surface charge on measured equilibrium constants can be calculated and (6) the intrinsic (i.e., charge and potential independent) equilibrium constants can then be extracted from experimental measurements (Dzombak and Morel, 1990 Koretsky, 2000). 

Saunders and Toran (1995) used an average of 0.9 wt.% amorphous Fe(OH)3 in soil around the Oak Ridge burial ground for calculating sorbent concentrations. The amorphous iron concentrations were obtained from the sequential extraction techniques. To calculate the surface sites in their surface complexation modeling, they used a soil porosity of 40% and dry density of 2.5 g cm-3, respectively. The calculation is recapped here ... 

Fig. 3 Experimental points of net proton surface excess amounts from the reversible backward titration cycles of sodium montmoril-lonite at different NaCl concentrations. The different lines represent the results of numerical fitting (FITEQL [28]) using the diffuse-double-layer option of the surface complexation model assuming reactions of and Na" ions with permanently charged ion-exchange sites in parallel with protonation/deprotonation reactions on amphoteric edge sites...

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. 

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. 

Bonded stationary phases for NPC are becoming increasingly popular in recent years owing to their virtues of faster column equilibration and being less prone to contamination by water. The use of iso-hydric (same water concentration) solvents is not needed to obtain reproducible results. However, predicting solute retention on bonded stationary phases is more difficult than when silica is used. This is largely because of the complexity of associations possible between solvent molecules and the chemically and physically heterogeneous bonded phase surface. Several models of retention on bonded phases have been advocated, but their validity, particularly when mixed solvent systems are used as mobile phase, can be questioned. The most commonly accepted retention mechanism is Snyder s model, which assumes the competitive adsorption between solutes and solvent molecules on active sites... 

The intrinsic constants are thermod3mamic constants written for reactions occurring at a hypothetical isolated site on the surface. Actual activities on the surface cannot be directly determined but Q or apparent stability quotients can be calculated based on measurable bulk concentrations. The intrinsic constants and apparent stability quotients are related by considering the electrostatic correction for an ion in solution near the surface compared to an isolated ion on the surface. In an idealized planar model, is the mean potential at the plane of surface charge created by the ionization of the surface functional groups and the formation of surface complexes and is the mean potential at the plane of adsorbed counter ions at a distance 3 from the surface (17). The electrostatic interaction energies at the surface and at a distance 3 are expressed as exponentials. Therefore ... 

In natural waters, other surface reactions will be occurring simultaneously. These include protonation and deprotonation of the >FeOH site at the inner o-plane and complexation of other cations and anions to either the inner (o) or outer (IS) surface planes. Expressions similar to Equation (5) above can be written for each of these reactions. In most studies, the activity coefficients of surface species are assumed to be equal to unity thus, the activities of the surface sites and surface species are equal to their concentrations. Different standard states for the activities of surface sites and species have been defined either explicitly or implicitly in different studies (Sverjensky, 2003). Sveijensky (2003) notes that the use of a hypothetical 1.0 M standard state or similar convention for the activities of surface sites and surface species leads to surface-complexation constants that are directly dependent on the site density and surface area of the sorbent. He defines a standard state for surfaces sites and species that is based on site occupancy and produces equilibrium constants independent of these properties of the solids. For more details about the properties of the electrical double layer, methods to calculate surface specia-tion and alternative models for activity coefficients for surface sites, the reader should refer to the reference cited above and other works cited therein. 

Dove and Elston, this interfadal layer can be described by a triple layer snrface com-plexation model (TLM) as shown in Fig. 4.31. The interface consists of three electrostatically charged regions, each with an associated electric potential and snrface charge these are termed the o, p, and d planes. Hydrogen ions are permitted to coordinate with the nnsatnrated sites of the interface at the innermost o layer. Sodinm is positioned at the P layer or the d layer. The surface silicon-oxygen complex may have a different chemical character depending on the adsorbed species, hi a sodium chloride solution the surface complexes can be represented as sSiOHaCl, sSiOHj, =SiOH, =SiO-Na, and SiO". The concentration of each species depends on pH and salt concentration, and the sum of the fractions of these surface species equals 1 ... 

Uptake curves in Cd-rutile system (at four different total Cd concentrations and one ionic strength) were modeled using seven types of surface sites reacting with for Cd [114]. This model involves fourteen adjustable parameters (concentrations of surface sites and stability constants of corresponding surface complexes) and it is valid only for one ionic strength. 

The rate-controlling step for dissolution of an oxide or primary silicate mineral generally involves a surface reaction. For surface-controlled dissolution, the rate-controlling step is either the detachment of silica or a metal ion from the surface or the attack of the surface to form precursor sites for detachment. Surface detachment controlled kinetics can be modelled using the surface complexation rate model (Wieland et al., 1988) that models rates as a function of the surface concentration of surface complexation sites that are precursors for dissolution. In this model, the formation of precursor sites is rapid compared to the rate of detachment and the concentration of sites can be described by surface complexation theory (Sposito, 1983). 

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