Surface complexation constants

The second and third method allow the measurement of surface complexation constants at various transition metal loadings and consequently yield apparent composition dependent constants. In the first method on the contrary a truly thermodynamic constant is obtained under standard state conditions. 

All three methods lead to consistent surface complexation constants in clay minerals (27, 28) and zeolites (82). The surface protonation constants of the organic compounds such as amines can be defined and determined similarly (41, 83) and are also found to be enhanced in the clay interface. 

As shown in Table V the rate of Mn(II) oxidation in 0.7M NaCl is about 3 times slower than in 0.7M NaC104, but the amount of Mn(II) adsorbed is only about a third less in the chloride solution. In the absence of surface complexation constants in these electrolyte matrices no model calculations can be made. 

Surface complexation constants. For reactions involving adsorption on surfaces the symbol =SOH is often used to designate a surface complexation site. So, for example, the reaction between lead and a surface can be written as... 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

An important development of the model presented by Davis al. (j.) was a method for estimating intrinsic surface ionization and complexation constants. It was suggested that intrinsic surface complexation constants, e.g. be... 

The results of a newly proposed model for adsorption at the oxide/water interface are discussed. The modeling approach is similar to other surface complexation schemes, but mass-law equations are corrected for the effect of the electrostatic field. In this respect, this model bridges the gap between those models that emphasize physical interactions. The general applicability of the model is demonstrated with comparisons of calculations and experimental data for adsorption of metal ions, anions, and metal-ligand complexes. Intrinsic ionization and surface complexation constants can be determined with an improved double extrapolation technique. 

D.A. Dzombak, F.M.M. Morel, Surface Complexatton Modelling Hydrous Ferric Oxide, Wiley (1990). (Compilation of surface complexation constants obtained after critical evaluation of literature data, using the method of the "generalized two-layer model".)... 

Sorption overview. Both empirical and mechanistic approaches have emerged since the 1970s to describe interactions between radionuclides and geomedia. These are based on conditional constants, which are valid for specific experimental conditions, or more robust intrinsic constants, which are valid over a wider range of conditions. The empirical approach involves measurements of conditional radionuclide distribution or sorption coefficients or Rsite-specific water-rock systems using synthetic or natural ground waters and crushed rock samples. Mechanistic-based approaches produce intrinsic, thermodynamic surface-complexation constants for simple electrolyte solutions with pure mineral phases. 

Figure 4 Comparison of sorption models. Several commonly used sorption models are compared with respect to the independent constants they require. These constants are vahd only under specific conditions, which must be specified in order to properly use them. In other words, the constants are conditional with respect to the experimental variables described in the third column of the figure. is the radionuclide distribution constant K and n are the Freundlich isotherm parameters and are surface complexation constants for protonation and deprotonation of surface sites K-, are surface complexation constants for sorption of cations and anions in the constant...

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. 

The generalized composite (GC) approach is an alternative approach in which surface-complexation constants are obtained by fitting experimental data for the natural mineral assemblage directly (KofS, 1988 Davis et al., 1998). A simplified form of this approach fits the pH-dependent sorption of the radionuclide without representation of the electrostatic interaction terms found in other SCMs. The disadvantages... 

There are an increasing number of data in the literature. A compilation for surface complexation constants for inorganic surface species on hydrous ferric oxide is available (Dzombak and Morel, 1990). Often correlations from solute complex formation constants with those of surface complex formation are possible. The good correlation obtained in this and similar linear free energy relationships (LFERs) is exemplified in Figure 14.9. 

Surface complexation is a typical multi-component reaction, similar to cation exchange. The database for surface complexation includes complexation constants for major elements in groundwater such as and S04 , but not for and HCOs". In the first instance, constants for these ions can be estimated with linear free energy relations (LFER s) in which the properties of similar chemical systems are compared and interpolated (Dzombak and Morel, 1990). Thus, the surface complexation constant for is expected to lie in between the ones for and for Zn, in line with the known differences of the association constants of these heavy metals with OH in water. For the weak sites, the LFER gives ... 

Figure 2. Sorption edge of Fe on ferrihydrite, data from Liger et at. (1999), model with surface complexation constants estimated from linear free energy relation fLFERj and model optimized on the data of Liger et al.

Table 2. Surface complexation constants of Fe and HC03 on ferrihydrite (Appelo et al., 2002).

Figure 10,26 Correlation plot for some metal cations, of their first hydrolysis constants ( /fii) versus intrinsic surface complex constants i Ku) for their adsorption by Si02(am) assuming the constant capacitance model. The equation of the solid line is log = 0.09 -( 0.62 log A. Hydrolysis and adsorption reactions are written A,i -t- H2O = +...

Smith, R. W., and E. A. Jenne. 1991. Recalculation, evaluation, and prediction of surface complexation constants for metal adsorption on iron and manganese oxides. Envir. Sci. Technol. 25 525-31. 

Figure 10.27 Correlation plot for some soft and borderline hard-soft acid cations, of their first hydrolysis constants ( Tn) versus intrinsic surface complex constants for their ad-...

Figure 6.6. Fit of the diffuse layer model to copper adsorption by hydrous ferric oxide. The solid line represents the optimal ht for these data. The dashed line represents the fit corresponding to the best overall estimate of the Cu surface complexation constant obtained from 10 Cu adsorption edges. (From Dzombak and Morel. 1990.)...

The charge balance expressions for the surface plane, CTo, and the intermediate plane, oi, are dependent on the charge distribution, as are the intrinsic surface complexation constants. The changes in charges in these planes are given by (Venema et al., 1997)... 

In the triple layer model, values for the intrinsic protonation and dissociation constants, as well as values for tlie intrinsic surface complexation constants for the background electrolyte, can be obtained from hnear, double, or electrokmetic extrapolations to zero surface charge and zero and infinite electrolyte concentration. Values of intrinsic protonation-dissociation constants and intrinsic surface complexation constants for background electrolytes obtamed for the triple layer model using the various extrapolations are compiled in Goldberg (1992). Use of graphical extrapolation methods has been criticized because the triple layer parameter values obtained are not unique (Koopal et al., 1987). 

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