Surface complexation models titrations

The surface complexation models used are only qualitatively correct at the molecular level, even though good quantitative description of titration data and adsorption isotherms and surface charge can be obtained by curve fitting techniques. Titration and adsorption experiments are not sensitive to the detailed structure of the interfacial region (Sposito, 1984) but the equilibrium constants given reflect - in a mean field statistical sense - quantitatively the extent of interaction. 

Hayes, K. F., Redden, G., Ela, W. Leckie, J. O. 1991. Surface complexation models an evaluation of model parameter estimation using FITEQL and oxide mineral titration data. Journal of Colloid and Interface Science, 142, 448-469. 

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). 

The characteristic properties of some soils, studied by potentiometric titration and the surface complexation model, are shown in Table 3.12. The mineral composition of some soils are also provided (Table 3.13). 

The proton-promoted surface complexation model was applied to silicates by several research groups in the late 1980s (e.g., Blum and Lasaga, 1988, 1991 Brady and Walther, 1989, 1992 Schott, 1990). For example, Blum and Lasaga (1988, 1991) performed dissolution and titration experiments for albite and found that the surface charge under acid conditions. 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

Spadini, L. et al., Hydrous ferric oxide Evaluation of Cd-HFO surface complexation models combining Cd EXAFS data, potentiometric titration results and surface site structures identified from mineralogical knowledge, J. Colloid Interf. Sci., 266, 1, 2003. 

A detailed discussion of the FITEQL optimization of TLM surface parameters for a-Al203 has been discussed previously (Hayes et al., 1991). The analysis demonstrated that the TLM version of the surface complexation model can fit titration data reasonably well using a range of TLM surface parameter sets (see Fig. 7-5 as an example). While it was not possible to identify a single optimum set of parameters, it was possible to narrow the range of parameters. Valid sets of surface parameters for the TLM were found for ApXa between 0 and 6, C, between 0.8 and 2.0 F m-2, and Ns between 1 and 10 sites nm 2. In all cases the value of C2 was set equal to 0.2 F m-2. 

Most applications in the regulatory environment have used speciation-solubility models. A few used surface complexation models. Applications of surface complexa-tion models mostly used the model and data from Dzombak and Morel (1990). Reaction path calculations are mostly limited to the titration and mixing calculations of two fluids. 

Hayes K. E, Redden G., Ela W., Leckie J. O. (1991) Surface Complexation Models - an Evaluation of Model Parsuneter-Estimation Using FITEQL md Oxide Mineral Titration Data. J. of Colloid and Interface Science. Vol. 142, N2, p. 448-469. 

Identification of the specific species of the adsorbed oxyanion as well as mode of bonding to the oxide surface is often possible using a combination of Fourier Transform Infrared (FTIR) spectroscopy, electrophoretic mobility (EM) and sorption-proton balance data. This information is required for selection of realistic surface species when using surface complexation models and prediction of oxyanion transport. Earlier, limited IR research on surface speciation was conducted under dry conditions, thus results may not correspond to those for natural systems where surface species may be hydrated. In this study we review adsorbed phosphate, carbonate, borate, selenate, selenite, and molybdate species on aluminum and iron oxides using FTIR spectroscopy in both Attenuated Total Reflectance (ATR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) modes. We present new FTIR, EM, and titration information on adsorbed arsenate and arsenite. Using these techniques we... 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

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 this survey, it is not attempted to show how successful surface complexation models are. Rather, it is attempted to show what can be done with them, what will be done with them in the future and what should not be done with them. The experimental aspects (e.g., the input data to the models) are discussed whenever judged important (certainly without full coverage but rather with focus on those aspects, which have not yet been addressed or details which have been of interest to the present author). In particular, the importance of combining methods and data is stressed. In recent years, there has been an increased interest in linking surface complexation models, which are traditionally based on macroscopic (adsorption, titration, and other) data, with structural information obtained with modem spectroscopic methods such as x-ray absorption spectroscopy (XAS). It is expected that the closer the agreement of the thermodynamic formulation of a surface chemical reaction with the actual structure of a surface complex is, the more reliable a prediction of the system behavior under more or less strongly varied conditions will be. 

Borrok et al. (2004a) used potentiometric titration to measure Cd sorption by different bacterial consortia, and a surface complexation approach to determine thermodynamic stability constants. When the data were modeled by adopting a single set of stability constants, a similar sorption behavior was shown by a wide range of bacterial species. Further, current models that rely on pure strains of laboratory-cultivated bacterial species appear to overestimate the extent of metal biosorption in natural systems. 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

---