Solution-phase reactions, surface complexation models

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. 

In surface complexation modeling, chemisorption of ions on mineral surfaces is described by assuming reactions analogous to those that occur among solutes. Reactive surface sites are represented as independent reactant species. Surface hydroxyl groups, for example, are represented by =SOH°, where =S indicates a surface metal atom having multiple bonds in the bulk solid phase. With this notation, the coordination of an ion by a surface hydroxyl group may be described by... 

To see how this process works, we construct a model in which reaction of a hypothetical drainage water with calcite leads to the precipitation of ferric hydroxide [Fe(OH)3, which we use to represent HFO] and the sorption of dissolved species onto this phase. We assume that the precipitate remains suspended in solution with its surface in equilibrium with the changing fluid chemistry, using the surface complexation model described in Chapter 8. In our... 

Chemical equilibrium models are used to predict the speciation of dissolved solutes in natural systems (e.g., MINTEQA [31]). These models attempt to incorporate all of the various processes that affect the speciation of solutes, including all known solution-phase reactions (e.g., acid-base, precipitation-dissolution, and complexation reactions) and adsorption to solid surfaces. Current models for inorganic chemicals have been successM in predicting speciation in aqueous systems containing well-characterized soUd particles. 

Accurate predictions of the transport of As in groundwater requires site specific data to model adsorption/desorption reactions. In complex mixtures of minerals, it may not be possible to quantify the adsorption properties of individual minerals. Therefore, it has been suggested that adsorption properties of composite materials should be characterized as a whole (Davis and Kent, 1990). Previously published data for adsorption by pure mineral phases such as the surface complexation database for adsorption by ferrihydrite (Dzombak and Morel, 1990) can be a useful starting point for modeling adsorption of solutes in groundwater however, these equilibrium constants may not reflect the adsorption properties of composite oxide coatings on aquifer solids. For example, incorporation of Si, and to a lesser extent, A1 into Fe oxyhydroxides has been shown to decrease adsorption reactivity towards anions (Ainsworth et al., 1989 Anderson and Benjamin, 1990 Anderson et al, 1985). Therefore, equilibrium constants will likely need to be modified for site-specific studies. 

Considering the mechanisms of sorption discussed in Section II, e.g. surface precipitation or formation of new phases involving the adsorbent and the adsorbate, the above kinetic model is not sufficient to describe isotope exchange in all relevant systems, although most experimental kinetic curves can be reproduced by proper adjustment of parameters (effective D in the solid and in the liquid film, rate constants of surface reactions) within the discussed above model. Spectroscopic studies (Section I and II) suggest that the uptake of adsorbate is often due to simultaneous formation of surface complex and surface precipitation. Single F t) curve based on the radioactivity of the solution is not sufficient to describe sorption kinetics in such systems. 

The early models yielded approximate concentrations that reflected the understanding of the soil solution at the time. Later models have yielded better predictions of the soil solution s composition, but they are still only approximate. That reflects the complexity of the soil more than the inadequacy of modeling. The models predict ion interactions in the aqueous solution quite well. Reactions at the surface of colloidal particles are more complex, less understood, slower, and hence are more difficult to formulate. In addition, the models are forced to use the solubility products of pure, simple solids. Soil inorganic particles are far from pure compounds, are often poorly crystalline to amorphous, are not at internal equilibrium, and may not be in equilibrium with the aqueous phase. In addition, the reactions of soil organic matter are not known quantitatively aud soils are open systems, meaning that matter is continually being added and removed. 

The purpose of this paper is to present a model for single metal-multiligand solution equilibrium which quantitatively describes the net binding of metal ions to the multiligand mixture as a whole. Equally important aspects of this model are that it yields a unified conceptual visualization of complexation in complicated multiligand mixtures, and provides a framework for interpreting the results of experiments on such systems. The model, as presented here, will be confined to mononuclear complexes, and it is assumed that no solid phases are present. The influence of pH on the complexation reactions is described. The model is illustrated by simulation of various metal-multiligand systems, and the properties of these systems are represented by means of three-dimensional plots, called stability surfaces. 

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