Types of Geochemical Models

Reaction-path calculations Reactive transport calculations 

Geochemical models have been extensively reviewed in the literature (Appelo and Postma, 1993 Nordstrom and Ball, 1984 Paschke and van der Heijde, 1996 and Plummer et al., 1992). Here we describe them only briefly. More detailed descriptions of models can be found in the above-mentioned reviews, the manuals for the geochemical modeling codes, and individual chapters for respective types of models in this book. 

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An important consideration in constructing certain types of geochemical models, especially those applied to environmental problems, is to account for the sorption of aqueous species onto sediment surfaces (e.g., Zhu and Anderson, 2002). Because of their large surface areas and high reactivities (e.g., Davis and Kent, 1990), many components of a sediment - especially clay minerals, zeolites, metal oxides and oxyhydroxides, and organic matter - can sorb considerable masses. 

Consider a combination of processes, such as the solution mentioned above dissolving limestone in one area, then flowing to another location where it loses some of its CO2 content, and precipitates calcite. The overall process is, of course, far from equilibrium. Nevertheless, the process can be considered in a number of separate steps, each of which is not far from equilibrium. Thus a state of local undersaturation might be calculated, then the calcium and carbonate content of the solution increased slightly, or the CO2 content decreased slightly, or whatever, and the calculation repeated until some final state is achieved. The overall process is simulated as a number of equilibrium steps. This is a type of geochemical modeling known as titration, and is discussed in Chapter 8. 

Reaction models, despite their simple conceptual basis (Fig. 2.1), can be configured in a number of ways to represent a variety of geochemical processes. Each type of model imposes on the system some variant of equilibrium, as described in the previous section, but differs from others in the manner in which mass and heat transfer are specified. This section summarizes the configurations that are commonly applied in geochemical modeling. 

Geochemical modelers currently employ two types of methods to estimate activity coefficients (Plummer, 1992 Wolery, 1992b). The first type consists of applying variants of the Debye-Hiickel equation, a simple relationship that treats a species activity coefficient as a function of the species size and the solution s ionic strength. Methods of this type take into account the distribution of species in solution and are easy to use, but can be applied with accuracy to modeling only relatively dilute fluids. 

The reaction rate Rj in these equations is a catch-all for the many types of reactions by which a component can be added to or removed from solution in a geochemical model. It is the sum of the effects of equilibrium reactions, such as dissolution and precipitation of buffer minerals and the sorption and desorption of species on mineral surfaces, as well as the kinetics of mineral dissolution and precipitation reactions, redox reactions, and microbial activity. 

In a solid-fluid reaction system, the fluid phase may have a chemistry of its own, reactions that go on quite apart from the heterogeneous reaction. This is particularly true of aqueous fluid phases, which can have acid-base, complexation, oxidation-reduction and less common types of reactions. With rapid reversible reactions in the solution and an irreversible heterogeneous reaction, the whole system may be said to be in "partial equilibrium". Systems of this kind have been treated in detail in the geochemical literature (1) but to our knowledge a partial equilibrium model has not previously been applied to problems of interest in engineering or metallurgy. 

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