Models activity model Geochemical

Francis Albarede has been a very active actor in this evolution towards quantitative science. His abundant scientific contributions published in the best international journals are all focussed on the goal of building a quantitative science. He is one of the leading scientists in this area and has now decided to broaden his approach by writing a book on geochemical modeling. This book has no equivalent in the present literature. It explains how we can build mathematical models to explain geochemical observations. 

The conceptual geochemical model of acidic waste after injection into the subsurface, proposed by Leenheer and Malcolm,102 involves a moving front of microbial activity with five zones as shown in Figure 20.10 ... 

In the simplest class of geochemical models, the equilibrium system exists as a closed system at a known temperature. Such equilibrium models predict the distribution of mass among species and minerals, as well as the species activities, the fluid s saturation state with respect to various minerals, and the fugacities of different gases that can exist in the chemical system. In this case, the initial equilibrium system constitutes the entire geochemical model. 

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. 

Geochemical models, as with the approach, are commonly formulated with a variant of the Freundlich isotherm based on a chemical reaction, like Reaction 9.1. In this approach, known as the reaction Freundlich model or the activity Freundlich model, the extent of sorption by the reaction can be expressed,... 

In the broadest sense, of course, no model is unique (see, for example, Oreskes et al., 1994). A geochemical modeler could conceptualize the problem differently, choose a different compilation of thermodynamic data, include more or fewer species and minerals in the calculation, or employ a different method of estimating activity coefficients. The modeler might allow a mineral to form at equilibrium with the fluid or require it to precipitate according to any of a number of published kinetic rate laws and rate constants, and so on. Since a model is a simplified version of reality that is useful as a tool (Chapter 2), it follows that there is no correct model, only a model that is most useful for a given purpose. 

The promoting and inhibiting species Aj are most commonly aqueous species, but may also be mineral, gas, or surface species. For aqueous, mineral, and surface species, mj is formally the volumetric concentration, in units such as mol cm-3 or mol l-1, but in geochemical modeling we commonly carry this variable as the species molality. Sometimes, especially when Aj is H+ or OH-, mj in this equation is understood to stand for the species activity rather than its molality. For a gas species, mj represents partial pressure or fugacity. 

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 the decade since I published the first edition of this book,1 the field of geochemical reaction modeling has expanded sharply in its breadth of application, especially in the environmental sciences. The descriptions of microbial activity, surface chemistry, and redox chemistry within reaction models have become more robust and rigorous. Increasingly, modelers are called upon to analyze not just geochemical but biogeochemical reaction processes. 

Once formed, H2S° would partially dissociate into HS- and H+. A small amount of H2As03 and H+ would form from the dissociation of H3As03°. Some H3As03° and H2S° could also react to produce thioarsenic species, such as AsS(OH)HS (see also Section 2.7.3). Depending on the accuracy and completeness of their thermodynamic databases, geochemical computer models may be able to identify the major reactions and estimate the activities of their products. 

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