Coupled reactive mass transport model

In general, geochemical models can be divided according to their levels of complexity (Figure 2.3). Speciation-solubility models contain no spatial or temporal information and are sometimes called zero-dimension models. Reaction path models simulate the successive reaction steps of a system in response to the mass or energy flux. Some temporal information is included in terms of reaction progress, f, but no spatial information is contained. Coupled reactive mass transport models contain both temporal and spatial information about chemical reactions, a complexity that is desired for environmental applications, but these models are complex and expensive to use. 

In this book, we use the term coupled model to describe models in which two sets of equations that describe two types of processes are solved together. For example, multi-component, multi-species coupled reactive mass transport models (this is a mouthful,... 

Steefel et al. ([23] and references therein) noted that the approach does not account for pH, competitive ion effects or oxidation-reduction reactions. As a consequence, values may vary by orders of magnitude from one set of conditions to another. Chen [25] also highlighted these limitations by comparing numerical modeling results of contaminant transport using a multi-component coupled reactive mass transport model and a based transport model. The conclusion from this work was that values vary with location and time and this variation could not be accounted for in the model. 

Freedman V. L. and Ibaraki M. (2003) Coupled reactive mass transport and fluid flow issues in model verification. Adv. Water Resour. 26, 117-127. 

In this chapter we shall limit our discussion of the codes to speciation-solubility and reaction path modeling codes. Coupled reactive mass transport codes are much more mathematically and computationally complex. The readers can find recent discussions in Lichtner (1996) and Steefel and MacQuarrie (1996). 

As discussed in Chapter 2, we reserve the term coupled transport model to multiple component-multiple species reactive mass transport models such as phreeqc described above. In coupled models, two set of equations are solved together through some coupling schemes. In the case of coupled reactive transport models, two sets of mathe-... 

Figure2 Ohnishi et al. (1985) and Chijimatsu et al. (2000)) and reactive-mass transport model (inside the box named Chemical in Figure 2). This is a system of governing equations composed of Equations (l)-(9), which couple heat flow, fluid flow, deformation, mass transport and geochemical reaction in terms of following primary variables temperature T, pressure head y/, displacement u total dissolved concentration of the n master species C< > and total dissolved and precipitated concentration of the n" master species T,. Here we set master species as the linear independent basis for geochemical reactions, and speciation in solution and dissolution/precipitation of minerals are calculated by a series of governing equations for geochemical reaction. Now we adopt equilibrium model for geochemical reaction (Parkhurst et al. (1980)), mainly because of reliability and abundance of thermodynamic data for geochemical reaction.

In the previous part of the book chemical interactions were described without any consideration of transport processes in aqueous systems. Models for reactive mass transport combine these chemical interactions with convective and dispersive transport, so that they can model the spatial distribution coupled to the chemical behavior. Requirement for every transport model is a flow model as accurate as possible. 

Coupled mass transport models can also include heat transport (e.g., Raffensperger and Garven, 1995) or fluid flow. Coupled reactive transport models represent the desired tools for evaluating fate and transport of contaminants. 

Since publication of the first edition, the held of reaction modeling has continued to grow and hnd increasingly broad application. In particular, the description of microbial activity, surface chemistry, and redox chemistry within reaction models has become broader and more rigorous. Reaction models are commonly coupled to numerical models of mass and heat transport, producing a classification now known as reactive transport modeling. These areas are covered in detail in this new edihon. 

At the same time, reaction modeling is now commonly coupled to the problem of mass transport in groundwater flows, producing a subfield known as reactive transport modeling. Whereas a decade ago such modeling was the domain of specialists, improvements in mathematical formulations and the development of more accessible software codes have thrust it squarely into the mainstream. 

As discussed in previous sections, coupled reactive transport models generate numerous data and the predicted mass transport is complex. Here, we only give some basic information from this modeling exercise. Interested readers are referred to Zhu and Burden (2001) and Zhu et al. (2001a, 2002) for details. 

While the mechanistic treatment of chemical reactions in the coupled multi-component, multi-species mass transport has obvious advantages over the empirical isotherm-based transport models, we can also easily compile a long list of shortcomings for coupled reactive transport models. We choose a few and list them here. 

Electroactive polymers have a number of attractive features that account for this continuing interest. First they present a distributed array of catalytic sites. Thus in contrast to monolayer chemically modified electrodes, there are potentially a much greater number of reactive sites that can contribute to the catalytic current. Since these sites are distributed throughout the film, it is essential to consider the mass transport of reagents into the film and the mass transport of products out of these films when studying the overall kinetics of these processes. The coupled mass transport and kinetics in redox polymer films have been investigated in some detail, and good models exist for these processes. ... 

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