Kinetics, geochemical

Geochemical kinetics can be viewed as applications of chemical kinetics to Earth sciences. Geochemists have borrowed many theories and concepts from chemists. Although fundamentally similar to chemical kinetics, geochemical kinetics distinguishes itself from chemical kinetics in at least the following ways ... 

Lasaga, A. C. (1980). The kinetic treatment of geochemical cycles. Geochim. Cosmoclhm. Acta 44, 815-828. 

Lasaga, A. C. (1981). Dynamic treatment of geochemical cycles global kinetics. In "Kinetics of Geochemical Processes" (A. C. Lasaga and R. J. Kirkpatrick, eds), pp. 69-110. Mineral. Soc. Amer., Washington, DC. 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

Hastings, D. and Emerson, S. (1986). Oxidation of manganese by spores of a marine Bacillus Kinetics and thermodynamic considerations. Geochem. Cosmochim. Acta 50,1819-1824. 

Quigley MS, Honeyman BD, Santschi PH (1996) Thorium sorption in the marine enviromnent equilibrium partitioning at the Hematite/water interface, sorption/desorption kinetics and particle tracing. Aquat Geochem 1 277-301... 

Langmuir, D., Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symposium, American Chemical Society, Washington, DC, 1979, pp. 353-387. 

Beginning in the late 1980s, a number of groups have worked to develop reactive transport models of geochemical reaction in systems open to groundwater flow. As models of this class have become more sophisticated, reliable, and accessible, they have assumed increased importance in the geosciences (e.g., Steefel et al., 2005). The models are a natural marriage (Rubin, 1983 Bahr and Rubin, 1987) of the local equilibrium and kinetic models already discussed with the mass transport... 

Do the kinetic rate constants and rate laws apply well to the system being studied Using kinetic rate laws to describe the dissolution and precipitation rates of minerals adds an element of realism to a geochemical model but can be a source of substantial error. Much of the difficulty arises because a measured rate constant reflects the dominant reaction mechanism in the experiment from which the constant was derived, even though an entirely different mechanism may dominate the reaction in nature (see Chapter 16). 

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. 

In geochemical kinetics, the rates at which reactions proceed are given (in units such as mol s-1 or mol yr 1) by rate laws, as discussed in the next section. Kinetic theory can be applied to study reactions among the species in solution. 

In this chapter we consider the problem of the kinetics of the heterogeneous reactions by which minerals dissolve and precipitate. This topic has received a considerable amount of attention in geochemistry, primarily because of the slow rates at which many minerals react and the resulting tendency of waters, especially at low temperature, to be out of equilibrium with the minerals they contact. We first discuss how rate laws for heterogeneous reactions can be integrated into reaction models and then calculate some simple kinetic reaction paths. In Chapter 26, we explore a number of examples in which we apply heterogeneous kinetics to problems of geochemical interest. 

As discussed already in Chapter 7, redox reactions constitute a second class of geochemical reactions that in many cases proceed too slowly in the natural environment to attain equilibrium. The kinetics of redox reactions, both homogeneous and those catalyzed on a mineral surface are considered in detail in the next chapter, Chapter 17, and the role microbial life plays in catalyzing redox reactions is discussed in Chapter 18. 

Reaction kinetics enter into a geochemical model, as we noted in the previous chapter, whenever a reaction proceeds quickly enough to affect the distribution of mass, but not so quickly that it reaches the point of thermodynamic equilibrium. In Part I of this book, we considered two broad classes of reactions that in geochemistry commonly deviate from equilibrium. 

It is in principle possible for a free enzyme to promote reaction in a geochemical system, but enzyme kinetics are invoked in geochemical modeling most commonly to describe the effect of microbial metabolism. Microbes are sometimes described from a geochemical perspective as self-replicating enzymes. This is of course a considerable simplification of reality, as we will discuss in the following chapter (Chapter 18), since even the simplest metabolic pathway involves a series of enzymes. 

Redox reactions in the geochemical environment, as discussed in previous chapters (Chapters 7 and 17), are commonly in disequilibrium at low temperature, their progress described by kinetic rate laws. The reactions may proceed in solution homogeneously or be catalyzed on the surface of minerals or organic matter. In a great many cases, however, they are promoted by the enzymes of the ambient microbial community. 

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 next chapter (Chapter 27) we show calculations of this type can be integrated into mass transport models to produce models of weathering in soils and sediments open to groundwater flow. In later chapters, we consider redox kinetics in geochemical systems in which a mineral surface or enzyme acts as a catalyst (Chapter 28), and those in which the reactions are catalyzed by microbial populations (Chapter 33). 

Various theories, ranging from qualitative interpretations to those rooted in irreversible thermodynamics and geochemical kinetics, have been put forward to explain the step rule. A kinetic interpretation of the phenomenon, as proposed by Morse and Casey (1988), may provide the most insight. According to this interpretation, Ostwald s sequence results from the interplay of the differing reactivities of the various phases in the sequence, as represented by Ts and k+ in Equation 26.1, and the thermodynamic drive for their dissolution and precipitation of each phase, represented by the (1 — Q/K) term. 

In this chapter, we consider how to construct quantitative models of the dynamics of microbial communities, building on our discussion of microbial kinetics in Chapter 18. In our modeling, we take care to account for how the ambient geochemistry controls microbial growth, and the effect of the growth on geochemical conditions. 

ABSTRACT Atmospheric carbon dioxide is trapped within magnesium carbonate minerals during mining and processing of ultramafic-hosted ore. The extent of mineral-fluid reaction is consistent with laboratory experiments on the rates of mineral dissolution. Incorporation of new serpentine dissolution kinetic rate laws into geochemical models for carbon storage in ultramafic-hosted aquifers may therefore improve predictions of the rates of carbon mineralization in the subsurface. 

Detailed interpretation of kinetic test data collected for environmental purposes has allowed criteria for ground and surface water geochemical exploration to be selected. Parameters predicted from kinetic testing to be anomalous in both ground and surface waters were observed to occur reflecting the presence of both molybdenum mineralization and natural acid rock drainage. Kinetic testing is very expensive and careful use of the acquired... 

Luther, G. W., (1990), The Frontier-Molecular-Orbital Theory Approach in Geochemical Processes", in W. Stumm, Ed., Aquatic Chemical Kinetics, Wiiey-lnterscience, New York, pp. 173-198. 

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