Kinetics of dissolution and precipitation

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. 

We might, for example, study the rate at which the ferrous ion Fe++ oxidizes by reaction with O2 to produce the ferric species Fe+++. Since the reaction occurs within a single phase, it is termed homogeneous. Reactions involving more than one phase (including the reactions by which minerals precipitate and dissolve and those involving a catalyst) are called heterogeneous. 

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. 

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Brantley, S. L., 1992, Kinetics of dissolution and precipitation - experimental and field results. In Y. K. Kharaka and A. S. Maest (eds.), Water-Rock Interaction. Balkema, Rotterdam, pp. 3-6. 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

Nagy K. L. and Lasaga A. C. (1990) The effect of deviation from equilibrium on the kinetics of dissolution and precipitation of kaolinite and gibbsite. Chem. Geol 84, 283 -285. 

To formulate a kinetic reaction path, we consider one or more minerals A whose rates of dissolution and precipitation are to be controlled by kinetic rate laws. We wish to avoid assuming that the minerals A- are in equilibrium with the... 

Bolton E. W., Lasaga A. C., and Rye D. M. (1999) Long-term flow/chemistry feedback in a porous medium with heterogeneous permeability kinetic control of dissolution and precipitation. Am. J. Sci. 299, 1-68. 

PRECIP (Noy 1990) was developed to examine changes in the mass-transfer properties of a system resulting from mineral reactions. The rates of these mineral reactions are explicitly included by using a kinetic formulation of dissolution and precipitation. The conceptual model is of a one-dimensional flow path along which the flow is Darcian and in which precipitation and dissolution reactions can take place amongst a number of components. The flow field is defined by fixed values of head at... 

Nagy, K. L. (1995). Dissolution and precipitation kinetics of sheef silicates. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America, Washington, DC, Reviews in Mineralogy 31, 173-233. 

Christy, A.G. and Putnis, A. (1993) The kinetics of barite dissolution and precipitation in water and sodium chloride barines at 44-850°C. Geochim. Cosmochim. Acta, 57, 2161-2168. 

Little is known about the kinetics of dissolution, precipitation, and oxidation-reduction reactions in the natural environment. Consequently, simulating the kinetics of even more complicated injection- zone chemistry is very difficult. 

In kinetic reaction paths (discussed in Chapter 16), the rates at which minerals dissolve into or precipitate from the equilibrium system are set by kinetic rate laws. In this class of models, reaction progress is measured in time instead of by the nondimensional variable . According to the rate law, as would be expected, a mineral dissolves into fluids in which it is undersaturated and precipitates when supersaturated. The rate of dissolution or precipitation in the calculation depends on the variables in the rate law the reaction s rate constant, the mineraTs surface area, the degree to which the mineral is undersaturated or supersaturated in the fluid, and the activities of any catalyzing and inhibiting species. 

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). 

We might take a purist s approach and attempt to use kinetic theory to describe the dissolution and precipitation of each mineral that might appear in the calculation. Such an approach, although appealing and conceptually correct, is seldom practical. The database required to support the calculation would have to include rate laws for every possible reaction mechanism for each of perhaps hundreds of minerals. Even unstable minerals that can be neglected in equilibrium models would have to be included in the database, since they might well form in a kinetic model (see Section 26.4, Ostwald s Step Rule). If we are to allow new minerals to form, furthermore, it will be necessary to describe how quickly each mineral can nucleate on each possible substrate. 

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. 

Retardation also arises when a fluid undersaturated or supersaturated with respect to a mineral invades an aquifer, if the mineral dissolves or precipitates according to a kinetic rate law. When the fluid enters the aquifer, a reaction front, which may be sharp or diffuse, develops and passes along the aquifer at a rate less than the average groundwater velocity. Lichtner (1988) has derived equations describing the retardation arising from dissolution and precipitation for a variety of reactive transport problems of this sort. 

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 build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

A number of factors contribute to the disparity between the predictions of kinetic theory and conditions observed in the field, as discussed in Section 16.2. In this case, we might infer the dissolution and precipitation of minerals such as opal CT (cristobalite and tridymite, Si02), smectite and other clay minerals, and zeolites help control silica concentration. The minerals may be of minor significance in the aquifer volumetrically, but their high rate constants and specific surface areas allow them to react rapidly. 

Nagy, K. L., 1995, Dissolution and precipitation kinetics of sheet silicates. Reviews in Mineralogy 31, 173-233. 

Bolton EW, Lasaga AC, Rye DM (1996) A model for the kinetic control of quartz dissolution and precipitation in porous media flow with spatially variable permeability Eormulation and examples of thermal convection. J Geophys Res 101 22,157-22,187 Bolton EW, Lasaga AC, Rye DM (1997) Dissolution and precipitation via forced-flux injection in the porous medium with spatially variable permeability Kinetic control in two dimensions. J Geophys Res 102 12,159-12,172... 

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