Minerals precipitation/dissolution

Dissolution of carbonate minerals does not lead to mineral trapping of C02 (Gunter et al. 1993). However, carbonate dissolution, and other mineral precipitation-dissolution reactions can impact sequestration capacity by altering the permeability of the aquifer near the injection site. 

Lichtner (2001) developed the computer code FLOTRAN, with coupled thermal-hydrologic-chemical (THC) processes in variably saturated, nonisothermal, porous media in 1, 2, or 3 spatial dimensions. Chemical reactions included in FLOTRAN consist of homogeneous gaseous reactions, mineral precipitation/dissolution, ion exchange, and adsorption. Kinetic rate laws and redox... 

Simulations of THC processes were performed using the TOUGHREACT code (Xu and Pruess, 2001 Xu et al 2003). TOUGHREACT is based on TOUGH2 (Pruess, 1991) and in addition to the coupling between heat, water, and vapor flow for multiphase systems, it solves aqueous and gaseous species transport, kinetic and equilibrium mineral-water reactions, and feedback of mineral precipitation/dissolution on porosity, permeability, and capillary pressure. 

In this chapter we consider how to construct reactions paths that account for the effects of simple reactants, a name given to reactants that are added to or removed from a system at constant rates. We take on other types of mass transfer in later chapters. Chapter 14 treats the mass transfer implicit in setting a species activity or gas fugacity over a reaction path. In Chapter 16 we develop reaction models in which the rates of mineral precipitation and dissolution are governed by kinetic rate laws. 

Despite the authority apparent in its name, no single rate law describes how quickly a mineral precipitates or dissolves. The mass action equation, which describes the equilibrium point of a mineral s dissolution reaction, is independent of reaction mechanism. A rate law, on the other hand, reflects our idea of how a reaction proceeds on a molecular scale. Rate laws, in fact, quantify the slowest or rate-limiting step in a hypothesized reaction mechanism. 

Comparing the development here to the accounting for the kinetics of mineral precipitation and dissolution presented in the previous chapter (Chapter 16), we see the mass transfer coefficients v and so on serve a function parallel to the coefficients v , etc., in Reaction 16.1. The rates of change in the mole number of each basis entry, accounting for the effect of each kinetic redox reaction carried in the simulation, for example,... 

Fig. 25.1. Mineralogical consequences of mixing the two fluids shown in Table 25.1 at 60 °C in the presence of microcline, muscovite, quartz, and dolomite. Results shown as the volume change for each mineral (precipitation is positive, dissolution negative), expressed per kg of pore water.

In calculating most of the reaction paths in this book, we have measured reaction progress with respect to the dimensionless variable . We showed in Chapter 16, however, that by incorporating kinetic rate laws into a reaction model, we can trace reaction paths describing mineral precipitation and dissolution using time as the reaction coordinate. 

The negative sign refers to mineral precipitation instead of dissolution. Such computations are done with PHREEQC (Parkhurst Appelo 1999). An inescapable conclusion of mass balances is that during the weathering of pyritiferous hydrothermally altered rock, iron and silica are precipitated. 

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

A significant amount of seawater is trapped in the open spaces that exist between the particles in marine sediments. This fluid is termed pore water or interstitial water. Marine sediments are the site of many chemical reactions, such as sulfate reduction, as well as mineral precipitation and dissolution. These sedimentary reactions can alter the major ion ratios. As a result, the chemical composition of pore water is usually quite different from that of seawater. The chemistry of marine sediments is the subject of Part 111. 

The Chemical Speciation of Iron Mineral Precipitation and Dissolution... 

The fact that the hydrogen ion is an important chemical species in these reactions is indicative of the major role that carbonic acid plays in influencing the pH and buffer capacity of natural waters. Furthermore, the activity of the carbonate anion in part determines the degree of saturation of natural waters with respect to carbonate minerals. Determination of the activity or concentration of CO32- is not an easy task nevertheless, it is necessary to the interpretation of a myriad of processes, including carbonate mineral and cement precipitation-dissolution and recrystallization reactions. 

In a subsequent wetting event the primary minerals, the precipitates, and the secondary minerals formed under dry conditions may react. As new low-concentration water films come in contact with the minerals, the dissolution processes are activated once again. 

Smectites are one of the most important soil clay minerals as regards cation exchange. The reversible exchange reactions of these minerals with metal cations may be pictured as kinetically-favored (i.e., rapid) precipitation-dissolution reactions (44). From this point of view, it is meaningful to write the "exchange half-reaction" ... 

One of the key objectives in estimating fresh rock compositions is to determine the chemical inventory present before alteration in a given reference volume of altered rock. This is different from the fresh-rock composition, because water-rock interaction occurs in an open system. The pitfalls arising from open-system behavior can be illustrated in two examples secondary mineral precipitation and rock dissolution. 

Empirical models predicting the rates of mineral-specific dissolution as a function of pH are summarized within the section on mineral composition in an attempt to provide a useful database for predicting dissolution rates for both laboratory and field systems. Equations describing near-equilibrium mineral dissolution and precipitation rates are summarized in the section on chemical affinity. 

S. S., and Balabin A. (1997) Change in the dissolution rates of alkali feldspars as a result of secondary mineral precipitation and approach to equilibrium. Geochim. Cosmochim. Acta 59, 19-31. 

The calculation of rates based on changes in solute species concentrations in soils, aquifers, and watersheds requires partitioning the reactant between sources produced by primary mineral dissolution and sinks created by secondary mineral precipitation. Calculation of weathering rates based on solute transport requires knowing the nature and rate of fluid flow through soils, aquifers, and watersheds. 

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