Kinetics mineral dissolution

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. 

It is further interesting to observe that the behavior of a system approaching a thermodynamic equilibrium differs little from one approaching a steady state. According to the kinetic interpretation of equilibrium, as discussed in Chapter 16, a mineral is saturated in a fluid when it precipitates and dissolves at equal rates. At a steady state, similarly, the net rate at which a component is consumed by the precipitation reactions of two or more minerals balances with the net rate at which it is produced by the minerals dissolution reactions. Thermodynamic equilibrium viewed from the perspective of kinetic theory, therefore, is a special case of the steady state. 

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. 

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. 

Bruno, J., W. Stumm, P. Wersin, and F. Brandberg (1991), The Influence of Carbonate in Mineral Dissolution. Part 1, The Thermodynamics and Kinetics of Hematite Dissolution in Bicarbonate Solution at T = 25° C", in preparation. 

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

Fig. 2.3 Rate-limiting steps in mineral dissolution (a) transport-controlled, (b) surface reaction-controlled, and (c) mixed transport and surface reaction control. Concentration (C) versus distance (r) from a crystal surface for three rate-controUing processes, where is the saturation concentration and is the concentration in an infinitely diluted solution. Reprinted from Sparks DL (1988) Kinetics of soil chemical processes. Academic Press New York 210 pp. Copyright 2005 with permission of Elsevier...

Bloesch, P.M. Bell, L.C. Hughes, J.D. (1987) Adsorption and desorption of boron by goethite. Aust. J. Soil Res. 25 377-390 Blomiley, E.R. Seebauer, E.G. (1999) New approach to manipulating and characterising powdered photo adsorbents. NO on Cl treated Ee20j. Langmuir 15 5970-5976 Bloom, P.R. Nater, E.A. (1991) Kinetics of dissolution of oxide and primary silicate minerals. In Sparks, D.L. Suarez, D.L. (eds.) Rates of soil chemical processes. Soil Sci. 

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surface Sd. 68 459—468 Bruno, J. Sturam, J.A. Wersin, P. Brand-berg, E. (1992) On the influence of carbonate on mineral dissolutions I. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25°C. Geo-chim. Cosmochim. Acta 56 1139—1147 Brusic.V. (1979) Ferrous passivation. In Corrosion Chemistry, 153—184 Bruun Hansen, H.C. Raben-Lange, R. Rau-lund-Rasmussen, K. Borggaard, O.K. 

Experimental investigations will continue to play a critical role in understanding heterogeneous reaction kinetics, such as mineral dissolution rates in silicate melts and in aqueous solutions, the melting rates at the interfaces of two... 

Bruno, J., Stumm, W., Wersin, P. Brandberg, F. 1992. On the influence of carbonate in mineral dissolution 1. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25 °C. Geochimica et Cosmochimica Acta, 56, 1139-1147. 

Hummel, W. 2000. Comment on On the influence of carbonate in mineral dissolution 1. The thermodynamics and kinetics of hematite dissolution in... 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

The fluidized bed reactor has been used by several researchers to study the kinetics of chemical weathering (Holdren and Speyer, 1985, 1987 Chou and Wollast, 1985). One of the advantages in using the fluidized bed reactors for studies of this type is that there are no strong concentration gradients in the aqueous and solid phases. Additionally, the concentration of the dissolved species can be maintained at levels well below saturation with respect to possible precipitates. This means, for example, that one could study mineral dissolution exclusively without secondary precipita-... 

Rate-Limiting Steps in Mineral Dissolution 146 Feldspar, Amphibole, and Pyroxene Dissolution Kinetics 148 Parabolic Kinetics 149 Dissolution Mechanism 155 Dissolution Rates of Oxides and Hydroxides 156 Supplementary Reading 161... 

There are basically three, rate-limiting mechanisms for mineral dissolution assuming a fixed degree of undersaturation. They are (1) transport of solute away from the dissolved crystal or transport-controlled kinetics. 

The third type of rate-limiting mechanism for mineral dissolution— mixed or partial surface reaction-controlled kinetics—exists when the surface detachment is fast enough that the surface concentration builds up to levels greater than the surrounding solution concentration but lower than that expected for saturation (Berner, 1978). 

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. 

The variety of mechanisms possible for reductive mineral dissolution processes that are mediated by organic ligands is discussed by J. G. Hering and W. Stumm, op. cit.,26 A. T. Stone and J. J. Morgan, op. cit.,28 andB. Sulzberger, D. Suter, C. Siffert, S. Banwart, and W. Stumm, Dissolution of Fe(III) (hydr)oxides in natural waters Laboratory assessment on the kinetics controlled by surface coordination, Mar. Chem. 28 127 (1989). The examples considered in this section are illustrative only. 

Stumm, W., Aquatic Chemical Kinetics, Wiley, New York, 1990. Chapters 11-15 and 17 of this edited monograph provide excellent surveys of research on the kinetics and mechanisms of mineral dissolution processes investigated in both laboratory and field settings. 

Stumm, W., and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981. Chapter 5 of this standard textbook gives many examples of the concepts discussed in the present chapter. A broad conceptual picture of mineral dissolution kinetics and mechanisms is developed in the celebrated three-part paper The Coordination Chemistry of Weathering ... 

Experimental features of mineral dissolution kinetics research are described in exemplary fashion in the following four papers ... 

There are many reactions in soil-water systems pertaining to nutrient availability, contaminant release, and nutrient or contaminant transformations. Two processes regulating these reactions are chemical equilibria (Chapter 2) and kinetics. The specific kinetic processes that environmental scientists are concerned with include mineral dissolution, exchange reactions, reductive or oxidative dissolution, reductive or oxidative precipitation, and enzymatic transformation. This chapter provides a quantitative description of reaction kinetics and outlines their importance in soil-water systems. 

The rate constants are estimated by the slopes of lines fitted by a least squares method and listed in Table 1. It is concluded that a two-step first-order kinetic behavior dominates in the surface sorption of U(VI). The relatively slower second-step may be due to such effects as a diffusion-controlled sorption onto the fracture surface of micropores and a mineral dissolution of the granite surface [19], It is noticed from Table 1 that the reaction rates do not greatly depend upon pH although the amount of U(VI) sorbed onto the granite surface is greatly dependent on pH. 

These computations describe the tendency of a water sample to be saturated, but they do not necessarily demonstrate whether mineral dissolution or precipitation is taking place. For dissolution to take place, the mineral must be present and it must dissolve at a rate that is fast enough relative to the flow rate of the water to affect the water chemistry (Berner, 1978). Likewise for a mineral to precipitate it must do so at a fast enough rate. The kinetics of precipitation and dissolution reactions must be applied to get a realistic interpretation of water-rock interactions. 

Mineral dissolution kinetics influence such phenomena as development of soil fertility, amelioration of the effects of acid rain, formation of karst, acid mine drainage, transport and sequestration of contaminants, sequestration of carbon dioxide at depth in the earth, ore deposition, and metamorphism. On a global basis, mineral weathering kinetics are also involved in the long-term sink for CO2 in the atmosphere ... 

Oelkers E. H., Schott J., and Devidal J.-L. (2001) On the interpretation of closed system mineral dissolution experiments Comment on Mechanism of kaolinite dissolution at room temperature and pressure Part II. Kinetic study by Huertas et al. (1999). Geochim. Cosmochim. Acta 65, 4429-4432. 

The effects of pH on sorption isotherms have been studied extensively particularly with oxide surfaces (Anderson and Rubin, 1981 Sposito, 1984), but pH effects in sorption kinetic studies have not received equal attention. In contrast, pH effects in mineral-dissolution kinetic experiments have received a great deal of attention (e.g., Chou and Wollast, 1984 Stumm, 1986 Stone, 1987a,b). 

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