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Systems at the surface of the Earth are continually responding to energy inputs - rived ultimately from radiation from the Sun or from the radiogenic heat in the - terior. These energy inputs drive plate movements and erosion, exposing metastable mineral phases at the Earth’s surface. In addition, these energy ?uxes are harvested and transformed by living organisms. As long as these processes persist, chemical disequilibrium at the Earth’s surface will be perpetuated. In addition, as human populations grow, the need to produce food, extract - ter, and extract energy resources increases. These processes continually contribute to chemical disequilibrium at the Earth surface. We therefore ?nd it necessary to predict how the surface regolith will change in response to anthropogenic processes as well as long-term climatic and tectonic forcings. To address these questions, we must understand the rates at which reactions occur and the chemical feedbacks that relate these reactions across extreme temporal and spatial scales. Scientists and - gineers who work on soil fertility, nuclear waste disposal, hydrocarbon production, and contaminant and CO sequestration are among the many researchers who need 2 to understand geochemical kinetics. Fundamental questions concerning the lo- term geological, climatic and biological evolution of the planet also rely on g- kinetic information. In this book, we summarize approaches toward measuring and predicting the - netics of water-rock interactions which contribute to the processes mentioned above.



1. Analysis of Rates of Geochemical Reactions

Over the last several billion years, rocks formed at equilibrium within the mantle of the Earth have been exposed at the surface and have reacted to move towards a new equilibrium with the atmosphere and hydrosphere. At the same time that minerals, liquids, and gases react abiotically and progress toward chemical equilibrium at the Earth’s surface, biological processes harvest solar energy and use it to store electrons in reservoirs which are vastly out of equilibrium with the Earth’s other surface reservoirs. In addition to these processes, over the last several thousand years, humans have produced and disseminated non-equilibrated chemical phases into the Earth’s pedosphere, hydrosphere, and atmosphere. To safeguard these mineral and fluid reservoirs so that they may continue to nurture ecosystems, we must understand the rates of chemical reactions as driven by tectonic, climatic, and anthropogenic forcings.
Susan L. Brantley, Christine F. Conrad

2. Transition State Theory and Molecular Orbital Calculations Applied to Rates and Reaction Mechanisms in Geochemical Kinetics

For much of the history of geochemistry, thermodynamics has dominated discussions on geological processes. Geologic time is so long that systems were generally thought to reach equilibrium, so only knowledge of the reactants and products were considered important. As an emphasis on lower temperature processes and environment geochemistry has increased, the need to understand reaction rates has become more obvious. As geochemists have become more aware of the role of kinetics, disequilibrium has been found to be common. Even in mantle rocks where high temperatures and long equilibration times are the norm, disequilibrium has been observed (Bell and Ihinger, 2000).
James D. Kubicki

3. The Mineral-Water Interface

The typical satellite view of Earth’s surface reveals ubiquitous contact between rocks and water (Fig. 3.1). Rocks are composed primarily of minerals, naturally crystallized materials having a periodic structure. The long-range structure of crystals, expressed internally as the periodic lattice, determines their fundamental physical and chemical properties. Water, in addition to supplying the basis for life on Earth, is also its critical solvent. It is the dominant medium through which rocks and minerals “communicate” during chemical precipitation and dissolution reactions. However, at room temperature the mobility of ions via diffusion to and from sites in the solid bulk crystal is extremely limited. Dissolution and precipitation reactions thus usually occur at the mineral-water interface (Fig. 3.2). This interface is the locus of exchange and interaction between the surface atoms of the solid and the overlying aqueous phase. In addition to water molecules, the fluid contains dissolved components: e.g., inorganic salts, hydrogen and hydroxyl ions, gases such as CO2 and O2, and organic molecules. These components interact with each other as well as with the mineral surface, yielding a complex distribution of species and functional groups (moieties) that characterize even compositionally “simple” solutions. This fluid-solid interaction alters both the surface layers of the crystal and the boundary layer of the fluid (Fig. 3.3). As used here, the term “boundary layer” applies to that fluid in direct contact with the mineral surface. Although the bulk fluid may be in turbulent motion (e.g., a stirred reactor), intermolecular attractive forces between the mineral surface and the fluid bring the fluid velocity to zero (“no-slip” condition). In classical theory, this constraint reduces advection and turbulent mixing within the boundary layer, whose thickness is a function of the flow characteristics prevailing in the overlying bulk fluid. The demands of reactive fluxes from precipitation or dissolution of the underlying mineral surface must be satisfied by the diffusive flux of components through the boundary layer. Much discussion is often devoted to the question of whether reactions are “controlled” by transport or surface reaction mechanisms (i.e., molecular detachment or attachment); because of the interplay between diffusion and reaction, the more pertinent question is whether the crystal surface is close to thermodynamic equilibrium with the fluid (see e.g., discussion in Lasaga, 1998). It is thus critical to recognize that at this interface neither the crystal nor the fluid is equivalent to its bulk counterpart. This central distinction is the subject of this chapter: the nature of the interfacial contact between a crystalline surface and an aqueous fluid, how this region of the crystal differs in terms of physical and chemical properties and behavior from its surroundings, how these differences are the basic engine for dynamic, scale-dependent interface processes, and how these atomic-scale processes express themselves as macroscopic phenomena.
A. Liittge, R. S. Arvidson

4. Kinetics of Sorption—Desorption

The fate of nutrients, pollutants and other solutes in natural waters is coupled to their distribution between solid, aqueous and gas phases. The processes of phase distribution are many, including penetration and absorption into one of the phases, or accumulation at the interface between them. The term sorption is defined here as the full range of processes whereby matter is partitioned between the gas, aqueous and solid phases. In geochemical systems, this includes adsorption of matter at the surfaces of solid particles (minerals and organic matter) or at the air—water interface, and absorption into the solids during surface precipitation or solid phase diffusion. The complexity of natural geomedia (Fig. 4.1) implies that both broad classes of “sorption” reaction may occur simultaneously. As discussed in this chapter, recent research into the kinetics and mechanisms of sorption for inorganic and organic species indicates that both processes are indeed important. The relative predominance of a given reaction and sorbate—sorbent structure is a function of time scale, system loading and geochemical conditions.
Jon Chorover, Mark L. Brusseau

5. Kinetics of Mineral Dissolution

The rates of mineral dissolution contribute to processes controlling soil fertility, porosity in aquifers and oil reservoirs, transport and sequestration of contaminants and CO2, cycling of metals and formation of ore deposits, and many other geochemical characteristics and phenomena. For example, the weathering rates of Ca- and Mg-silicates influence the concentrations of CO2 in the atmosphere over 105–106 y timescales, impacting the global carbon cycle. Mineral dissolution thus influences the chemical and physical nature of our landscape as well as the quality and quantity of potable water and fertile soil available to sustain ecosystems. The rates of mineral dissolution (Fig. 5.1) determine the lifetimes of minerals in soil environments. Especially since the 1970s, researchers have focused on measurement of mineral dissolution rates in order to promote quantitative prediction of the evolution of our environment (Stumm, 1997). In this chapter, we discuss many of the concepts and models used to predict mineral dissolution rates for oxide, carbonate, and silicate minerals.
Susan L. Brantley

6. Data Fitting Techniques with Applications to Mineral Dissolution Kinetics

A common problem in chemical kinetics is the development of a rate law that describes the dependence of the reaction rate on the surrounding conditions such as concentrations of reacting species or temperature of the reacting media (see Chap. 1). The most direct approach to solving this problem is to measure reaction rates under systematically varied conditions and then to perform mathematical analyses on these data to determine the form of the rate law and to generate estimates of any unknown constants, or parameters, that make up the proposed rate law. Other chapters in this book provide information both for designing kinetics experiments and for selecting appropriate rate laws for a variety of geochemical reactions. In this chapter we describe the mathematical analyses — known collectively as curve fitting or regression analysis — that can be used to select a rate equation that matches a given data set, to generate estimates for any unknown parameters in the rate equation (e.g., rate constants or reaction orders), and to quantify the uncertainty associated with the estimated values for the parameters. As we traverse this entirely quantitative process we will attempt to describe the underlying, qualitative process of looking at kinetic data: plots to make, features of these plots to examine, and conceptual sketches to draw.
Joel Z. Bandstra, Susan L. Brantley

7. Nucleation, Growth, and Aggregation of Mineral Phases: Mechanisms and Kinetic Controls

The formation of any phase, whether natural or synthetic (Fig. 7.1), is usually a disequilibrium process that follows a series of steps until a thermodynamically stable state (equilibrium) is achieved. The first step in the process of creating a new solid phase from a supersaturated solution (either aqueous or solid) is called nucleation. A particle formed by the event of nucleation usually has a poorly ordered and often highly hydrated structure. This particle is metastable with respect to ordering into a well-defined phase, which can accompany growth of the particle. This process of initiation of a new phase is defined as a first order transition and can follow various pathways involving a host of mechanisms. One of these pathways occurs when individual nuclei coalesce into larger clusters, a process defined as aggregation, which itself can follow a series of different pathways. The new phase is thermodynamically defined when the growing nucleus or aggregate has distinct properties relative to its host matrix; for example, a well-defined crystal structure, composition and/or density. These processes depend on a plethora of chemical and physical parameters that control and strongly affect the formation of new nuclei, the growth of a new crystal, or the aggregation behavior of clusters, and it is these issues that will be the focus of this chapter. We will discuss the mechanisms and rates of each process as well as the methods of quantification or modeling from the point of view of existing theoretical understanding. Each step will be illustrated with natural examples or laboratory experimental quantifications. Complementary to the information in this chapter, a detailed analysis of the mechanisms and processes that govern dissolution of a phase are discussed in detail in Chap. 5 and more detailed information about molecular modeling approaches are outlined in Chap. 2.
Liane G. Benning, Glenn A. Waychunas

8. Microbiological Controls on Geochemical Kinetics 1: Fundamentals and Case Study on Microbial Fe(III) Oxide Reduction

The pervasive influence of microorganisms (abbreviated hereafter as “morgs”; see Table 8.1 for a list of abbreviations) on the geochemistry of low-temperature environments is well-recognized and has been the subject of voluminous experimental and observational research (Banfield and Nealson, 1997; Brezonik, 1994; Canfield et al., 2005; Chapelle, 2001; Ehrlich, 2002; Lovley, 2000b). Many of the foundational insights into the role of morgs as agents of geochemical reaction can be traced to basic discoveries in microbiology which took place in the 19th and early 20th centuries. Perhaps the most important contribution of all was Louis Pasteur’s definitive demonstration that decomposition of OM does not proceed in the absence of living morgs (Pasteur, 1860). Though not made in the context of geochemistry, his decisive defeat of the theory of spontaneous generation was a key step toward recognizing the role of microbial life as a direct agent of chemical transformation in natural, medical, and industrial settings. A long series of discoveries followed in which the participation of morgs in various aspects of elemental cycling and mineral transformation was revealed, many in the context of soil and aquatic microbiology (Clarke, 1985; Ehrlich, 2002; Gorham, 1991). These early discoveries, together with developments in the fields of general microbiology and biochemistry (e.g., as embodied in Kluyver (1957)’s synthesis of unity and diversity in microbial metabolism) laid the groundwork for our current understanding of microbial metabolism based on principles of biochemical energetics (thermodynamics) and enzymatic reaction kinetics.
Eric E. Roden

9. Microbiological Controls on Geochemical Kinetics 2: Case Study on Microbial Oxidation of Metal Sulfide Minerals and Future Prospects

Sulfide minerals are components of magmatic, igneous, and sedimentary rocks, as well as hydrothermal deposits (ore bodies). They are exploited economically as sources of sulfuric acid and metals, such as Co, Cu, Au, Ni, and Zn that are present either as discrete sulfide phases (e.g., CuS, Gu2S, NiS, ZnS) or coprecipitates with major iron-sulfide phases such as FeS2 (e.g., chalcopyrite, CuFeS2). However, in many situations, sulfide minerals (in particular FeS2, the most abundant sulfide phase in most mineral deposits) are waste (“gangue”) phases associated with mining and metallurgical operations. Exposure of these phases to atmospheric O2 during and after such operations causes acid mine/acid rock drainage (referred to hereafter simply as acid mine drainage or AMD; see Table 9.1 for a list of abbreviations), an environmental problem of major environmental concern (Evangelou and Zhang, 1995). A vast amount of research has been conducted on the mechanisms and controls on sulfide mineral oxidation for the purposes of (i) understanding and predicting environmental risk associated with AMD; (ii) understanding sulfur and metal cycling associated with natural sulfide mineral weathering; and (iii) optimizing commercial recovery of metals from low-grade ores and wastes. Microorganisms play a pivotal role in sulfide mineral oxidation, and this phenomenon represents another premier example of how microbial activity exerts fundamental control on water—rock interaction phenomena.
Eric E. Roden

10. Quantitative Approaches to Characterizing Natural Chemical Weathering Rates

Silicate minerals, constituting more than 90% of the rocks exposed at the earth’s surface, are commonly formed under temperature and pressure conditions that make them inherently unstable in surficial environments. Undoubtedly, the most significant aspect of chemical weathering resulting from this instability is the formation of soils which makes life possible on the surface of the earth. Many soil macronutrients in this “critical zone” are directly related to the rate at which primary minerals weather (Huntington, 1995; Chadwick et al., 2003). Chemical weathering also creates economically significant ore deposits, such as those for Al and U (Samma, 1986; Misra, 2000), as well as potentially releasing high concentrations of toxic trace elements such as Se and As (Frankenberger and Benson, 1994). Silicate weathering is a significant buffer to acidification caused by atmospheric deposition (Driscoll et al., 1989) and from land use practices (Farley and Werritty, 1989). Atmospheric CO2 levels have been primarily controlled by the balance between silicate weathering and the rate of volcanic inputs from the Earth’s interior, a relationship which may explain long-term climate stability (Ruddiman, 1997)
Art F. White

11. Geochemical Kinetics and Transport

The kinetics of geochemical and biogeochemical processes can be studied in isolation in the laboratory, but only rarely is it possible to separate these processes from those of transport when considering their importance in natural systems at the field scale. This is because the driving force for most reactions of interest in water—rock interaction is transport. The most intense geochemical and biogeochemical activity occurs at the interface between global compartments like the oceans, the atmosphere, and the Earth’s crust where elemental and nutrient fluxes provide the maximum driving force for reactions to take place. The important role of transport in these settings makes it critical to consider these time-dependent processes in conjunction with those processes we think of as more purely biogeochemical. In other words, these global interfaces are open systems, where both mass and energy transfers must be accounted for.
Carl I. Steefel

12. Isotope Geochemistry as a Tool for Deciphering Kinetics of Water-Rock Interaction

What have isotopes added to our understanding of the kinetics of water-rock interaction processes at the surface of the Earth? Since the development of routine techniques of mass spectrometry, an impressive number of studies have used isotopes to constrain water-rock interaction processes. A number of these initial studies were not purposefully designed to determine the kinetics of water-rock interaction, but they demonstrated that isotopic kinetic effects were both sufficiently large and specific so that they could be used to constrain the kinetics of water-rock interactions. Here we provide a brief synthesis of the different approaches where isotopes have been applied to constrain the rates and timescales of water-rock interaction. We have adopted a very broad definition of the word “kinetics” for this review and have not limited the studies to reaction rates of particular reactions. Because most of the reactions occurring at the Earth’s surface are occurring at low temperatures, they do not proceed rapidly enough to reach thermodynamic equilibrium; therefore, we face the problem of determining the rates at which they proceed. Physical processes, such as climate variability, glacier dynamics, landsliding and tectonic processes often proceed more rapidly than chemical weathering reactions. For example, mineral weathering and the subsequent formation of secondary minerals in soils occurs over timescales on the order of tens to hundreds of thousands of years, while the characteristic timescales of climate variability are much shorter. Isotopic approaches have proven to be very powerful in constraining rates of water-rock interaction in natural systems, where physical and chemical processes are inevitably coupled.
Jérôme Gaillardet

13. Kinetics of Global Geochemical Cycles

Geochemical systems of the Earth’s surface and interior are often studied by means of conceptual models that represent them as geochemical or biogeochemical cycles of chemical elements. Such models usually address the various geological, geochemical, geophysical, and biological processes within the cycle or system, and they focus on the model’s ability to evaluate the system changes at different time scales, often extending from the remote past into the future. The time dimension of changes taking place in the different parts of the Earth System makes it necessary to understand the mechanisms and rates of the numerous processes that control the element interactions in geochemical systems of different physical structures and degrees of complexity.
Abraham Lerman, Lingling Wu


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