Geochemical system

Fig. 8. Steady-state model for the earth s surface geochemical system. The kiteraction of water with rocks ki the presence of photosynthesized organic matter contkiuously produces reactive material of high surface area. This process provides nutrient supply to the biosphere and, along with biota, forms the array of small particles (sods). Weatheriag imparts solutes to the water, and erosion brings particles kito surface waters and oceans.

To this point, we have emphasized that the cycle of mobilization, transport, and redeposition involves changes in the physical state and chemical form of the elements, and that the ultimate distribution of an element among different chemical species can be described by thermochemical equilibrium data. Equilibrium calculations describe the potential for change between two end states, and only in certain cases can they provide information about rates (Hoffman, 1981). In analyzing and modeling a geochemical system, a decision must be made as to whether an equilibrium or non-equilibrium model is appropriate. The choice depends on the time scales involved, and specifically on the ratio of the rate of the relevant chemical transition to the rate of the dominant physical process within the physical-chemical system. 

My first attempt to calculate the time history of a geochemical system (Section 2.3) used the obvious approach (the direct Euler method) of evaluating the time derivatives and stepping forward. But it was not sue-... 

Aqueous geochemists work daily with equations that describe the equilibrium points of chemical reactions among dissolved species, minerals, and gases. To study an individual reaction, a geochemist writes the familiar expression, known as the mass action equation, relating species activities to the reaction s equilibrium constant. In this chapter we carry this type of analysis a step farther by developing expressions that describe the conditions under which not just one but all of the possible reactions in a geochemical system are at equilibrium. 

In this chapter we develop a description of the equilibrium state of a geochemical system in terms of the fewest possible variables and show how the resulting equations can be applied to calculate the equilibrium states of natural waters. We reserve for the next two chapters discussion of how these equations can be solved by using numerical techniques. 

The ancient categories of water, earth, and air persist in classifying the phases that make up geochemical systems. For purposes of constructing a geochemical model, we assume that our system will always contain a fluid phase composed of water and its dissolved constituents, and that it may include the phases of one or more minerals and be in contact with a gas phase. If the fluid phase occurs alone, the system is homogeneous the system when composed of more than one phase is heterogeneous. 

At this point we can derive a set of governing equations that fully describes the equilibrium state of the geochemical system. To do this we will write the set of independent reactions that can occur among species, minerals, and gases in the system and set forth the mass action equation corresponding to each reaction. Then we will derive a mass balance equation for each chemical component in the system. Substituting the mass action equations into the mass balance equations gives a set... 

To derive the governing equations we need to identify each independent chemical reaction that can occur in the system. It is possible to write many more reactions than are independent in a geochemical system. The remaining or dependent reactions, however, are linear combinations of the independent reactions and need not be considered. 

We have considered a large number of values (including the molality of each aqueous species, the mole number of each mineral, and the mass of solvent water) to describe the equilibrium state of a geochemical system. In Equations 3.32-3.35, however, this long list has given way to a much smaller number of values that constitute the set of independent variables. Since there is only one independent variable per chemical component, and hence per equation, we have succeeded in reducing the number of unknowns in the equation set to the minimum possible. In addition,... 

Complicating matters further is the fact that the platinum electrode, the standard tool for measuring Eh directly, does not respond to some of the most important redox couples in geochemical systems. The electrode, for example, responds incorrectly or not at all to the couples SO -HS-, O2-H2O, CO2-CH4, NOJ-N2, and N2-NH4 (Stumm and Morgan, 1996 Hostettler, 1984). In a laboratory experiment, Runnells and Lindberg (1990) prepared solutions with differing ratios of selenium in the Se4+ and Se6+ oxidation states. They found that even under controlled conditions the platinum electrode was completely insensitive to the selenium composition. The meaning of an Eh measurement from a natural water, therefore, may be difficult or impossible to determine (e.g., Westall, 2002). 

I to represent the solution. As such, it can be applied readily to study a variety of geochemical systems, simple and complex. 

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. 

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). 

We take two cases in which mineral surfaces catalyze oxidation or reduction, and one in which a consortium of microbes, modeled as if it were a simple enzyme, promotes a redox reaction. In Chapter 33, we treat the question of modeling the interaction of microbial populations with geochemical systems in a more general way. 

Bethke, C. M., 1997, Modelling transport in reacting geochemical systems. Comptes Rendus de TAcademie des Sciences 324,513-528. 

Karpov, I. K., L. A. Kaz min and S. A. Kashik, 1973, Optimal programming for computer calculation of irreversible evolution in geochemical systems. Geochemistry International 10,464 170. 

Lee, M.-K. and C. M. Bethke, 1996, A model of isotope fractionation in reacting geochemical systems. American Journal of Science 296,965-988. 

Thorstenson, D. C., 1984, The concept of electron activity and its relation to redox potentials in aqueous geochemical systems. US Geological Survey Open File Report 84—072,45 p. 

Earth surface geochemical system, 26 9 Earthy odor, 3 228t Easiest next heuristic, for simple distillation, 22 300 Easily dispersible (ED) pigments, 14 316-317... 

The immobilization of dissolved chemical species by adsorption and ion exchange onto mineral surfaces is an important process affecting both natural and environmentally perturbed geochemical systems. However, sorption of even chemically simple alkali elements such as Cs and Sr onto common rocks often does not achieve equilibrium nor is experimentally reversible (l). Penetration or diffusion of sorbed species into the underlying matrix has been proposed as a concurrent non-equilibration process (2). However, matrix or solid state diffusion is most often considered extremely slow at ambient temperature based on extrapolated data from high tem-... 

In physical chemistry, the world is divided in two parts the system, containing the portion of the world of particular interest, and the surroundings, comprising the region ontside the system (Atkins and de Paula 2002). A geochemical system is an open system that may be studied within two basic frameworks ... 

As we have already seen, the mole is the mass unit in the SI system. The molar concentrations of components in geochemical systems assume different significances as a function of the adopted reference unit ... 

Siderophile elements in the zonal structure of ore geochemical systems ... 

Keywords exploration, geochemical, system, siderophile, anomaly... 

A model of an ore geochemical system has been developed (Goldberg et al, 2003), which can be applied to ore entities of various categories ore bodies, deposits and ore regions. The nuclear section of the system contains a zone of accumulation of the principal ore and associated elements. The peripheral areas contain zones of depletion of ore-forming elements. Anomalies of siderophile elements (Ni, Co, Mn, Ti, V, Cr), which are the subject of this paper, are located on the periphery of the nuclear sections of these systems. 

Despite the individual features of formation of the deposits and differences in ore composition and morphology, a general pattern of distribution of elements of the siderophiie group can be observed. They accumulate in the peripheral areas of ore objects (the nuclear parts of ore geochemical systems) of different categories, often forming a picture of concentric zonality. 

This regional-scale geochemical survey has defined two main polar geochemical systems the Leninogorsk and... 

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