Water natural, chemical equilibria

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

Garrels, R. M. and Mackenzie, F. T. (1967). Origin of the chemical compositions of some springs and lakes. In "Equilibrium Concepts in Natural Water Systems" (W. Stumm, ed.). Advances in Chemistry Series 67, pp. 222-274. American Chemical Society, Washington. 

More than 50 computer codes that calculate chemical equilibrium in natural waters or similar aqueous systems are described in the literature.122 Most are not suitable for modeling the deep-well... 

Bioconcentration Factor - Fish/Water (BCF). The partitioning of a chemical between water and fish is yet another expression of the hydrophobic nature of the chemical. The ratio of chemical in the fish to that in the water at equilibrium is defined as the bioconcentration factor. 

Geochemical models can be conceptualized in terms of certain false equilibrium states (Barton et al., 1963 Helgeson, 1968). A system is in metastable equilibrium when one or more reactions proceed toward equilibrium at rates that are vanishingly small on the time scale of interest. Metastable equilibria commonly figure in geochemical models. In calculating the equilibrium state of a natural water from a reliable chemical analysis, for example, we may find that the water is supersaturated with respect to one or more minerals. The calculation predicts that the water exists in a metastable state because the reactions to precipitate these minerals have not progressed to equilibrium. 

Thirteen minerals appear supersaturated in the first block of results produced by the chemical model (Table 6.6). These results, therefore, represent an equilibrium achieved internally within the fluid but metastable with respect to mineral precipitation. It is quite common in modeling natural waters, especially when working at low temperature, to find one or more minerals listed as supersaturated. Unfortunately, the error sources in geochemical modeling are large enough that it can be difficult to determine whether or not a water is in fact supersaturated. 

The equilibrium model, despite its limitations, in many ways provides a useful if occasionally abstract description of the chemical states of natural waters. However, if used to predict the state of redox reactions, especially at low temperature, the model is likely to fail. This shortcoming does not result from any error in formulating the thermodynamic model. Instead, it arises from the fact that redox reactions in natural waters proceed at such slow rates that they commonly remain far from equilibrium. 

Greenberg, J. P. and N. Mpller, 1989, The prediction of mineral solubilities in natural waters, a chemical equilibrium model for the Na-K-Ca-Cl-S04-H20 system to high concentration from 0 to 250 °C. Geochimica Cosmochimica Acta 53,2503-2518. 

Parks, G. A. (1967), "Aqueous Surface Chemistry of Oxides and Complex Oxide Minerals Isoelectric Point and Zero Point of Charge," in Equilibrium Concepts in Natural Water Systems, Advances in Chemistry Series, No. 67, American Chemical Society, Washington, DC. 

Garrels, R.M. Mackenzie, F.T. In "Equilibrium Concepts in Natural Water Systems" Stumm, W., Ed. ADVANCES IN CHEMISTRY SERIES No. 67, American Chemical Society Washington, D.C., 1967 pp. 222-42. 

Although the stoichiometry for reaction (9.1) suggests that one only needs 1 mol of water per mole of methane, excess steam must be used to favor the chemical equilibrium and reduce the formation of coke. Steam-to-carbon ratios of 2.5-3 are typical for natural gas feed. Carbon and soot formation in the combustion zone is an undesired reaction which leads to coke deposition on downstream tubes, causing equipment damage, pressure losses and heat transfer problems [21]. 

Naturally, the principles of chemical equilibrium can be applied to any reaction or process. When a solid substance, such as CaSO, is dissolved in water, the reaction initially proceeds towards the right side. As a result, the concentrations of ions in the solvent increase. But, as time passes, the reverse reaction will start to occur and an equilibrium (dissolution-precipitation) is established. 

In natural waters organisms and their abiotic environment are interrelated and interact upon each other. Such ecological systems are never in equilibrium because of the continuous input of solar energy (photosynthesis) necessary to maintain life. Free energy concepts can only describe the thermodynamically stable state and characterize the direction and extent of processes that are approaching equilibrium. Discrepancies between predicted equilibrium calculations and the available data of the real systems give valuable insight into those cases where chemical reactions are not understood sufficiently, where nonequilibrium conditions prevail, or where the analytical data are not sufficiently accurate or specific. Such discrepancies thus provide an incentive for future research and the development of more refined models. 

Thermodynamic systems are parts of the real world isolated for thermodynamic study. The parts of the real world which are to be isolated here are either natural water systems or certain regions within these systems, depending upon the physical and chemical complexity of the actual situation. The primary objects of classical thermodynamics are two particular kinds of isolated systems adiabatic systems, which cannot exchange either matter or thermal energy with their environment, and closed systems, which cannot exchange matter with their environment. (The closed system may, of course, consist of internal phases which are each open with respect to the transport of matter inside the closed system.) Of these, the closed system, under isothermal and iso-baric conditions, is the one particularly applicable for constructing equilibrium models of actual natural water systems. 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

The need to abstract from the considerable complexity of real natural water systems and substitute an idealized situation is met perhaps most simply by the concept of chemical equilibrium in a closed model system. Figure 2 outlines the main features of a generalized model for the thermodynamic description of a natural water system. The model is a closed system at constant temperature and pressure, the system consisting of a gas phase, aqueous solution phase, and some specified number of solid phases of defined compositions. For a thermodynamic description, information about activities is required therefore, the model indicates, along with concentrations and pressures, activity coefficients, fiy for the various composition variables of the system. There are a number of approaches to the problem of relating activity and concentrations, but these need not be examined here (see, e.g., Ref. 11). 

Kinetics of Reactions in Natural Waters. In considering equilibria and kinetics in natural water systems, it is usually necessary to recall that widely different time scales need to be identified with chemical reactions in different systems. Relatively short times (days to weeks) are available for approach to equilibrium in rivers, smaller lakes, reservoirs, and estuaries. Times for reaction in large lakes, seas, and perhaps typical ground waters are of the order of tens to hundreds of years. In ocean waters, the reaction time may range from thousands of years to... 

In view of all of the preceding observations concerning the formal differences between closed and open systems, what general conclusions can be drawn about the applicability of equilibrium concepts in understanding and describing the chemical behavior of the elements in natural water systems Since equilibrium is the time-invariant state of a closed system, the question is under what conditions do open systems approximate closed systems. A simple example will illustrate the relationships, which are already implicit in Equation 35. If one considers the case of a simple reaction... 

Since natural waters are generally in a dynamic rather than an equilibrium condition, even the concept of a single oxidation-reduction potential characteristic of the aqueous system cannot be maintained. At best, measurement can reveal an Eh value applicable to a particular system or systems in partial chemical equilibrium and then only if the systems are electrochemically reversible at the electrode surface at a rate that is rapid compared with the electron drain or supply by way of the measuring electrode. Electrochemical reversibility can be characterized... 

Biological Activity in Relation to the Chemical Equilibrium Composition of Natural Waters... 

Eh-pH diagrams are sometimes used to predict or describe the major dissolved species and precipitates that should exist at equilibrium in aqueous solutions, including groundwaters, surface waters, laboratory solutions, and porewaters from soils, sediments, or rocks. However, as previously described, many natural aqueous systems are not at equilibrium and they often contain metastable species that are not predicted by Eh-pH diagrams. Metastable species refer to compounds, other substances, or ions that are present under redox, pH, pressure, temperature, or other conditions where chemical equilibrium indicates that they should be unstable and absent. Many metastable species (such as As(III) in oxygenated seawater) result from biological activity. 

Eh Reduction-oxidation (redox) potential. A value measured in millivolts or volts when compared with a H2 — 2H+ + 2e standard of 0.00 V at 25 °C and one bar pressure. Eh describes the reduction or oxidation of an element. Most natural waters are chemically complex and not at redox equilibrium. However, unless all redox reactions are at equilibrium, a single accurate Eh value cannot be obtained for a water sample (compare with oxidation and reduction). 

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