Natural water systems, chemical thermodynamics

Applications and Limitations of Chemical Thermodynamics in Natural Water Systems... 

This paper outlines the basis for applying thermodynamic principles in studying the chemistry of natural water systems of all kinds, discusses the kinds of thermodynamic models available, and indicates some important limitations of such thermodynamic approaches. The general ideas will be illustrated by considering a few examples of chemical reactions of some interest in various kinds of natural water systems. 

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. 

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). 

J. J. Morgan, Applications and limitations of chemical thermodynamics in natural water systems, pp. 1-29 in Equilibrium Concepts in Natural Water Systems, ed. by W. Sturnm, American Chemical Society, Washington, DC, 1967. 

Natural waters obtain their equilibrium composition through a variety of chemical reactions and physicochemical processes. In this chapter we consider principles and applications of two alternative models for natural water systems thermodynamic models and kinetic models. Thermodynamic, or equilibrium, models for natural waters have been developed more extensively than kinetic models. They are simpler in that they require less information, but they are nevertheless powerful when applied within their proper limits. Equilibrium models for aquatic systems receive the greater attention in this book. However, kinetic interpretations are needed in description of natural waters when the assumptions of equilibrium models no longer apply. Because rates of different chemical reactions in water and sediments can differ enormously, kinetic and equilibrium are often needed in the same system. 

In summary, thermodynamic models of natural water systems require manipulation of chemical potential expressions in which three concentration scales may be involved mole fractions, partial pressures, and molalities. For aqueous solution species, we will use the moial scale for most solutes, with an infinite dilution reference state and a unit molality standard state (of unit activity), l or the case of nonpolar organic solutes, the pure liquid reference and standard states are used. Gaseous species will be described on the partial pressure (atm — bar) scale. Solids will be described using the mole fraction scale. Pure solids (and pure liquids) have jc, = 1, and hence p, = pf. 

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. 

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. 

Due to the large number of components, natural waters are rather complex systems. The relative concentrations of many components, as well as the pH and Eh, are controlled by chemical equilibria. However, there are also components, in particular colloids and microorganisms, for which thermodynamic equilibrium conditions are not applicable. The complexity of the chemistry in natural waters and the non-applicability of thermodynamics are the main reasons for the fact that calculations are very difficult and problematic. The same holds for laboratory experiments with model waters results obtained with a special kind of water are, in general, not applicable for other natural waters of different origin. 

Many natural aquatic systems have a chemical composition close to saturation with respect to calcite or even dolomite. This is the case, for instance, for seawater, which is usually slightly oversaturated in the upper part of the water column and slightly undersaturated at greater depths. Under these conditions, the rates of both precipitation and dissolution contribute significantly to the overall rate of reaction. Even though the reaction paths may be very complex, there is a very direct and important link between the kinetic rate constants, according to which the rates of forward and reverse microscopic processes are equal for every elementary reaction. The fundamental aspect of this principle forms the essential aspect of the theory of irreversible thermodynamics (Frigogine, 1967). 

Even though little thermodynamic data are available for low temperature SSAS systems, the equations and techniques presented in this paper can be used to estimate the importance of solid-solution aqueous-solution interactions on the chemical evolution of natural waters. 

A revised, updated suinmary of equilibrium constants and reaction enthalpies for aqueous ion association reactions and mineral solubilities has been compiled from the literature for common equilibria occurring in natural waters at 0-100 C and 1 bar pressure. The species have been limited to those containing the elements Na, K, Li, Ca, Mg, Ba, Sr, Ra, Fe(II/III), Al, Mn(II,III,IV), Si, C, Cl, S(VI) and F. The necessary criteria for obtaining reliable and consistent thermodynamic data for water chemistry modeling is outlined and limitations on the application of equilibrium computations is described. An important limitation is that minerals that do not show reversible solubility behavior should not be assumed to attain chemical equilibrium in natural aquatic systems. 

Chemical modeling results for aqueous systems is dependent on the primary thermodynamic and kinetic data needed to perform the calculations. For aqueous equilibrium computations, a large number of thermodynamic properties of solute-solute, solute-gas and solute-solid reactions are available for application to natural waters and other aqueous systems. Unfortunately, an internally consistent thermodynamic data base that is accurate for all modeling objectives, has not been achieved. Nor is it likely to be achieved in the near future. The best that can be hoped for is a tolerable level of inconsistency, with continuing progress toward the utopian goal through national and international consensus. 

The recent extension of these thermodynamic models to include the kinetics and mechanisms of organo-metallic interactions has made it possible (1) to quantify the electrochemical availability of these metal complexes to voltammetric systems (Whitfield and Turner, 1980) (2) to examine diffusion and dissociation models for the tremsport of chelated iron to biological cells (Jackson and Morgan, 1978) and (3) to estimate the significance of adsorptive and convective removal processes on the equilibrium specia-tion of metals in natural waters (Lehrman and Childs, 1973). Thus both equOibrium and dynamic models have become an indispensable tool in the identification of the important chemical forms and critical reaction pathways of interactive elements in aquatic environments. 

The difficulties of experimentally determining the speciation of actinides present at very low concentrations in natural waters have encouraged the use of computer simulations, based on thermodynamic data, as a means of predicting their speciation and hence their environmental behaviour. The use of modelling techniques to describe the speciation, sorption, solubility and kinetics of inorganic systems in aqueous media has been reviewed in the papers given at an international conference in 1978. Both chemical equilibrium models, exemplified by computer programs such as MINEQL and SOLMNQ, and dynamic reaction path models, exemplified by EQ6, have been developed. Application of the equilibrium models to radioactive waste disposal... 

---