Equilibrium in natural waters

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... 

There is every justification for using true equilibrium constants determined in the 10"3M range for calculations in the 10"r,M range. However, I question how many true equilibrium constants are known at this time, and more importantly whether or not we are dealing with solutions at equilibrium in natural waters. 

Bicarbonate and carbonate ions are common OH scavengers that exist in equilibrium in natural waters and react with OH with second-order rate constants of 8.5 x 106 and 3.9 x 108 Mr1 s respectively. Of the waters tested, potable water had a pH of about 7 and secondary wastewater a pH of about 9. As the pH increased, OH scavenging increased due to the presence of increased alkalinity species, bicarbonate and carbonate ions, thereby reducing removal efficiency. The effect of these ions on OH can be predicted according to the reactions ... 

Stumm, W. and Morgan, J. J., "Aquatic Chemistry An introduction emphasizing chemical equilibrium in natural waters, 583 p., Wiley-Interscience, New York, 1970. 

Paces T. (1972) Chemical characteristics and equilibrium in natural water—felsic rock—carbon dioxide system. Geochim. Cosmochim. Acta 36, 217 -240. 

The extent of redox equilibrium in natural waters has been the cause of considerable discussion. In the case of arsenic. Cherry et al. (1979) suggested that redox equilibrium was sufficiently rapid for As(V)/As(III) ratios to be... 

O Connor, J. J., and C. E. Rbnn. 1964. Soluble-adsorbed zinc equilibrium in natural waters. J. Am. Water Works Assoc. 56 1055-61. 

Truesdell, A.H. and Jones B.F., 1974. WATEQ a computer program for calculating chemical equilibrium in natural waters. J. Res. U.S. Geol. Surv, 2 233—248. 

Dissolved arsenic concentrations can be limited either by the solubility of minerals containing arsenic as a constituent element (or in solid solution) or by sorption of arsenic onto various mineral phases. For both the precipitation-dissolution of arsenic-containing minerals and sorption-desorption of arsenic onto solid phases, equilibrium calculations can indicate the level of control over dissolved arsenic concentrations that can be exerted by these processes. However, neither of these types of reactions is necessarily at equilibrium in natural waters. The kinetics of these reactions can be very sensitive to a variety of environmental parameters and to the level of microbial activity. In particular, a pronounced effect of the prevailing redox conditions is expected because potentially important sorbents (e.g., Fe(III) oxyhydroxides) are unstable under reducing conditions and because of the differing solubilities of As(V) and As(III) solids. 

Value characterizes total oxidation-reduction potential of the solution at the complete chemical equilibrium under standard conditions when activities of its components are equal to 1. However, such equilibrium in natural waters, as a rule, is absent due to a great difference in rates of redox-reactions. In this connection for evaluating real values of Eh, it is necessary to account for real activities (concentrations) of the components participating in redox-reactions of the solution. Then, according to equation (2.5), Eh of the solution on the whole is equal to... 

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. 

As an application of these computational helpers I shall also introduce the carbon system and the equilibrium relationships among the species of carbon dissolved in natural waters. 

The species dissolved in a fluid may be in partial equilibrium, as well. Many redox reactions equilibrate slowly in natural waters (e.g., Lindberg and Runnells, 1984). The oxidation of methane... 

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. 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

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. 

Several computer-based techniques have been developed for more specific applications. Truesdell (45) describes a computer program for calculating equilibrium distributions in natural water systems, given concentrations and pH. Edwards, et al. (31, Z2) have developed computer programs for treating volatile weak electrolytes such as ammonia, carbon dioxide and hydrogen sulfide systems however, in their present state these programs (presumably) do not accommodate metallic species in solutions. 

Truesdell (45) Equilibrium constants for reactions important in natural water systems. 

Particle size distributions of natural sediments and soils are undoubtedly continuous and do not drop to zero abundance in the region of typical centrifugation or filtration capabilities. Additionally, there is some evidence to indicate that dissolved and particulate organic carbon in natural waters are in dynamic equilibrium, causing new particles or newly dissolved molecules to be formed when others are removed. Experiments with soil columns have shown that natural soils can release large quantities of DOC into percolating fluids [109]. 

Once the spontaneous direction of a natural process is determined, we may wish to know how far the process will proceed before reaching equilibrium. For example, we might want to find the maximum yield of an industrial process, the equilibrium solubility of atmospheric carbon dioxide in natural waters, or the equilibrium concentration of a group of metabolites in a cell. Thermodynamic methods provide the mathematical relations required to estimate such quantities. 

In Goulded RF, Equilibrium concepts in natural water systems. Adv Chem Ser 67 161-172 Stumm W, Furrer G, Wieland E, Zinder B (1985) The effects of complex-forming ligands on the dissolution of oxides and aluminosilicates In Drever JI (ed) The chemistry of weathering. Reidel, Dordrecht, pp 55-74... 

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