Ion-association model

The saturation indices discussed previously can be calculated based upon total analytical values for all possible reactants. Ions in water, however, do not tend to exist totally as free ions [21]. Calcium, for example, may be paired with sulfate, bicarbonate, carbonate, phosphate, and other species. Bound ions are not readily available for scale formation and such binding, or reduced availability of the reactants, decreases the effective ion-activity product Saturation indices such as the LSI are based upon total analytical values rather than free species primarily because of the intense calculation requirements for determining the distribution of species in water. Speciation breakdown of all species in a given water requires numerous computer iterations to achieve the following [22]  

Indices based upon ion-association models provide a common denominator for comparing results between systems. For example, calcite saturation level calculated using free calcium and carbonate 

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The best-known of these current programs is French Creek Water Cycle, which uses the concept of Ion Association Model in the calculation of saturation level indices. This software series is available in a range of editions suitable for research laboratories, product managers, or field water treaters and operators with access to a laptop computer. 

Ferguson, Robert J. Computerized Ion Association Model Profiles Complete Range of Cooling System Parameters. 52nd Annual Meeting, IWC, USA, October 1991. 

Ferguson, Robert J. Freedman, A. J. Fowler, G. Kulik, A. J. Robson, J. Weintritt, D. J. The Practical Application of Ion Association Model Saturation Level Indices to Commercial Water Treatment Problem Solving. American Chemical Society, USA, August 1994. 

G. Sposito and S. J. Traina, An ion-association model for highly saline, sodium chloride-dominated waters, J. Environ. Qual. 16 80 (1987). 

Figure 2.4. Relationship between experimental and computer-simulated CaSO H Os solubility at different concentrations of various salt solutions employing an ion association model (from Evangelou et a]., 1987, with permission).

We have utilized a chemical model in this investigation to interpret the equilibrium behavior of iron in acid mine waters. A successful correlation between calculated and measured Eh values has been found, using WATEQ2, the computerized ion association model. This correlation supports the basic assumption of homogeneous solution equilibrium in these waters and simultaneously corroborates both the validity of the aqueous model and the quantitative interpretation of Eh measurements in these waters. This interpretation makes it possible to calculate the distribution of iron... 

Whitfield, M. The ion-association model and the buffer capacity of the carbon dioxide system in sea water at 25 C and 1 atmosphere total pressure. Limnol. Oceanogr. 19,... 

Pytkovicz, R. M. and Kester, D. R. Hamed s rule behavior of NaCl-Na2S04 solutions explained by an ion association model. Amer. J. Sci. 267, 217-229 (1969). 

Dickson A. G. and Whitfield M. (1981) An ion-association model for estimating acidity constants (at 25 °C and 1 atm total pressure) in electrolyte mixtures related to seawater (ionic strength < 1 mol Kg H2O). Mar. Chem. 10, 315-333. 

Figure 12 Pore-water chemistry and saturation indices versus depth at the tailings site of the Heath Steele mine. lA represents saturation indices calculated using an ion-association model, and SII represents saturation indices calculated using a specific ion-interaction model (after Ptacek and Blowes, 2000).

Our discussion here is based on Bjemim s ion-association model. An alternative treatment of short-range interaction, the specific interaction model, will be discussed in Appendix 6.2 to this chapter. 

The Specific Ionic Interaction Model as an Alternative and Complement to the Ion Association Model... 

Pitzer and co-workers (1973, 1974) have proposed a more detailed, but at the same time more complex, approach. Whitfield (1973, 1975) has applied these equations to seawater and has shown that this model gives good agreement with available experimental data for the osmotic coefficient and for the mean ion activity coefficient of the major electrolyte components. The results obtained yield numerical results similar to the predictions of the ion association model (see Table A6.2). 

Parkhurst, D. L. 1990. Ion-association models and mean activity coefficients of various salts. In Chemical modeling of aqueous systems fl, ed D. C. Melchior and R. L. Bassett, Am. Chem. Soc. Symp. Ser. 416, pp. 30-43. Washington DC Am. Chem. Soc. 

We attempted to attack this problem by extending the technique and the ion association model for ion-exchange chromatography. Here, we describe the mechanism of chromatographic separation of enantiomers of facial and meridional tris(aminoacidato)cobalt(lIl) chelates (See Figure l). 

Ion-Association Models and Mean Activity Coefficients of Various Salts... 

The components of an ion-association aqueous model are (1) The set of aqueous species (free ions and complexes), (2) stability constants for all complexes, and (3) individual-ion activity coefficients for each aqueous species. The Debye-Huckel theory or one of its extensions is used to estimate individual-ion activity coefficients. For most general-purpose ion-association models, the set of aqueous complexes and their stability constants are selected from diverse sources, including studies of specific aqueous reactions, other literature sources, or from published tabulations (for example, Smith and Martell, (13)). In most models, stability constants have been chosen independently from the individual-ion, activity-coefficient expressions and without consideration of other aqueous species in the model. Generally, no attempt has been made to insure that the choices of aqueous species, stability constants, and individual-ion activity coefficients are consistent with experimental data for mineral solubilities or mean-activity coefficients. 

The derived model reproduced the experimental mean activity coefficients for a fixed set of salts to concentrations of about 2 molal. Thus, it is possible to construct an ion-association model that is consistent with the experimental data. However, the fitting process for the derived model could not determine uniquely all of the parameters of the model. Alternative choices for the complexes included in the model and for the individual-ion, activity-coefficient parameters could fit the experimental data equally well. Further work is needed to incorporate other physical evidence for the existence of aqueous complexes into the fitting process to insure that the ion-association model provides an accurate physical description of aqueous solutions in addition to reproducing experimental mean activity coefficients. 

Hie parameters ois, oos and at, reflect the disposition of ions in the soil solution after they have become incorporated into the interfacial region. Therefore, these surface charge densities represent the net charging effects of the surface speciation of the ions. By analogy with the use of speciation models (ion-association models) to estimate the distribution of ionic charge in aqueous phases like soil. solutions, surface speciation models (surface... 

Calculation of saturation levels based upon fiee concentrations of ions estimated using the ion-association model (ion pairing). 

Some of the analyses were run through an ion-association model computer program to determine the susceptibility of the brine to halite precipitation. If a halite precipitation problem was predicted, the ion-association model was run in a "mixing" mode to determine if mixing the connate water with boiler feedwater would prevent the problem. This approach has been used successfully to control salt deposition in the well with the composition outlined in Table 8.18. The ion-assodation model evaluation of the bottom-hole chemistry indicated that the water was slightly supersaturated with sodium chloride under the bottom-hole conditions of pressure and temperature. As the fluids cooled in the well bore, the production of copious amounts of halite was predicted. 

In this case, the ion-association model predicted that the connate water would require a minimum dilution with boiler feedwater of 15 percent to prevent halite precipitation (Fig. 8.23). The model also predicted that over-injection of dilution water would promote barite (barium sulfate) formation (Fig. 8.24). Although the well produced F1,S at a concentration of 50 mg/L, the program did not predict the formation of iron sulfide because of the combination of low pH and high temperature. Boiler feedwater was injected into the bottom of the well using the downhole injection valve normally used for corrosion inhibitor injection. Injection of dilution water at a rate of 25 to 30 percent has allowed the well to produce successfully since startup. Barite and iron sulfide precipitation have not been observed, and plugging with salt has not occurred. 

F erguson RJ, Freedman AJ, Fowler G, Kulik AJ, Robson J, Weintritt DJ. The practical application of ion association model saturation level indices to commercial water treatmant problem solving. In Amjad Z, ed. Mineral Scale Formation and Inhibition. New York, N.Y. Plenum Press, 1995 323 0. 

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