Calculation of multicomponent ionic diffusion from zero to high concentration: I. The system Na-K-Ca-Mg-Cl-SO 4 -H 2 O at 25°C

Details Calculation of multicomponent ionic diffusion from zero to high concentration: I. The system Na-K-Ca-Mg-Cl-SO 4 -H 2 O at 25°C Calculation of multicomponent ionic diffusion from zero to high concentration: I. The system Na-K-Ca-Mg-Cl-SO 4 -H 2 O at 25°C

Jan 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..113f&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 55, Issue 1, pp.113-131

Mathematics

Logic

5

Scientific paper

A model is described for the calculation of diffusion coefficients and other vector transport processes for the six component seawater system Na-K-Ca-Mg-Cl-SO 4 -H 2 O at 25 °C, from low to high ionic strengths. The model is based upon an extension of the empirical equations of (1967a) for the calculation of Onsager phenomenological coefficients and upon an accurate thermodynamic model utilizing the ion-interaction method of (1979) and coworkers. The necessary parameters for the activity derivatives and the Onsager coefficients may be evaluated from binary and ternary data. Additional data are not required for applications in more complex systems. The present model parameterization accurately reproduces all of the relevant, binary mutual diffusion coefficients to high ionic strength. In common-ion ternary systems, the model with the present parameterization does well for solutions in which the concentrations of the component electrolytes are approximately equal and for the dominant electrolyte in systems in which one of the electrolytes is in excess. The model may not be as accurate for the minor components of mixed electrolyte systems, because the required ternary experimental data are not available; however, even in these cases errors do not exceed 20%. The model is also used to calculate diffusion coefficients at the composition of seawater. Our calculated values are supported by the measured conductivities, partial molal conductivities, and mutual diffusion coefficients of neutral components at seawater composition. The importance of coupling between diffusive flows is illustrated by application to a hypothetical geochemical problem which could result from a salt dome penetrating an overlying formation saturated with seawater. The large diffusive flux of NaCl creates significant gradients in all of the minor species (i.e., K + , Ca 2+ , Mg 2+ , SO 4 2- ), even though there were no initial concentration gradients for these species. The use of tracer diffusion coefficients is also discussed. The use of such coefficients in simplified forms of Fick's law is shown to be fundamentally inapplicable for electrolyte systems even at infinite dilution. When used in more comprehensive electrolyte diffusion models, isotope-isotope coupling effects complicate the use of tracer diffusion coefficients, especially at higher concentrations.

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