CALCULATION OF MULTICOMPONENT IONIC-DIFFUSION FROM ZERO TO HIGH-CONCENTRATION .1. THE SYSTEM NA-K-CA-MG-CL-SO4-H2O AT 25-DEGREES-C

A model is described for the calculation of diffusion of coefficients and other vector transport processes for the six component seawater system Na-K-Ca-Mg-Cl-SO4-H2O at 25-degrees-C, from the low to high ionic strengths. The model is based upon an extension of the empirical equations of MILLER (1967a) for the calculation of Onsager phenomenological coefficients and upon an accurate thermodynamic model utilizing the ion-interaction method of PITZER (1979) and coworkers. The necessary parameters for the activity derivatives and the Onsager coefficient 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 solution in which the concentrations of th 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 the minor species (i.e., K+, Ca2+, Mg2+, SO4(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.