Sodium-calcium ion exchange on clay minerals at moderate to high ionic strengths

Sodium-calcium ion exchange on several clay minerals was studied at ionic strengths ranging from O.Ol to above 1.0. The minerals studied included attapulgite, illite, kaolin, and several montmorillonites. Distribution coefficients of calcium and sodium were obtained for the minerals over a wide range of solution conditions at pH five and equilibrium constants were calculated.

The distribution coefficient of calcium, ^DCa was studied as a function of time, solution pH, loading, sodium concentration, and ionic strength fraction of sodium in constant ionic strength solutions. Exchange was found to be rapid and only slightly dependent on pH between pH three and nine. The distribution coefficient of calcium was independent of loading of calcium in solutions of constant sodium concentration at low calcium loadings. When the logarithm of the distribution coefficient of calcium at low loading of calcium was plotted as a function of the logarithm of the concentration of sodium, a straight line of slope -l.55 resulted. Values of ^D Ca as a function of ionic strength fraction of sodium in solutions of constant total ionic strength were tabulated for the various clays.

The distribution coefficient of sodium, ^D Na was also studied as a function of time, loading, and sodium ionic strength fraction in constant total ionic strength solutions. The exchange of sodium ions was also found to be rapid, but the value of ^D Na varied widely with small changes in loading of sodium at very low sodium loadings. The values of ^D Na obtained as a function of sodium ionic strength fraction in constant total ionic strength solutions were also tabulated.

Values of equilibrium constants calculated from distribution coefficients for solutions of constant fonic strength scattered between 2 and 10 kg/kg for the montmorillonites and attapulgite while equilibrium constants for illite ranged from 5 to l0 kg/kg. No equilibrium constants for kaolin were calculated since distribution coefficients of sodium on this clay were too small to be measured. It was found that equilibrium constants at trace sodium loading were generally lower than those for higher sodium loadings by an order of magnitude or more due to the sensitivity of sodium distribution coefficients to the concentration of sodium in the clay at low loadings.

Theoretical and experimental treatments of ion exclusion were also included.