To a series of calcium or strontium saturated clays suspended in 50 symmetry concentrations of the saturating ion, variable amounts of Sr85Cl2 or Ca45Cl2 were added. From the radiochemical analysis of the clay at equilibrium, the amount of exchange was determined.
Plotting the data as [Cation-Clay]/[Cation-Solution] vs. [Cation-Clay] yielded a curved line composed of at least two adsorption isotherms. This is interpreted to mean two independent sites of adsorption or that two mass action equations are necessary to describe the system.
Following the cation desorbed by tracer technique resulted in almost identical curves as when the adsorbing cation was followed. This evidence showed that no precipitation or reaction other than the one described took place. Also, the sites were shown to be independent by introducing a third cation such as barium. Barium ions at an appropriate concentration prevented the adsorption of the cation being studied on the “tight site” but had no effect on the adsorption of the cation on “loose site.”
The two sites were separated graphically and labeled “tight” and “loose” sites according to the bonding energies for the cation being studied. An apparent equailibrium constant was calculated from the slope of the “loose site.” The slope of the “tight site” could not be determined accurately enough to calculate an equilibrium constant, being less than 1% of the exchange capacity. Strontium is adsorbed more tightly than calcium on either site, but the difference becomes less as the exchange capacity of the clay decreases.
It was necessary to define the data in terms of two mass action equations if the exchange reaction is to be stoichiometric at either site. Using one equation to compare the adsorption of Sr and Ca resulted in considerable hysteresis, largely due to the influence that the sites have on each other.