Thermodynamics of Potassium‐Calcium and Magnesium‐Calcium Exchange Reactions on a Kaolinitic Soil Clay

Thermodynamic parameters derived from experimental results were determined for the potassium‐calcium and magnesium‐calcium exchange reactions on a kaolinitic soil clay separated from the B horizon of a strongly weathered soil profile. The clay samples were equilibrated at 10° and 30°C with a mixed solution of KCl and CaCl 2 for the K‐Ca system or MgCl 2 and CaCl 2 for the Mg‐Ca system. For the K‐Ca system, the selectivity coefficients were high indicating a greater affinity of the clay for K than for Ca. The high equilibrium constant and the negative values of standard free energy and enthalpy changes for the exchange of Ca for K also revealed a preference of the clay for K relative to Ca ions. In the Mg‐Ca system, the exchange isotherms and the low selectivity coefficients indicated a preference of Ca ions for the clay phase. This exchange of Ca for Mg was accompanied by an increase in the free energy and enthalpy changes also pointing to a more stable Ca‐clay than Mg‐clay.