Flow-through and batch methods for determining calcium-magnesium and magnesium-calcium selectivity

Conducting binary-exchange experiments is a common way to identify cationic preferences of exchanger phases and the two most common approaches are the flow-through and batch methods. The objectives of this research were to provide the materials and methods for both flow-through and batch equilibration techniques and to compare Ca-Mg and Mg-Ca selectivity when using these two methods on a montmorillonitic soil. The methods were evaluated by comparing both the Gibbs free energy values (Delta G(ex)) and selectivity diagrams derived from the flow-through and batch exchange reactions. The Delta G(ex) values for the Ca-Mg reaction were 634 and 444 J mol(-1) as determined by the flow-through and batch methods, respectively, indicating an exchanger preference for Ca. Exchanger preference for Ca was also evident in the Mg-Ca reaction with Delta G(ex) values of -882 and -784 J mol(-1) for the flow-through and batch methods, respectively. The flow-through and batch methods worked very well for determining cation selectivity and results indicate no significant differences existed between the two methods.