Cation Exchange Selectivity of Some Clay‐Sized Minerals and Soil Materials

Abstract

Abstract Cation exchange selectivity (CES) values for montmorillonite, vermiculite, biotite, muscovite, and micaceous vermiculitic clays from Triangle and Harpster soils were determined by equilibration of each exchanger with a mixed, equinormal solution containing two competing cations. The cation affinity of Na‐saturated materials increased in the order K < Mg < Ca < Sr < Ba for montmorillonite and the soil clays, Mg < K < Ca < Sr < Ba for biotite, Mg < Ca < Sr < K < Ba for muscovite, and K < Ba < Sr < Ca < Mg for vermiculite. Sodium‐saturated micaceous vermiculite gave a K/Ca CES value of 0.2, whereas K‐saturated, dried vermiculite gave a K/Ca CES value of 0.4, indicating that the large number of interlayer wedge sites produced by the structural collapse in K‐saturated and dried vermiculite increased the selectivity for K ions. Potassium‐saturated and dried muscovite having a CEC equal to the similarly treated vermiculite gave a K/Ca CES value of 4.0 compared to 0.4 for the vermiculite, suggesting that layer charge density has a large influence upon K selectivity. The higher CES values for muscovite relative to those for biotite are explained by the smaller ditrigonal holes and the oblique orientation of hydroxyl groups in the muscovite structure.