Predicting Cation Mobility in Montmorillonitic Media Based on Exchange Selectivities of Montmorillonite

Cation‐exchange reactions influence the distribution of species between soil solution and sorbed phases and, in turn, the rate of movement of species through the soil. This study was undertaken to determine whether cation‐exchange selectivity coefficients of montmorillonite could be used to adequately describe cation movement in a montmorillonitic soil. Literature data for Ca‐Mg, Ca‐Na, and Mg‐Na exchange on pure montmorillonite in ClO 4 and Cl anionic backgrounds were used to calculate average values for binary exchange selectivity coefficients. These selectivity coefficients were subsequently used in transport models in attempts to describe experimental binary (Ca‐Mg or Ca‐Na) and ternary (Ca‐Mg‐Na) miscible displacement in columns of bulk montmorillonite (admixed with sand) and Sharkey clay soil (a very‐fine montmorillonitic, non‐acid, thermic Vertic Haplaquept) from Louisiana. In general, use of Ca‐Mg exchange selectivity coefficients produced transport simulations in good agreement with experimental results. Use of heterovalent exchange coefficients tended to predict somewhat later breakthroughs of Na than were observed. Although cation‐exchange selectivities of montmorillonite could be used to adequately describe Ca, Mg, and Na transport in Sharkey soil, generalization of these results requires additional confirmation using other montmorillonitic soils.