Ion Exchange Behavior of the Febex Bentonite. 1. Na/K, Na/Mg and Na/Ca Experimental Exchange Isotherms

Abstract

Na/K, Na/Mg, and Na/Ca exchange isotherms have been experimentally determined for the FEBEX bentonite. Na-homoionized FEBEXbentonite was reacted at room temperature with mixedsalt dissolution of NaCl/KCl, NaCl/MgCl₂, or NaCl/CaCl₂, while keeping a total cation normality of 0.5 eq L^-1. Isotherm exchange experiments were performed using ten (duplicated)experimental points, which cover the complete range of the corresponding binary equivalent fractions. Results indicate that for the Na/K exchange reaction, Vanselow coefficients are larger than one, what is in agreement with the tendency of the smectite of having greater affinity for K than for Na. The exchange constant decreases as K progressively replaces Na in the smectite. This tendency ends when the equivalent fraction of potassium, E _K, reaches a value of around 0.250.3. From this point to higher K contents, it remains nearly constant irrespective of E _K but slightly decreasing again at values near one. The Vanselow selectivity coefficient for the Na/Mg isotherm indicates a preference for the divalent cation. It is nearly constant (K_v ≍ 5.6) for E _Mg < 0.6, but increases up to 10.2 for a nearly Mg-saturated smectite.

The Na/Ca exchange resembles that of Na/Mg, although the selectivity coefficients are larger (K_v ≍ 7.0 for E _Ca < 0.6). The comparison of the selectivity coefficients for the Na/Mg and Na/Ca exchange reactions indicates that the smectite has a slightly higher affinity for Ca than for Mg. This result is consistent with those observed for the Wyoming bentonite.