Cation-Exchange Kinetics on Montmorillonite Using Pressure-Jump Relaxation

Batch and flow methods are usually not suitable for measuring kinetics of ion-exchange reactions on soils and soil constituents. These reactions are usually very fast. Thus, a rapid kinetic method must be employed. A pressure-jump relaxation technique was used in this study to investigate the kinetics of Ca-Na and Ca-K exchange on montmorillonite. Single relaxations were observed in both systems. Forward (k,) and backward (£_,) rate constants for the cation-exchange reactions were determined and were greater for Ca-Na exchange (k, = 953 L mols~', *., = 643 L mol' s-) than those for Ca-K exchange (k, = 385 L mols-, t, = 432 L mol5-'). The values of the exchange equilibrium constant obtained kinetically were in good agreement with those obtained statically. The exchange isotherms from the static studies showed a preference of K over Ca on montmorillonite, but no preference was observed in the Ca-Na exchange system. A VOLUMINOUS AMOUNT of research has been conducted on cation exchange on soils and soil constituents. Since the early work of Way (1850), who studied NH4-Ca exchange on soils, it has been generally assumed that exchange processes on soils and soil colloids are rapid, often instantaneous. This may explain in part why most studies on cation exchange have dealt with equilibrium aspects, and only recently have studies appeared on the kinetics and mechanisms of exchange phenomena (Sparks, 1989). Another reason that the kinetics of cation exchange have not been extensively investigated is the lack of appropriate techniques to measure rapid reaction rates. With most batch and flow techniques, one cannot measure the kinetics of chemical reactions that are faster than about 15 s (Sparks, 1989). Moreover, traditional batch and flow methods primarily measure transport-controlled reactions and, thus, apparent rate parameters are determined. Consequently, chemical kinetics are not determined. Cation-exchange reactions on some clay minerals are particularly rapid, usually occurring within minutes or seconds. Malcom and Kennedy (1969), using a specific ion electrode technique, found that >70% of Ba-K exchange on kaolinite, illite, and montmorillonite occurred in 3 s. Sparks and Jardine (1984), using a continuous flow method, found that adsorption of K on kaolinite and montmorillonite was completed in 40 and 120 min, respectively. With the technique of Sparks and Jardine (1984), a combination of chemical exchange and diffusion phenomena were being measured. Pressure-jump relaxation is a useful technique for Dept. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. Received 3 Feb. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:42-46 (1993). measuring rapid reaction rates on soil constituents (Sparks, 1989; Sparks and Zhang, 1991). This technique is based on the principle that chemical equilibrium is pressure dependent. The pressure pulse created by an autoclave perturbs a system from equilibrium by a small amount, and then the system decays back to the original equilibrium. The time required for the decay process (from the nonequilibrium to equilibrium state) is referred to as the relaxation time (T), which is related to the specific rates of the elementary reactions involved. With the pressure-jump method, one can measure the chemical kinetics of reactions in millisecond or microsecond time scales. Pressure-jump, as well as other chemical relaxation techniques, have been used to study cation adsorption-desorption kinetics (Hachiya et al., 1979; Negishi et al., 1984; Hayes and Leckie, 1986) and, recently, to investigate anion adsorption-desorption on goethite (Zhang and Sparks, 1989, 1990a, b). In most of these studies, reactions on the oxide-water interface were investigated. No one has used pressure-jump relaxation to study cation-exchange kinetics on clay surfaces. Accordingly, the objective of this study was to investigate the rates of Ca-Na and Ca-K exchange on montmorillonite using pressure-jump relaxation, and to compare the equilibrium constants calculated via kinetics with those determined using a traditional thermodynamic approach. MATERIALS AND METHODS Theoretical Considerations For the Ca-Na and Ca-K exchange reaction,