Analyses of Adsorption Kinetics Using a Stirred-Flow Chamber: II. Potassium-Calcium Exchange on Clay Minerals

Potassium-calcium adsorption kinetics on a Llano vermiculite and an Arizona montmorillonite were studied using a stirred-flow technique. Various experiments were conducted to distinguish between (i) instantaneous equilibrium and time-dependent reactions, and (ii) solution-concentration-dependent kinetic models vs. those that are independent of solution concentration. These experiments included varying the flow rate and influent concentration and stopping the flow for a period of time. It was found that K-Ca exchange on montmorillonite was too rapid to be measured with the stirred-flow technique, while exchange rates on vermiculite could be ascertained. Adsorption kinetic parameters should be included in transport models to more accurately predict the fate of ions in soils dominated by vermiculitic and micaceous clay minerals. "DOTASSIUM is AN ESSENTIAL ELEMENT for plant X^ growth and is a dynamic ion in the soil system. Soil K can be divided into four major phases: solution, exchangeable, fixed, and structural (Sparks and Huang, 1985). To fully understand the physical chemistry of soil K and to predict its fate with time, one must have a knowledge of the rates and mechanisms of K reactions. The application of kinetics to soil surface reactions, however, is arduous and has often been limited by the difficulty in separating the solution from the solid phase quickly enough to measure rapid reactions (Sparks, 1989). Also, problems of correctly interpretM.J. Eick and D.L. Sparks, Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303; A. Bar-Tal and S. Feigenbaum, Inst. of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel. Published with the approval of the Delaware Agric. Exp. Stn. as Miscellaneous Paper no. 1323, Contribution no. 255 of the Dep. of Plant and Soil Sciences, Univ. of Delaware. Received 2 Oct. 1989. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 54:1278-1282 (1990). ing experimental results and comparing different kinetic methods have not been solved. Kinetics of K adsorption on clay minerals have been studied using various methods: batch, stirred batch, vortex batch, and miscible displacement (Keay and Wild, 1961; Jardine and Sparks, 1984; Ogwada and Sparks, 1986). The rate of K adsorption on vermiculite was reported to be much slower than on kaolinite, regardless of the experimental method (Ogwada and Sparks, 1986). Similar results were obtained by Jardine and Sparks (1984), who found K adsorption to be slowest on vermiculite, followed by montmorillonite and kaolinite. Bar-Tal et al. (1990) presented mathematical solutions of the mass-balance equation in combination with equilibrium and kinetic models. They also proposed experimental tests to distinguish between three types of models: (i) instantaneous equilibrium, (ii) kinetic models in which the adsorbed fraction is a function of solution concentration, and (iii) kinetic models in which the adsorbed fraction is independent of the solution concentration. The objective of this study is to determine the utility of different experimental tests (under specific experimental conditions) for distinguishing between kinetic and instantaneous equilibrium processes using a stirred-flow technique for vermiculite and montmorillonite clays. MATERIALS AND METHODS A Llano, Texas, vermiculite and an Arizona montmorillonite obtained from the University of Missouri Source Clays Depository were used in this study. The clays were fractionated by standard procedures (Jackson, 1969), based on a gravimetric method, to an equivalent diameter of 1 to 2 jam. The cation-exchange capacity (CEC) of the Llano vermiculite and the Arizona montmorillonite as determined by Ba-Ca exchange was 210 and 105 cmol kg", respectively. These values are in good agreement with published values (Foster, 1963; Sparks and Jardine, 1984). EICK ET AL.: ANALYSES OF ADSORPTION KINETICS: II. 1279 Prior to the equilibrium and kinetic studies, the clays were made homoionic with respect to Ca by saturating four times with 500 mol nr of Ca(ClO4)2. The excess salt was removed by repeated washing with deionized water until the electrical conductivity (EC) of the supernatant was near that of the deionized water. Complete saturation of the exchanger was tested by replacing the sprbed Ca with Ba. Perchlorate salts were used in both kinetic and equilibrium studies to minimize CaCl* formation (Sposito et al., 1981). Calcium and K concentrations were measured in solution using atomic absorption spectrophotometry.