Potassium-Calcium Exchange in a Multireactive Soil System: I. Kinetics

The kinetics of K exchange were investigated in Ca-saturated samples from the Ap horizon of an Evesboro soil from Delaware. Biphasic kinetics characterized the first-order plots for K adsorption and desorption at 283 and 298K with the two simultaneous reactions being attributed to exchange sites with varying reactivity for K and Ca ions. The rapid reaction was ascribed to exchange sites of the soil that are readily accessible to cation exchange reactions, whereas the slow reaction was attributed to exchange sites that are difficultly accessible to cation exchange reactions. Confirmation of the biphasic kinetics was achieved through the use of cetyltrimethylammonium bromide (CTAB). Parabolic diffusion plots for K adsorption and desorption at 283 and 298 K indicated that an intraparticle diffusion process may be rate limiting for the difficultly accessible sites of the soil. The initial deviation from linearity of the parabolic plots for K desorption suggested that film diffusion may be rate limiting for desorption on readily accessible sites of the soil that form strong bonds with Ca ions. At 313 K the initial rapid kinetics of exchange was no longer present, and the exchange process was described by a single first-order reaction. The parabolic plots at 313 K suggested that this phenomenon could possibly be attributed to the finite rate at which the polymer structure of soil organic matter changes in response to the adsorption and desorption of the two reacting cations. Additional Index Words: biphasic kinetics, adsorption by entropy effect, film diffusion, intraparticle diffusion, surface diffusion. Jardine, P.M., and D.L. Sparks. 1984. Potassium-calcium exchange in a multireactive soil system: I. Kinetics. Soil Sci. Soc. Am. J. 48:39-45. A NUMBER OF RESEARCHERS have investigated kinetic reactions on pure clays and soils (Way, 1850; Gedroiz, 1914; Mortland, 1958; Sawhney, 1966; Kuo and Lotse, 1974a, 1974b; Sparks et al., 1980a, 1980b). Sawhney (1966) described the uptake of Cs on vermiculitic clay minerals as a pseudo first-order reaction where diffusion of the cation into the interlayer space was the rate-controlling process. First-order kinetics were also found to describe the desorption of phosphate from kaolinite (Bar-Yosef and Kafkafi, 1978). Keay and Wild (1961) investigated the exchange of numerous univalent and divalent cations with Na on a vermiculite and suggested that intraparticle diffusion was the rate-controlling mechanism governing the adsorption velocity of the cations. Numerous other investigators have noted that K release from micaceous minerals was diffusion controlled and the kinetics of exchange conformed to the parabolic diffusion law (Barshad, 1951; Rausell-Colom et al., 1965; Chute and Quirk, 1967; Feigenbaum et al., 1981). The kinetics of K adsorption and desorption in heterogeneous soil systems have also been shown to con1 Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Miscellaneous Paper no. 1026. Contribution no. 160 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711. Received 2 Mar. 1982. Approved 10 Aug. 1983. 2 Graduate Research Assistant and 1982 Potash and Phosphate Institute Fellow, and Associate Professor of Soil Chemistry, respectively. The address for the senior author is now Dep. of Agronomy, Virginia Polytechnic Inst. & State Univ., Blacksburg, VA 24061. form to first-order kinetics and to the parabolic law (Sivasubramaniam and Talibudeen, 1972; Sparks et al., 1980a, 1980b; Sparks and Jardine, 1981). Salim and Cooksey (1980), investigating the adsorption of Pb on river muds, found that the rate of exchange conformed to first-order kinetics. Application of the parabolic law indicated that the adsorption process could not be fully explained by either film diffusion or by intraparticle diffusion. The kinetics of phosphate adsorption and desorption in soils has also been shown to conform to first-order kinetics and to exhibit a diffusion-controlled exchange (Amer et al., 1955; Kuo and Lotse, 1974b; Evans and Jurinak, 1976; Vig and Dev, 1979). Chien and Clayton (1980), investigating phosphate sorption and release in soils, found that the Elovich equation was superior to the first-order rate approach, since the former method tended to combine simultaneous first-order reactions into one linear slope. It has been theorized that soils contain sites of differing reactivity for K ions (Bolt et al., 1963; Sawhney, 1966; Goulding and Talibudeen, 1979). Bolt et al. (1963) theorized the existence of three different types of binding sites for K exchange on illite. The authors attributed rapid kinetics of exchange to external planar sites, slow kinetics of exchange to interlattice exchange sites, and intermediate kinetics of exchange to interlattice edge sites. Goulding and Talibudeen (1979) also alluded to three different types of K binding sites in their soil system. Accessible K was attributed to external surface reactions, peripheral K was ascribed to that in the interlayers of clay minerals, and matrix K was theorized to be from mica interstratifications. Beckett and Nafady (1967) characterized different binding sites in a soil system using various organic compounds. Cetyltrimethylammonium bromide (CTAB) was used to block external surface sites and leave sites associated with clay mineral interlayers intact. The different sites associated with the soil matrix should exhibit different reaction rates; thus, multiple rate coefficients may exist. The objective of this study is to evaluate the kinetics of K-Ca exchange in a multireactive soil system. MATERIALS AND METHODS Theoretical Considerations With the use of a miscible displacement technique, the apparent adsorption and desorption rate coefficients (ka and kd, respectively) can be determined from first-order kinetic equations as derived below. Apparent Adsorption Rate Coefficient (k^