DETERMINATION OF RATE COEFFICIENTS FOR POTASSIUM-CALCIUM EXCHANGE ON VERMICULITE USING A STIRRED-FLOW CHAMBER

A stirred-flow technique has been widely used to study the rate of ion adsorption on clays and soils. However, no mathematical analysis of c hemical kinetics combined with mass transport has been used to derive rate coefficients for ion exchange phenomena. Accordingly, the objecti ves of this study were to: (i) develop a mathematical model of the kin etics of an elementary cation-exchange reaction combined with transpor t processes in a stirred-flow chamber and (ii) determine rate coeffici ents based on this model for K-Ca exchange on a vermiculite. The kinet ics of exchange were described by a second-order mechanistic rate law from which the equilibrium exchange equation was derived. The second-o rder mechanistic rate law combined with an equation of mass balance de scribing transport were solved numerically. The equilibrium K-Ca excha nge data were described by the Gaines and Thomas approach with a therm odynamic exchange equilibrium constant K-ex = 754.6 L mol(-1). The val ues of the apparent adsorption rate coefficient (k(a)) ranged from 84 to 185 L(2) mol(-2) min(-1), as influent rate increased from 0.3 to 2. 0 mL, min(-1). The calculated K-Ca exchange half-life (t(1/2)) ranged from 0.5 to 1.5 min, in agreement with published data based on batch m ethods.