Thermodynamic parameters of potassium exchange as characterized by equilibrium and kinetic approaches in chloride and perchlorate background anions

Thermodynamics of potassium exchange were compared using equilibrium and kinetic approaches in Ca-saturated samples of a Tulewal loamy sand soil profile from Punjab. The classical Argersinger theory was employed for the equilibrium approach, while Arrhenius and Vonʼt Hoff theories were used for the kinetic approach.The curves of Vanselow selectivity coefficients (kv) against equivalent fractions of K-adsorbed (XK), Inkv vs. XK, showed perference for K over Ca. The values of kv varied for each corresponding value of XK, which suggests non-ideal behavior of the exchanger phase. The isotherms of equivalent fraction of K on the exchanger phase (qK/qO, where qK is the amount of K adsorbed in equivalents per unit weight of the soil and qO is the total amount of K and Ca adsorbed in equivalents per unit weight of the soil) and in the equilibrium solution (cK/cO) also exhibited preference of K over Ca when compared with nonpreferential curves. The values of standard free energy, Δ, for K+-Ca exchange ranged from −3.96 to −3.10 kJmol at 298K and from −3.62 to −2.86 kJmol at 313K. This suggested the exothermic nature of K+-Ca exchange. The negative values (−12.02 to −6.79 kJmol) of standard enthalpy change (ΔH) further confirmed the exothermic nature of K+-Ca2+ exchange.The thermodynamic parameters in perchlorate anionic media were higher than those observed in chloride media, suggesting and increase in the spontaneity of the K-Ca excgabge process in perchlorate media.As with the equilibrium approach, the kinetic approach also exhibited negative values of ΔG that were higher in ClO4- (−2.32 to 2.71 kJmol) anionic background compared with Cl (−2.21 to −2.60 kJmol) as the background anion. These negative values also decreased with an increase in temperature. Although the magnitude of thermodynamic parameters for the two approaches compared poorly, they compared very well in trend and showed similar inference of ion behavior in the soil. It seems reasonable, therefore, to use kinetic approaches to describe adsorption-desorption wherever possible. The kinetic approach has the added advantage that, in addition to the thermodynamic parameters, one can study the mechanisms and rates of exchange in soil and clay minerals.