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 a nd kinetic approaches in Ca-saturated samples of a Tulewal loamy sand soil profile from Punjab. The classical Argersinger theory was employe d for the equilibrium approach, while Arrhenius and Von't Hoff theorie s were used for the kinetic approach. The curves of Vanselow selectivi ty coefficients (kv) against equivalent fractions of K-adsorbed (X(K)B AR), lnkv vs. X(K)BAR, showed preference for K over Ca. The values of kv varied for each corresponding value of X(K)BAR, which suggests non- ideal behavior of the exchanger phase. The isotherms of equivalent fra ction 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 tota l amount of K and Ca adsorbed in equivalents per unit weight of the so il) 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, DELTAG0, for K+-Ca2+ exchange ranged from -3.96 to -3.10 kJmol-1 at 298K and from -3.62 to -2.86 kJmol-1 at 313K. This suggested the exothermic nature of K+-Ca2+ exchange. The negative val ues (-12.02 to -6.79 kJmol-1) of standard enthalpy change (DELTAH0) fu rther confirmed the exothermic nature of K+-Ca2+ exchange. The thermod ynamic parameters in perchlorate anionic media were higher than those observed in chloride media, suggesting an increase in the spontaneity of the K+-Ca2+ exchange process in perchlorate media. As with the equi librium approach, the kinetic approach also exhibited negative values of DELTAG0 that were higher in ClO4-(-2.32 to 2.71 kJmol-1) anionic ba ckground compared with Cl- (-2.21 to -2.60 kJmol-1) as the background anion. These negative values also decreased with an increase in temper ature. Although the magnitude of thermodynamic parameters for the two approaches compared poorly, they compared very well in trend and showe d 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 mechan isms and rates of exchange in soil and clay minerals.