THERMODYNAMIC STUDY FOR THE (NH4-K+) EXCHANGE ON K-SATURATED CLINOPTILOLITE()

Homoionic K-clinoptilolite was evaluated as both sink for NH4+ and as a source of nutrient such as K+ through a thermodynamic model. In this model, the isotherm for the exchange reaction, K-z + (NH4+)(s) revers ible arrow = (NH4)(z) + K-s(+) for different size fraction of clinopti lolite (20-50 and 50-75 mu m) was obtained at various charge fractions of NH4+ + K+ solution but at total variable concentration of 0.01, 0. 1 and 1.0 M. From the thermodynamic data obtained, the mass action quo tient (K-m) values, corrected with activity coefficient (K-c) were cal culated. These data were plotted as log K-c vs. the equivalent fractio n of (NH4)(z) in zeolite to derive the thermodynamic equilibrium const ants (K-a). The standard Gibbs free energy Delta G(0) was calculated. From the obtained K-a and Delta G(0) values, it was found that the exc hange reaction was favoured at lower concentration (0.01 M) and for sm all particle size fraction (20-50 mu m) of clinoptilolite. The maximum selectivity coefficient (alpha) values of clinoptilolite (20-50 and 5 0-75 mu m) to NH4+ from aqueous solution were found to be 2.4 and 1.9, respectively at lower concentration of 0.01 M. The effect of anions b ackground on the selectivity of natural clinoptilolite to K+ and NH4was studied by calculating the Gapon selectivity coefficient (K-G). Th e activities of NH4Cl and KCl used in the exchange solutions were calc ulated by using Ion Speciation Model (MINTEQ A(2)/PRODEFA(2)). The use of CsCl together with the solution mixture of KCl and NH4Cl helps the exchange of NH4+ to K+ due to the role of Cs+ in displacing K+ ions. The importance of this study stems from the fact that it throws light on thermodynamics exchange process of NH4+ to K+ and vice versa on the surface of clinoptilolite and consequently between environment contai ning NH4+ and soil amendment containing K+. (C) 1997 Elsevier Science B.V.