Kinetics and energetics of phosphate release from tropical soils determined by mixed ion-exchange resins

Kinetic information on soil P release is required to optimize P fertilizer use efficiency in agricultural production and develop guidelines for the disposal of P-rich wastes onto the land. We determined the kinetics and energy changes of P release from tropical soils by mixed ion-exchange resin. Phosphate release patterns were determined with or without shaking and at three temperatures: 298, 308, and 333 K. Phosphate release patterns determined as P uptake by the exchange resins were best described by diffusion models. The nature of diffusional limitation was discriminated on the basis of activation energies (E-a), which ranged from 42.03 to 48.72 kJ mol(-1). The range of E-a was consistent with, and close to, 42.1 kJ mol(-1) for intraparticle diffusion-controlled ion-exchange reactions. The P diffusion coefficients ranged from 2.81 to 4.71 X to 10(-13) m(2) s(-1) under shaking conditions, but decreased to 1.42 to 2.68 x 10(-13) m(2) s(-1) for nonshaking conditions probably because of large films that developed around the resin beads under static or nonshaking conditions. The free energy of activation DeltaG(0 double dagger) and entropy of activation DeltaS(0 double dagger) were lower than those of soils studied by means of resin bugs indicating favorable P exchange reactions and less stearic inhibition for the resins. The enthalpy of activation DeltaH(0 double dagger) for the overall P exchange reactions was strongly endothermic.