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 × to 10 −13 m 2 s −1 under shaking conditions, but decreased to 1.42 to 2.68 × 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 Δ G O‡ and entropy of activation Δ S O‡ were lower than those of soils studied by means of resin bags indicating favorable P exchange reactions and less stearic inhibition for the resins. The enthalpy of activation Δ H O‡ for the overall P exchange reactions was strongly endothermic.