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DIVISION S-2—SOIL CHEMISTRY

Kinetics and Energetics of Phosphate Release from Tropical Soils Determined by Mixed Ion-Exchange Resins

^a Dep. of Soil Science, Faculty of Agriculture, Ahmadu Bello University, Zaria, Nigeria
^b Embrapa Meio Ambiente, Caixa Postal 69, 13820-000 Jaguariuna (SP), Brazil

* Corresponding author (Agbenin{at}abu.edu.ng)

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 m² s^-1 under shaking conditions, but decreased to 1.42 to 2.68 x 10^-13 m² 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 G^O and entropy of activation 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 H^O for the overall P exchange reactions was strongly endothermic.