Phosphate Sorption/Desorption Kinetics in Some Acidic Soils

The kinetics of phosphate sorption/desorption in eight acidic soils were studied. The data were fitted to a modified Freundlich-type of kinetic equation and the modified Elovich equation to yield several kinetic parameters. The kinetic experiments were conducted at 35 ± 1°C and for some soil, also at 45 ± 1°C. The phosphate sorption rate coefficients and the Elovich kinetic parameters were in agreement with the trend of phosphate sorption equilibrium behaviour of the present soils. The phosphate sorption kinetic parameters showed significant correlations with soil organic matter and oxalate-extractable aluminium and iron of the soils. The possibility that an association of organic matter in acidic soils with the hydrous sesquioxides of iron and aluminium leads to the formation of chelate complexes having large active surface area for phosphate sorption has been considered for the present soils. The sorption rate coefficients increased with temperature, in agreement with the simple Arrhenius behaviour. The activation energies for phosphate sorption suggested the chemical nature of the given sorption process.

During desorption, the sorption rate parameter was lower than that during the corresponding sorption run, indicating the possibility that the sorted phosphate might have undergone transformations that render it less susceptible to release.