Kinetics of DTPA extraction of zinc from calcareous soils

The kinetics of Zn extraction by diethylenetriaminepentaacetic acid (DTPA) from the ≤ 2 mm fraction of 12 calcareous soils was investigated using surface soil (0–30 cm) samples. Soils were equilibrated with 0.005 M, DTPA solution for 0.25 to 192 h. Zero-, first-, second-, third-order, parabolic diffusion and simple Elovich equations did not adequately describe Zn extraction kinetics. The best model for describing extraction data for all soils was the exponential rate equation ( q = a t b ). q is the amount of Zn desorption (mg Zn kg − 1 ), a is the Zn desorption magnitude constant (mg Zn kg − 1 h − 1 ) and b is the desorption rate constant. Constant a ranged from 21 to 79.8 with a mean of 41.4 and b from 0.19 to 0.33 with a mean of 0.25. The differences in a value presumably reflect the quite different properties and constituents of these soil types. The magnitude constant a, was positively correlated ( r = 0.76⁎⁎) to organic carbon content and negatively to pH ( r = − 0.67⁎). The rate constant b was not related to any of the measured soil properties such as pH, cation exchange capacity, carbonate calcium equivalent, active calcium carbonate equivalent, clay content, organic carbon and specific surface area or to any combination of these properties.