Kinetics of Zinc Chelates Reaction in Some Calcareous Soils

The kinetics of reaction of Zn-DTPA, Zn-EDTA and zincfulvate with calcareous soils between five minutes and 120 minutes conformed to a set of parallel pseudo-first order reaction. As predicted by the kinetic model, the kinetic constant for the loss of Zn from soil solution was equal to the sum of kinetic constant of the formation of DTPA, EDTA and fulvate chelates of competing cations. The kinetic constant for the loss of Zn from Zn-DTPA, Zn-EDTA and zinc-fulvate varied from 2.9 to 9.1 x 10⁻³ h⁻¹, 2.0 to 6.4 x 10⁻³ h⁻¹ and 9.2 to 0.89 x l0⁻³ h⁻¹, respectively.

The subsequent reaction of Zn-DTPA, Zn-EDTA and zinc-fulvate (Zn-FA) beyond four hours was marked by a slow diffusion controlled release of labile Fe²⁺, Mn²⁺ and Cu²⁺ from solid matrix into ambient soil solution. The effectiveness of different Zn-chelates varied in the order: Zn-DTPA > Zn-EDTA > Zn-FA in keeping zinc in soluble form in soils. The fulvate macro molecules were adsorbed very rapidly by soils. The subsequent displacement of Zn from surface bound Zn fulvate by Fe²⁺, Cu²⁺ and Mn²⁺ may occur slowly as reaction proceeds.