Thermal decomposition characterization and kinetics of copper, iron, manganese and zinc chelates of ethylenediaminetetraacetic acid in air atmosphere

Abstract

The analysis of the thermal stability of Cu, Fe, Mn, and Zn-EDTA (copper, iron manganese, and zinc chelates of ethylenediaminetetraacetic acid) chelates indicates a high complexity of occurring decomposition processes, ranging from two to three simplified stages of the decomposition process. Obtained results reports that the thermal stability decreased in the following order: Zn-EDTA > Mn-EDTA > Cu-EDTA > Fe-EDTA. The initial model-free kinetic analysis suggested that three out of four chelates had a multi-step first stage of the decomposition. Model-fitting methodology resulted in the best fit in the first stage of decomposition of the studied EDTA chelates that corresponded to An (nth-dimensional nucleation) or Cn (nth-order autocatalytic reaction) models with correlation coefficients within 0.9951 to 0.9991. The activation energy values were within the range 126.27–253.25 kJ·mol−1. The least stable chelates, Cu and Fe, were defined by using competitive An and Cn reaction types, while the most stable, Zn, has a single-step Cn first stage of the decomposition process. The study showed the initial possibility of using EDTA chelates of zinc, manganese, and possibly copper in fertilizer production.