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

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.