Study on thermal decomposition and the non-isothermal decomposition kinetics of glyphosate

The thermal decomposition process and non-isothermal decomposition kinetic of glyphosate were studied by the Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA). The results showed that the thermal decomposition temperature of glyphosate was above 198 °C. And the decomposition process was divided into three stages: The zero stage is the decomposition of impurities, and the mass loss in the first and second stage may be methylene and carbonyl, respectively. The mechanism function and kinetic parameters of non-isothermal decomposition of glyphosate were obtained from the analysis of DTA–TG curves by the methods of Kissinger, Flynn–Wall–Ozawa, Distributed activation energy model, Doyle and Šatava-Šesták, respectively. In the first stage, the kinetic equation of glyphosate decomposition obtained showed that the decomposition reaction is a Valensi equation of which is two-dimensional diffusion, 2D. Its activation energy and pre-exponential factor were obtained to be 201.10 kJ mol−1 and 1.15 × 1019 s−1, respectively. In the second stage, the kinetic equation of glyphosate decomposition obtained showed that the decomposition reaction is a Avrami–Erofeev equation of which is nucleation and growth, and whose reaction order (n) is 4. Its activation energy and pre-exponential factor were obtained to be 251.11 kJ mol−1 and 1.48 × 1021 s−1, respectively. Moreover, the results of thermodynamical analysis showed that enthalpy change of ΔH≠, entropy change of ΔS≠ and the change of Gibbs free energy of ΔG≠ were, respectively, 196.80 kJ mol−1,107.03 J mol−1 K−1, and 141.77 kJ mol−1 in the first stage of the process of thermal decomposition; and 246.26 kJ mol−1,146.43 J mol−1 K−1, and 160.82 kJ mol−1 in the second stage.