In Situ ATR-FTIR Nonisothermal Kinetic Analysis of Struvite–Dittmarite Thermal Transformation

The thermal transformation of magnesium ammonium phosphate hydrates is highly relevant to environmental processes including physicochemical mechanisms associated with nutrient recovery and release. In this study, a nonisothermal kinetic framework was developed using in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy to quantitatively describe the struvite (NH₄MgPO₄·6H₂O) to dittmarite (NH₄MgPO₄·H₂O) transformation in the magnesium ammonium phosphate hydrate compound. A linear least-squares (LLS) spectral decomposition approach was applied to temperature-resolved ATR-FTIR data sets to extract the degree of conversion (α) across multiple heating rates. Complementarily, in situ X-ray diffraction (XRD), thermogravimetric analysis, and ex situ Raman spectromicroscopy provided structural and compositional validation of the chemical and crystalline transformations. The resulting kinetics information derived from spectral analysis of phosphate (PO₄ ^3-) and water/hydroxyl/ammonium (H₂O/OH^-/NH₄ ⁺) vibrational regions identified four thermally distinct stages, corresponding to the release of surface water (adsorbed water), followed by crystalline dehydration associated with struvite to dittmarite transformation, and two subsequent amorphization steps yielding magnesium hydrogen phosphate (MgHPO₄). Activation energies of 129.3 ± 17.9 and 126.3 ± 16.8 kJ/mol were obtained using the Kissinger analysis for the struvite-dittmarite transformation, while isoconversional Kissinger-Akahira-Sunose (KAS) evaluation indicated an average activation energy of approximately 143.9 ± 5.5 and 140.1 ± 10.9 kJ/mol across multiple α values. These results show that temperature-programmed ATR-FTIR, coupled with LLS spectral analysis, provides a surface-sensitive route for deriving nonisothermal kinetics and identifying coupled structural-vibrational mechanisms in hydrated ammonium phosphate systems.