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

## TL;DR

This study uses infrared spectroscopy to analyze how a magnesium ammonium phosphate compound changes when heated, revealing new insights into its thermal behavior and transformation stages.

## Contribution

A novel nonisothermal kinetic framework using in situ ATR-FTIR and LLS spectral decomposition to study struvite–dittmarite transformation.

## Key findings

- Four thermally distinct stages were identified during the transformation, including dehydration and amorphization.
- Activation energies for the struvite–dittmarite transformation were calculated using Kissinger and KAS methods.
- Temperature-programmed ATR-FTIR with LLS analysis proved effective for tracking structural and vibrational changes.

## Abstract

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.