An investigation of the thermal behavior of magnesium ammonium phosphate hexahydrate

Struvite (MgNH4PO4 center dot 6H(2)O) was heated to temperatures from 55 to 300 A degrees C. X-ray diffraction analysis revealed struvite was stable at 55 A degrees C, partially decomposed to dittmarite (MgNH4PO4 center dot H2O) at 100-200 A degrees C, and formed an amorphous phase at 250-300 A degrees C. Thermogravimetric analysis confirmed sample mass loss consistent with dittmarite formation at 100-200 A degrees C and evolution of all volatiles at 250-300 A degrees C. Fourier transform infrared (FTIR) spectroscopy detected the nu(4) NH (4) (+) band in 55-200 A degrees C solids, as expected for struvite and dittmarite. This band decreased in intensity at 250 A degrees C, and was not evident at 300 A degrees C, confirming loss of NH (4) (+) (s) at these temperatures. FTIR spectra also showed changes in the vibrations of the nu(3) PO (4) (3-) band. At 55 A degrees C, splitting in the band indicated destabilization of the PO (4) (3-) (s) group despite no change in mineralogy. Vibrations at 100-200 A degrees C were associated with dittmarite and MgHPO4, and at 250-300 A degrees C, MgHPO4 and Mg2P2O7. Analysis of acid-digested solids indicated the presence of P other than P-PO4 at 200-250 A degrees C, confirming Mg2P2O7 formation. Overall, heat treatment of struvite produces several decomposition products, complete identification of which requires the use of multiple approaches. Temperature-induced phase transformations along with emission of NH3(g) have implications for use of struvite in multiple applications.