Thermal decomposition of struvite in water: qualitative and quantitative mineralogy analysis

ABSTRACT

Struvite (MgNH₄PO₄·6H₂O) is a potential fertilizer mineral that can be obtained from wastewaters. When the ambient temperature changes, struvite may decompose in water and other phosphate-bearing minerals form instead. The wet decomposition may include complex mineralization, as the struvite crystal structure releases both water molecules and ammonia. An in-situ x-ray measurement for the wet transformation of the struvite is needed to get insight into the mineral formed and into the influence of the water temperature on the decomposition/remineralization. In this study, the X-ray diffraction (XRD) sample holder containing struvite and water in a sealed condition was heated to temperatures of 55 to 120°C for 24 h. Later the still sealed sample holder was exposed to the X-ray beam with the Debye-Scherrer transmission technique, and the diffraction pattern was analyzed by the XRD Rietveld method. With increasing temperature (<100°C), struvite first dehydrated to dittmarite (MgNH₄PO₄·H₂O). Moreover, a decomposition of struvite into an amorphous form of magnesium hydrogen phosphate has occurred as the XRD background increased dramatically and showed a structured profile with very broad intensity maxima. Furthermore, struvite transforms into dittmarite, newberyite, and bobierrite when the sample was heated above 100°C. The outcome of this work is expected to add knowledge on the instability of struvite, which may occur in the fields of the wastewater treatment and in the bio-mineralization in the urine of animals and humans.

KEYWORDS:

• XRD method
• thermal decomposition
• struvite
• dittmarite
• newberyite

Acknowledgments

The study was supported by the KAAD (Katholischer Akademischer Austausch Dienst) Bonn, Germany for a sabbatical on leave study program at LMU Munich. The authors also wish to thank Miss. Stefanie Hoser for helping to perform lab experiments.