The Transformation of Magnesium Phosphate Minerals at Atmospheric Conditions - Mechanisms, Kinetics and Environmental Applications

Struvite (MgNH₄PO₄·6H₂O), a magnesium phosphate mineral, is a promising recoverable phosphorous (P) and nitrogen (N) source and is employed as a slow-release agricultural fertilizer. However, struvite is unstable in air and transforms into two distinct crystalline phases - newberyite (MgHPO₄·3H₂O) and dittmarite (MgNH₄PO₄·H₂O), before breaking down into an amorphous phase. The transformation products of struvite have been indicated in the literature. However, published studies quantifying those processes focused on struvite decomposition in solutions, and investigations in air were conducted only at high temperatures, far from the conditions under which this mineral would be stored for its use as a fertilizer. Furthermore, the kinetics including transformation rates and mechanisms are unknown, and thus we present complementary X-ray diffraction (XRD), vibrational spectroscopic and light- to electron microscopic data from which we determined the temperature-dependent (22–60 °C) kinetics and mechanisms for the transformation and breakdown of struvite in air. Our results revealed that its transformation into both newberyite and dittmarite follows a coupled dissolution-reprecipitation mechanism, with the reaction kinetics dominated by the diffusion of H₂O and NH₃ out of the struvite structure. The reaction pathways proved to be temperature-dependent, with newberyite being the only transformation product at room temperature even after 10 months, a transformation resulting from the loss of ammonia and three of the six water molecules. On the contrary, at higher temperatures, dittmarite was the major transformation product of struvite, forming through the release of five of the six molecules of water, but retaining its ammonia. At 60 °C and after 3 months, dittmarite breaks down into an amorphous magnesium phosphate phase. This study provides comprehensive insights into the kinetics and the underlying mechanisms governing the transformation of struvite at atmospheric conditions. Our findings have significant implications for the long-term storage of struvite, and its subsequent utilization as a slow-release fertilizer.