dc.description.abstract
The magnesium phosphate mineral struvite (MgNH4PO4·6H2O) has gained scientific significance in the field of phosphorous and nitrogen recovery from wastewater. The recovery of these elements has become critical due to the limited availability of natural P resources like phosphorites and as a tool to reduce the environmental harm linked to mining such deposits. Furthermore, recovered struvite is employed as a slow-release fertiliser for agriculture.
Fertilisers from wastewater-derived struvite are produced through intentional precipitation in wastewater treatment plants. However, it is fundamental to gain an in-depth understanding of the conditions necessary for the formation of struvite to maximise the efficiency of this process. This includes the formation conditions of struvite and the kinetics involved in its nucleation and growth. While the formation conditions of struvite-forming wastewater have been widely examined, a significant scientific knowledge gap still exists regarding the formation of struvite from pure salt solutions and the kinetic parameters of nucleation and growth. Aspects such as the nucleation and growth mechanisms of struvite and the role of temperature in the formation of struvite remain poorly understood. Better understanding these parameters is paramount to predicting struvite nucleation in complex solutions like the ones found in residual wastewater. As a result, my first aim for this study was to investigate the nucleation and growth kinetics of struvite from pure salt solutions at different temperatures.
The use of struvite fertiliser is limited by the fact that struvite is unstable under atmospheric conditions. The precise conditions that trigger the breakdown of struvite are not fully understood, which limits its widespread use as a fertiliser. The secure transport and storage of struvite for its later industrial use requires fully comprehending the conditions in which it transforms into secondary phases. This constitutes the second objective of my thesis, where I addressed the stability of struvite by looking at its crystal structure and the transformation of struvite in air as a function of temperature.
I used an experimental approach to address these knowledge gaps, including synthetic struvite solutions and crystals. To evaluate kinetic parameters governing the formation of struvite, I conducted nucleation and growth experiments at different solution concentrations and temperatures, employing in-situ spectroscopy and X-ray diffraction techniques. To provide a more fundamental understanding of the stability of struvite, I used single-crystal X-ray diffraction and evaluated a more accurate crystal structure of struvite by analysis at low temperatures (100 K), providing less temperature-induced influence on the atomic bonds. Furthermore, I evaluated the conditions and the mechanism of struvite decomposition into secondary phases through dry transformation experiments at different temperatures. I used powder XRD, Infrared and Raman spectroscopy, and scanning electron microscopy to follow the decomposition kinetics and characterise the resulting products to elucidate the mechanisms of struvite decomposition.
The investigation of struvite nucleation and growth kinetics as a function of temperature revealed a reaction order (n) of ~ 0.4–0.8, indicating a diffusion-controlled nucleation mechanism and rate constants (k) of ~ 0.3–0.8 s-1. The activation energy of crystallisation was calculated as ~ 17 kJ/mol. No significant changes in n and k values across temperatures were observed. However, the induction period of struvite nucleation decreased up to 25 °C and increased beyond this temperature, supporting previous assumptions about the reverse temperature-dependence of struvite solubility.
The improved crystal structure of struvite revealed that ammonia plays a key role in its stability. At low temperatures, NH3 is retained within the structure of struvite as ammonium, stabilised by three hydrogen bonds connected to phosphate anions and magnesium cations. This configuration is critical for the structural stability of struvite at low temperatures. In contrast, at room temperature, NH4+ forms fewer hydrogen bonds and exhibits rotational behaviour, which leads to a decrease in the stability of struvite.
Finally, I evaluated kinetic parameters for the decomposition and transformation of struvite with temperature during the struvite transformation experiments. Struvite transformed into two different minerals: newberyite (MgHPO4·3H2O) and dittmarite (MgNH4PO4·H2O), and one amorphous product, depending on the reaction temperature. The transformation reactions comprise similar reaction orders n of ~ 0.5 and rate constants k of 0.5–8.1·10-4 s-1 for the reaction to newberyite, and 2.9·10-3 s-1 for the reaction to dittmarite, indicative of a diffusion-dominated mechanism that was interpreted as the transport of NH3 and water out of the struvite structure. Electron microscopy and Raman spectroscopy revealed that the following nucleation and growth of the new phases occur via a coupled dissolution-reprecipitation reaction.
In summary, through my thesis, I could extend the knowledge of struvite nucleation and growth kinetics from aqueous solution and give new information on this mineral's decomposition and transformation pathways. This work probed the critical role played by temperature in the formation and decomposition of struvite. My findings contribute to improving struvite precipitation processes in wastewater treatment plants for P recovery. Furthermore, the new insights into the decomposition and transformation parameters of struvite in air have significant implications for struvite fertiliser storage and transportation, as understanding the transformation to secondary phases helps ensure the fertiliser properties and quality.
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