Nucleation, Growth and Transformation Phenomena of Vivianite

Abstract

Vivianite, a hydrated Fe(II) phosphate (Fe3(PO4)2·8H2O), is a mineral found globally in anoxic lacustrine or marine sediments, soils, associated with organic matter and in groundwater streams. It is a key phase controlling the cycling and immobilization of Fe(II) and phosphorus in these systems. Vivianite formation can aid in phosphorus recovery from wastewater; therefore, it is an important mineral for circular economy applications. This dissertation presents novel insights into the intricate processes governing the nucleation, growth, and phase transformations of vivianite. The nucleation of vivianite occurs via a transient intermediate phase – amorphous ferrous phosphate (AFEP), via a nonclassical pathway. AFEP is a metastable precursor phase which has a distinctly different local structure and composition (Fe3(PO4)2·4.7H2O), compared to vivianite. Through laboratory experiments and ex situ techniques, I demonstrated that AFEP precipitates from supersaturated solutions, undergoes subsequent hydration and structural transformations to transform into thermodynamically stable vivianite. The crucial role of water in this transformation is also highlighted. The stability of AFEP is inversely proportional to systemic (vivianite) supersaturation and temperature, suggesting AFEP occurrence to be a widely prevalent phenomenon in near-equilibrium (environmental) systems. Synchrotron-based small and wide-angle X-ray scattering experiments provide novel insights into the temporal evolution of vivianite crystallization via AFEP nanoparticles, with observations indicating concomitant processes of AFEP transformation and vivianite growth in solution – revealing a direct link between these two processes. Furthermore, the investigation of vivianite growth kinetics shows temperature-dependent dynamics, and a direct correlation between temperature and transformation rate. The rate of vivianite growth could be modeled via the JMAK model to derive the apparent activation energy (~ 51 kJ/mol) of this transformation. Temperature-dependent variations in crystal splitting also reveal new insights into varying vivianite crystal morphologies. Additionally, the thermally induced phase transformations of vivianite (amorphization and recrystallization into a distinctly new mineral phase), particularly under high-temperature conditions, including the critical influence of atmosphere on the resultant transformed phase compositions, is also reported here for the first time. This research provides new and significant insights for vivianite/mineral nucleation and growth mechanisms in aqueous solutions, materials synthesis and thermal stability of iron phosphates.