REVERSALS IN PHASE STABILITY OF Al-O MINERALS AT THE NANOSCALE

It is important to establish the crystallization sequences in common oxide minerals, both for furthering understanding of fundamental mineralogy, and also for its application to pressing environmental issues. For example, nanoparticles of aluminum oxide minerals can be found as pollutants in surface waters such as wastewater treatment facilities. The stability of mineral phases in a certain system may differ between bulk vs. nanoscale particle sizes. This is displayed in a crossover in the total Gibbs free energy as a function of the surface area of the crystals. For example, previous research has established a switch in stability from the typically stable mineral phase rutile to anatase in the crystallization of the TiO₂ system at a nanoscale. Within the Al-O system, there may be similar controls on the sequence of crystallization. However, there is a lack of data available on surface energies and stabilities as a function of size in the Al-O system at the nanoscale.

The goal of this study is to create a detailed model of phase stability for these contaminant particles. We conduct crystallization experiments with Al-O minerals from an AlCl₃-H₂O solution with buffered pH, and analyze produced phases via XRD and SEM. These experiments indicate a crossover in stability between the stable bulk phase of corundum, or α-Al₂O₃, and other Al-O minerals at the nanoscale, such as gibbsite, boehmite, diaspore, and γ-Al₂O₃. Additionally, other variables alongside crystal size, such as pH and salinity, affect the timing of the crystallization sequence, making these dependencies highly relevant to a variety of environmental settings. Remediation techniques with Al-bearing wastewater must therefore carefully account for solution conditions to regulate the form of contaminant particles.