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

Establishing the crystallization sequences in common oxide minerals aids in application to pressing environmental issues and furthering understanding of fundamental mineralogy. For example, nanoparticles of aluminum oxide minerals, sourced from acid mine drainage, can be found in surface waters such as wastewater treatment facilities and contribute to the secondary contaminant level for aluminum in the water system. 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 formation enthalpy as a function of the crystals’ surface area. 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-(H) 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-(H) system at the nanoscale.

The goal of this study is to create a detailed phase stability diagram for these contaminant particles. We conduct crystallization experiments of Al-O-(H) minerals from an AlCl3-H2O solution with buffered pH, and produced phases will be analyzed via XRD, SEM-EDS, TEM, FTIR, and synchrotron micro-XRD methods. These experiments indicate a crossover in stability between the stable bulk phase of corundum, or α-Al2O3, and other Al-O-(H) minerals at the nanoscale, such as gibbsite, boehmite, diaspore, and γ-Al2O3. Additionally, other variables alongside crystal size, such as pH and salinity, will affect the crystallization sequence's timing, 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.