Disordered interfaces of alkaline aluminate salt hydrates provide glimpses of Al³⁺ coordination changes

TR Graham and M Pouvreau and R Gorniak and HW Wang and ET Nienhuis and QRS Miller and J Liu and MP Prange and GK Schenter and CI Pearce and KM Rosso and AE Clark, JOURNAL OF COLLOID AND INTERFACE SCIENCE, 637, 326-339 (2023).

DOI: 10.1016/j.jcis.2023.01.003

Hypothesis: The precipitation and dissolution of aluminum-bearing mineral phases in aqueous systems often proceed via changes in both aluminum coordination number and connectivity, complicating molecular- scale interpretation of the transformation mechanism. Here, the thermally induced transformation of crystalline sodium aluminum salt hydrate, a phase comprised of monomeric octahedrally coordinated aluminate which is of relevance to industrial aluminum processing, has been studied. Because intermediate aluminum coordination states during melting have not previously been detected, it is hypothesized that the transition to lower coordinated aluminum ions occurs within a highly disordered quasi-two-dimensional phase at the solid-solution interface. Experiments and simulations: In situ X-ray diffraction (XRD), Raman and Al-27 nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na-9Al(OH)(6)(2)center dot 3(OH)center dot 6H(2)O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. A reactive forcefield was developed to bridge speciation in the solution and in the solid phase. Findings: In contrast to classical dissolution, aluminum coordination change proceeds through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. In both melting and dissolution of NSA, octahedral, monomeric aluminum transition through an intermediate of pentahedral coordination. The intermediate dehydroxylates to form tetrahedral aluminate (Al(OH)(4)(-)) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ion linkages. The solution phase Al(OH)(4)(-) ions subsequently polymerize into polynuclear aluminate ions. However, there are some differences between bulk melting and interfacial dissolution, with the onset of the surface- controlled process occurring at a lower temperature (similar to 30 degrees C) and the coordination change taking place more gradually as a function of temperature. This work to determine the local structure and dynamics of aluminum in the disordered layer provides a new basis to understand mechanisms controlling aluminum phase transformations in highly alkaline solutions. (C) 2023 Elsevier Inc. All rights reserved.

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