PNNL

Abstract

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 melting of sodium aluminum salt hydrate crystals comprised of monomers of octahedrally coordinated aluminate, a phase with relevance to a range of industrial processes, is examined. Because intermediate aluminum coordination states during melting had not previously been detected we hypothesized the transition to lower coordinated aluminum ions occurs within a highly disordered quasi-two-dimensional phase at the solid-solution interface. Experiments In situ x-ray diffraction (XRD), Rama,n and 27Al nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2·3(OH)·6H2O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. Reactive forcefields were developed to bridge speciation in the solution and in the solid phase. Findings In contrast to classical dissolution, aluminum coordination changes proceed through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. CMD simulations identified similarities between the homogenous melting of pure NSA, and the dissolution of NSA at an interface in contact with a concentrated sodium hydroxide solution. These findings indicate that both melting and dissolution of octahedral, monomeric aluminum in NSA proceeds through a transient monomeric intermediate of pentahedral coordination, which dehydroxylates to form tetrahedral aluminate (Al(OH4-) in the liquid phase, and subsequently polymerizes into polynuclear aluminate ions. 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.