Pacific Northwest National Laboratory, Richland, WA 99354, USA.
Pacific Northwest National Laboratory, Richland, WA 99354, USA; Department of Chemistry, Washington State University, Pullman, WA 99163, USA.
J Colloid Interface Sci. 2023 May;637:326-339. doi: 10.1016/j.jcis.2023.01.003. Epub 2023 Jan 5.
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 ahighly disordered quasi-two-dimensional phase at the solid-solution interface.
In situ X-ray diffraction (XRD), Raman andAl nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na[Al(OH)]·3(OH)·6HO). 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.
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)) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ionlinkages. The solution phase Al(OH) 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 (∼30 °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.
在水相体系中，含铝矿物相的沉淀和溶解通常通过铝配位数和连接性的变化来进行，这使得对转化机制的分子尺度解释变得复杂。在此，研究了晶态一水合铝酸钠盐的热诱导相变，该相由单体八面体配位的铝酸根组成，与工业铝加工有关。由于以前没有检测到熔融过程中的中间铝配体状态，因此假设在固溶界面的高度无序准二维相中，较低配位数的铝离子发生转变。
使用原位 X 射线衍射（XRD）、拉曼和 Al 核磁共振（NMR）光谱监测九水合铝酸钠（NSA，Na[Al(OH)]·3(OH)·6H₂O）的熔融转变。基于互补的经典分子动力学（CMD）模拟，包括增强采样，开发了一种机理解释。开发了一个反应力场来桥接溶液和固相中物种的形成。
与经典溶解不同，铝配体变化通过固溶界面无序层中动态稳定的中间态 ensemble 进行。在 NSA 的熔融和溶解过程中，八面体、单体铝通过五配位的中间态过渡。中间态去羟基化形成液体相中的四面体铝酸根（Al(OH)₄）。这种配位变化伴随着离子铝酸钠-钠离子键的断裂。溶液相中的 Al(OH)₄离子随后聚合形成多核铝酸根离子。然而，在 bulk melting 和 interfacial dissolution 之间存在一些差异，表面控制过程的开始温度（约 30°C）较低，配位变化随温度的变化更加渐进。这项确定无序层中铝的局部结构和动力学的工作为理解高碱性溶液中铝相转变的控制机制提供了新的基础。
