Unraveling Gibbsite Transformation Pathways into LiAl-LDH in Concentrated Lithium Hydroxide

Abstract

Gibbsite (α-Al(OH)₃) transformation into layered double hydroxides, such as lithium aluminum hydroxide dihydrate (LiAl-LDH), is generally thought to occur by solid-state intercalation of Li⁺, in part because of the intrinsic structural similarities in the quasi-2D octahedral Al³⁺ frameworks of these two materials. However, in caustic environments where gibbsite solubility is high relative to LiAl-LDH, a dissolution-reprecipitation pathway is conceptually enabled, proceeding via precipitation of tetrahedral (<i>T</i>_<i>d</i>) aluminate anions (Al(OH)₄^-) at concentrations held below 150 mM by rapid LiAl-LDH nucleation and growth. In this case, the relative importance of solid-state versus solution pathways is unknown because it requires <i>in situ</i> techniques that can distinguish Al³⁺ in solution and in the solid phase (gibbsite and LiAl-LDH), simultaneously. Here, we examine this transformation in partially deuterated LiOH solutions, using multinuclear, magic angle spinning, and high field nuclear magnetic resonance spectroscopy (²⁷Al and ⁶Li MAS NMR), with supporting X-ray diffraction and scanning electron microscopy. <i>In situ</i> ²⁷Al MAS NMR captured the emergence and decline of metastable aluminate ions, consistent with dissolution of gibbsite and formation of LiAl-LDH by precipitation. High field, <i>ex situ</i> ⁶Li NMR of the the progressively reacted solids resolved an <i>O</i>_<i>h</i> Li⁺ resonance that narrowed during the transformation. This is likely due to increasing local order in LiAl-LDH, correlating well with observations in high field, <i>ex situ</i> ²⁷Al MAS NMR spectra, where a comparatively narrow LiAl-LDH <i>O</i>_<i>h</i> ²⁷Al resonance emerges upfield of gibbsite resonances. No intermediate pentahedral Al³⁺ is resolvable. Quantification of aluminate ion concentrations suggests a prominent role for the solution pathway in this system, a finding that could help improve strategies for manipulating Al³⁺ concentrations in complex caustic waste streams, such as those being proposed to treat the high-level nuclear waste stored at the U.S. Department of Energy's Hanford Nuclear Reservation in Washington State, USA.