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

Gibbsite (α-Al(OH)) 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 Al 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 () aluminate anions (Al(OH)) 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 techniques that can distinguish Al 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 (Al and Li MAS NMR), with supporting X-ray diffraction and scanning electron microscopy. Al 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, Li NMR of the the progressively reacted solids resolved an 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, Al MAS NMR spectra, where a comparatively narrow LiAl-LDH Al resonance emerges upfield of gibbsite resonances. No intermediate pentahedral Al 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 Al 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.