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

Area of Science:

• Materials Science
• Geochemistry
• Nuclear Waste Management

Background:

• Gibbsite (α-Al(OH)₃) transformation to lithium aluminum hydroxide dihydrate (LiAl-LDH) is typically attributed to solid-state intercalation.
• Caustic environments enhance gibbsite solubility, enabling a potential dissolution-reprecipitation pathway.
• Distinguishing between solid-state and solution pathways requires advanced *in situ* analytical techniques.

Purpose of the Study:

• To investigate the transformation mechanism of gibbsite to LiAl-LDH in caustic solutions.
• To determine the relative importance of solid-state intercalation versus dissolution-reprecipitation pathways.
• To provide insights for managing aluminum concentrations in industrial waste streams.

Main Methods:

• Utilized *in situ* multinuclear magic angle spinning (MAS) NMR spectroscopy (²⁷Al and ⁶Li MAS NMR) in partially deuterated LiOH solutions.
• Supported *in situ* NMR with X-ray diffraction and scanning electron microscopy.
• Quantified aluminate ion concentrations to assess pathway contributions.

Main Results:

• *In situ* ²⁷Al MAS NMR revealed the transient formation and disappearance of metastable aluminate ions, indicating gibbsite dissolution and LiAl-LDH precipitation.
• High-field NMR of reacted solids showed spectral changes consistent with increasing order in LiAl-LDH, with no resolvable pentahedral Al³⁺ intermediates.
• Quantification of aluminate ions strongly suggests a dominant role for the solution-mediated pathway.

Conclusions:

• The transformation of gibbsite to LiAl-LDH in caustic solutions proceeds predominantly via a dissolution-reprecipitation mechanism.
• This finding challenges the conventional view of solid-state intercalation as the sole transformation pathway.
• Understanding this pathway is crucial for optimizing strategies for treating high-level nuclear waste at facilities like the Hanford Reservation.