Crystallization and Phase Transformations of Aluminum (Oxy)hydroxide Polymorphs in Caustic Aqueous Solution

Area of Science:

• Materials Science
• Geochemistry
• Environmental Science

Background:

• Gibbsite, bayerite, and boehmite are key aluminum (oxy)hydroxide minerals with significant industrial uses.
• These minerals are major components in stored caustic nuclear wastes, necessitating an understanding of their behavior.
• Existing knowledge on their phase transformations in alkaline solutions is limited, especially regarding reaction mechanisms.

Purpose of the Study:

• To investigate the effects of sodium hydroxide concentration, temperature, and aluminum concentration on the crystallization and phase transformation of aluminum (oxy)hydroxides.
• To elucidate the underlying reaction mechanisms governing these transformations in highly alkaline solutions.
• To provide a basis for controlling solid phase selection in waste treatment processes.

Main Methods:

• Systematic variation of reaction parameters: sodium hydroxide (0.1-3 M), temperature (60-100 °C), and aluminum concentration (0.1-1 M).
• Characterization techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectrometry.
• Analysis of phase formation, crystallinity, morphology, and transformation pathways.

Main Results:

• Phase transformation processes are significantly influenced by reaction temperature and the Al/OH^- ratio.
• High-crystallinity precipitates form when 1 ≤ Al/OH^- ≤ 2.5; precipitation halts at Al/OH^- ≥ 2.5 unless Al concentration exceeds 1 M.
• Intermediate phases (pseudoboehmite, bayerite, gibbsite) transform to final products (bayerite, gibbsite, boehmite, respectively) via dissolution-reprecipitation.
• Gibbsite transforms to boehmite above 80 °C in acidic/weakly caustic conditions, but adopts a distinct "bar-shaped" morphology in 3 M NaOH.

Conclusions:

• The study provides critical insights into the controlled crystallization and phase transformation of aluminum (oxy)hydroxides under varying alkaline conditions.
• Findings offer a robust framework for optimizing solid phase selection in industrial applications, particularly for nuclear waste management.
• Tuning reaction conditions, specifically temperature and Al/OH^- ratio, allows for targeted manipulation of mineral phase formation and morphology.