Unraveling Anion-Specific Inhibition and Structural Modulation of Gibbsite Crystallization: Implications for Aluminum Mobility in Natural and Engineered Systems

Gibbsite (α-Al(OH)₃, sometimes designated as γ-Al(OH)₃) plays a crucial role in the chemistry of aluminum in the environment and industry, yet its crystallization behavior under multianionic conditions is not well understood. In this study, we investigate how six common anions─fluoride (F^-), chloride (Cl^-), bromide (Br^-), nitrate (NO₃^-), sulfate (SO₄^2-), and phosphate (PO₄^3-)─influence the mineralization, structure, and morphology of gibbsite at room temperature. The results show that PO₄^3-, SO₄^2-, and F^- strongly inhibit gibbsite formation, stabilizing amorphous or alternative crystalline phases such as nordstrandite and cryolite. On the contrary, Cl^-, Br^-, and NO₃^- allow partial to complete crystallization of gibbsite without significant morphological changes. Solid-state ²⁷Al magic angle spinning nuclear magnetic resonance provides crucial insight into aluminum coordination environments in both crystalline and amorphous phases, distinguishing between octahedral, pentahedral, and tetrahedral Al species. The density functional theory calculations reveal a direct correlation between the Al-X bond strength and the inhibition of crystallization, following the order: PO₄^3- > SO₄^2- > F^- > NO₃^- > Cl^- > Br^-. These findings offer molecular-scale insights into anion-specific effects on aluminum hydroxide nucleation and transformation, improving the understanding of gibbsite formation and aluminum cycling in soils, phosphate retention, contaminant immobilization, and waste treatment strategies in nuclear and industrial settings. These insights can also guide the controlled synthesis of aluminum hydroxide materials with tailored crystallinity and morphology via liquid-assisted methods.