PNNL

Abstract

Gibbsite (a-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 (SO42-) and phosphate (PO43-) - influence the mineralization, structure and morphology of gibbsite at room temperature. The results show that PO43-, SO42-, 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: PO43- > SO42- > 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.