Morphology-Controlled γ-Alumina Adsorbents: Fluoride Removal Performance and Cyclic Regeneration Stability in Aqueous Media.

Excessive fluoride in groundwater and industrial wastewater poses serious risks to human health. Although commercial γ-alumina is a low-cost adsorbent recommended for defluoridation, its limited adsorption capacity and poor regeneration stability hinder its practical application. In this work, three template-free γ-alumina materials with distinct morphologies (nanosheets, rhombic flakes, and nanofibers) were synthesized by a mild hydrothermal method and systematically compared with commercial granular γ-alumina. To isolate the effect of morphology, all four γ-alumina adsorbents were prepared with comparable BET surface areas (180-230 m2·g-1). Their structural, morphological, and surface properties were comprehensively characterized, and their fluoride adsorption performance, adsorption mechanism, and regeneration behavior were systematically evaluated. Despite their similar BET surface areas, the four samples exhibited markedly different adsorption performances. At an initial fluoride concentration of 600 mg·L-1, the adsorption capacity followed the order: nanosheets (87.2 mg·g-1) > nanofibers (49.0 mg·g-1) > rhombic flakes (46.1 mg·g-1) > commercial alumina (35.0 mg·g-1). The superior adsorption performance of the nanosheet morphology suggests that the accessibility and local chemical environment of surface hydroxyl species, rather than their total abundance alone, play an important role in fluoride adsorption. In contrast, nanofibrous XW-600 exhibited superior resistance to repeated acid-alkali regeneration, preserving its crystal structure and fibrous morphology while maintaining the highest residual adsorption capacity among the synthesized alumina samples after five regeneration cycles. Overall, the results suggest that alumina morphology strongly influences fluoride adsorption performance and regeneration stability. This work provides a simple, template-free strategy for preparing morphology-controlled γ-alumina and offers new insights into the relationship between alumina morphology and fluoride adsorption, providing guidance for the rational design of high-performance, regenerable alumina adsorbents.