Enhanced removal of perfluorooctanoic acid by aluminum-based metal–organic frameworks prepared by bauxite residue

Upcycling solid waste into advanced adsorbents is a sustainable approach in the field of waste valorization and wastewater treatment. In this study, we developed a phase-controlled synthesis method for a single phase of an aluminum-based metal–organic framework (MOF) using an aluminum source (Al³⁺) in red mud (RM), and demonstrated its potential for aqueous perfluorooctanoic acid (PFOA) removal. By optimizing the pre-treatment process, the selective extraction of aluminum ion from RM was achieved. Subsequently, three distinct aluminum-based MOFs (i.e., MIL-53(Al), MIL-96(Al), and MIL-100(Al)) were synthesized by controlling the hydrothermal synthesis conditions and using specific organic linkers (terephthalic acid and trimesic acid). For MOFs based on trimesic acid, the initial Al³⁺: trimesic acid ratio and duration of hydrothermal synthesis exerted an observable influence on the formation of the second building unit of the MOF. By manipulating these factors, we could precisely control isolated MIL-96(Al) and MIL-100(Al). The PFOA adsorption results revealed a remarkable increase in the adsorption capacity (Q_max: 131.58 mg/g) on MIL-100(Al) compared with that on MIL-96(Al). This was due to its large surface area (1189.15 m²/g) and the presence of numerous hydrophilic sites favorable for interaction with the carboxylic group of PFOA. Furthermore, a computational investigation revealed that in addition to direct Lewis acid–base interaction between PFOA and aluminum sites, the major mechanism involved the formation of a complex induced by ion exchange between coordinated NO₃^- and PFOA anions.