(197j) Functionalized Porous Aromatic Frameworks for Rapid Boron Removal from Aqueous Solutions

Boron is a contaminant of considerable interest for seawater desalination, existing in seawater predominantly as boric acid at concentrations of ~5 ppm. Current state-of-the-art reverse osmosis membranes are inefficient at excluding boric acid largely because of its small size and electrically neutral state at ambient conditions. Boron selective resins, often made from macroporous polymers bearing chelating functional groups, have emerged as a promising technology for boron removal from water owing to their low cost and excellent selectivity. However, current commercial resins suffer from low capacities, slow boron uptake kinetics, and poor stability/regenerability. Porous aromatic frameworks (PAFs), a relatively new class of porous polymers with exceptionally high surface areas (up to 5600 m²/g), remarkable chemical stability in harsh environments, and chemical structures that are amenable to diverse functionalization, have the potential to overcome these drawbacks. Due to these desirable properties, PAFs have been extensively explored for use in gas separation applications, but their utility as materials for water purification applications remains relatively unexplored.

In this study, novel boron-selective PAFs were synthesized by post-synthetically appending N-methyl-D-glucamine, an effective boron chelating moiety, to the parent framework. The performance of the boron-selective PAFs was evaluated via batch adsorption measurements using aqueous boric acid as well as synthetic seawater solutions, and the material properties were compared with those of a commercial boron chelating resin, Amberlite IRA743. Compared with the commercial resin, the boron-selective PAFs demonstrated higher adsorption capacities due to greater functional group loading. All materials explored exhibited selective boron uptake in the presence of other ions due to the highly specific nature of the interactions. However, relative to the amount of PAF used, nearly twice the amount of commercial resin was required to reduce the boron concentration in synthetic seawater to values acceptable for human consumption and irrigation. Due to their highly porous nature, the boron-selective PAFs demonstrated substantially improved boron uptake kinetics compared with the commercial resin, and, to the best of our knowledge, the boron adsorption rate constants of the PAFs represent the highest values reported in the open literature. The PAFs were regenerated by an acid/base treatment and reused for several cycles with no loss in performance.