Amine-functionalized porous aromatic frameworks for efficient adsorption of hazardous organic micropollutants in water

Amongst the most hazardous organic micropollutants (OMPs), β-naphthol, para-chloro-meta-xylenol, and bisphenol-A are highlighted due to their toxicity and persistent nature in aquatic environments. This work presents a series of cost-effective novel amine-functionalized porous aromatic frameworks (PAFs) for the adsorption of OMPs: PAF-82-NH₂, PAF-81-NH₂, and PAF-80-NH₂. PAF-82-NH₂ shows outstanding adsorption performance by exhibiting the highest Brunauer-Emmett-Teller surface area, polarity match with pollutants, and structural conjugation, with no loss in removal efficiency after five cycles of regeneration. The Langmuir adsorption capacity of PAF-82-NH₂ for three OMPs outstrips the most previously reported adsorbents: 461 mg·g^-1 for β-naphthol, 497 mg·g^-1 for para-chloro-meta-xylenol, and 763 mg·g^-1 for bisphenol-A. Despite having lower surface areas, amine-functionalized PAFs exhibit higher adsorption capacities than hydroxyl-functionalized PAFs (PAF-82,81,80). In comparison, PAF-1 (5,600 m^2·g-1) cannot even beat the adsorption capacity of PAF-81-NH₂, carrying the lowest surface area (564 m^2·g-1) among three amine-functionalized PAFs. This study reveals that the (-NH₂) group has a higher potential to enhance the adsorption capacity of OMPs than the (-OH) group. The specific surface area is not enough as a single factor preliminary without pore polarity. However, once polarity is established at pore channels, specific surface area, degree of conjugation, polarity, and size of pore channels work in a team fashion for efficient adsorption.