Covalent Organic Framework with Triazine and Hydroxyl Bifunctional Groups for Efficient Removal of Lead(II) Ions

Removal of toxic heavy-metal ions from water is of great concern owing to their potential hazards to the ecosystem and humans. A covalent organic framework (COF) based adsorbent with good porosity and triazine (Tz) and hydroxyl (OH) bifunctional groups was rationally designed and prepared using a simple predesigned ligand method. The crystalline structure, porous property, and stability of COF-Tz-OH material were systematically investigated, and it then served as a scaffold for lead(II) ions adsorption from aqueous solution. The obtained maximum uptake capacity for Pb2+ reaches as high as 476 mg g–1, surpassing those of most nanoporous materials and layered double hydroxides reported thus far. Meanwhile, the kinetic studies showed that COF-Tz-OH achieved over 99% adsorption capacity at an equilibrium within 5 min and was discovered to have a high distribution coefficient value (Kd) value of 4.998 × 106 mL g–1. The investigation of adsorption mechanism based on experiment results and theoretical quantum calculation reflected that the excellent performance should be attributed to the high density of the available chelating sites arising from bifunctional groups within ordered mesopores that provide a strong binding interaction between Pb2+ and polymer skeleton. Furthermore, the used COF-Tz-OH material also displayed cyclic utilization ability and selectivity for Pb2+ in the presence of background metal ions Na+, K+, and Ca2+. This research provides a fundamental understanding of the adsorption performance for heavy-metal ions and suggests the great promise of COFs as a designable porous material for water purification.