Pyridine-based covalent organic framework for efficient and selective removal of Hg(II) from water: Adsorption behavior and adsorption mechanism investigations

The high toxicity and carcinogenicity of mercury ions (Hg2+) motivates the development of efficient technologies for their removal from water. In this work, a pyridine-based covalent organic framework (Pyridine-COF) was synthesized via the Schiff base condensation reaction of 1,3,5-tris(4-aminophenyl)benzene (TAP) and 2,6-difor-mylpyridine (DFP) at room temperature. The obtained Pyridine-COF was crystalline, microporous and possessed a high specific surface area (348.5 m2 g-1). The close association of the pyridine linker and imine groups in the COF structure created an N-rich pore space that showed high selectivity towards Hg2+ ions. Adsorption exper-iments showed Pyridine-COF to have very fast Hg2+ adsorption kinetics, a high adsorption capacity for Hg2+ (719.4 mg g-1 at 293 K, 840.3 mg g-1 at 303 K, and 1000 mg g-1 at 313 K), and very good Hg2+ removal performance in the presence of other metal cations. Further, Pyridine-COF maintained a high adsorption capacity over five cycles of adsorption-desorption, confirming good reusability as an adsorbent. Mechanistic studies using X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that the adsorption of Hg2+ in the pores of Pyridine-COF was mainly chelation not covalent interactions. Pyridine-COF represents one of the best COF-based adsorbents developed to date for the removal of Hg2+ from water.