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

A pyridine-based COF successfully synthesized by Schiff base reactions at room temperature achieves efficient and selective removal of Hg(II). • A pyridine-based COF was synthesized by Schiff base reactions at room temperature. • Pyridine-COF was crystalline and possessed N-rich chelating sites. • Pyridine-COF rapidly adsorbed Hg 2+ to a very high capacity (714.85 mg g -1 at 293 K). • The COF adsorbent was very robust and able to be recycled multiple times. The high toxicity and carcinogenicity of mercury ions (Hg 2+ ) 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-diformylpyridine (DFP) at room temperature. The obtained Pyridine-COF was crystalline, microporous and possessed a high specific surface area (348.5 m 2 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 Hg 2+ ions. Adsorption experiments showed Pyridine-COF to have very fast Hg 2+ adsorption kinetics, a high adsorption capacity for Hg 2+ (714.85 mg g -1 at 293 K, 804.66 mg g -1 at 303 K, and 976.01 mg g -1 at 313 K), and very good Hg 2+ 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 Hg 2+ 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 Hg 2+ from water.