Sponge-supported monolithic materials of porphyrin covalent organic frameworks for selective recognition, convenient removal and extraction of Cd2+

Abstract

Adsorption is a cheap, energy-efficient, and easy-to-operate method for heavy metal pollution treatment. As highly porous and crystalline polymers, covalent organic frameworks (COFs) hold great promise as the adsorbents of metal ions. Herein, two hydroxyl-functionalized porphyrin COFs (2,3-DhaTph and 2,5-DhaTph) were synthesized, characterized and demonstrated to be good adsorbents of cadmium(II) ion (Cd2+). Via the chemical coordination of abundant phenolic hydroxyls to Cd2+ and the fluorescent response of porphyrin moieties to Cd2+, 2,3-DhaTph realized bifunction of specific optical recognition and highly efficient adsorption of Cd2+. To further simplify the removal operation of Cd2+, bulk monolithic materials (PS@COFs) were prepared via the in-situ growth of 2,3-DhaTph or 2,5-DhaTph on the pore wall of a dopamine-treated heat-resistant polyurethane sponge. Compared to the COFs, PS@COFs showed faster adsorption kinetics and shorter adsorption equilibrium time due to the interconnected large porous channels and the uniform distribution of thin COF layer on the pore wall. The characteristics of easy operation and good reusability (≥ 6 times) endow PS@COFs with great potential in practical applications, more than 99% Cd2+ could be easily removed from water samples by them. PS@COFs were also demonstrated to work well for the extraction of various heavy metal ions from polluted food samples (rice and wheat), achieving the detection of these metal ions.