Metal ion-assisted carboxyl-containing covalent organic frameworks for the efficient removal of Congo red

Covalent organic frameworks (COFs) have been utilized as molecular sieves to adsorb or remove or separate a wide range of substances. Herein, a new carboxyl-containing COF (COF-COOH) is fabricated from the polymerization of 1,3,5-triformylphloroglucinol (TP) and 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxylic acid (DBA). COF-COOH displays good adsorption performance on Congo red (CR) through hydrogen bonds and π-π stacking interactions. Through post-modification with Ca²⁺/Ni²⁺ ions, the adsorption capacity of COF-COOCa/COF-COONi to CR is improved, which is due to the coordinated metal cations having a positive contribution to the electrostatic interactions. At 25 °C, the maximum adsorption amount of COF-COOCa and COF-COONi to CR is 704.23 mg g^-1 and 781.25 mg g^-1, respectively. The removal efficiency of COF-COOCa to CR is 95% and that of COF-COONi is 96%. This demonstrates that the new metal ion-assisted COFs are viable adsorbents to remove dye pollutants, which are harmful to the environment and to human health, from wastewater.