Superb adsorption capacity of ferrocene-based covalent organic frameworks towards Congo red with high-pH resistance

Porous materials are excellent adsorbents for the removal of organic dyes from sewage and play a significant role in environmental restoration. Herein, two ferrocene (Fc)-based covalent organic frameworks (Fc-COFs), namely FcTF-COF and FcBD-COF, are successfully synthesized for the first time through a solvothermal method, and the obtained Fc-COFs powders are used to adsorb Congo red (CR) from water. The results show that both FcTF-COF and FcBD-COF have superb adsorption performance towards CR with ultrahigh adsorption capability of 1672.2 mg g –1 and 1983.7 mg g –1 at pH = 4.0, respectively, outperforming the majority of the reported solid porous adsorbents. The maximum adsorption of both Fc-COFs agrees with the Sips adsorption isothermal model, indicating that their adsorption was dominated by heterogeneous adsorption. The Coulombic interactions, hydrogen bonding, π-π interactions and ion-dipolar interactions should all contribute to their ultrahigh CR adsorption capability and high-pH resistance performance regardless of the pH in the range of 4 to 9. In addition, after five cycles, both COFs still remain their exceptional high CR adsorption capabilities. This study offers a prospective organic porous adsorbent with promising applications for organic dye removal in sewage processing. Two ferrocene (Fc)-based covalent organic frameworks are successfully synthesized for the first time and the obtained Fc-COFs exhibit ultrahigh adsorption capacity and good pH-resistance towards Congo red, and rank top among all porous materials reported previously, showing promising applications for organic dye removal in sewage processing. • Ferrocene derivative, a class of important functional organic molecules, was first used as building unit to construct two ferrocene-based covalent organic frameworks (Fc-COFs) with high specific surface area and good crystallinity. • The synthesized Fc-COFs were used as adsorbents for the removal of Congo red from water and both Fc-COFs exhibited the good pH-resistance and ultrahigh Congo red adsorption capability, ranking among the top among all porous materials reposted previously. • The adsorption mechanism analyses reveal that electrostatic interaction, hydrogen bonding, π-π interactions, and ion-dipole interactions may all contribute to their ultrahigh adsorption capability towards Congo red.