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.