Monolithic melamine foam@COF composite: Preparation, characterization and applications in ultrafast removal and on-site quick assessment of Cd2+ pollution

The assessment and treatment of heavy metal pollution are eternal subjects of environmental protection. Facile on-site assessment of heavy metal pollution and efficient, rapid removal of pollutes are still challenging. Covalent organic frameworks (COFs) hold great promise as absorbents for heavy metal pollution treatment. To overcome the application limits of COF powders, a COF-based monolithic composite is prepared by direct in-situ growth of an N/O-rich COF on the surface of a melamine foam (MF). The resultant MF@COF inherits the macro-porous channels of MF, the good crystallinity of COF, and integrates the attractive features of monolithic materials such as ease of operation, facile and highly efficient recovery, and good reusability. The abundant N/O groups endow MF@COF with powerful capture capability towards various heavy metal ions. Its maximum adsorption capacity for Cd2+ is 278.45 mg/g, and the adsorption equilibrium can be reached within 1–3 s. The adsorption kinetic of Cd2+ by MF@COF follows the pseudo-second-order kinetic model, and the adsorption isotherm conforms to Freundlich model. Rapid adsorption, together with the easy-to-operate feature of monolithic material, make the ultrafast water purification possible. Meanwhile, MF@COF works well for the highly efficient enrichment of Cd2+ to realize the naked-eye detection of Cd2+ pollution at low levels, thus may be developed as an important tool for both on-site assessment and rapid treatment of heavy metal pollution.