β-Ketoenamine covalent organic frameworks with abundant carboxyl groups for the efficient removal of malachite green in actual water environments

In this study, imine-based COF (TpPa-2COOH) with abundant carboxyl groups is constructed by 1,3,5-triisopropylchloroglucitol (Tp) and 2,5-diaminoterephthalic acid (Pa-2COOH) via "one-step" approach with solvent stability and resistance to acidity and alkalinity. TpPa-2COOH COF achieves highly efficient malachite green (MG) adsorption with a maximum adsorption of 296.56 mg·g−1 at pH = 5, and the adsorption capacity exceeds many reported adsorbents. Meanwhile, TpPa-2COOH COF shows outstanding adsorption performance to MG, such as 97.67 % removal rate in actual water environments, unique selective adsorption of MG in kinds of contaminants and good adsorption capacity in highly concentrated salt solutions. Additionally, TpPa-2COOH COF shows 78.4 % removal rate after six cyclic desorption-adsorption, demonstrating its excellent recyclability. Furthermore, the processes of monolayer adsorption and chemisorption are confirmed by the isotherms and kinetics of adsorption experiments. The adsorption mechanisms of π-π interaction, electrostatic attraction, hydrogen bonding and channel adsorption are further verified by XPS, EDX, Zeta potential and DFT calculations. This work hopes the synthesis and adsorption application of carboxylated COFs provide new ideas for developing novel adsorbents.