High-performance adsorption of chromate by hydrazone-linked guanidinium-based ionic covalent organic frameworks: Selective ion exchange

Abstract

Graphical abstract illustrates the Cr (VI) adsorption mechanism of Tp-DG Cl in aqueous body. The waste water containing Cr (VI) oxoanions could be purified when contacting the material. At the time, the Cr (VI) oxoanions were adsorbed on Tp-DG Cl owing to the directional hydrogen bonds, and Cl - of Tp-DG Cl would be released into the water because of the ion exchange between Cr (VI) oxoanions and Cl - . • Guanidinium groups of Tp-DG Cl were employed to remove chromate selectively. • The adsorption ability of Tp-DG Cl remained at a high level subsequent to five cycles. • Tp-DG Cl showed remarkable selectivity and high adsorption capacity of 336.04 mg/g for Cr (VI). • The excellent adsorption performance of Tp-DG Cl was systematically studied. • The adsorption mechanism is dominant by ion exchange and electrostatic attraction. In consideration of the serious environmental issues caused by heavy metals, novel nanomaterials have been developed and applied as highly selective adsorbents with the capacity to eliminate heavy metal ions in ambient ecosystems. Ionic covalent organic frameworks (I-COFs) serve as an emerging species of functional nanomaterials that inherently possess ionic interfaces that interact strongly with oppositely charged ionic species. In this work, a guanidinium-based COF with a β-ketoenamine linkage (Tp-DG Cl ) was synthesized using a solvothermal method. The obtained Tp-DG Cl colligated four advantages, namely easy synthesis, high selectivity (for Cr (VI)), excellent adsorption (360.02 mg/g calculated by Langmuir model), and good recyclability (five cycles), which enables it to serve as a reliable sorbent for wastewater treatment. Owing to the stable and selective binding affinities between tetrahedral oxoanions and diiminoguanidinium groups, Tp-DG Cl exhibited an outstanding adsorption capacity for Cr (VI). Subsequent to five cycles, the adsorption performance of Tp-DG Cl remained at a high level (85%), which demonstrated an excellent regenerative capacity by supplementing Cl - to the Tp-DG Cl after adsorption. Consequently, Tp-DG Cl revealed a strong potential for broad applications in wastewater remediation.