Bottom-up construction of betaine-grafted iCOF: Selective adsorption of Cr(VI) via precise host-guest interaction

A novel ionic COF was synthesized firstly using betaine molecule as the quaternary ammonium source. • This iCOF enables facile synthesis and low cost ($88.31/kg), showing promising potential for practical applications. • Abundant binding sites and confined pores enable the material to achieve excellent Cr(VI) adsorption. • The iCOF achieves superior adsorption selectivity for Cr(VI) against competing anions ( Cl − ${\mathrm{Cl}}^{-}$ , Br − ${\mathrm{Br}}^{-}$ , NO 3 − ${\mathrm{NO}}_3^{-}$ , SO 4 2 − ${\mathrm{SO}}_4^{2-}$ , and CO 3 2 − ${\mathrm{CO}}_3^{2-}$ ). Due to the high toxicity of Cr(VI), the development of adsorbents for effective elimination of Cr(VI) from wastewater is urgently required. Covalent organic frameworks(COFs) show great promise in heavy metal remediation. However, most COFs suffer from high costs and poor anti-interference performance, which severely restricts their practical applications. To address these limitations, natural betaine was firstly utilized as precursor to construct a novel quaternary ammonium-grafted COF structure (iCOF-BHC) via a bottom-up strategy. Grafted betaine introduces abundant quaternary ammonium sites and extra amide sites. Combined with the hydrazone groups and confined pore of the framework, these functional moieties exert multi-synergistic effects to construct precise host-guest interactions, thereby realizing superior selective adsorption toward Cr(VI). The equilibrium adsorption capacity of iCOF-BHC at 10 °C is 132.73 mg g −1 (under ambient winter conditions in Qinghai Province)，and further heating significantly improves adsorption rate and uptake capacity. iCOF-BHC exhibits outstanding selectivity for Cr(VI) in the presence of competing anions ( Cl − ${\mathrm{Cl}}^{-}$ , Br − ${\mathrm{Br}}^{-}$ , NO 3 − ${\mathrm{NO}}_3^{-}$ , SO 4 2 − ${\mathrm{SO}}_4^{2-}$ and CO 3 2 − ${\mathrm{CO}}_3^{2-}$ ), as well as excellent recyclability, with negligible attenuation in adsorption capacity after six cycles. Furthermore, compared with the vast majority of existing COFs, it exhibits significant cost advantages and is more suitable for practical industrial applications. This work develops a facile, low-cost, and innovative strategy for the fabrication of cationic COFs, providing a viable strategy and experimental basis for the efficient adsorption of Cr(VI) from wastewater. Betaine-grafted cationic hydrazone iCOF for effective removal of Cr (VI) in wastewater. Download: Download high-res image (224KB) Download: Download full-size image