Imine-Linked Covalent Organic Frameworks Tuned by Quaternary-Ammonium-Alkyl for Highly Effective and Selective Adsorption of Perchlorate from Water

Perchlorate (ClO₄ ^-) contamination in surface water is an escalating issue for drinking water safety. Herein, an imine-linked covalent organic framework (COF) tuned by quaternary ammonium alkyls (R₄N⁺) is developed for ClO₄ ^- adsorption. The hydrophobic and cationic COF adsorbent achieves a record-breaking adsorption capacity of ClO₄ ^- at 912.7 mg g^-1, demonstrating remarkable selectivity for ClO₄ ^- over other environmentally relevant anions and exhibiting rapid adsorption kinetics. Furthermore, the adsorbent maintains excellent recycling performance (removal efficiency ≥ 80% after 10 cycles) using tetrachloroferrate for regeneration. In dynamic flow-through experiments with real water samples, the adsorbent effectively treats ≈3200-bed volumes of feed streams (≈500 µg L^-1), with an enrichment factor of 15.2. The hydrophobicity of the COF adsorbent is identified as a premise for its interaction with ClO₄ ^-. Molecular dynamic simulations and density functional theory calculations reveal that R₄N⁺ anchored in COF cores enriches ClO₄ ^- via electrostatic attraction and bonds with ClO₄ ^- via unconventional hydrogen bonds (C─H─O). These key insights pave the way for future design and optimization of adsorbents for removing oxo-anion from water, especially for ClO₄ ^-.