Hollow Covalent Organic Framework with "Shell-Confined" Environment for the Effective Removal of Anionic Per- and Polyfluoroalkyl Substances

The ubiquity of per- and polyfluoroalkyl substances (PFAS) has become an emerging challenge for modern water treatment and public health. The development of high-performance adsorbents remains at the forefront of PFAS remediation. Covalent organic frameworks (COFs) with structural regularity and task-specific functionality are promising candidates yet have not been fully explored. Herein, cystamine-grafted hollow COF nanospheres (hollow Cys-COF) are synthesized via the hard-template method and functionalized by the thiol-ene "click" reaction. The well-defined hollow structure provides a specific "shell-confined" environment, which ensures sufficient modification in a short period and retains the crystallinity of the original material, circumventing the dilemma between functionality and crystallinity in traditional post-synthetic modification protocols. The as-prepared hollow Cys-COF is highly functionalized and shows remarkable removal performance to various anionic PFAS under environmentally relevant conditions. Twenty kinds of anionic PFAS containing carboxylic, sulfonic, and phosphoric individuals are removed by 90% within several minutes, and a column-test further verified its superior performance under the dynamic situation. An integrated analysis from both experimental results and theoretical simulations provides a mechanistic illustration of how the morphology and functionality can jointly contribute to effective PFAS adsorption. These findings provide a multiscale understanding of high-performance adsorbent design.