Energy-autonomous KCl extraction from brine via biomimetic covalent-organic-framework membranes and redox-driven ion transport

Selective extraction of KCl from complex salt lake brines remains a challenging task due to the presence of competing ions and the high energy requirements of conventional separation methods. To overcome these limitations, we have developed a biomimetic separation system that integrates three-dimensional (3D) covalent organic framework (COF) membranes with redox-mediated energy harvesting. These COF membranes are designed with sub-nanometer cavity channels decorated with oxygen-containing groups, which enable fine-tuning of electrostatic potential. This design allows for the simultaneous separation of anions and cations through valence-dependent short-range interactions. By coupling the COF membranes with Ag/AgCl redox pairs, the system converts the salinity gradient across the membrane into an internal electric field, facilitating autonomous ion transport without the need for external energy input. The system achieves over fivefold higher flux rates for K+ and Cl⁻ (2.6 and 3.2 mol m–2 h–1, respectively) and demonstrates outstanding separation performance with Cl–/SO42– = 332, K+/Mg2+ = 60, K+/Na+ = 7, and K+/Li+ = 11, substantially exceeding the passive diffusion ratios of 253, 27, 3, and 5, respectively. This work presents a sustainable, scalable approach for extracting valuable resources from complex brines, combining innovative biomimetic membrane design with redox-assisted process engineering, offering a promising solution to the energy and selectivity challenges in industrial ion separation. Natural salt lake brines contain various ions, making the selective extraction of KCl challenging. Here, the authors combine 3D covalent organic framework membranes with redox-mediated salinity gradient harvesting to enable the low-energy separation of anions and cations.