Tunable Hydrated Channels in Covalent Organic Framework Membrane for Seawater Desalination

Nanofiltration and reverse osmosis (RO) are instances of pressure-driven membrane desalination processes (PMDs), which have been extensively employed for seawater desalination due to their great efficiency and environmental friendliness. However, the PMD process is usually limited to thin-film composite polyamide membranes, despite enormous research efforts in recent decades. Here, light-controlled RO COF membranes are developed by using a defect-engineered strategy to chemically rivet spiropyran units into COF channels. The spatial arrangement of spiropyran provides the defect-engineered COF membranes with manageable apertures spanning from 6.9 to 11.1 Å. The COF membrane featuring ordered ultramicropores (6.9 Å, TAPA-TFP-SP-25% COFs) exhibits a preeminent desalinization performance with a NaCl rejection of 91.2%. Furthermore, under light stimulation, the COF channels decorated with spiropyran units are capable of self-regulating the framework structure and hydration conformation by controlling the interconnectivity of confined water clusters, thus achieving hydrated pore size tuning from 11.1 to ∼4.0 Å (from TAPA-TFP-SP-50% to TAPA-TFP-MC-50% COF membrane). Under dark conditions, zwitterionic COF membranes after photoisomerization (TAPA-TFP-MC-50%) exhibit an enhanced KCl rejection (96.2%), representing a 24.1% increase when compared to the COF membrane without interconnected hydrated channels (TAPA-TFP-SP-50%). This membrane channel design concept exploits a viable avenue for developing RO membranes to achieve efficient water purification.