High boron removal polyamide reverse osmosis membranes by swelling induced embedding of a sulfonyl molecular plug

The removal of small and neutral boron molecules from seawater remains a critical challenge in energy-saving and low-cost desalination. In this paper, a high boron removal reverse osmosis (RO) membrane was designed by embedding sulfonyl moleculars in the polyamide (PA) network. The swelling PA structure has enlarged free volumes. It would induce more facile penetration of 4-nitrobenzenesulfonyl chloride (NBS). Evaporation of solvent molecules contracts the PA network and leads to the fixation of the NBS molecules into the PA network. The swelling of the polyamide network was studied systematically. The most suitable swelling solvent was selected based on the solubility parameter and the restoration capability. The "swelling-embedding-shrinking (SES)" strategy reconstruct the free volume of PA chains. It resulted in significantly higher rejection of neutral boron molecules due to enhanced steric hindrance by embedded NBS plugs and synergistic exclusion effect by -SO3H groups. Meanwhile, the alteration of membrane composition and structure were kept minimal. Further, the impacts of key determining factors (polarity, steric hindrance, and affinity) on boron separation performance were systematically analyzed by boron-containing compounds with different numbers of hydrogen bonding sites (or dipole moments) or molecular volumes. Under simulated seawater conditions, the boron rejection of the SES modified reverse osmosis membranes increased from 82.12% to 93.10%, and the salt rejection rate increased to 99.57%. Our study shows that the swelling induced embedding of NBS molecules can be readily adopted to regulate the permselectivity of the PA layer towards removal of boron species.