Quaternary Phosphonium Salt Functionalization of Polyamide Reverse Osmosis Membranes: Effects on Separation Performance

Abstract Biofouling is a persistent challenge in reverse osmosis (RO) desalination and water reuse because it accelerates flux decline, increases energy consumption, and drives chemical cleaning requirements. Developing durable antibiofouling interfaces is therefore critical for improving the sustainability of membrane-based water treatment. Here, we show that covalent installation of quaternary phosphonium functionality on polyamide thin-film composite (TFC) membranes provides a promising route to enhanced biofouling resistance while largely preserving desalination performance. Using a mild surface-grafting strategy, we immobilized quaternary phosphonium salts (QPS) on RO membrane surfaces and directly compared their behavior with that of structurally analogous quaternary ammonium salts (QAS). Surface analyses confirmed successful functionalization while preserving the characteristic morphology of the polyamide selective layer. Under optimized conditions, the QPS-modified membrane maintained NaCl rejection of ∼99% with only a limited permeance penalty. More importantly, QPS outperformed QAS in suppressing Escherichia coli, achieving 72.6% antibacterial efficiency versus 51.2%, and reduced surface-associated biomass more effectively under static conditions. In 24 h cross-flow filtration, the QPS-modified membrane retained ∼95–97% of its initial flux, compared with ∼66–68% for the pristine membrane. Comparative transcriptomic profiling revealed distinct bacterial response states on QPS- and QAS-functionalized membranes, with QPS eliciting a stress-responsive, membrane-adaptive program and QAS promoting a low-activity, translation-repressed state. These results identify phosphonium-based interfaces as an effective design strategy for reducing RO biofouling and advancing more energy-efficient, chemically resilient water treatment systems.