Enhancing the Rejection of Small Neutral N-Heterocyclic Molecules with SO₄^•--Modified RO Membranes

Conventional polyamide reverse osmosis (RO) membranes often exhibit limited rejection of nonionized, low-molecular-weight trace organic contaminants. In this study, we demonstrate that modifying polyamide membranes using sulfate radicals significantly enhances the rejection of benzothiazole, benzotriazole, and their derivatives (BTHs and BTRs, MW < 200 Da), while more than doubling the water flux. The modified membranes achieved a 35-44% reduction in permeate concentrations of these compounds under identical transmembrane pressure (14 bar) or a 49-52% reduction in compound passage at the same permeate flux (35 L m^-2 h^-1), compared to the pristine membrane. Comprehensive evaluation of compound-membrane interactions revealed that the enhanced rejection stems from tailored surface chemistry─particularly the incorporation of abundant oxygenated functional groups in the polyamide layer. These modifications reduced the affinity of BTHs and BTRs for the membrane surface, as evidenced by density functional theory (DFT) calculations showing increased transmembrane activation energies. This work provides fundamental insights into the rejection mechanisms of small neutral N-heterocyclic compounds by RO membranes and introduces a practical modification strategy for improving their removal. The findings may inform future development of advanced RO membranes with improved selectivity toward challenging organic contaminants.