Overlooked Role of Coulombic and Dipole–Surface Interactions in the Rejection of Polar Contaminants by Polyamide Membranes

Size exclusion is commonly believed to be the major mechanism governing the rejection of antibiotics by nanofiltration (NF), while both size exclusion and Coulombic interactions affect the rejection of negatively charged polyfluoroalkyl substances (PFASs). Nevertheless, the potential interactions between polar antibiotics (e.g., zwitterionic fluoroquinolones and nonionized sulfonamides) and the membrane are often overlooked. Herein, this study identifies the key impacts of Coulombic and dipole-surface interactions in the rejection of polar antibiotics by polyamide NF membranes with various pore sizes and charges. Membranes with identical pore sizes yet different charges showed distinguished rejection behavior of such antibiotics. Further analysis reveals a strong relevance between the molecular polarity and the passage of antibiotics. This result indicates the non-negligible impact of Coulombic and dipole-surface attraction for the passage of polar zwitterionic fluoroquinolones and nonionized sulfonamides, respectively. In comparison, the dipole-surface interaction had a negligible impact on the rejection of low polar and charged PFASs compared to the dominant size exclusion and Coulombic interaction. Leveraging on these results, a mechanistic framework integrating with size exclusion, Coulombic interaction, and dipole-surface interaction toward efficient rejection of various contaminants were developed. Such improved fundamental understanding promotes the rational design of NF membranes for highly effective water decontamination.