Use of polyamide nanofiltration membranes with varied active layer chemistry for treating pharmaceuticals of emerging concern from saline water

The recent emergence of pollutants of concern in the water bodies is becoming a challenge for the water treatment community. In such a scenario, the deployment of the polyamide (PA) nanofiltration (NF) has proved to be a successful solution. Hence, the current study focused on applying a set of PA NF membranes for their potential to treat saline water containing organic pharmaceutical pollutants of emerging concern. The membranes have a varied active layer chemistry owing to the use of a set of aliphatic amines of varying chain lengths during interfacial polymerization. Five membranes, ranging from M1 to M5 [ M1 (2% w/v PIP + 0.15% w/v TMC), M2 (1.8% w/v PIP and 0.2% w/v EDA+ 0.15% w/v TMC), M3 (1.8% w/v PIP and 0.2% w/v DETA + 0.15% w/v TMC), M4 (1.8% w/v PIP and 0.2% w/v BAEP + 0.15% w/v TMC), and M5 (1.8% w/v PIP and 0.2% w/v TEPA+ 0.15% w/v TMC)], were fabricated, exhibiting varied physical and chemical features, including different surface charges, roughness, and wettability. The M4 membrane was found to be the best-performing membrane for rejecting the majority of pharmaceutical pollutant drugs and having higher permeate flux compared to other membranes. The M4 membrane has an entirely different surface morphology of larger-sized PA globules observed in the scanning electron microscopy (SEM) analysis of the M4 membrane. In addition, the M4 membrane possessed a hydrophilic surface with a water contact angle of 24.5°, resulting in higher clean water permeability compared to other membranes. It was observed that the rejection of the pharmaceutical pollutant drugs was mainly governed by size exclusion, augmented by the Donnan effect, where the positively charged drugs were rejected almost entirely by the membranes. The M4 membrane rejected 66.9% of 4-hydroxyacetanilide, 82.0% of sulfamethoxazole, 96.9% of caffeine, and >99% of amitriptyline and ranolazine, where amitriptyline and ranolazine have positive charges. The presence of monovalent and divalent salts affected the rejection of the drugs, where the rejection of the drugs increased with increasing concentration of the salts. Moreover, the long-term stability tests revealed that the membranes exhibited stable rejection performance and a stable permeate flux, with only slight variations. The ultrahigh-performance liquid chromatography (UHPLC) analyses confirmed rejection of drugs from the water. This study demonstrated that variations in the chemistry of the PA active layer can yield promising membranes for removing organic pharmaceutical pollutants from saline water bodies, enabling safe reuse of treated water.

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