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Nanofiltration (NF) is considered as a promising technology for the removal of trace organic compounds (TrOCs) for wastewater reclamation, while a simultaneous low rejection of salts is desirable for the ease of using the reclaimed water. This study was conducted to exploit the potential of polypiperazine-amide NF membranes in achieving selective separation of TrOCs and NaCl by varying the concentration of the diamine monomer, piperazine (PIP), for interfacial polymerization and by modifying the nascent membrane surface with hydrophilic monomers, diethanolamine (DEA) and monoethanolamine (MEA). Results showed that increasing PIP concentration could decrease the membrane pore size and reduce the surface negative charge density, which led to an increased rejection of xylose and a reduced rejection of NaCl. The grafting of DEA or MEA molecules could further decrease the NaCl rejection, substantially improve the water permeability and maintain an effective removal for eight pharmaceuticals (PhACs). The optimal membrane prepared with a PIP concentration of 2.0 wt% and subsequently modified by MEA exhibited a rejection of NaCl at 33.1% and an average rejection of PhACs at 90.8% in a 10 mmol/L NaCl solution under an applied pressure of 6 bar. The membrane could have an even higher selectivity when the NaCl concentration was increased to higher levels. These results demonstrated the feasibility and versatility of the membrane in separating TrOCs from NaCl for wastewater reclamation. Image 1 • Increasing PIP concentration decreased membrane pore size and surface charge density. • Membrane properties were further tailored by surface grafting of hydrophilic monomers. • The modified membrane exhibited low rejection of NaCl and high rejection of TrOCs. • The membrane had a higher selectivity for TrOCs and NaCl at higher NaCl concentration. [ABSTRACT FROM AUTHOR]

Abstract A simultaneous high rejection of trace organic compounds (TrOCs) and low rejection of divalent cations (Ca2+/Mg2+) would be desirable for nanofiltration (NF) applied for drinking water treatment. The target was however not attainable by using the currently available commercial NF membranes. In this study, 3,5-diaminobenzoic acid (BA) was used as an additional diamine monomer besides piperazine (PIP) to prepare novel thin film composite NF membranes via interfacial polymerization with trimesoyl chloride (TMC). A series of NF membranes were prepared by varying BA to PIP concentration ratios in order to obtain a proper membrane pore size and a sufficiently high negative surface charge density. Membrane characterizations and performance tests showed that the incorporation of BA could increase the pore size, enhance the negative surface charge and change the structural properties of the membrane, which in turn affected the membrane water permeability and separation performance. The optimal membrane prepared with a BA/PIP ratio of 50%:50% exhibited a low rejection of CaCl 2 (25.8%) and MgCl 2 (32.0%) in single salt solutions at 10 mmol/L and a moderate rejection of Ca2+/Mg2+ ions (<60%) in a real tap water, while maintaining a reasonably high water permeability and an effective removal (>80%) for six out of eight selected pharmaceuticals (PhACs). The ATR-FTIR spectra and XPS analysis showed that BA could interfere the polymerization between PIP and TMC in forming the membrane active layer while the cross-linking degree between BA and TMC was relatively low. This implied that the proposed treatment target could also be achieved by using some other acidic diamines with similar functionalities of BA. Graphical abstract fx1 Highlights • 3,5-diaminobenzoic acid (BA) in addition to PIP was used to prepare novel NF membranes. • Addition of BA increased membrane pore size and negative surface charge density. • The optimal membrane exhibited low rejection of Ca2+/Mg2+ and high rejection of TrOCs. • The role of BA in affecting PA layer structure and properties was elucidated. [ABSTRACT FROM AUTHOR]