Precise Regulation of Monomer Reactive Sites Enhances the Water Permeance and Membrane Selectivity of Polyamide Nanofiltration Membranes

Polyamide structure and chemistry play critical roles in the separation performance of thin-film composite (TFC) nanofiltration (NF) membranes. Typically, polyamide formation is based on the reactive sites on monomers (e.g., amino groups on piperazine (PIP)) during interfacial polymerization (IP). To precisely tailor polyamide properties (e.g., cross-linking degree) and membrane performance, we regulated PIP reactive sites by pH adjustment to vary the dominant species in forms of non-, mono-, and diprotonated PIP. Specifically, at pH values between p K a1 (i.e., 5.3) and p K a2 (i.e., 9.7), the dominant monoprotonated PIP with relatively fewer nonprotonated PIP resulted in a reduced cross-linking degree of polyamide. Such reduced cross-linked polyamide exhibited simultaneously improved water permeance and better solute-to-solute selectivity (e.g., CaCl 2 /Na 2 SO 4 and CaCl 2 /PFOS selectivity), thanks to their looser structure and more negative charge. For example, membrane NF-pH9, prepared at pH 9, exhibited simultaneously improved water permeance (20.2 L m –2 h –1 bar –1 ) and higher CaCl 2 /PFOS selectivity (12.6). The dominant diprotonated PIP with little nonprotonation at pH < p K a1 resulted in ineffective cross-linked polyamide with low salt rejection (e.g., 0.9 ± 0.3% of Na 2 SO 4 ). This study investigated a facile strategy to tailor membrane permeance and selectivity by regulating monomer reactive sites, which provides new insights into the development of high-performance NF membranes.