Nitrate rejection of sulfonic acid group incorporated polyethersulfone-type nanofiltration membrane and reutilization of the concentrated solution

The nanofiltration (NF) membrane technology has attracted considerable attention for the rejection removal of various salts. However, the practical application of this membrane separation process has been severely retarded due to the ineffective rejection of monovalent ions (such as nitrate and chloride), undesirable filtration flux, and insufficient disposal of the concentrated solution. In this work, three polyethersulfone-type NF membranes were fabricated and their morphologies, functional groups, chemical components, wettabilities, chargeabilities, and rejection molecular weights were characterized. Among them, the membrane of NF_M3 bearing sulfonic acid group (  SO 3 H) shows the desirable rejection efficiency of nitrate (90.32%), acceptable solution filtration flux (10.15 L m −2 h −1 at 0.6 MPa), and the decent pure water permeation flux (50.8 L m −2 h −1 at 0.6 MPa). The introduction of  SO 3 H can significantly enhance the performances of NF_M3 membrane in hydrophilicity, chargeability, anti-fouling, and the oxidation resistance. Promisingly, the concentrated nitrate was ingeniously recycled for the electrochemical intercalation of flake graphite to prepare expandable graphite, and the obtained expanded graphite has an acceptable expanded volume (243 mL g −1 via the microwave expansion process), thereby achieving the resource utilization of NF concentrated solution.