3-[[3-(Triethoxysilyl)-propyl] amino] propane-1-sulfonic acid zwitterion grafted polyvinylidene fluoride antifouling membranes for concentrating greywater in direct contact membrane distillation

Abstract

Direct contact membrane distillation (DCMD) is attractive for wastewater reuse where low-grade heat is highly abundant, but membrane fouling is of critical concerns. In this work, a zwitterion surface-grafted composite polyvinylidene fluoride membrane (PVDF-T) was introduced for DCMD treatment of greywater, a lightly polluted wastewater. Surface grafting was achieved by hydroxylation of a pristine PVDF membrane followed by silanization with the zwitterionomer 3-[[3-(triethoxysilyl)-propyl] amino] propane-1-sulfonic acid (TPAPS). The resulting composite PVDF-T membrane showed a significantly reduced water contact angle but nearly the same liquid entry pressure (LEP) indicating that silanization took place mostly at the surface, and the porous matrix remained hydrophobic. Elemental analysis confirmed that the TPAPS surface grafting was successful. Gas permeability tests indicated increased transport resistance, indicating the extra resistance imposed the zwitterionic modification. In concentrating the synthetic greywater, the PVDF-T membrane exhibited much more stable DCMD flux and constant permeate conductivity than the virgin PVDF membrane. Visual and scanning electron microscopy analysis after MD experiments showed that the PVDF-T membrane was fouled less than the pristine membrane. Improvement in surface hydrophilicity, electronegative charge and steric hindrance were ascribed as the main factors for the fouling resistance of a surface grafted zwitterionic composite PVDF membrane.