Combing three-dimensional water channels and ultra-thin skin layer enable high flux and stability of loose polyimide/SiO2 nanofiltration membranes at low operating pressure via one step in-situ modification

Abstract

Desalination of brackish water and purification of drinking water still pose several challenges. Combination of in-situ hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in polyamic acid (PAA) solution along with imidization reaction provide a novel polyimide/SiO2 (PI/SiO2) membrane. This is the first synthesis of a PI nanofiltration (NF) membrane with three-dimensional (3D) water channels, ultra-thin skin layer (30 ± 2 nm), and high flux at low operating pressure, by one-step phase inversion method. There was no additional requirement of an acid, since PAA served as a catalyst. The SiO2 cross-linked network provided the skeletal support, due to hydrogen bonding between PAA-PI and silicon hydroxyl groups. The fabricated PI/SiO2 membrane demonstrated a water contact angle of 45°, pore size of 1.05 ± 0.15 nm, Zeta potential of −30 mV, and tensile strength of 2.24 MPa when under optimized conditions. The loose PI/SiO2 NF membrane exhibited salt rejection in the sequence of Na2SO4 (87%)>MgSO4 (67%)>NaCl (41%)>MgCl2 (37%), which was consistent with size exclusion and Donnan exclusion effects. In particular, the water flux of 20.4 L/(m2 h bar) was about 4–15 times higher than that of universal PI-NF membranes. The membrane also exhibited excellent stability for over six days when used for separation. Furthermore, the membrane showed excellent dye (Rose Bengal (RB), Brilliant Blue G (BBG), Amido Black (AB)) rejection of over 90%. To conclude, this work paves a new pathway for the fabrication of PI-NF membranes for efficient NF separation.