Pore size regulation of polyamide composite membrane via a sol-gel process confined within the selective layer

Membrane with pores of homogeneous size distribution is vital to high fraction efficiency of nanofiltration process. In this study, a novel approach of space-confined sol-gel was put forward to regulate the size and distribution of the pores of interfacially synthesized polyamide composite membrane for enhanced fraction efficiency. Tetraethyl orthosilicate (TEOS) was introduced into the porous polyamide network of the selective layer and employed to in situ generate a new silica network through space-confined hydrolysis and condensation. Membrane property including pore size distribution and separation performance was tuned by regulating the content of TEOS and systematically characterized by ATR-FTIR, XPS, SEM, AFM and cross-flow permeation test. The sol-gel of TEOS molecules confined within the selective layer was found to form a new polyamide-silica based separation layer with decreased pore size and homogenized pore size distribution. The mean size in diameter and geometric standard deviation of size distribution of the pores of the virgin polyamide based membrane were about 0.64 nm and 1.32, while those of the desired polyamide-silica based membrane obtained with a TEOS content of 3.0 v/v% were 0.59 nm and 1.26, respectively. The glucose rejection increased significantly from 58.0% of virgin polyamide membrane to 81.5% of the desired polyamide-silica based membrane. Permeation tests with concentrated mixed solution also revealed that, the desired polyamide-silica based membrane possessed better efficiency in the removal of sodium sulfate from the concentrated brine compared to the commercial membrane NF270, showing an average Na2SO4 rejection above 98.3%, a water flux around 120.0 l/m2 h, and nearly no NaCl rejection to a concentrated brine containing 200.0 g/L NaCl and 10.0 g/L Na2SO4 under 30.0 bar and 40.0 °C.